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Table of contents

The principle of relativity

The Principle of Relativity
Frames of reference, events and transformations
Special relativity
The postulates of special relativity


The spacetime interpretation of special relativity
The lightcone
The Lorentz transformation equations
The relativity of simultaneity and the Andromeda paradox
The twin paradox
Addition of velocities

Relativistic dynamics

Mass and energy

Light propagation and the aether

The aether drag hypothesis
The Michelson-Morley experiment

Mathematical approach to special relativity

Mathematical techniques
Linear Transformations
Indicial Notation
Analysis of curved surfaces and transformations
Four vectors
The Lorentz transformation
The linearity and homogeneity of spacetime
The Lorentz transformation
Length contraction, time dilation and phase
Hyperbolic geometry
Addition of velocities
Acceleration transformation
Relativistic dynamics
Relativistic Mass


Mathematics of the Lorentz Transformation Equations

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The Special Theory of Relativity is a theory of classical physics that was developed at the end of the nineteenth century and the beginning of the twentieth century. It changed our understanding of older physical theories such as Newtonian Physics and led to early Quantum Theory and later the Theory of General Relativity. Special Relativity is one of the foundation blocks of physics.

This book will introduce the reader to, perhaps, the most profound discovery of the twentieth century and the modern world: the universe has at least four dimensions.

Historical Development

Special Relativity is not a theory about light, it is a theory about space and time, but it was the strange behaviour of light that first alerted scientists to the possibility that the universe had an unexpected geometry. The short history of Special Relativity given here will start with light but will end with the discovery that the behaviour of light is related to the geometry of the universe.

In the nineteenth century it was widely accepted that light travelled as waves in a substance called the “aether”. Light was thought to travel in this aether in a similar way to how other types of waves travel in material substances; for example, sound waves travel in air (and other substances). Light would travel to our eyes as waves through the aether like sound travels to our ears as waves in the air.

The nature of the aether was unknown but a possible link between the aether and electrical and magnetic fields became apparent during the first half of the nineteenth century. Faraday demonstrated that the polarisation of light was affected by magnetic fields and Weber showed that electrical effects could be transmitted across non-conducting materials so there was a strong suggestion that light might be some sort of electromagnetic effect.

James Clerk Maxwell

In 1865 the Scottish physicist James Clerk Maxwell drew together the various experiments on electricity and magnetism into an electromagnetic theory of light based on the idea of an aether. He identified that electricity and magnetism, previously considered to be separate forces, were two aspects of one force. He was able to calculate the speed at which electromagnetic waves moved as a simple ratio between the strength of the electric field and the strength of the magnetic field. From this, one of his key observations was that electromagnetic effects seemed to propagate at nearly light speed. He wrote of the velocity of electrical interactions that:

“This velocity is so nearly that of light that it seems we have strong reason to conclude that light itself (including radiant heat and other radiations, if any) is an electromagnetic disturbance in the form of waves propagated through the electromagnetic field according to electromagnetic laws.”

Maxwell's theory explained radio, heat radiation, light and many other phenomena as electromagnetic waves travelling in an aether. The velocity of these waves depended upon the properties of the aether itself. Someone who was stationary within the aether would measure the speed of light to be constant as a result of the constant properties of the aether. A light ray going from one stationary observer to another in the aether would take the same amount of time to make the journey no matter which stationary observer measured it. However, although stationary observers would all observe the same velocity for light, moving observers would measure the velocity of light as the sum of their velocity relative to the aether and the velocity of light in the aether.

If space were indeed full of an aether then the motion of objects through this aether should be detectable by measuring the velocity of light rays. In practice it is difficult to measure the velocity of light with sufficient precision. Maxwell suggested that an instrument called an "interferometer" would provide the required accuracy. He proposed that if an interferometer were moved through the aether the addition of the velocity of the equipment to the velocity of the light in the aether would cause a distinctive interference pattern. Maxwell's idea was submitted as a letter to Nature in 1879 (posthumously).

Albert Michelson read Maxwell's paper and in 1887 Michelson and Morley performed an 'interferometer' experiment to test whether the observed velocity of light is indeed the sum of the speed of light in the aether and the velocity of the observer. Michelson and Morley discovered that the measured velocity of light did not change with the velocity of the observer. To everyone's surprise the experiment showed that the speed of light was independent of the speed of the destination or source of the light in the proposed aether.
Albert Abraham Michelson
How might this "null result" of the interferometer experiment be explained? How could the speed of light in a vacuum be constant for all observers no matter how they are moving themselves? It was possible that Maxwell's theory was correct but the theory about the way that velocities add together (known as Galilean Relativity) was wrong. Alternatively it was possible that Maxwell's theory was wrong and Galilean Relativity was correct. However, the most popular interpretation at the time was that both Maxwell and Galileo were correct and something was happening to the measuring equipment. Perhaps the instrument was being squeezed in some way by the aether or some other material effect was occurring.

Various physicists attempted to explain the Michelson and Morley experiment. George Fitzgerald (1889) and Hendrik Lorentz (1895) suggested that objects tend to contract along the direction of motion relative to the aether and Joseph Larmor (1897) and Hendrik Lorentz (1899) proposed that moving objects are contracted and that moving clocks run slow as a result of motion in the aether. Fitzgerald, Larmor and Lorentz's contributions to the analysis of light propagation are of huge importance because they produced the "Lorentz Transformation Equations". The Lorentz Transformation Equations were developed to describe how physical effects would need to change the length of the interferometer arms and the rate of clocks to account for the lack of change in interference fringes in the interferometer experiment. It took the rebellious streak in Einstein to realise that the equations could also be applied to changes in space and time itself.

Albert Einstein
By the late nineteenth century it was becoming clear that aether theories of light propagation were problematic. Any aether would have properties such as being massless, incompressible, entirely transparent, continuous, devoid of viscosity and nearly infinitely rigid. In 1905 Albert Einstein realised that Maxwell's equations did not require an aether. On the basis of Maxwell's equations he showed that the Lorentz Transformation was sufficient to explain that length contraction occurs and clocks appear to go slow provided that the old Galilean concept of how velocities add together was abandoned. Einstein's remarkable achievement was to be the first physicist to propose that Galilean relativity might only be an approximation to reality. He came to this conclusion by being guided by the Lorentz Transformation Equations themselves and noticing that these equations only contain relationships between space and time without any references to the properties of an aether.

In 1905 Einstein was on the edge of the idea that made relativity special. It remained for the mathematician Hermann Minkowski to provide the full explanation of why an aether was entirely superfluous. He announced the modern form of Special Relativity theory in an address delivered at the 80th Assembly of German Natural Scientists and Physicians on September 21, 1908. The consequences of the new theory were radical, as Minkowski put it:

"The views of space and time which I wish to lay before you have sprung from the soil of experimental physics, and therein lies their strength. They are radical. Henceforth space by itself, and time by itself, are doomed to fade away into mere shadows, and only a kind of union of the two will preserve an independent reality."

What Minkowski had spotted was that Einstein's theory was actually related to the theories in differential geometry that had been developed by mathematicians during the nineteenth century. Initially Minkowski's discovery was unpopular with many physicists including Poincaré, Lorentz and even Einstein. Physicists had become used to a thoroughly materialist approach to nature in which lumps of matter were thought to bounce off each other and the only events of any importance were those occurring at some universal, instantaneous, present moment. The possibility that the geometry of the world might include time as well as space was an alien idea. The possibility that phenomena such as length contraction could be due to the physical effects of spacetime geometry rather than the increase or decrease of forces between objects was as unexpected for physicists in 1908 as it is for the modern high school student. Einstein rapidly assimilated these new ideas and went on to develop General Relativity as a theory based on differential geometry but many of the earlier generation of physicists were unable to accept the new way of looking at the world.

The adoption of differential geometry as one of the foundations of relativity theory has been traced by Walter (1999). Walter's study shows that by the 1920's modern differential geometry had become the principal theoretical approach to relativity, replacing Einstein's original electrodynamic approach.

Henri Poincaré
It has become popular to credit Henri Poincaré with the discovery of the theory of Special Relativity, but Poincaré got many of the right answers for some of the wrong reasons. He even came up with a version of . In 1904 Poincaré had gone as far as to enunciate the "principle of relativity" in which "The laws of physical phenomena must be the same, whether for a fixed observer, as also for one dragged in a motion of uniform translation, so that we do not and cannot have any means to discern whether or not we are dragged in a such motion." Furthermore, in 1905 Poincaré coined the term "Lorentz Transformation" for the equation that explained the null result of the Michelson Morley experiment. Although Poincaré derived equations to explain the null result of the Michelson Morley experiment, his assumptions were still based upon an aether. It remained for Einstein to show that an aether was unnecessary.

It is also popular to claim that Special Relativity and aether theories such as those due to Poincaré and Lorentz are equivalent and only separated by Occam's Razor. This is not strictly true. Occam's Razor is used to separate a complex theory from a simple theory, the two theories being different. In the case of Poincare's and Lorentz's aether theories both contain the Lorentz Transformation which is already sufficient to explain the Michelson and Morley Experiment, length contraction, time dilation etc. without an aether. The aether theorists simply failed to notice that this is a possibility because they rejected spacetime as a concept for reasons of philosophy or prejudice. In Poincaré's case he rejected spacetime because of philosophical objections to the idea of spatial or temporal extension (see note 1).

It is curious that Einstein actually returned to thinking based on an aether for philosophical reasons similar to those that haunted Poincaré (See Granek 2001). The geometrical form of Special Relativity as formalised by Minkowski does not forbid action at a distance and this was considered to be dubious philosophically. This led Einstein, in 1920, to reintroduce some of Poincaré's ideas into the theory of General Relativity. Whether an aether of the type proposed by Einstein is truly required for physical theory is still an active question in physics. However, such an aether leaves the spacetime of Special Relativity almost intact and is a complex merger of the material and geometrical that would be unrecognised by 19th century theorists.

Intended Audience

This book presents special relativity (SR) from first principles and logically arrives at the conclusions. There will be simple diagrams and some thought experiments. Although the final form of the theory came to use Minkowski spaces and metric tensors, it is possible to discuss SR using nothing more than high school algebra. That is the method used here in the first half of the book. That being said, the subject is open to a wide range of readers. All that is really required is a genuine interest.

For a more mathematically sophisticated treatment of the subject, please refer to the Advanced Text in Wikibooks.

The book is designed to confront the way students fail to understand the relativity of simultaneity. This problem is well documented and described in depth in: Student understanding of time in special relativity: simultaneity and reference frames by Scherr et al.

What's so special?

The special theory was suggested in 1905 in Einstein's article "On the Electrodynamics of Moving Bodies", and is so called because it applies in the absence of non-uniform gravitational fields.

In search of a more complete theory, Einstein developed the general theory of relativity published in 1915. General relativity (GR), a more mathematically demanding subject, describes physics in the presence of gravitational fields.

The conceptual difference between the two is the model of spacetime used. Special relativity makes use of a Euclidean-like (flat) spacetime. GR lives in a spacetime that is generally not flat but curved, and it is this curvature which represents gravity. The domain of applicability for SR is not so limited, however. Spacetime can often be approximated as flat, and there are techniques to deal with accelerating special relativistic objects.

Common Pitfalls in Relativity

Here is a collection of common misunderstandings and misconceptions about SR. If you are unfamiliar with SR then you can safely skip this section and come back to it later. If you are an instructor, perhaps this can help you divert some problems before they start by bringing up these points during your presentation when appropriate.

Beginners often believe that special relativity is only about objects that are moving at high velocities. Strictly speaking, this is a mistake. Special relativity applies at all velocities but at low velocity the predictions of special relativity are almost identical to those of the Newtonian empirical formulae. As an object increases its velocity the predictions of relativity gradually diverge from Newtonian Mechanics.

There is sometimes a problem differentiating between the two different concepts "relativity of simultaneity" and "signal latency/delay." This book text differs from some other presentations because it deals with the geometry of spacetime directly and avoids the treatment of delays due to light propagation. This approach is taken because students would not be taught Euclid's geometry using continuous references to the equipment and methods used to measure lengths and angles. Continuous reference to the measurement process obscures the underlying geometrical theory whether the geometry is three dimensional or four dimensional.

If students do not grasp that, from the outset, modern Special Relativity proposes that the universe is four dimensional, then, like Poincaré, they will consider that the constancy of the speed of light is just an event awaiting a mechanical explanation and waste their time by pondering the sorts of mechanical or electrical effects that could adjust the velocity of light to be compatible with observation.

A Word about Wiki

This is a wikibook. That means it has great potential for improvement and enhancement. The improvement can be in the form of refined language, clear mathematics, simple diagrams, and better practice problems and answers. The enhancement can be in the form of artwork, historical context of SR, anything. Feel free to improve and enhance Special Relativity and other wikibooks as you see necessary.



  1. The modern philosophical objection to the spacetime of Special Relativity is that it acts on bodies without being acted upon, however, in General Relativity spacetime is acted upon by its content.

Further Reading

  • Feynman Lectures on Physics. Symmetry in Physical Laws. (World Student) Vol 1. Ch 52.
  • Gross, D.J. The role of symmetry in fundamental physics. PNAS December 10, 1996 vol. 93 no. 25 14256-14259 http://www.pnas.org/content/93/25/14256.full
  • Stephen Hawking and Leonard Mlodinow; The grand design book ; Chapter 5 The Theory of Everything.

The principle of relativity

The principle of relativity

Galileo Galilei
Principles of relativity address the relationship between observations made at different places. This problem has been a difficult theoretical challenge since the earliest times and involves physical questions such as how the velocities of objects can be combined and how influences are transmitted between moving objects.

One of the most fruitful approaches to this problem was the investigation of how observations are affected by the velocity of the observer. This problem had been tackled by classical philosophers but it was the work of Galileo that produced a real breakthrough. Galileo (1632), in his Dialogues Concerning the Two Chief World Systems, considered observations of motion made by people inside a ship who could not see the outside:

"have the ship proceed with any speed you like, so long as the motion is uniform and not fluctuating this way and that. You will discover not the least change in all the effects named, nor could you tell from any of them whether the ship was moving or standing still. "

According to Galileo, if the ship moved smoothly then someone inside it would be unable to determine whether they were moving. If people in Galileo's moving ship were eating dinner they would see their peas fall from their fork straight down to their plate in the same way as they might if they were at home on dry land. The peas move along with the people and do not appear to the diners to fall diagonally. This means that the peas continue in a state of uniform motion unless someone intercepts them or otherwise acts on them. It also means that simple experiments that the people on the ship might perform would give the same results on the ship or at home. This concept led to “Galilean Relativity” in which it was held that things continue in a state of motion unless acted upon and that the laws of physics are independent of the velocity of the laboratory.

This simple idea challenged the previous ideas of Aristotle. Aristotle had argued in his Physics[1] that objects must either be moved or be at rest. According to Aristotle, on the basis of complex and interesting arguments about the possibility of a 'void', objects cannot remain in a state of motion without something moving them. As a result Aristotle proposed that objects would stop entirely in empty space. If Aristotle were right the peas that you dropped whilst dining aboard a moving ship would fall in your lap rather than falling straight down on to your plate. Aristotle's idea had been believed by everyone so Galileo's new proposal was extraordinary and, because it was nearly right, became the foundation of physics.

Galilean Relativity contains two important principles: firstly it is impossible to determine who is actually at rest and secondly things continue in uniform motion unless acted upon. The second principle is known as Galileo’s Law of Inertia or Newton's First Law of Motion.

Special relativity

Until the nineteenth century it appeared that Galilean relativity treated all observers as equivalent no matter how fast they were moving. If you throw a ball straight up in the air at the North Pole it falls straight back down again and this also happens at the equator even though the equator is moving at almost a thousand miles an hour faster than the pole. Galilean velocities are additive so that the ball continues moving at a thousand miles an hour when it is thrown upwards at the equator and continues with this motion until it is acted on by an external agency.

This simple scheme became questioned in 1865 when James Clerk Maxwell discovered the equations that describe the propagation of electromagnetic waves such as light. His equations showed that the speed of light depended upon constants that were thought to be simple properties of a physical medium or “aether” that pervaded all space. If this were the case then, according to Galilean relativity, it should be possible to add your own velocity to the velocity of incoming light so that if you were travelling at a half the speed of light then any light approaching you would be observed to be travelling at 1.5 times the speed of light in the aether. Similarly, any light approaching you from behind would strike you at 0.5 times the speed of light in the aether. Light itself would always go at the same speed in the aether so if you shone a light from a torch whilst travelling at high speed the light would plop into the aether and slow right down to its normal speed. This would spoil Galileo's Relativity because all you would need to do to discover whether you were in a moving ship or on dry land would be to measure the speed of light in different directions. The light would go slower in your direction of travel through the aether and faster in the opposite direction.

If the Maxwell equations are valid and the simple classical addition of velocities applies then there should be a preferred reference frame, the frame of the stationary aether. The preferred reference frame would be considered the true zero point to which all velocity measurements could be referred.

Special relativity restored a principle of relativity in physics by maintaining that Maxwell's equations are correct but that classical velocity addition is wrong: there is no preferred reference frame. Special relativity brought back the interpretation that in all inertial reference frames the same physics is going on and there is no phenomenon that would allow an observer to pinpoint a zero point of velocity. Einstein preserved the principle of relativity by proposing that the laws of physics are the same regardless of the velocity of the observer. According to Einstein, whether you are in the hold of Galileo's ship or in the cargo bay of a space ship going at a large fraction of the speed of light the laws of physics will be the same.

Einstein's idea shared the same philosophy as Galileo's idea, both men believed that the laws of physics would be unaffected by motion at a constant velocity. In the years between Galileo and Einstein it was believed that it was the way velocities simply add to each other that preserved the laws of physics but Einstein adapted this simple concept to allow for Maxwell's equations.

Frames of reference, events and transformations

Before proceeding further with the analysis of relative motion the concepts of reference frames, events and transformations need to be defined more closely.

Physical observers are considered to be surrounded by a reference frame which is a set of coordinate axes in terms of which position or movement may be specified or with reference to which physical laws may be mathematically stated.

An event is something that happens independently of the reference frame that might be used to describe it. Turning on a light or the collision of two objects would constitute an event.

Suppose there is a small event, such as a light being turned on, that is at coordinates in one reference frame. What coordinates would another observer, in another reference frame moving relative to the first at velocity along the axis assign to the event? This problem is illustrated below:


What we are seeking is the relationship between the second observer's coordinates for the event and the first observer's coordinates for the event . The coordinates refer to the positions and timings of the event that are measured by each observer and, for simplicity, the observers are arranged so that they are coincident at . According to Galilean Relativity:


This set of equations is known as a Galilean coordinate transformation or Galilean transformation.

These equations show how the position of an event in one reference frame is related to the position of an event in another reference frame. But what happens if the event is something that is moving? How do velocities transform from one frame to another?

The calculation of velocities depends on Newton's formula: . The use of Newtonian physics to calculate velocities and other physical variables has led to Galilean Relativity being called Newtonian Relativity in the case where conclusions are drawn beyond simple changes in coordinates. The velocity transformations for the velocities in the three directions in space are, according to Galilean relativity:

where are the velocities of a moving object in the three directions in space recorded by the second observer and are the velocities recorded by the first observer and is the relative velocity of the observers. The minus sign in front of the means the moving object is moving away from both observers.

This result is known as the classical velocity addition theorem and summarises the transformation of velocities between two Galilean frames of reference. It means that the velocities of projectiles must be determined relative to the velocity of the source and destination of the projectile. For example, if a sailor throws a stone at 10 km/hr from Galileo's ship which is moving towards shore at 5 km/hr then the stone will be moving at 15 km/hr when it hits the shore.

In Newtonian Relativity the geometry of space is assumed to be Euclidean and the measurement of time is assumed to be the same for all observers.

The derivation of the classical velocity addition theorem is as follows.
If the Galilean transformations are differentiated with respect to time:


But in Galilean relativity, and so therefore:

If we write etc. then:

The postulates of special relativity

The previous section described transformations from one frame of reference to another, using the simple addition of velocities that was introduced in Galileo's time. These transformations are consistent with Galileo's main postulate, which was that the laws of physics would be the same for all inertial observers, so that no-one could tell who was at rest. Aether theories had threatened Galileo's postulate, because the aether would be at rest and observers could determine that they were at rest simply by measuring the speed of light in the direction of motion. Einstein preserved Galileo's fundamental postulate, that the laws of physics are the same in all inertial frames of reference. But to do so, he had to introduce a new postulate, that the speed of light would be the same for all observers. These postulates are listed below:

1. First postulate: the principle of relativity

Formally: the laws of physics are the same in all inertial frames of reference.

Informally: every physical theory should look the same mathematically to every inertial observer. Experiments in a physics laboratory in a spaceship or planet orbiting the sun and galaxy will give the same results, no matter how fast the laboratory is moving.

2. Second postulate: the invariance of the speed of light

Formally: the speed of light in free space is a constant in all inertial frames of reference.

Informally: the speed of light in a vacuum, commonly denoted c, is the same for all inertial observers; is the same in all directions; and does not depend on the velocity of the object emitting the light.

Using these postulates, Einstein was able to calculate how the observation of events depends upon the relative velocity of observers. He was then able to construct a theory of physics that led to predictions such as the equivalence of mass and energy and early quantum theory.

Einstein's formulation of the axioms of relativity is known as the electrodynamic approach to relativity. It has been superseded in most advanced textbooks by the space-time approach, in which the laws of physics themselves are due to symmetries in space-time and the constancy of the speed of light is a natural consequence of the existence of space-time. However, Einstein's approach is equally valid and represents a tour de force of deductive reasoning; one that provided the insights required for the modern treatment of the subject.

Einstein's Relativity - the electrodynamic approach

Einstein asked how the lengths and times that are measured by the observers might need to vary if both observers found that the speed of light was constant. He looked at the formulae for the velocity of light that would be used by the two observers, and , and asked what constants would need to be introduced to keep the measurement of the speed of light at the same value even though the relative motion of the observers meant that the axis was continually advancing. His working is shown in detail in the appendix. The result of this calculation is the Lorentz Transformation Equations:

Where the constant . These equations apply to any two observers in relative motion but note that the sign within the brackets changes according to the direction of the velocity - see the appendix.

The Lorentz Transformation is the equivalent of the Galilean Transformation with the added assumption that everyone measures the same velocity for the speed of light no matter how fast they are travelling. The speed of light is a ratio of distance to time (ie: metres per second) so for everyone to measure the same value for the speed of light the length of measuring rods, the length of space between light sources and receivers and the number of ticks of clocks must dynamically differ between the observers. So long as lengths and time intervals vary with the relative velocity of two observers (v) as described by the Lorentz Transformation the observers can both calculate the speed of light as the ratio of the distance travelled by a light ray divided by the time taken to travel this distance and get the same value.

Einstein's approach is "electrodynamic" because it assumes, on the basis of Maxwell's equations, that light travels at a constant velocity. As mentioned above, the idea of a universal constant velocity is strange because velocity is a ratio of distance to time. Do the Lorentz Transformation Equations hide a deeper truth about space and time? Einstein himself[1] gives one of the clearest descriptions of how the Lorentz Transformation equations are actually describing properties of space and time itself. His general reasoning is given below.

If the equations are combined they satisfy the relation:


Einstein (1920) describes how this can be extended to describe movement in any direction in space:


Equation (2) is a geometrical postulate about the relationship between lengths and times in the universe. It suggests that there is a constant s such that:

This equation was recognised by Minkowski as an extension of Pythagoras' Theorem (ie: ), such extensions being well known in early twentieth century mathematics. What the Lorentz Transformation is telling us is that the universe is a four dimensional spacetime and as a result there is no need for any "aether". (See Einstein 1920[1], appendices, for Einstein's discussion of how the Lorentz Transformation suggests a four dimensional universe but be cautioned that "imaginary time" has now been replaced by the use of "metric tensors").

Einstein's analysis shows that the x-axis and time axis of two observers in relative motion do not overlie each other, The equation relating one observer's time to the other observer's time shows that this relationship changes with distance along the x-axis ie:

This means that the whole idea of "frames of reference" needs to be re-visited to allow for the way that axes no longer overlie each other.

  1. a b Einstein, A. (1920). Relativity. The Special and General Theory. Methuen & Co Ltd 1920. Written December, 1916. Robert W. Lawson (Authorised translation). http://www.bartleby.com/173/

Inertial reference frames

The Lorentz Transformation for time involves a component which results in time measurements being different along the x-axis of relatively moving observers. This means that the old idea of a frame of reference that simply involves three space dimensions with a time that is in common between all of the observers no longer applies. To compare measurements between observers the concept of a "reference frame" must be extended to include the observer's clocks.

An inertial reference frame is a conceptual, three-dimensional latticework of measuring rods set at right angles to each other with clocks at every point that are synchronised with each other (see below for a full definition). An object that is part of, or attached to, an inertial frame of reference is defined as an object which does not disturb the synchronisation of the clocks and remains at a constant spatial position within the reference frame. The inertial frame of reference that has a moving, non-rotating body attached to it is known as the inertial rest frame for that body. An inertial reference frame that is a rest frame for a particular body moves with the body when observed by observers in relative motion.


This type of reference frame became known as an "inertial" frame of reference because, as will be seen later in this book, each system of objects that are co-moving according to Newton's law of inertia (without rotation, gravitational fields or forces acting) have a common rest frame, with clocks that differ in synchronisation and rods that differ in length, from those in other, relatively moving, rest frames.

There are many other definitions of an "inertial reference frame" but most of these, such as "an inertial reference frame is a reference frame in which Newton's First Law is valid" do not provide essential details about how the coordinates are arranged and/or represent deductions from more fundamental definitions.

The following definition by Blandford and Thorne(2004)[1] is a fairly complete summary of what working physicists mean by an inertial frame of reference:

"An inertial reference frame is a (conceptual) three-dimensional latticework of measuring rods and clocks with the following properties: (i ) The latticework moves freely through spacetime (i.e., no forces act on it), and is attached to gyroscopes so it does not rotate with respect to distant, celestial objects. (ii ) The measuring rods form an orthogonal lattice and the length intervals marked on them are uniform when compared to, e.g., the wavelength of light emitted by some standard type of atom or molecule; and therefore the rods form an orthonormal, Cartesian coordinate system with the coordinate x measured along one axis, y along another, and z along the third. (iii ) The clocks are densely packed throughout the latticework so that, ideally, there is a separate clock at every lattice point. (iv ) The clocks tick uniformly when compared, e.g., to the period of the light emitted by some standard type of atom or molecule; i.e., they are ideal clocks. (v) The clocks are synchronized by the Einstein synchronization process: If a pulse of light, emitted by one of the clocks, bounces off a mirror attached to another and then returns, the time of bounce as measured by the clock that does the bouncing is the average of the times of emission and reception as measured by the emitting and receiving clock:

¹For a deeper discussion of the nature of ideal clocks and ideal measuring rods see, e.g., pp. 23-29 and 395-399 of Misner, Thorne, and Wheeler (1973)."

Special Relativity demonstrates that the inertial rest frames of objects that are moving relative to each other do not overlay one another. Each observer sees the other, moving observer's, inertial frame of reference as distorted. This discovery is the essence of Special Relativity and means that the transformation of coordinates and other measurements between moving observers is complicated. It will be discussed in depth below.


  1. Blandford, R.D. and Thorne, K.S.(2004). Applications of Classical Physics. California Institute of Technology. See: http://www.pma.caltech.edu/Courses/ph136/yr2004/


The modern approach to relativity

Although the special theory of relativity was first proposed by Einstein in 1905, the modern approach to the theory depends upon the concept of a four-dimensional universe, that was first proposed by Hermann Minkowski in 1908.

Minkowski's contribution appears complicated but is simply an extension of Pythagoras' Theorem:

In 2 dimensions:

In 3 dimensions:

in 4 dimensions:

(where k = )

The modern approach uses the concept of invariance to explore the types of coordinate systems that are required to provide a full physical description of the location and extent of things. The modern theory of special relativity begins with the concept of "length". In everyday experience, it seems that the length of objects remains the same no matter how they are rotated or moved from place to place. We think that the simple length of a thing is "invariant". However, as is shown in the illustrations below, what we are actually suggesting is that length seems to be invariant in a three-dimensional coordinate system.


The length of a thing in a two-dimensional coordinate system is given by Pythagoras's theorem:

This two-dimensional length is not invariant if the thing is tilted out of the two-dimensional plane. In everyday life, a three-dimensional coordinate system seems to describe the length fully. The length is given by the three-dimensional version of Pythagoras's theorem:

The derivation of this formula is shown in the illustration below.


It seems that, provided all the directions in which a thing can be tilted or arranged are represented within a coordinate system, then the coordinate system can fully represent the length of a thing. However, it is clear that things may also be changed over a period of time. Time is another direction in which things can be arranged. This is shown in the following diagram:


The length of a straight line between two events in space and time is called a "space-time interval".

In 1908 Hermann Minkowski pointed out that if things could be rearranged in time, then the universe might be four-dimensional. He boldly suggested that Einstein's recently-discovered theory of Special Relativity was a consequence of this four-dimensional universe. He proposed that the space-time interval might be related to space and time by Pythagoras' theorem in four dimensions:

Where i is the imaginary unit (sometimes imprecisely called ), c is a constant, and t is the time interval spanned by the space-time interval, s. The symbols x, y and z represent displacements in space along the corresponding axes. In this equation, the 'second' becomes just another unit of length. In the same way as centimetres and inches are both units of length related by centimetres = 'conversion constant' times inches, metres and seconds are related by metres = 'conversion constant' times seconds. The conversion constant, c has a value of about 300,000,000 meters per second. Now is equal to minus one, so the space-time interval is given by:

Minkowski's use of the imaginary unit has been superseded by the use of advanced geometry that uses a tool known as the "metric tensor". The metric tensor permits the existence of "real" time and the negative sign in the expression for the square of the space-time interval originates in the way that distance changes with time when the curvature of spacetime is analysed (see advanced text). We now use real time but Minkowski's original equation for the square of the interval survives so that the space-time interval is still given by:

Space-time intervals are difficult to imagine; they extend between one place and time and another place and time, so the velocity of the thing that travels along the interval is already determined for a given observer.

If the universe is four-dimensional, then the space-time interval (rather than the spatial length) will be invariant. Whoever measures a particular space-time interval will get the same value, no matter how fast they are travelling. In physical terminology the invariance of the spacetime interval is a type of Lorentz Invariance. The invariance of the spacetime interval has some dramatic consequences.

The first consequence is the prediction that if a thing is travelling at a velocity of c metres per second, then all observers, no matter how fast they are travelling, will measure the same velocity for the thing. The velocity c will be a universal constant. This is explained below.

When an object is travelling at c, the space time interval is zero, this is shown below:

The distance travelled by an object moving at velocity v in the x direction for t seconds is:
If there is no motion in the y or z directions the space-time interval is
But when the velocity v equals c:
And hence the space time interval

A space-time interval of zero only occurs when the velocity is c (if x>0). All observers observe the same space-time interval so when observers observe something with a space-time interval of zero, they all observe it to have a velocity of c, no matter how fast they are moving themselves.

The universal constant, c, is known for historical reasons as the "speed of light in a vacuum". In the first decade or two after the formulation of Minkowski's approach many physicists, although supporting Special Relativity, expected that light might not travel at exactly c, but might travel at very nearly c. There are now few physicists who believe that light in a vacuum does not propagate at c.

The second consequence of the invariance of the space-time interval is that clocks will appear to go slower on objects that are moving relative to you. Suppose there are two people, Bill and John, on separate planets that are moving away from each other. John draws a graph of Bill's motion through space and time. This is shown in the illustration below:


Being on planets, both Bill and John think they are stationary, and just moving through time. John spots that Bill is moving through what John calls space, as well as time, when Bill thinks he is moving through time alone. Bill would also draw the same conclusion about John's motion. To John, it is as if Bill's time axis is leaning over in the direction of travel and to Bill, it is as if John's time axis leans over.

John calculates the length of Bill's space-time interval as:
whereas Bill doesn't think he has travelled in space, so writes:

The space-time interval, , is invariant. It has the same value for all observers, no matter who measures it or how they are moving in a straight line. Bill's equals John's so:


So, if John sees Bill measure a time interval of 1 second () between two ticks of a clock that is at rest in Bill's frame, John will find that his own clock measures between these same ticks an interval , called coordinate time, which is greater than one second. It is said that clocks in motion slow down, relative to those on observers at rest. This is known as "relativistic time dilation of a moving clock". The time that is measured in the rest frame of the clock (in Bill's frame) is called the proper time of the clock.

John will also observe measuring rods at rest on Bill's planet to be shorter than his own measuring rods, in the direction of motion. This is a prediction known as "relativistic length contraction of a moving rod". If the length of a rod at rest on Bill's planet is , then we call this quantity the proper length of the rod. The length of that same rod as measured from John's planet, is called coordinate length, and given by


This equation can be derived directly and validly from the time dilation result with the assumption that the speed of light is constant.

The last consequence is that clocks will appear to be out of phase with each other along the length of a moving object. This means that if one observer sets up a line of clocks that are all synchronised so they all read the same time, then another observer who is moving along the line at high speed will see the clocks all reading different times. In other words observers who are moving relative to each other see different events as simultaneous. This effect is known as Relativistic Phase or the Relativity of Simultaneity. Relativistic phase is often overlooked by students of Special Relativity, but if it is understood then phenomena such as the twin paradox are easier to understand.

The way that clocks go out of phase along the line of travel can be calculated from the concepts of the invariance of the space-time interval and length contraction.


In the diagram above John is conventionally stationary. Distances between two points according to Bill are simple lengths in space (x) all at t=0 whereas John sees Bill's measurement of distance as a combination of a distance (X) and a time interval (T):

Notice that the quantities represented by capital letters are proper lengths and times and in this example refer to John's measurements.

Bill's distance, x, is the length that he would obtain for things that John believes to be X metres in length. For Bill it is John who has rods that contract in the direction of motion so Bill's determination "x" of John's distance "X" is given from:


This relationship between proper and coordinate lengths was seen above to relate Bill's proper lengths to John's measurements. It also applies to how Bill observes John's proper lengths.


Clocks that are synchronised for one observer go out of phase along the line of travel for another observer moving at metres per second by : seconds for every metre. This is one of the most important results of Special Relativity and should be thoroughly understood by students.

The net effect of the four-dimensional universe is that observers who are in motion relative to you seem to have time coordinates that lean over in the direction of motion and consider things to be simultaneous that are not simultaneous for you. Spatial lengths in the direction of travel are shortened, because they tip upwards and downwards, relative to the time axis in the direction of travel, akin to a rotation out of three-dimensional space.


What velocity would cause events A and B to be simultaneous?
An observer records an event A next to herself then an event B a millisecond later and 600 kilometres away. How fast would another observer need to travel in the direction of event B to record it as simultaneous with event A?
The phase difference between moving clocks is given by:
The velocity needed to create a phase difference of 10-3 secs is:
This means that another observer who is travelling at 0.5c relative to the first observer in the direction of the line joining events A and B will consider that these events are simultaneous.

Interpreting space-time diagrams

Great care is needed when interpreting space-time diagrams. Diagrams present data in two dimensions, and cannot show faithfully how, for instance, a zero length space-time interval appears.


When diagrams are used to show both space and time it is important to be alert to space and time being related by Minkowski's equation and not by simple Euclidean geometry. The diagrams are only aids to understanding the approximate relation between space and time and it must not be assumed, for instance, that simple trigonometric relationships can be used to relate lines that represent spatial displacements and lines that represent temporal displacements.

It is sometimes mistakenly held that the time dilation and length contraction results only apply for observers at x=0 and t=0. This is untrue. An inertial frame of reference is defined so that length and time comparisons can be made anywhere within a given reference frame.

Time dilation applies to time measurements taken between corresponding planes of simultaneity

Time differences in one inertial reference frame can be compared with time differences anywhere in another inertial reference frame provided it is remembered that these differences apply to corresponding pairs of lines or pairs of planes of simultaneous events.


Spacetime diagram showing an event, a world line, and a line of simultaneity.

In order to gain an understanding of both Galilean and Special Relativity it is important to begin thinking of space and time as being different dimensions of a four-dimensional vector space called spacetime. Actually, since we can't visualize four dimensions very well, it is easiest to start with only one space dimension and the time dimension. The figure shows a graph with time plotted on the vertical axis and the one space dimension plotted on the horizontal axis. An event is something that occurs at a particular time and a particular point in space. ("Julius X. wrecks his car in Lemitar, NM on 21 June at 6:17 PM.") A world line is a plot of the position of some object as a function of time (more properly, the time of the object as a function of position) on a spacetime diagram. Thus, a world line is really a line in spacetime, while an event is a point in spacetime. A horizontal line parallel to the position axis (x-axis) is a line of simultaneity; in Galilean Relativity all events on this line occur simultaneously for all observers. It will be seen that the line of simultaneity differs between Galilean and Special Relativity; in Special Relativity the line of simultaneity depends on the state of motion of the observer.

In a spacetime diagram the slope of a world line has a special meaning. Notice that a vertical world line means that the object it represents does not move -- the velocity is zero. If the object moves to the right, then the world line tilts to the right, and the faster it moves, the more the world line tilts. Quantitatively, we say that


Notice that this works for negative slopes and velocities as well as positive ones. If the object changes its velocity with time, then the world line is curved, and the instantaneous velocity at any time is the inverse of the slope of the tangent to the world line at that time.

The hardest thing to realize about spacetime diagrams is that they represent the past, present, and future all in one diagram. Thus, spacetime diagrams don't change with time -- the evolution of physical systems is represented by looking at successive horizontal slices in the diagram at successive times. Spacetime diagrams represent the evolution of events, but they don't evolve themselves.

The lightcone

Things that move at the speed of light in our four dimensional universe have surprising properties. If something travels at the speed of light along the x-axis and covers x meters from the origin in t seconds the space-time interval of its path is zero.

but so:

Extending this result to the general case, if something travels at the speed of light in any direction into or out from the origin it has a space-time interval of 0:

This equation is known as the Minkowski Light Cone Equation. If light were travelling towards the origin then the Light Cone Equation would describe the position and time of emission of all those photons that could be at the origin at a particular instant. If light were travelling away from the origin the equation would describe the position of the photons emitted at a particular instant at any future time 't'.


At the superficial level the light cone is easy to interpret. Its backward surface represents the path of light rays that strike a point observer at an instant and its forward surface represents the possible paths of rays emitted from the point observer. Things that travel along the surface of the light cone are said to be light- like and the path taken by such things is known as a null geodesic.

Events that lie outside the cones are said to be space-like or, better still space separated because their space time interval from the observer has the same sign as space (positive according to the convention used here). Events that lie within the cones are said to be time-like or time separated because their space-time interval has the same sign as time.

However, there is more to the light cone than the propagation of light. If the added assumption is made that the speed of light is the maximum possible velocity then events that are space separated cannot affect the observer directly. Events within the backward cone can have affected the observer so the backward cone is known as the "affective past" and the observer can affect events in the forward cone hence the forward cone is known as the "affective future".

The assumption that the speed of light is the maximum velocity for all communications is neither inherent in nor required by four dimensional geometry although the speed of light is indeed the maximum velocity for objects if the principle of causality is to be preserved by physical theories (ie: that causes precede effects).

The Lorentz transformation equations

The discussion so far has involved the comparison of interval measurements (time intervals and space intervals) between two observers. The observers might also want to compare more general sorts of measurement such as the time and position of a single event that is recorded by both of them. The equations that describe how each observer describes the other's recordings in this circumstance are known as the Lorentz Transformation Equations. (Note that the symbols below signify coordinates.)


The table below shows the Lorentz Transformation Equations.

See mathematical derivation of Lorentz transformation.

Notice how the phase ( (v/c2)x ) is important and how these formulae for absolute time and position of a joint event differ from the formulae for intervals.

A spacetime representation of the Lorentz Transformation

Spacetime representation of Lorentz Transformation for time
Bill and John are moving at a relative velocity, v, and synchronise clocks when they pass each other. Both Bill and John observe an event along Bill's direction of motion. What times will Bill and John assign to the event? It was shown above that the relativistic phase was given by: . This means that Bill will observe an extra amount of time elapsing on John's time axis due to the position of the event. Taking phase into account and using the time dilation equation Bill is going to observe that the amount of time his own clocks measure can be compared with John's clocks using:


This relationship between the times of a common event between reference frames is known as the Lorentz Transformation Equation for time.

Simultaneity, time dilation and length contraction

More about the relativity of simultaneity

Most physical theories assume that it is possible to synchronise clocks. If you set up an array of synchronised clocks over a volume of space and take a snapshot of all of them simultaneously, you will find that the one closest to you will appear to show a later time than the others, due to the time light needs to travel from each of the distant clocks towards you. However, if the correct clock positions are known, by taking the transmission time of light into account, one can easily compensate for the differences and synchronise the clocks properly. The possibility of truly synchronising clocks exists because the speed of light is constant and this constant velocity can be used in the synchronisation process (the use of the predictable delays when light is used for synchronising clocks is known as "Einstein synchronisation").

The Lorentz transformation for time compares the readings of synchronised clocks at any instant. It compares the actual readings on clocks allowing for any time delay due to transmitting information between observers and answers the question "what does the other observer's clock actually read now, at this moment". The answer to this question is shocking. The Lorentz transformation for time shows that the clocks in any frame of reference moving relative to you cease to be synchronised!


The desynchronisation between relatively moving observers is illustrated below with a simpler diagram:


The effect of the relativity of simultaneity is for each observer to consider that a different set of events is simultaneous. The relativistic phase difference between clocks ("relativistic phase") means that observers who are moving relative to each other have different sets of things that are simultaneous, or in their "present moment". It is this discovery that time is no longer absolute that profoundly unsettles many students of relativity.


The amount by which the clocks differ between two observers depends upon the distance of the clock from the observer (). Notice that if both observers are part of inertial frames of reference with clocks that are synchronised at every point in space then the phase difference can be obtained by simply reading the difference between the clocks at the distant point and clocks at the origin. This difference will have the same value for both observers.

Example: flashing lights

Two lights that are stationary with respect to each other are separated by 3000 metres. The lights flash one after another with a time interval of 4 microseconds between the flashes. How fast would you need to travel along the line of the lights to see the lights flashing simultaneously?

This problem is a straightforward example of relativistic phase, events that are simultaneous for a moving observer being successive events in their own frame of reference. The phase difference between clocks with distance is given by:
Two successive flashes


Dividing by to express as a fraction of the speed of light gives:



(See the "explosion" example in Scherr et al. (2001))

Discuss the relationship between the times that a stationary and a moving observer will assign to the flashes in the example: the Lorentz transformation for the time of an event is , it is composed of an elapsed time, and a phase difference, . The flashing lights example is arranged so that when the observers in the two inertial frames of reference pass each other at the first flash and so that and is the time of the second flash.

If then so, according to the Lorentz transformation the moving observer finds events to be simultaneous in the stationary inertial reference frame when , ie: when the elapsed time in the stationary frame is equal to the phase difference between the frames.

Scherr, R.E., Shaffer, P.S. and Vokos, S. Student understanding of time in special relativity: simultaneity and reference frames. Physics Education Research, American Journal of Physics Supplement, 69, S24-35 (2001)

Example: Relativistic Bug Capture

How fast would a 1000 m long spaceship need to travel to observe a nanosecond difference between stationary clocks aligned with its bow and stern?

From :

Therefore or 90 km per second.

This effect could be used to bring together bugs of different ages that are twins in their own inertial frame of reference:

How the relativistic phase difference between clocks in moving and stationary inertial frames provides a moving observer with simultaneous access to different times in the stationary frame.

Bill travels past Jim in a very long spaceship and simultaneously captures two bugs, one coincident with the bow of the spaceship and one coincident with the stern. Although the bugs are twins in their own frame of reference they are different ages in Bill's reference frame. The captured bugs are free to wander up and down Bill's spaceship and could meet each other.

What length of spaceship travelling at 90 km per second would be needed to capture the bugs in the picture?

The Andromeda Paradox

What do we mean when we say that events are occurring "now"? If we are looking out over a cityscape, watching the traffic, lots of things appear to be happening all at once. We can take a snapshot with a camera and the scene on the photo consists of all those things that happened at very nearly the same time. They are only "very nearly" at the same time because the events on the photo that were furthest away actually occurred slightly earlier than those that were nearby because of the time taken for light to reach the camera. If we want to discover the events that really happened at the same time we would need to subtract the time taken for the light to get to us. This would be highly necessary if you were observing events on the Moon: if you were on the Earth and saw the time on a lunar clock you would know that the real time on the moon was more than a second later. But would this be enough? What about the relativistic phase differences between clocks due to motion?

Special Relativity introduces yet another factor, in addition to the travel time of light, that upsets our knowledge of which events are simultaneous. The relativistic phase differences between clocks are tiny at the distance of the moon but have the startling consequence that at distances as large as our separation from nearby galaxies an observer who is driving on the earth can have a radically different set of events that are simultaneous with her "present moment" from another person who is standing on the earth. The classic example of this effect of relativistic phase is the "Andromeda Paradox", also known as the "Rietdijk-Putnam-Penrose" argument. Penrose described the argument:

"Two people pass each other on the street; and according to one of the two people, an Andromedean space fleet has already set off on its journey, while to the other, the decision as to whether or not the journey will actually take place has not yet been made. How can there still be some uncertainty as to the outcome of that decision? If to either person the decision has already been made, then surely there cannot be any uncertainty. The launching of the space fleet is an inevitability." (Penrose 1989).

The argument is illustrated below:


Notice that neither observer can actually "see" what is happening on Andromeda now. The argument is not about what can be "seen", it is purely about what different observers consider to be contained in their instantaneous present moment. The two observers observe the same, two million year old events in their telescopes but the moving observer must assume that events at the present moment on Andromeda are a day or two in advance of those in the present moment of the stationary observer. (Incidentally, the two observers see the same events in their telescopes because length contraction of the distance from Earth to Andromeda compensates exactly for the time difference on Andromeda.)

This "paradox" has generated considerable philosophical debate on the nature of time and free-will. The advanced text of this book provides a discussion of some of the issues surrounding this geometrical interpretation of special relativity.

A result of the relativity of simultaneity is that if the car driver launches a space rocket towards the Andromeda galaxy it might have a several days head start compared with a space rocket launched from the ground. This is because the "present moment" for the moving car driver is progressively advanced with distance compared with the present moment on the ground. The present moment for the car driver is shown in the illustration below:


The net effect of the Andromeda paradox is that when someone is moving towards a distant point there are later events at that point than for someone who is not moving towards the distant point. There is a time gap between the events in the present moment of the two people.

The twin paradox

The "Twin Paradox" derives from an article by Langevin (1911) who used travel to a distant star and back to describe the relationships between times in different inertial reference frames. Langevin's original example was called the "Clock Paradox" and showed that a space traveller who travels to a distant star and back finds that he has aged less than the people who stay on Earth. Benguigui (2012) provides a detailed account of how, over time, this example became the story of two twins, one who travels out into space and one who stays at home, and how it became described by Weyl in 1922 as the "Twin Paradox". The "Twin Paradox" is an interesting example of the relativity of simultaneity and time dilation and deserves close study so that these features of Special Relativity can be understood. However, be warned, the reason that the "twin paradox" has attracted so much puzzlement is that, although superficially a simple problem, the analysis of the various inertial frames of reference is complex.

A one way trip

The twin "paradox" consists of two journeys, an outbound journey and a return journey. Much can be learnt about the relativity of simultaneity by considering just the outbound journey without any return. The single journey without any return might consist of the following scenario: Jim stays at home on Earth and Bill goes off in a spaceship, Bill flies past Jim at a velocity of 0.8c, they both set all of their clocks to zero as they pass each other and Bill flies straight to Mars where he drops off a record of his clock reading. Jim, who stays on Earth, finds that Bill's clocks record less time than his own for the journey.

The journey to Mars consists of two inertial frames of reference. In Jim's frame of reference Bill is moving and Jim is stationary. In Bill's frame of reference Jim, Earth and Mars move and Bill is stationary. There are only two reference frames so questions such as "what does Jim observe if he considers himself to be moving" are equivalent to asking "how does Jim move in Bill's frame of reference?".

The start of the journey is shown below:

Jim and Bill at the start of the journey

Suppose for ease of calculation Mars is assumed to be 100 light seconds away from Earth. Jim's view of the journey is straightforward and is shown below:

Jim's view of the journey from Earth to Mars

If Mars is 100 light seconds away, Jim will time Bill's journey to take 125 seconds (). Relativistic time dilation will cause Bill's clocks to read 75 seconds ie:

Bill's clocks will read:

where gamma is:

so seconds.

In Jim's inertial frame of reference Jim has aged by 125 seconds when Bill gets to Mars but Bill, as a result of time dilation, has only aged by 75 seconds.

The reason that this lack of aging by Bill seems to be paradoxical is that it might be thought that Jim must also have aged less than Bill, after all, Jim travels away from Bill as much as Bill travels away from Jim so Bill should find Jim to have aged less than himself in the same way as Jim finds Bill to have aged less than himself. In fact there is no paradox because Bill and Jim have very different ideas of the journey. Bill's view of the journey is shown below.

Bill's view of the journey from Earth to Mars

As a result of relativistic phase, Bill finds that the clocks on Mars start the journey 80 seconds ahead of his own. The Martian clocks record another 45 seconds for the journey. The 45 seconds elapsed time for the journey is what Bill expects when time dilation is taken into account (ie: seconds). More surprising still, when Bill considers Jim's clock readings back on Earth he finds that only 45 seconds have elapsed on Jim's earth-based clocks during the journey because the 80 seconds of relativistic phase difference mean that Bill's clocks as he passes Mars are simultaneous with events on Earth that are only 45 seconds later than when he passed Earth.

If Bill had an absurdly long rocket he could use a version of "relativistic bug capture" (see above) to net one of Jim's clocks the moment Bill reaches Mars. Bill could then show there was a Jim on Earth whose clocks had only progressed by 45 seconds for Bill's journey in Bill's inertial frame of reference. If Bill had captured one of Jim's clocks, the Jim who is simultaneous in his own frame of reference with the moment that Bill passes Mars would remember a giant net coming down from the tail of a spaceship and taking one of his clocks 45 seconds after Bill passed him.

Notice that there are two Jims in this story, one who is simultaneous with the Mars that Bill visits and another who is simultaneous with Bill in Bill's frame of reference. The Jim who is simultaneous with Bill in Bill's frame of reference would be an earlier Jim than the Jim who is simultaneous in his own frame of reference with Mars when Bill arrives.

The one way journey is symmetrical in the sense that Jim observes Bill to age less than himself during the journey and Bill also observes an earlier Jim to have aged less than himself.

The one way journey would become fully symmetrical if Bill continued past Mars until his clocks read that 125 seconds had elapsed, at this point he would assess that Jim had experienced 75 seconds of elapsed time. When Jim's clock's read 125 seconds Jim finds Bill's clocks read 75 seconds and, symmetrically, when Bill's clocks read 125 seconds Bill finds that Jim's clocks read 75 seconds. This symmetry is emphasised in the schematic diagram below which compares Jim and Bill's clock readings:

Schematic of Jim and Bill's clock readings

What happens when there is a change of velocity?

In the previous section it was shown that time dilation is symmetrical when observers separate at a constant velocity. The symmetry is evident in the way that, during the one way journey, Jim observes Bill's clocks to go slow and Bill also observes Jim's clocks to go slow. The symmetry between the observers means that Bill can regard himself as stationary and observing Jim departing or vice versa.

Jim and Bill's view of the turn

Bill finds that there are two Jims at two separate times involved in going to Mars then suddenly changing direction to return to Earth: these are labelled A and C in the diagram. The first Jim has clocks that read 45 seconds since Bill passed Earth, this is the Jim who is simultaneous with Bill in Bill's frame of reference as Bill passes Mars. The second Jim has clocks that read 205 seconds since meeting Bill, this Jim is simultaneous with Bill, in Bill's frame of reference, the moment after he has turned around at Mars and reached a velocity of -0.8c for the return journey.

The lack of any single Jim that is simultaneous with Bill when Bill changes velocity introduces an asymmetry into Special Relativity. Bill turning around at Mars to come back to Earth is not equivalent to Earth turning to meet Bill because it is not clear, until Bill has made the turn, which Earth and which Jim is making the journey. Langevin (1911), who first proposed the example of a traveller departing and returning younger, was well aware of the asymmetry and stated that: "Thus the asymmetry – which occurred because only the traveler, in the middle of his journey, has undergone an acceleration that changes the direction of his velocity".

In Special Relativity the laws of physics are the same for each observer in an inertial frame of reference. An inertial frame of reference might be all the clocks and measuring rods in a room on board a ship or in an entire city, the crucial feature of an inertial reference frame being that the clocks and measuring rods are stationary with respect to each other. Motions are measured so that the rest of the universe moves relative to the observer's inertial frame of reference. There are three inertial frames of reference in the example of Bill going to Mars and returning to Earth: Jim, outbound Bill and inbound Bill. Outbound and inbound Bill are separated by a period of varying velocity when Bill turns around. This period of changing velocity can also be regarded as a non-inertial hiatus in Bill's single inertial reference frame after which the relations between Bill's clocks and measuring rods and those in the rest of the universe have changed.

Questions such as "how does the Jim who is simultaneous with Bill after the turn view events if he regards himself as moving towards Bill?" are really about the inbound Bill's reference frame. In Jim's own inertial frame of reference Bill just goes to Mars, turns round and comes back again. Jim always regards himself as stationary unless his reference frame becomes non-inertial by experiencing a change in velocity in which case Jim would regard himself as moving from one stationary state to another that has a different set of relationships with the universe. Special Relativity holds that the laws of physics are the same in all inertial reference frames, it does not hold that all motion is relative, even in non-inertial changes.

The gap in time between the Jim who is simultaneous with Bill as Bill reaches Mars and the Jim who is simultaneous with Bill as he starts the journey back to Earth is known as the "Time Gap". The time gap in the "twin paradox" consists of the sum of the outgoing and incoming phase differences and in this case the time gap is 160 seconds .

The time gap description of the "twin paradox"

Once it is accepted that Bill and Jim have very different views of the journey these views can be summarised in the "Time Gap" description of the journey. In this description Bill flies to Mars and discovers that the clocks there are reading a later time than his own clock. He turns round to fly back to Earth and realises that the relativity of simultaneity means that, for Bill, the clocks on Earth will have jumped forward and are ahead of those on Mars, yet another "time gap" appears. When Bill gets back to Earth the time gaps and time dilations mean that people on Earth have recorded more clock ticks that he did.

For ease of calculation suppose that Bill is moving at a truly astonishing velocity of 0.8c in the direction of a distant point that is 100 light seconds away (about 30 million kilometres). The illustration below shows Jim and Bill's observations:


From Bill's viewpoint there is both a time dilation and a phase effect. It is the added factor of "phase" that explains why, although the time dilation occurs for both observers, Bill observes the same readings on Jim's clocks over the whole journey as does Jim.

To summarise the mathematics of the twin paradox using the example:

Jim observes the distance as 100 light seconds and the distant point is in his frame of reference. According to Jim it takes Bill the following time to make the journey:

Time taken = distance / velocity therefore according to Jim:
Again according to Jim, time dilation should affect the observed time on Bill's clocks:

So for Jim the round trip takes 250 secs and Bill's clock reads 150 secs.

Bill measures the distance as:

light seconds.
For Bill it takes seconds.

Bill observes Jim's clocks to appear to run slow as a result of time dilation:


But there is also a time gap of seconds.

So for Bill, Jim's clocks register 125 secs have passed from the start to the distant point. This is composed of 45 secs elapsing on Jim's clocks at the turn round point plus an 80 secs time gap from the start of the journey. Bill sees 250 secs total time recorded on Jim's clocks over the whole journey, this is the same time as Jim observes on his own clocks.

Further reading:

Benguigui, L (2012). A tale of two twins. arXiv:1212.4414v1

Bohm, D. The Special Theory of Relativity (W. A. Benjamin, 1965).

D’Inverno, R. Introducing Einstein’s Relativity (Oxford University Press, 1992).

Eagle, A. A note on Dolby and Gull on radar time and the twin "paradox". American Journal of Physics. 2005, VOL 73; NUMB 10, pages 976-978. http://arxiv.org/PS_cache/physics/pdf/0411/0411008v2.pdf

Langevin, P. (1911) L'Evolution de l'espace et du temps Scientia 10 (1911), 31-34

The nature of length contraction

According to special relativity items such as measuring rods consist of events distributed in space and time and a three dimensional rod is the events that compose the rod at a single instant. However, from the relativity of simultaneity it is evident that two observers in relative motion will have different sets of events that are present at a given instant. This means that two observers moving relative to each other will usually be observing measuring rods that are composed of different sets of events. If the word "rod" means the three dimensional form of the object called a rod then these two observers in relative motion observe different rods.

The way that measuring rods differ between observers can be seen by using a Minkowski diagram. The area of a Minkowski diagram that corresponds to all of the events that compose an object over a period of time is known as the worldtube of the object. It can be seen in the image below that length contraction is the result of individual observers having different sections of an object's worldtube in their present instant.


(It should be recalled that the longest lengths on space-time diagrams are often the shortest in reality).

It is sometimes said that length contraction occurs because objects rotate into the time axis. This is actually a half truth, there is no actual rotation of a three dimensional rod, instead the observed three dimensional slice of a four dimensional rod is changed which makes it appear as if the rod has rotated into the time axis. In special relativity it is not the rod that rotates into time, it is the observer's slice of the worldtube of the rod that rotates.

There can be no doubt that the three dimensional slice of the worldtube of a rod does indeed have different lengths for relatively moving observers. The issue of whether or not the events that compose the worldtube of the rod are always existent is a matter for philosophical speculation.

Further reading:

Vesselin Petkov. (2005) Is There an Alternative to the Block Universe View?

Dragan V Redžić (2010). Relativistic length agony continued

More about time dilation

The term "time dilation" is applied to the way that observers who are moving relative to you record fewer clock ticks between events than you. In special relativity this is not due to properties of the clocks, such as their mechanisms getting heavier. Indeed, it should not even be said that the clocks tick faster or slower because what is truly occurring is that the clocks record shorter or longer elapsed times and this recording of elapsed time is independent of the mechanism of the clocks. The differences between clock readings are due to the clocks traversing shorter or longer distances between events along an observer's path through spacetime. This can be seen most clearly by re-examining the Andromeda Paradox.

Suppose Bill passes Jim at high velocity on the way to Mars. Jim has previously synchronised the clocks on Mars with his Earth clocks but for Bill the Martian clocks read times well in advance of Jim's. This means that Bill has a head start because his present instant contains what Jim considers to be the Martian future. Jim observes that Bill travels through both space and time and expresses this observation by saying that Bill's clocks recorded fewer ticks than his own. Bill achieves this strange time travel by having what Jim considers to be the future of distant objects in his present moment. Bill is literally travelling into future parts of Jim's frame of reference.

In special relativity time dilation and length contraction are not material effects, they are physical effects due to travel within a four dimensional spacetime. The mechanisms of the clocks and the structures of measuring rods are irrelevant.

It is important for advanced students to be aware that special relativity and General Relativity differ about the nature of spacetime. General Relativity, in the form championed by Einstein, avoids the idea of extended space and time and is what is known as a "relationalist" theory of physics. Special relativity, on the other hand, is a theory where extended spacetime is pre-eminent. The brilliant flowering of physical theory in the early twentieth century has tended to obscure this difference because, within a decade, special relativity had been subsumed within General Relativity. The interpretation of special relativity that is presented here should be learnt before advancing to more advanced interpretations.

The Pole-barn paradox

The length contraction in relativity is symmetrical. When two observers in relative motion pass each other they both measure a contraction of length.


(Note that Minkowski's metric involves the subtraction of displacements in time, so what appear to be the longest lengths on a 2D sheet of paper are often the shortest lengths in a (3+1)D reality).

The symmetry of length contraction leads to two questions. Firstly, how can a succession of events be observed as simultaneous events by another observer? This question led to the concept of de Broglie waves and quantum theory. Secondly, if a rod is simultaneously between two points in one frame how can it be observed as being successively between those points in another frame? For instance, if a pole enters a building at high speed how can one observer find it is fully within the building and another find that the two ends of the rod are opposed to the two ends of the building at successive times? What happens if the rod hits the end of the building? The second question is known as the "pole-barn paradox" or "ladder paradox".


The pole-barn paradox states the following: suppose a superhero running at 0.75c and carrying a horizontal pole 15 m long towards a barn 10m long, with front and rear doors. When the runner and the pole are inside the barn, a ground observer closes and then opens both doors (by remote control) so that the runner and pole are momentarily captured inside the barn and then proceed to exit the barn from the back door.

One may be surprised to see a 15-m pole fit inside a 10-m barn. But the pole is in motion with respect to the ground observer, who measures the pole to be contracted to a length of 9.9 m (check using equations).

The “paradox” arises when we consider the runner’s point of view. The runner sees the barn contracted to 6.6 m. Because the pole is in the rest frame of the runner, the runner measures it to have its proper length of 15 m. Now, how can our superhero make it safely through the barn?

The resolution of the “paradox” lies in the relativity of simultaneity. The closing of the two doors is measured to be simultaneous by the ground observer. However, since the doors are at different positions, the runner says that they do not close simultaneously. The rear door closes and then opens first, allowing the leading edge of the pole to exit. The front door of the barn does not close until the trailing edge of the pole passes by.

What happens if the rear door is kept closed and made out of some impenetrable material? Can we or can we not trap the rod inside the barn by closing the front door while the whole rod is inside according to a ground observer? When the front end of the rod hits the rear door, information about this impact will travel backwards along the rod in the form of a shock wave. The information cannot travel faster than c, so the rear end of the rod will continue to travel forward at its original speed until the wave reaches it. Even if the shock wave is traveling at the speed of light, it will not reach the rear end of the rod until after the rear end has passed through the front door even in the runner's frame. Therefore the whole rod (albeit quite scrunched up) will be inside the barn when the front door closes. If it is infinitely elastic, it will end up compressed and "spring loaded" against the inside of the closed barn.

Evidence for length contraction, the field of an infinite straight current

Length contraction can be directly observed in the field of an infinitely straight current. This is shown in the illustration below.


Non-relativistic electromagnetism describes the electric field due to a charge using:

and describes the magnetic field due to an infinitely long straight current using the Biot Savart law:

Or using the charge density (from where is the amount of charges per unit length):

Using relativity it is possible to show that the formula for the magnetic field given above can be derived using the relativistic effect of length contraction on the electric field and so what we call the "magnetic" field can be understood as relativistic observations of a single phenomenon. The relativistic calculation is given below.

If Jim is moving relative to the wire at the same velocity as the negative charges he sees the wire contracted relative to Bill:

Bill should see the space between the charges that are moving along the wire to be contracted by the same amount but the requirement for electrical neutrality means that the moving charges will be spread out to match those in the frame of the fixed charges in the wire.

This means that Jim sees the negative charges spread out so that:

The net charge density observed by Jim is:


Using the binomial expansion:

Therefore, allowing for a net positive charge, the positive charges being fixed:

The electric field at Jim's position is given by:

The force due to the electrical field at Jim's position is given by which is:

Now, from classical electromagnetism:

So substituting this into :


This is the formula for the relativistic electric force that is observed by Bill as a magnetic force. How does this compare with the non-relativistic calculation of the magnetic force? The force on a charge at Jim's position due to the magnetic field is, from the classical formula:

Which from the Biot-Savart law is:


which shows that the same formula applies for the relativistic excess electrical force experienced by Jim as the formula for the classical magnetic force.

It can be seen that once the idea of space-time is understood the unification of the two fields is straightforward. Jim is moving relative to the wire at the same speed as the negatively charged current carriers so Jim only experiences an electric field. Bill is stationary relative to the wire and observes that the charges in the wire are balanced whereas Jim observes an imbalance of charge. Bill assigns the attraction between Jim and the current carriers to a "magnetic field".

It is important to notice that, in common with the explanation of length contraction given above, the events that constitute the stream of negative charges for Jim are not the same events as constitute the stream of negative charges for Bill. Bill and Jim's negative charges occupy different moments in time.

Incidently, the drift velocity of electrons in a wire is about a millimetre per second but a huge charge is available in a wire (See link below).

Further reading:

Purcell, E. M. Electricity and Magnetism. Berkeley Physics Course. Vol. 2. 2nd ed. New York, NY: McGraw-Hill. 1984. ISBN: 0070049084.

Useful links:



De Broglie waves

De Broglie noticed that the differing three dimensional sections of the universe would cause oscillations in the rest frame of an observer to appear as wave trains in the rest frame of observers who are moving.


He combined this insight with Einstein's ideas on the quantisation of energy to create the foundations of quantum theory. De Broglie's insight is also a round-about proof of the description of length contraction given above - observers in relative motion have differing three dimensional slices of a four dimensional universe. The existence of matter waves is direct experimental evidence of the relativity of simultaneity.


Further reading: de Broglie, L. (1925) On the theory of quanta. A translation of : RECHERCHES SUR LA THEORIE DES QUANTA (Ann. de Phys., 10e s´erie, t. III (Janvier-F ´evrier 1925).by: A. F. Kracklauer. http://replay.web.archive.org/20090509012910/http://www.ensmp.fr/aflb/LDB-oeuvres/De_Broglie_Kracklauer.pdf

Bell's spaceship paradox

Bell devised a thought experiment called the "Spaceship Paradox" to enquire whether length contraction involved a force and whether this contraction was a contraction of space. In the Spaceship Paradox two spaceships are connected by a thin, stiff string and are both equally and linearly accelerated to a velocity relative to the ground, at which, in the special relativity version of the paradox, the acceleration ceases. The acceleration on both spaceships is arranged to be equal according to ground observers so, according to observers on the ground, the spaceships will stay the same distance apart. It is asked whether the string would break.

It is useful when considering this problem to investigate what happens to a single spaceship. If a spaceship that has rear thrusters is accelerated linearly, according to ground observers, to a velocity then the ground observers will observe it to have contracted in the direction of motion. The acceleration experienced by the front of the spaceship will have been slightly less than the acceleration experienced by the rear of the spaceship during contraction and then would suddenly reach a high value, equalising the front and rear velocities, once the rear acceleration and increasing contraction had ceased. From the ground it would be observed that overall the acceleration at the rear could be linear but the acceleration at the front would be non-linear.

In Bell's thought experiment both spaceships are artificially constrained to have constant acceleration, according to the ground observers, until the acceleration ceases. Sudden adjustments are not allowed. Furthermore no difference between the accelerations at the front and rear of the assembly are permitted so any tendency towards contraction would need to be borne as tension and extension in the string.

The most interesting part of the paradox is what happens to the space between the ships. From the ground the spaceships will stay the same distance apart (the experiment is arranged to achieve this) whilst according to observers on the spaceships they will appear to become increasingly separated. This implies that acceleration is not invariant between reference frames (see Part II) and the force applied to the spaceships will indeed be affected by the difference in separation of the ships observed by each frame.

The section on the nature of length contraction above shows that as the string changes velocity the observers on the ground observe a changing set of events that compose the string. These new events define a string that is shorter than the original. This means that the string will indeed attempt to contract as observed from the ground and will be drawn out under tension as observed from the spaceships. If the string were unable to bear the extension and tension in the moving frame or the tension in the rest frame it would break.

Another interesting aspect of Bell's Spaceship Paradox is that in the inertial frames of the ships, owing to the relativity of simultaneity, the lead spaceship will always be moving slightly faster than the rear spaceship so the spaceship-string system does not form a true inertial frame of reference until the acceleration ceases in the frames of reference of both ships. The asynchrony of the cessation of acceleration shows that the lead ship reaches the final velocity before the rear ship in the frame of reference of either ship. However, this time difference is very slight (less than the time taken for an influence to travel down the string at the speed of light ).

It is necessary at this stage to give a warning about extrapolating special relativity into the domain of general relativity (GR). SR cannot be applied with confidence to accelerating systems which is why the comments above have been confined to qualitative observations.

Further reading

Bell, J. S. (1976). Speakable and unspeakable in quantum mechanics. Cambridge University Press 1987 ISBN 0-521-52338-9

Hsu, J-P and Suzuki, N. (2005) Extended Lorentz Transformations for Accelerated Frames and the Solution of the “Two-Spaceship Paradox” AAPPS Bulletin October 2005 p.17 http://www.aapps.org/archive/bulletin/vol15/15-5/15_5_p17p21%7F.pdf

Matsuda, T and Kinoshita, A (2004. A Paradox of Two Space Ships in Special Relativity. AAPPS Bulletin February 2004 p3. http://www.aapps.org/archive/bulletin/vol14/14_1/14_1_p03p07.pdf

The transverse doppler effect

The existence of time dilation means that the frequency of light emitted from a source that is moving towards or away from an observer should be red shifted in directions that are perpendicular to the direction of motion. The transverse doppler effect is given by:

Where is the observed frequency and is the frequency if the source were stationary relative to the observer (the proper frequency).

This effect was first confirmed by Ives and Stillwell in 1938. The transverse doppler effect is a purely relativistic effect and has been used as an example of proof that time dilation occurs.

Relativistic transformation of angles

If a rod makes an angle with its direction of motion toward or away from an observer the component of its length in the direction of motion will be contracted. This means that observed angles are also transformed during changes of frames of reference. Assuming that motion occurs along the x-axis, suppose the rod has a proper length (rest length) of metres and makes an angle of degrees with the x'-axis in its rest frame. The tangent of the angle made with the axes is:

Tangent in rest frame of rod =
Tangent in observer's frame =

Showing that angles with the direction of motion are observed to increase with velocity.

The angle made by a moving object with the x-axis also involves a transformation of velocities to calculate the correct angle of incidence.

Addition of velocities

How can two observers, moving at v m/sec relative to each other, compare their observations of the velocity of a third object?


Suppose one of the observers measures the velocity of the object as where:

The coordinates and are given by the Lorentz transformations:




and hence:

Notice the role of the phase term . The equation can be rearranged as:

given that :

This is known as the relativistic velocity addition theorem, it applies to velocities parallel to the direction of mutual motion.

The existence of time dilation means that even when objects are moving perpendicular to the direction of motion there is a discrepancy between the velocities reported for an object by observers who are moving relative to each other. If there is any component of velocity in the x direction (, ) then the phase affects time measurement and hence the velocities perpendicular to the x-axis. The table below summarises the relativistic addition of velocities in the various directions in space.

Notice that for an observer in another reference frame the sum of two velocities (u and v) can never exceed the speed of light. This means that the speed of light is the maximum velocity in any frame of reference. Simultaneity, time dilation and length contraction



The way that the velocity of a particle can differ between observers who are moving relative to each other means that momentum needs to be redefined as a result of relativity theory.

The illustration below shows a typical collision of two particles. In the right hand frame the collision is observed from the viewpoint of someone moving at the same velocity as one of the particles, in the left hand frame it is observed by someone moving at a velocity that is intermediate between those of the particles.


If momentum is redefined then all the variables such as force (rate of change of momentum), energy etc. will become redefined and relativity will lead to an entirely new physics. The new physics has an effect at the ordinary level of experience through the relation whereby it is the tiny deviations in gamma from unity that are expressed as everyday kinetic energy so that the whole of physics is related to "relativistic" reasoning rather than Newton's empirical ideas.


In physics momentum is conserved within a closed system, the law of conservation of momentum applies. Consider the special case of identical particles colliding symmetrically as illustrated below:


The momentum change by the red ball is:

The momentum change by the blue ball is:

The situation is symmetrical so the Newtonian conservation of momentum law is demonstrated:

Notice that this result depends upon the y components of the velocities being equal, that is, .

The relativistic case is rather different. The collision is illustrated below, the left hand frame shows the collision as it appears for one observer and the right hand frame shows exactly the same collision as it appears for another observer moving at the same velocity as the blue ball:


The configuration shown above has been simplified because one frame contains a stationary blue ball (ie: ) and the velocities are chosen so that the vertical velocity of the red ball is exactly reversed after the collision ie:. Both frames show exactly the same event, it is only the observers who differ between frames. The relativistic velocity transformations between frames is:

given that .

Suppose that the y components are equal in one frame, in Newtonian physics they will also be equal in the other frame. However, in relativity, if the y components are equal in one frame they are not necessarily equal in the other frame (time dilation is not directional so perpendicular velocities differ between the observers). For instance if then:

So if then in this case .

If the mass were constant between collisions and between frames then although it is found that:

So momentum defined as mass times velocity is not conserved in a collision when the collision is described in frames moving relative to each other. Notice that the discrepancy is very small if and are small.

To preserve the principle of momentum conservation in all inertial reference frames, the definition of momentum has to be changed. The new definition must reduce to the Newtonian expression when objects move at speeds much smaller than the speed of light, so as to recover the Newtonian formulas.

The velocities in the y direction are related by the following equation when the observer is travelling at the same velocity as the blue ball ie: when :

If we write for the mass of the blue ball) and for the mass of the red ball as observed from the frame of the blue ball then, if the principle of relativity applies:




This means that, if the principle of relativity is to apply then the mass must change by the amount shown in the equation above for the conservation of momentum law to be true.

The particle velocities were chosen so that . was selected so that . This allows to be expressed in terms of :

and hence:

So substituting for in :

The blue ball is at rest so its mass is sometimes known as its rest mass, and is given the symbol . As the balls were identical at the start of the boost the mass of the red ball is the mass that a blue ball would have if it were in motion relative to an observer; this mass is sometimes known as the relativistic mass symbolised by . These terms are now infrequently used in modern physics, as will be explained at the end of this section. The discussion given above was related to the relative motions of the blue and red balls, as a result corresponds to the speed of the moving ball relative to an observer who is stationary with respect to the blue ball. These considerations mean that the relativistic mass is given by:

The relativistic momentum is given by the product of the relativistic mass and the velocity .

The overall expression for momentum in terms of rest mass is:

and the components of the momentum are:

So the components of the momentum depend upon the appropriate velocity component and the speed.

Since the factor with the square root is cumbersome to write, the following abbreviation is often used, called the Lorentz gamma factor:

The expression for the momentum then reads .

It can be seen from the discussion above that we can write the momentum of an object moving with velocity as the product of a function of the speed and the velocity :

The function must reduce to the object's mass at small speeds, in particular when the object is at rest .

There is a debate about the usage of the term "mass" in relativity theory. If inertial mass is defined in terms of momentum then it does indeed vary as for a single particle that has rest mass, furthermore, as will be shown below the energy of a particle that has a rest mass is given by . Prior to the debate about nomenclature the function , or the relation , used to be called 'relativistic mass', and its value in the frame of the particle was referred to as the 'rest mass' or 'invariant mass'. The relativistic mass, , would increase with velocity. Both terms are now largely obsolete: the 'rest mass' is today simply called the mass, and the 'relativistic mass' is often no longer used since, as will be seen in the discussion of energy below, it is identical to the energy but for the units.


Newton's second law states that the total force acting on a particle equals the rate of change of its momentum. The same form of Newton's second law holds in relativistic mechanics. The relativistic 3 force is given by:

If the relativistic mass is used:

By Leibniz's law where :

This equation for force will be used below to derive relativistic expressions for the energy of a particle in terms of the old concept of "relativistic mass".

The relativistic force can also be written in terms of acceleration. Newton's second law can be written in the familiar form

where is the acceleration.

here m is not the relativistic mass but is the invariant mass.

In relativistic mechanics, momentum is

again m being the invariant mass and the force is given by

This form of force is used in the derivation of the expression for energy without relying on relativistic mass.

It will be seen in the second section of this book that Newton's second law in terms of acceleration is given by:


The debate over the use of the concept "relativistic mass" means that modern physics courses may forbid the use of this in the derivation of energy. The newer derivation of energy without using relativistic mass is given in the first section and the older derivation using relativistic mass is given in the second section. The two derivations can be compared to gain insight into the debate about mass but a knowledge of 4 vectors is really required to discuss the problem in depth. In principle the first derivation is most mathematically correct because "relativistic mass" is given by: which involves the constants and .

Derivation of relativistic energy using the relativistic momentum

In the following, modern derivation, m means the invariant mass - what used to be called the "rest mass". Energy is defined as the work done in moving a body from one place to another. We will make use of the relativistic momentum . Energy is given from:

so, over the whole path:

Kinetic energy (K) is the energy used to move a body from a velocity of 0 to a velocity . Restricting the motion to one dimension:

Using the relativistic 3 force:

substituting for and using :

Which gives:

The Lorentz factor is given by:

meaning that :

So that

Alternatively, we can use the fact that:


So, rearranging:

In which case:

As goes from 0 to , the Lorentz factor goes from 1 to , so:

and hence:

The amount is known as the total energy of the particle. The amount is known as the rest energy of the particle. If the total energy of the particle is given the symbol :

So it can be seen that is the energy of a mass that is stationary. This energy is known as mass energy.

The Newtonian approximation for kinetic energy can be derived by using the binomial theorem to expand .

The binomial expansion is:

So expanding :

So if is much less than :

which is the Newtonian approximation for low velocities.

Derivation of relativistic energy using the concept of relativistic mass

Energy is defined as the work done in moving a body from one place to another. Energy is given from:

so, over the whole path:

Kinetic energy (K) is the energy used to move a body from a velocity of 0 to a velocity . So:

Using the relativistic force:


substituting for and using :

Which gives:

The relativistic mass is given by:

Which can be expanded as:


So, rearranging:

In which case:

is simplified to:

But the mass goes from to so:

and hence:

The amount is known as the total energy of the particle. The amount is known as the rest energy of the particle. If the total energy of the particle is given the symbol :

So it can be seen that is the energy of a mass that is stationary. This energy is known as mass energy and is the origin of the famous formula that is iconic of the nuclear age.

The Newtonian approximation for kinetic energy can be derived by substituting the rest mass for the relativistic mass ie:




The binomial theorem can be used to expand :

The binomial theorem is:

So expanding :

So if is much less than :

Which is the Newtonian approximation for low velocities.

Nuclear Energy

When protons and neutrons (nucleons) combine to form elements the combination of particles tends to be in a lower energy state than the free neutrons and protons. Iron has the lowest energy and elements above and below iron in the scale of atomic masses tend to have higher energies. This decrease in energy as neutrons and protons bind together is known as the binding energy. The atomic masses of elements are slightly different from that calculated from their constituent particles and this difference in mass energy, calculated from , is almost exactly equal to the binding energy.

The binding energy can be released by converting elements with higher masses per nucleon to those with lower masses per nucleon. This can be done by either splitting heavy elements such as uranium into lighter elements such as barium and krypton or by joining together light elements such as hydrogen into heavier elements such as deuterium. If atoms are split the process is known as nuclear fission and if atoms are joined the process is known as nuclear fusion. Atoms that are lighter than iron can be fused to release energy and those heavier than iron can be split to release energy.

When hydrogen and a neutron are combined to make deuterium the energy released can be calculated as follows:

The mass of a proton is 1.00731 amu, the mass of a neutron is 1.00867 amu and the mass of a deuterium nucleus is 2.0136 amu. The difference in mass between a deuterium nucleus and its components is 0.00238 amu. The energy of this mass difference is:

So the energy released is joules or about joules per gram of protons (ionised hydrogen).

(Assuming 1 amu = Kg, Avogadro's number = and the speed of light is metres per second)

Present day nuclear reactors use a process called nuclear fission in which rods of uranium emit neutrons which combine with the uranium in the rod to produce uranium isotopes such as 236U which rapidly decay into smaller nuclei such as Barium and Krypton plus three neutrons which can cause further generation of 236U and further decay. The fact that each neutron can cause the generation of three more neutrons means that a self sustaining or chain reaction can occur. The generation of energy results from the equivalence of mass and energy; the decay products, barium and krypton have a lower mass than the original 236U, the missing mass being released as 177 MeV of radiation. The nuclear equation for the decay of 236U is written as follows:

Nuclear explosion
If a large amount of the uranium isotope 235U (the critical mass) is confined the chain reaction can get out of control and almost instantly release a large amount of energy. A device that confines a critical mass of uranium is known as an atomic bomb or A-bomb. A bomb based on the fusion of deuterium atoms is known as a thermonuclear bomb, hydrogen bomb or H-bomb.



Many students confuse Relativity Theory with a theory about the propagation of light. According to modern Relativity Theory the constancy of the speed of light is a consequence of the geometry of spacetime rather than something specifically due to the properties of photons; but the statement "the speed of light is constant" often distracts the student into a consideration of light propagation. This confusion is amplified by the importance assigned to interferometry experiments, such as the Michelson-Morley experiment, in most textbooks on Relativity Theory.

The history of theories of the propagation of light is an interesting topic in physics and was indeed important in the early days of Relativity Theory. In the seventeenth century two competing theories of light propagation were developed. Christiaan Huygens published a wave theory of light which was based on Huygen's principle whereby every point in a wavelike disturbance can give rise to further disturbances that spread out spherically. In contrast Newton considered that the propagation of light was due to the passage of small particles or "corpuscles" from the source to the illuminated object. His theory is known as the corpuscular theory of light. Newton's theory was widely accepted until the nineteenth century.

In the early nineteenth century Thomas Young performed his Young's slits experiment and the interference pattern that occurred was explained in terms of diffraction due to the wave nature of light. The wave theory was accepted generally until the twentieth century when quantum theory confirmed that light had a corpuscular nature and that Huygen's principle could not be applied.

The idea of light as a disturbance of some medium, or aether, that permeates the universe was problematical from its inception (US spelling: "ether"). The first problem that arose was that the speed of light did not change with the velocity of the observer. If light were indeed a disturbance of some stationary medium then as the earth moves through the medium towards a light source the speed of light should appear to increase. It was found however that the speed of light did not change as expected. Each experiment on the velocity of light required corrections to existing theory and led to a variety of subsidiary theories such as the "aether drag hypothesis". Ultimately it was experiments that were designed to investigate the properties of the aether that provided the first experimental evidence for Relativity Theory.

The aether drag hypothesis

The aether drag hypothesis was an early attempt to explain the way experiments such as Arago's experiment showed that the speed of light is constant. The aether drag hypothesis is now considered to be incorrect.

According to the aether drag hypothesis light propagates in a special medium, the aether, that remains attached to things as they move. If this is the case then, no matter how fast the earth moves around the sun or rotates on its axis, light on the surface of the earth would travel at a constant velocity.

Stellar Aberration. If a telescope is travelling at high speed, only light incident at a particular angle can avoid hitting the walls of the telescope tube.

The primary reason the aether drag hypothesis is considered invalid is because of the occurrence of stellar aberration. In stellar aberration the position of a star when viewed with a telescope swings each side of a central position by about 20.5 seconds of arc every six months. This amount of swing is the amount expected when considering the speed of earth's travel in its orbit. In 1871, George Biddell Airy demonstrated that stellar aberration occurs even when a telescope is filled with water. It seems that if the aether drag hypothesis were true then stellar aberration would not occur because the light would be travelling in the aether which would be moving along with the telescope.

The "train analogy" for the absence of aether drag.

If you visualize a bucket on a train about to enter a tunnel and a drop of water drips from the tunnel entrance into the bucket at the very centre, the drop will not hit the centre at the bottom of the bucket. The bucket is the tube of a telescope, the drop is a photon and the train is the earth. If aether is dragged then the droplet would be travelling with the train when it is dropped and would hit the centre of bucket at the bottom.

The amount of stellar aberration, α is given by:


The speed at which the earth goes round the sun, v = 30 km/s, and the speed of light is c = 300,000,000 m/s which gives α = 20.5 seconds of arc every six months. This amount of aberration is observed and this contradicts the aether drag hypothesis.

In 1818, Augustin Jean Fresnel introduced a modification to the aether drag hypothesis that only applies to the interface between media. This was accepted during much of the nineteenth century but has now been replaced by special theory of relativity (see below).

The aether drag hypothesis is historically important because it was one of the reasons why Newton's corpuscular theory of light was replaced by the wave theory and it is used in early explanations of light propagation without relativity theory. It originated as a result of early attempts to measure the speed of light.

In 1810, François Arago realised that variations in the refractive index of a substance predicted by the corpuscular theory would provide a useful method for measuring the velocity of light. These predictions arose because the refractive index of a substance such as glass depends on the ratio of the velocities of light in air and in the glass. Arago attempted to measure the extent to which corpuscles of light would be refracted by a glass prism at the front of a telescope. He expected that there would be a range of different angles of refraction due to the variety of different velocities of the stars and the motion of the earth at different times of the day and year. Contrary to this expectation he found that there was no difference in refraction between stars, between times of day or between seasons. All Arago observed was ordinary stellar aberration.

In 1818 Fresnel examined Arago's results using a wave theory of light. He realised that even if light were transmitted as waves the refractive index of the glass-air interface should have varied as the glass moved through the aether to strike the incoming waves at different velocities when the earth rotated and the seasons changed.

Fresnel proposed that the glass prism would carry some of the aether along with it so that "...the aether is in excess inside the prism". He realised that the velocity of propagation of waves depends on the density of the medium so proposed that the velocity of light in the prism would need to be adjusted by an amount of 'drag'.

The velocity of light in the glass without any adjustment is given by:

The drag adjustment is given by:

Where is the aether density in the environment, is the aether density in the glass and is the velocity of the prism with respect to the aether.

The factor can be written as because the refractive index, n, would be dependent on the density of the aether. This is known as the Fresnel drag coefficient.

The velocity of light in the glass is then given by:

This correction was successful in explaining the null result of Arago's experiment. It introduces the concept of a largely stationary aether that is dragged by substances such as glass but not by air. Its success favoured the wave theory of light over the previous corpuscular theory.

The Fresnel drag coefficient was confirmed by an interferometer experiment performed by Fizeau. Water was passed at high speed along two glass tubes that formed the optical paths of the interferometer and it was found that the fringe shifts were as predicted by the drag coefficient.


The special theory of relativity predicts the result of the Fizeau experiment from the velocity addition theorem without any need for an aether.

If is the velocity of light relative to the Fizeau apparatus and is the velocity of light relative to the water and is the velocity of the water:

which, if v/c is small can be expanded using the binomial expansion to become:

This is identical to Fresnel's equation.

It may appear as if Fresnel's analysis can be substituted for the relativistic approach, however, more recent work has shown that Fresnel's assumptions should lead to different amounts of aether drag for different frequencies of light and violate Snell's law (see Ferraro and Sforza (2005)).

The aether drag hypothesis was one of the arguments used in an attempt to explain the Michelson-Morley experiment before the widespread acceptance of the special theory of relativity.

The Fizeau experiment is consistent with relativity and approximately consistent with each individual body, such as prisms, lenses etc. dragging its own aether with it. This contradicts some modified versions of the aether drag hypothesis that argue that aether drag may happen on a global (or larger) scale and stellar aberration is merely transferred into the entrained "bubble" around the earth which then faithfully carries the modified angle of incidence directly to the observer.


The Michelson-Morley experiment

The Michelson-Morley experiment, one of the most important and famous experiments in the history of physics, was performed in 1887 by Albert Michelson and Edward Morley at what is now Case Western Reserve University, and is considered to be the first strong evidence against the theory of a luminiferous aether.

Physics theories of the late 19th century postulated that, just as water waves must have a medium to move across (water), and audible sound waves require a medium to move through (air), so also light waves require a medium, the "luminiferous aether". The speed of light being so great, designing an experiment to detect the presence and properties of this aether took considerable thought.

Measuring aether

A depiction of the concept of the “aether wind”.

Each year, the Earth travels a tremendous distance in its orbit around the sun, at a speed of around 30 km/second, over 100,000 km per hour. It was reasoned that the Earth would at all times be moving through the aether and producing a detectable "aether wind". At any given point on the Earth's surface, the magnitude and direction of the wind would vary with time of day and season. By analysing the effective wind at various different times, it should be possible to separate out components due to motion of the Earth relative to the Solar System from any due to the overall motion of that system.

The effect of the aether wind on light waves would be like the effect of wind on sound waves. Sound waves travel at a constant speed relative to the medium that they are travelling through (this varies depending on the pressure, temperature etc (see sound), but is typically around 340 m/s). So, if the speed of sound in our conditions is 340 m/s, when there is a 10 m/s wind relative to the ground, into the wind it will appear that sound is travelling at 330 m/s (340 - 10). Downwind, it will appear that sound is travelling at 350 m/s (340 + 10). Measuring the speed of sound compared to the ground in different directions will therefore enable us to calculate the speed of the air relative to the ground.

If the speed of the sound cannot be directly measured, an alternative method is to measure the time that the sound takes to bounce off of a reflector and return to the origin. This is done parallel to the wind and perpendicular (since the direction of the wind is unknown before hand, just determine the time for several different directions). The cumulative round trip effects of the wind in the two orientations slightly favors the sound travelling at right angles to it. Similarly, the effect of an aether wind on a beam of light would be for the beam to take slightly longer to travel round-trip in the direction parallel to the “wind” than to travel the same round-trip distance at right angles to it.

“Slightly” is key, in that, over a distance such as a few meters, the difference in time for the two round trips would be only about a millionth of a millionth of a second. At this point the only truly accurate measurements of the speed of light were those carried out by Albert Abraham Michelson, which had resulted in measurements accurate to a few meters per second. While a stunning achievement in its own right, this was certainly not nearly enough accuracy to be able to detect the aether.

The experiments

Michelson, though, had already seen a solution to this problem. His design, later known as an interferometer, sent a single source of white light through a half-silvered mirror that was used to split it into two beams travelling at right angles to one another. After leaving the splitter, the beams travelled out to the ends of long arms where they were reflected back into the middle on small mirrors. They then recombined on the far side of the splitter in an eyepiece, producing a pattern of constructive and destructive interference based on the length of the arms. Any slight change in the amount of time the beams spent in transit would then be observed as a shift in the positions of the interference fringes. If the aether were stationary relative to the sun, then the Earth's motion would produce a shift of about 0.04 fringes.

Michelson had made several measurements with an experimental device in 1881, in which he noticed that the expected shift of 0.04 was not seen, and a smaller shift of about 0.02 was. However his apparatus was a prototype, and had experimental errors far too large to say anything about the aether wind. For a measurement of the aether wind, a much more accurate and tightly controlled experiment would have to be carried out. The prototype was, however, successful in demonstrating that the basic method was feasible.

A Michelson interferometer

He then combined forces with Edward Morley and spent a considerable amount of time and money creating an improved version with more than enough accuracy to detect the drift. In their experiment the light was repeatedly reflected back and forth along the arms, increasing the path length to 11m. At this length the drift would be about .4 fringes. To make that easily detectable the apparatus was located in a closed room in the basement of a stone building, eliminating most thermal and vibrational effects. Vibrations were further reduced by building the apparatus on top of a huge block of marble, which was then floated in a pool of mercury. They calculated that effects of about 1/100th of a fringe would be detectable.

The mercury pool allowed the device to be turned, so that it could be rotated through the entire range of possible angles to the "aether wind". Even over a short period of time some sort of effect would be noticed simply by rotating the device, such that one arm rotated into the direction of the wind and the other away. Over longer periods day/night cycles or yearly cycles would also be easily measurable.

During each full rotation of the device, each arm would be parallel to the wind twice (facing into and away from the wind) and perpendicular to the wind twice. This effect would show readings in a sine wave formation with two peaks and two troughs. Additionally if the wind was only from the earth's orbit around the sun, the wind would fully change directions east/west during a 12 hour period. In this ideal conceptualization, the sine wave of day/night readings would be in opposite phase.

Because it was assumed that the motion of the solar system would cause an additional component to the wind, the yearly cycles would be detectable as an alteration of the magnitude of the wind. An example of this effect is a helicopter flying forward. While on the ground, a helicopter's blades would be measured as travelling around at 50 km/h at the tips. However, if the helicopter is travelling forward at 50 km/h, there are points at which the tips of the blades are travelling 0 km/h and 100 km/h with respect to the air they are travelling through. This increases the magnitude of the lift on one side and decreases it on the other just as it would increase and decrease the magnitude of an ether wind on a yearly basis.

The most famous failed experiment

Ironically, after all this thought and preparation, the experiment became what might be called the most famous failed experiment to date. Instead of providing insight into the properties of the aether, Michelson and Morley's 1887 article in the American Journal of Science reported the measurement to be as small as one-fortieth of the expected displacement but "since the displacement is proportional to the square of the velocity" they concluded that the measured velocity was approximately one-sixth of the expected velocity of the Earth's motion in orbit and "certainly less than one-fourth". Although this small "velocity" was measured, it was considered far too small to be used as evidence of aether, it was later said to be within the range of an experimental error that would allow the speed to actually be zero.

Although Michelson and Morley went on to different experiments after their first publication in 1887, both remained active in the field. Other versions of the experiment were carried out with increasing sophistication. Kennedy and Illingsworth both modified the mirrors to include a half-wave "step", eliminating the possibility of some sort of standing wave pattern within the apparatus. Illingsworth could detect changes on the order of 1/300th of a fringe, Kennedy up to 1/1500th. Miller later built a non-magnetic device to eliminate magnetostriction, while Michelson built one of non-expanding invar to eliminate any remaining thermal effects. Others from around the world increased accuracy, eliminated possible side effects, or both. All of these with the exception of Dayton Miller also returned what is considered a null result.

Morley was not convinced of his own results, and went on to conduct additional experiments with Dayton Miller. Miller worked on increasingly large experiments, culminating in one with a 32m (effective) arm length at an installation at the Mount Wilson observatory. To avoid the possibility of the aether wind being blocked by solid walls, he used a special shed with thin walls, mainly of canvas. He consistently measured a small positive effect that varied, as expected, with each rotation of the device, the sidereal day and on a yearly basis. The low magnitude of the results he attributed to aether entrainment (see below). His measurements amounted to only ~10 kps instead of the expected ~30 kps expected from the earth's orbital motion alone. He remained convinced this was due to partial entrainment, though he did not attempt a detailed explanation.

Though Kennedy later also carried out an experiment at Mount Wilson, finding 1/10 the drift measured by Miller, and no seasonal effects, Miller's findings were considered important at the time, and were discussed by Michelson, Hendrik Lorentz and others at a meeting reported in 1928 (ref below). There was general agreement that more experimentation was needed to check Miller's results. Lorentz recognised that the results, whatever their cause, did not quite tally with either his or Einstein's versions of special relativity. Einstein was not present at the meeting and felt the results could be dismissed as experimental error (see Shankland ref below).

Name Year Arm length (meters) Fringe shift expected Fringe shift measured Experimental Resolution Upper Limit on Vaether
Michelson 1881 1.2 0.04 0.02
Michelson and Morley 1887 11.0 0.4 < 0.01 8 km/s
Morley and Morley 1902–1904 32.2 1.13 0.015
Miller 1921 32.0 1.12 0.08
Miller 1923–1924 32.0 1.12 0.03
Miller (Sunlight) 1924 32.0 1.12 0.014
Tomascheck (Starlight) 1924 8.6 0.3 0.02
Miller 1925–1926 32.0 1.12 0.088
Mt Wilson) 1926 2.0 0.07 0.002
Illingworth 1927 2.0 0.07 0.0002 0.0006 1 km/s
Piccard and Stahel (Rigi) 1927 2.8 0.13 0.006
Michelson et al. 1929 25.9 0.9 0.01
Joos 1930 21.0 0.75 0.002

In recent times versions of the MM experiment have become commonplace. Lasers and masers amplify light by repeatedly bouncing it back and forth inside a carefully tuned cavity, thereby inducing high-energy atoms in the cavity to give off more light. The result is an effective path length of kilometers. Better yet, the light emitted in one cavity can be used to start the same cascade in another set at right angles, thereby creating an interferometer of extreme accuracy.

The first such experiment was led by Charles H. Townes, one of the co-creators of the first maser. Their 1958 experiment put an upper limit on drift, including any possible experimental errors, of only 30 m/s. In 1974 a repeat with accurate lasers in the triangular Trimmer experiment reduced this to 0.025 m/s, and included tests of entrainment by placing one leg in glass. In 1979 the Brillet-Hall experiment put an upper limit of 30 m/s for any one direction, but reduced this to only 0.000001 m/s for a two-direction case (ie, still or partially entrained aether). A year long repeat known as Hils and Hall, published in 1990, reduced this to 2x10-13.


This result was rather astounding and not explainable by the then-current theory of wave propagation in a static aether. Several explanations were attempted, among them, that the experiment had a hidden flaw (apparently Michelson's initial belief), or that the Earth's gravitational field somehow "dragged" the aether around with it in such a way as locally to eliminate its effect. Miller would have argued that, in most if not all experiments other than his own, there was little possibility of detecting an aether wind since it was almost completely blocked out by the laboratory walls or by the apparatus itself. Be this as it may, the idea of a simple aether, what became known as the First Postulate, had been dealt a serious blow.

A number of experiments were carried out to investigate the concept of aether dragging, or entrainment. The most convincing was carried out by Hamar, who placed one arm of the interferometer between two huge lead blocks. If aether were dragged by mass, the blocks would, it was theorised, have been enough to cause a visible effect. Once again, no effect was seen.

Walter Ritz's Emission theory (or ballistic theory), was also consistent with the results of the experiment, not requiring aether, more intuitive and paradox-free. This became known as the Second Postulate. However it also led to several "obvious" optical effects that were not seen in astronomical photographs, notably in observations of binary stars in which the light from the two stars could be measured in an interferometer.

The Sagnac experiment placed the MM apparatus on a constantly rotating turntable. In doing so any ballistic theories such as Ritz's could be tested directly, as the light going one way around the device would have different length to travel than light going the other way (the eyepiece and mirrors would be moving toward/away from the light). In Ritz's theory there would be no shift, because the net velocity between the light source and detector was zero (they were both mounted on the turntable). However in this case an effect was seen, thereby eliminating any simple ballistic theory. This fringe-shift effect is used today in laser gyroscopes.

Another possible solution was found in the Lorentz-FitzGerald contraction hypothesis. In this theory all objects physically contract along the line of motion relative to the aether, so while the light may indeed transit slower on that arm, it also ends up travelling a shorter distance that exactly cancels out the drift.

In 1932 the Kennedy-Thorndike experiment modified the Michelson-Morley experiment by making the path lengths of the split beam unequal, with one arm being very long. In this version the two ends of the experiment were at different velocities due to the rotation of the earth, so the contraction would not "work out" to exactly cancel the result. Once again, no effect was seen.

Ernst Mach was among the first physicists to suggest that the experiment actually amounted to a disproof of the aether theory. The development of what became Einstein's special theory of relativity had the Fitzgerald-Lorentz contraction derived from the invariance postulate, and was also consistent with the apparently null results of most experiments (though not, as was recognised at the 1928 meeting, with Miller's observed seasonal effects). Today relativity is generally considered the "solution" to the MM null result.

The Trouton-Noble experiment is regarded as the electrostatic equivalent of the Michelson-Morley optical experiment, though whether or not it can ever be done with the necessary sensitivity is debatable. On the other hand, the 1908 Trouton-Rankine experiment that spelled the end of the Lorentz-FitzGerald contraction hypothesis achieved an incredible sensitivity.


Mathematical analysis of the Michelson Morley Experiment

The Michelson interferometer splits light into rays that travel along two paths then recombines them. The recombined rays interfere with each other. If the path length changes in one of the arms the interference pattern will shift slightly, moving relative to the cross hairs in the telescope. The Michelson interferometer is arranged as an optical bench on a concrete block that floats on a large pool of mercury. This allows the whole apparatus to be rotated smoothly.

If the earth were moving through an aether at the same velocity as it orbits the sun (30 km/sec) then Michelson and Morley calculated that a rotation of the apparatus should cause a shift in the fringe pattern. The basis of this calculation is given below.


Consider the time taken for light to travel along Path 1 in the illustration:

Rearranging terms:

further rearranging:


Considering Path 2, the light traces out two right angled triangles so:



It is now easy to calculate the difference ( between the times spent by the light in Path 1 and Path 2:

If the apparatus is rotated by 90 degrees the new time difference is:

because and exchange roles.

The interference fringes due to the time difference between the paths will be different after rotation if and are different.

This difference between the two times can be calculated if the binomial expansions of and are used:


If the period of one vibration of the light is then the number of fringes (), that will move past the cross hairs of the telescope when the apparatus is rotated will be:

Inserting the formula for :

But for a light wave is the wavelength of the light ie: so:

If the wavelength of the light is and the total path length is 20 metres then:

So the fringes will shift by 0.4 fringes (ie: 40%) when the apparatus is rotated.

However, no fringe shift is observed. The null result of the Michelson-Morley experiment is nowdays explained in terms of the constancy of the speed of light. The assumption that the light would have a velocity of and depending on the direction relative to the hypothetical "aether wind" is false, the light always travels at between two points in a vacuum and the speed of light is not affected by any "aether wind". This is because, in {special relativity} the Lorentz transforms induce a {length contraction}. Doing over the above calculations we obtain:

(taking into consideration the length contraction)

It is now easy to recalculate the difference between the times spent by the light in Path 1 and Path 2:


If the apparatus is rotated by 90 degrees the new time difference is:

The interference fringes due to the time difference between the paths will be different after rotation if and are different.

Note: if the rest length then and then and and, more importantly, . This is why Michelson took great pains in equalizing the arms of the interferometer.

Wave propagation in moving medium

To date, it is pointed out that the medium of light in Michelson-Morley experiment is the air. And the velocity of medium is zero. Therefore,

After apparatus rotated 90°, there is no interference movement. [1]

Coherence length

The coherence length of light rays from a source that has wavelengths that differ by is:

If path lengths differ by more than this amount then interference fringes will not be observed. White light has a wide range of wavelengths and interferometers using white light must have paths that are equal to within a small fraction of a millimetre for interference to occur. This means that the ideal light source for a Michelson Interferometer should be monochromatic and the arms should be as near as possible equal in length.

The calculation of the coherence length is based on the fact that interference fringes become unclear when light rays are about 60 degrees (about 1 radian or one sixth of a wavelength ()) out of phase. This means that when two beams are:

metres out of step they will no longer give a well defined interference pattern. Suppose a light beam contains two wavelengths of light, and , then in:

cycles they will be out of phase.

The distance required for the two different wavelengths of light to be this much out of phase is the coherence length. Coherence length = number of cycles x length of each cycle so:

coherence length = .

Lorentz-Fitzgerald Contraction Hypothesis

After the first Michelson-Morley experiments in 1881 there were several attempts to explain the null result. The most obvious point of attack is to propose that the path that is parallel to the direction of motion is contracted by in which case and would be identical and no fringe shift would occur. This possibility was proposed in 1892 by Fitzgerald. Lorentz produced an "electron theory of matter" that would account for such a contraction.

Students sometimes make the mistake of assuming that the Lorentz-Fitzgerald contraction is equivalent to the Lorentz transformations. However, in the absence of any treatment of the time dilation effect the Lorentz-Fitzgerald explanation would result in a fringe shift if the apparatus is moved between two different velocities. The rotation of the earth allows this effect to be tested as the earth orbits the sun. Kennedy and Thorndike (1932) performed the Michelson-Morley experiment with a highly sensitive apparatus that could detect any effect due to the rotation of the earth; they found no effect. They concluded that both time dilation and Lorentz-Fitzgerald Contraction take place, thus confirming relativity theory.

If only the Lorentz-Fitzgerald contraction applied then the fringe shifts due to changes in velocity would be: . Notice how the sensitivity of the experiment is dependent on the difference in path length and hence a long coherence length is required.

Recent Michelson-Morley experiments

Optical tests of the isotropy of the speed of light have become commonplace. New technologies, including the use of lasers and masers, have significantly improved measurement precision.

Author Year Description Upper bounds
Essen[2] 1955 The frequency of a rotating microwave optical cavity resonator is compared with that of a quartz clock ~3 km/s
Jaseja et al.[3] 1964 The frequencies of two Helium–neon lasers, mounted on a rotating table, placed perpendicular to each other. ~30 m/s
Shamir and Fox[4] 1969 Both arms of the interferometer were contained in a transparent solid (Poly(methyl methacrylate). The light source was a Helium–neon laser. ~7 km/s

More recent experiments still, using other types of experiment such as optical resonators (Eisele et al.[5]), have shown that the speed of light is constant to within m/s .

External links


  1. The model of wave propagation in classical physics.
  2. Essen, L. (1955). "A New Æther-Drift Experiment". Nature 175 (4462): 793–794. doi:10.1038/175793a0. Bibcode1955Natur.175..793E. 
  3. Jaseja, T. S.; Javan, A.; Murray, J.; Townes, C. H. (1964). "Test of Special Relativity or of the Isotropy of Space by Use of Infrared Masers". Phys. Rev. 133 (5a): 1221–1225. doi:10.1103/PhysRev.133.A1221. Bibcode1964PhRv..133.1221J. 
  4. Shamir, J.; Fox, R. (1969). "A new experimental test of special relativity". Il Nuovo Cimento B 62 (2): 258–264. doi:10.1007/BF02710136. Bibcode1969NCimB..62..258S. 
  5. Eisele, Ch.; Nevsky, A. Yu.; Schiller, S. (2009). "Laboratory Test of the Isotropy of Light Propagation at the 10−17 level". Physical Review Letters 103 (9): 090401. doi:10.1103/PhysRevLett.103.090401. PMID 19792767. Bibcode2009PhRvL.103i0401E. http://www.exphy.uni-duesseldorf.de/Publikationen/2009/Eisele%20et%20al%20Laboratory%20Test%20of%20the%20Isotropy%20of%20Light%20Propagation%20at%20the%2010-17%20Level%202009.pdf. 

mathematical approach


Physical effects involve things acting on other things to produce a change of position, tension etc. These effects usually depend upon the strength, angle of contact, separation etc of the interacting things rather than on any absolute reference frame so it is useful to describe the rules that govern the interactions in terms of the relative positions and lengths of the interacting things rather than in terms of any fixed viewpoint or coordinate system. Vectors were introduced in physics to allow such relative descriptions.

The use of vectors in elementary physics often avoids any real understanding of what they are. They are a new concept, as unique as numbers themselves, which have been related to the rest of mathematics and geometry by a series of formulae such as linear combinations, scalar products etc.

Vectors are defined as "directed line segments" which means they are lines drawn in a particular direction. The introduction of time as a geometric entity means that this definition of a vector is rather archaic, a better definition might be that a vector is information arranged as a continuous succession of points in space and time. Vectors have length and direction, the direction being from earlier to later.

Vectors are represented by lines terminated with arrow symbols to show the direction. A point that moves from the left to the right for about three centimetres can be represented as:


If a vector is represented within a coordinate system it has components along each of the axes of the system. These components do not normally start at the origin of the coordinate system.


The vector represented by the bold arrow has components a, b and c which are lengths on the coordinate axes. If the vector starts at the origin the components become simply the coordinates of the end point of the vector and the vector is known as the position vector of the end point.

Addition of Vectors

If two vectors are connected so that the end point of one is the start of the next the sum of the two vectors is defined as a third vector drawn from the start of the first to the end of the second:


c is the sum of a and b:

c = a + b

If a components of a are x1, y1, z1 and the components of b are x2, y2, z2 then the components of the sum of the two vectors are (x1+x2), (y1+y2) and (z1+z2). In other words, when vectors are added it is the components that add numerically rather than the lengths of the vectors themselves.

Rules of Vector Addition

1. Commutativity a + b = b + a

2. Associativity (a + b) + c = a + (b + c)

If the zero vector (which has no length) is labelled as 0

3. a + (-a) = 0

4. a + 0 = a

Rules of Vector Multiplication by a Scalar

The discussion of components and vector addition shows that if vector a has components a,b,c then qa has components qa, qb, qc. The meaning of vector multiplication is shown below:


The bottom vector c is added three times which is equivalent to multiplying it by 3.

1. Distributive laws q(a + b) = qa + qb and (q + p)a = qa + pa

2. Associativity q(pa) = qpa

Also 1 a = a

If the rules of vector addition and multiplication by a scalar apply to a set of elements they are said to define a vector space.

Linear Combinations and Linear Dependence

An element of the form:

is called a linear combination of the vectors.

The set of vectors multiplied by scalars in a linear combination is called the span of the vectors. The word span is used because the scalars (q) can have any value - which means that any point in the subset of the vector space defined by the span can contain a vector derived from it.

Suppose there were a set of vectors () , if it is possible to express one of these vectors in terms of the others, using any linear combination, then the set is said to be linearly dependent. If it is not possible to express any one of the vectors in terms of the others, using any linear combination, it is said to be linearly independent.

In other words, if there are values of the scalars such that:


the set is said to be linearly dependent.

There is a way of determining linear dependence. From (1) it can be seen that if is set to minus one then:

So in general, if a linear combination can be written that sums to a zero vector then the set of vectors ( are not linearly independent.

If two vectors are linearly dependent then they lie along the same line (wherever a and b lie on the line, scalars can be found to produce a linear combination which is a zero vector). If three vectors are linearly dependent they lie on the same line or on a plane (collinear or coplanar).


If n+1 vectors in a vector space are linearly dependent then n vectors are linearly independent and the space is said to have a dimension of n. The set of n vectors is said to be the basis of the vector space.

Scalar Product

Also known as the 'dot product' or 'inner product'. The scalar product is a way of removing the problem of angular measures from the relationship between vectors and, as Weyl put it, a way of comparing the lengths of vectors that are arbitrarily inclined to each other.

Consider two vectors with a common origin:


The projection of on the adjacent side is:

Where is the length of .

The scalar product is defined as:


Notice that is zero if and are perpendicular. This means that if the scalar product is zero the vectors composing it are orthogonal (perpendicular to each other).

(2) also allows to be defined as:

The definition of the scalar product also allows a definition of the length of a vector in terms of the concept of a vector itself. The scalar product of a vector with itself is:

cos 0 (the cosine of zero) is one so:

which is our first direct relationship between vectors and scalars. This can be expressed as:


where a is the length of .


1. Linearity

2. symmetry

3. Positive definiteness is greater than or equal to 0

4. Distributivity for vector addition

5. Schwarz inequality

6. Parallelogram equality

From the point of view of vector physics the most important property of the scalar product is the expression of the scalar product in terms of coordinates.


This gives us the length of a vector in terms of coordinates (Pythagoras' theorem) from:


The derivation of 7 is:

where are unit vectors along the coordinate axes. From (4)



etc. are all zero because the vectors are orthogonal, also and are all one (these are unit vectors defined to be 1 unit in length).

Using these results:


Matrices are sets of numbers arranged in a rectangular array. They are especially important in linear algebra because they can be used to represent the elements of linear equations.

11a + 2b = c

5a + 7b = d

The constants in the equation above can be represented as a matrix:

The elements of matrices are usually denoted symbolically using lower case letters:

Matrices are said to be equal if all of the corresponding elements are equal.

Eg: if


Matrix Addition

Matrices are added by adding the individual elements of one matrix to the corresponding elements of the other matrix.


Matrix addition has the following properties:

1. Commutativity

2. Associativity




From matrix addition it can be seen that the product of a matrix and a number p is simply where every element of the matrix is multiplied individually by p.

Transpose of a Matrix

A matrix is transposed when the rows and columns are interchanged:

Notice that the principal diagonal elements stay the same after transposition.

A matrix is symmetric if it is equal to its transpose eg: .

It is skew symmetric if eg: . The principal diagonal of a skew symmetric matrix is composed of elements that are zero.

Other Types of Matrix

Diagonal matrix: all elements above and below the principal diagonal are zero.

Unit matrix: denoted by I, is a diagonal matrix where all elements of the principal diagonal are 1.

Matrix Multiplication

Matrix multiplication is defined in terms of the problem of determining the coefficients in linear transformations.

Consider a set of linear transformations between 2 coordinate systems that share a common origin and are related to each other by a rotation of the coordinate axes.

Two Coordinate Systems Rotated Relative to Each Other

If there are 3 coordinate systems, x, y, and z these can be transformed from one to another:

By substitution:


The coefficient matrices are:

From the linear transformation the product of A and B is defined as:

In the discussion of scalar products it was shown that, for a plane the scalar product is calculated as: where a and b are the coordinates of the vectors a and b.

Now mathematicians define the rows and columns of a matrix as vectors:

A Column vector is

And a Row vector

Matrices can be described as vectors eg:


Matrix multiplication is then defined as the scalar products of the vectors so that:

From the definition of the scalar product etc.

In the general case:

This is described as the multiplication of rows into columns (eg: row vectors into column vectors). The first matrix must have the same number of columns as there are rows in the second matrix or the multiplication is undefined.

After matrix multiplication the product matrix has the same number of rows as the first matrix and columns as the second matrix:

times has 2 rows and 1 column

ie: first row is and second row is

times has 2 rows and 3 columns

Notice that cannot be determined because the number of columns in the first matrix must equal the number of rows in the second matrix to perform matrix multiplication.

Properties of Matrix Multiplication

1. Not commutative

2. Associative

3. Distributative for matrix addition

matrix multiplication is not commutative so is a separate case.

4. The cancellation law is not always true:

does not mean or

There is a case where matrix multiplication is commutative. This involves the scalar matrix where the values of the principle diagonal are all equal. Eg:

In this case . If the scalar matrix is the unit matrix: .

Linear Transformations

A simple linear transformation such as:

can be expressed as:




Using the associative law:

and so:

as before.

Indicial Notation

Consider a simple rotation of coordinates:


is defined as ,

is defined as ,

The scalar product can be written as:


and is called the metric tensor for this 2D space.

Now, , , , so:

If there is no rotation of coordinates the scalar product is:

Which is Pythagoras' theorem.

The Summation Convention

Indexes that appear as both subscripts and superscripts are summed over.

by promoting to a superscript it is taken out of the summation ie:.

Matrix Multiplication in Indicial Notation


Columns times rows:

times =

Matrix product Where i = 1, 2 j = 1, 2

There being no summation the indexes are both subscripts.

Rows times columns: times =

Matrix product

Where is known as Kronecker delta and has the value 0 when and 1 when . It is the indicial equivalent of the unit matrix:

There being summation one value of i is a subscript and the other a superscript.

A matrix in general can be specified by any of:

, , , depending on which subscript or superscript is being summed over.

Vectors in Indicial Notation

A vector can be expressed as a sum of basis vectors.

In indicial notation this is:

Linear Transformations in indicial notation

Consider where is a coefficient matrix and and are coordinate matrices.

In indicial notation this is:

which becomes: