Talk:Special Relativity

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Contents 1 wrong PDF 2 diagrams and dimensionality 3 Page names 4 Still wrong PDF 5 Contributors, List 6 Print Version 7 Twins Paradox 8 I think there's a mistake in the pdf 9 "Undergraduate" reading level? 10 PDF Corrupt 11 Introduction

[edit] wrong PDF

when I downloaded and opened the PDF file, though its filename is "Special_Relativity.pdf", it turned out that the PDF is on "Wikijunior Solar System"....

It seems that there are no problems with Special_Relativity.pdf now. Though, I found that the link captioned Part1 is actually linked to a blank file whilst the link captioned Part2 is actually linked to Special_Relativity.pdf, which is Part1:Introductory text 02:32, 15 April 2007 (UTC)

Thanks for noticing this. I hope it is now corrected. RobinH 08:48, 17 May 2007 (UTC)

[edit] diagrams and dimensionality

The structure of special relativity book is like this. There is a beginners part which comes first and a more advanced part which comes second. The advanced part seems to aim to use the full mathematical theory behind SR including the 4D vector spaces etc. I think SR should be easier to understand for beginners so heres my suggestion. The beginners section should only have 2D diagrams, and should only have variables x and t, no y or z except at the end of the lorentz transformations (as an aside). So the concepts of SR should be easy to tell to beginners with just 2 dimensions and easier diagrams. During the beginning section it can be hinted at that a more complete description is given in terms of 4 dimensions. Then in the advanced section, it starts out by revisting the old formulas from the beginners section but adding in the missing variables real fast. This would be a prelude to developing the whole vector space formulation of SR.

The reasoning for this is beginners need simpler diagrams to understand basic concepts in SR. Also, it is pretty much useless to have all 4 dimensions unless we are going all the way to minkowski spaces. That is not to say we totally separate it into 2D and 4D. in the beginners we should mention the extension to more dimensions somewhat, and in the advanced you start off with the 2D familiar stuff and expand it to 4 immediately.

This does mean a redrawing of many diagrams, but the current diagrams are too confusing for the most part anyway. I just wanted to see what people thought of this before I go ahead and do it. Does anyone even talk at this project? it seems dead. maybe its just new? SR's been around for 100 years! EvanR 06:33, 18 July 2006 (UTC)

It is very important to introduce the concept of invariance and Minkowski space as soon as possible when teaching SR. If this is not done then students get lost in the problem of how photons of light could have a constant velocity and end up missing the whole beauty of the theory. At least 90% of students who are taught SR as a subsidiary option have got absolutely no idea of what it is about because they were introduced to the subject with the old-fashioned approach. In the old-fashioned approach Einstein's postulates alone are used.

The SR book was produced over the last year. RobinH 08:39, 18 July 2006 (UTC)

if the book is targetted at undergrads in physics thats one thing. they have heard all about the nature of light and have the math background to dive right into the full theory. for others i think there should be a more down to earth explanation supported by historical anecdotes and experimental results. these 'others' probably dont have the math background to comprehend the kind of geometry that SR lives in, but you are right- it should still be emphasized that this structure is there and what mathematical form it takes. I think it is possible to do what you said, introduce invariance of spacetime intervals and other important geometry concepts, but at the same time as present it in the 'old fashioned' way. As far as confusing readers goes, there is absolutely no reason for this as long as there is a clear and precise explanation of the approach taken and the possible other approaches. If were writing a book with a problem of widespread confusing in mind, certainly there is a way to address it in the text! Obviously it would be certainly necessary to say in both sections the entire story about presentation of SR, including which is better and why. Such a thing would be necessary in any comprehensive text on SR dont you agree EvanR 20:22, 18 July 2006 (UTC)

Your inclusion of a historical section looks good. I am worried about the "approaches" section however and have removed this. There is only one modern special relativity theory and this is the theory of the world as a four dimensional manifold of events although there have been variations within this: see Approaches to SR. The various approaches to SR can be included in the existing section on spacetime and the misnamed "approaches" that are found in elementary textbooks can be included in a section on the applications of the theory.

Einstein's original (1905) approach was really a set of empirical formulae. He proposed that if c were assumed constant there were interesting relations between velocity and measurement. The modern theory of relativity is different, it has a real ontology. It proposes that if differential geometry applies to events in the world then the world is like a four dimensional manifold of signature (---+) and the physics of the world is then due to this geometry.

It is really important to remember that most people who are taught the Lorentz transformations using no more than algebra and the postulate that the speed of light is constant never really accept relativity theory. This is because this approach has no ontology. It has no understanding within it. It is actually not an explanation at all. When taught in this way the constancy of the speed of light appears to be an arbitrary assumption and students often believe that we are waiting for quantum theory to explain it! The introduction of elementary four dimensionalism overcomes this problem because it becomes apparent why the speed of light is constant and clear that special relativity is a geometrical theory.

I would propose that the existing explanation in terms of invariance and pythagoras' theorem is given pride of place and algebraical examples using people walking in trains etc. are included in a chapter containing examples of how the theory may be applied. Einstein's original approach can be included in the historical section (it was entirely superceded by 1909 although it still haunts every elementary textbook). RobinH 09:34, 19 July 2006 (UTC)

[edit] Page names

I would like to move pages to the slash convention as suggested by Wikibooks:Naming policy. Is this alright? --hagindaz 00:16, 13 May 2007 (UTC)

[edit] Still wrong PDF

Someone wrote the comment 'Wrong PDF' a while back. The comment notes that part II of the book is unavailable (the part II link points to part I, while the part I link points to a blank file).

This is still true, so... bump! :)

[edit] Contributors, List

I removed the list of contributors from the TOC. I don't feel that lists of this sort belong in the main content areas of pages. This kind of a list certainly does warrant a place, but I dont think the TOC of the book is the correct place. Maybe a coverpage or something? --Whiteknight (Page) (Talk) 02:07, 14 August 2007 (UTC)

• Carandol
• EvanR
• Flonejek
• RobinH

[edit] Print Version

I updated all the links in the print version when i moved all the pages to the new naming convention. However, I noticed that not all of the pages from the book are included in the PV (and one page was even included twice!) I've got a script that can redo the PV in one click, and hopefully be more aesthetic and more complete. I don't want to do anything radical to the PV without getting some approval first. Maybe I could put together a demonstration? --Whiteknight (Page) (Talk) 02:15, 14 August 2007 (UTC)

[edit] Twins Paradox

Einstein's relativity is a holy grail and the establishment will go to any lengths to protect it. The standard resolution by introducing an acceleration into the argument is a fallacy. My point is this.

All attempts to resolve the paradox involve making the voyager twin follow a curved path and thus experience an acceleration which the earthbound twin does not experience. However, the Time Dilation equation does not contain an acceleration term. Therefore time dilation is independent of any acceleration. As long as two inertial frames are in relative non-accelerated motion then the effect is/should be present.

Thus, if the voyager twin sets off in a straight line at near light speed (although the actual speed is irrelevant) then after a lapse of time (again the length of time does not matter) he should be younger than his earthbound twin - or rather his clock should have ticked slower so he hasn't aged as quickly. It is not necessary for the voyager to return to earth to compare the different ages and it is not necessary to follow a curved path. I seem to remember an example I got given as an undergraduate of a cosmic ray particle(?) that did not live long enough to reach earth unless you applied the time dilation equation to extend its life! This particle did not travel in a curved path and did not suffer any acceleration - you just applied the acceleration-independent equation and got the result (the result you wanted). I didn't believe it then and I don't believe it now. However, since the equation sorted out this problem then its interpretation must be correct - obviously - no question about it. Having established that the equation is correct you then apply to the twins and get the paradox.

There is no paradox. If the only way to resolve an apparent paradox is to introduce a fictitious element like Einstein's cosmological constant - in this case an acceleration which does not appear in the original equation - then either there is something wrong with the original equation or with the interpretation of it.

So then, how should one interpret the time dilation equation? As follows. The voyager twin's clock does not actually tick slower. It's just that the earthbound twin observes that it ticks slower - rather like the way an astronomer observes a redshift. A receding galaxy doesn't actually change the colour of the light it emits but to the oberver it just looks redder. Similarly, the voyager twin doesn't actually age slower - it just looks that way to the earthling. If you want to make a round trip then the argument goes like this. On the outward journey, the clock ticks slower (redshift) and on the return journey it ticks faster (blueshift); the overall result being no gain, no loss. The same applies to the voyager twin. To him the earthbound clock just appears to run slower but actually it keeps going at the same rate. Hence no paradox. To use the astronomical analogy, if the redshift galaxy appeared on the doorstep of the Milky Way you would not see a redshift.

The whole problem is blighted by the unwillingness of the establishment to acknowledge the problem with the interpretation of the equation(s). It will go to any lengths to protect the holy grail - just like the way the religious establishment refused to accept the heliocentric solar system until it had no choice. The fallacy is self-evident (blindingly obvious) and until someone who commands sufficient authority challenges the orthodoxy then this fallacious paradox will continue to exist.

Modern Special Relativity is the proposal that the universe is a four dimensional spacetime in which all observers have the same physical laws. Now, it is possible, although unlikely, that this proposal is wrong. Suppose for the moment that the proposal is correct, what physical predictions would arise? One prediction is that the elapsed time recorded on clocks that are moved away from Earth and then returned to Earth will be less than the elapsed time measured by clocks on Earth. The reasoning behind this prediction is given in depth in the section on the twin paradox:

http://en.wikibooks.org/wiki/Special_Relativity/Simultaneity%2C_time_dilation_and_length_contraction#The_twin_paradox

Notice that clocks do not tick slower or run faster in the twin paradox, all we can say is that the elapsed time for the whole journey is recorded as less by the travelling twin (the one who turns round).

Your objection about acceleration does not apply because the twin paradox can also be analysed without including any turnaround by Bill. Suppose that when Bill passes Mars he meets another traveller coming towards Earth. If the two travellers synchronise clocks as they pass each other they will obtain the same elapsed times for the whole journey to Mars and back as Bill would have recorded himself had he turned around. This shows that the "paradox" is independent of any acceleration effects at the turnaround point.

The reason that textbooks contain this result as an "orthodoxy" is that it is consistent with the proposal of a 4D universe. The 4D universe is the basis of modern physics, being the underpinning of Quantum Electrodynamics and QCD and hence at the bottom of all physics, chemistry and biology. In fact, because it does not involve proposals about the constancy of physical laws, the twin paradox is solely about 4D geometry rather than relativity per se and would apply in any manifold with the signature ---+ or +++-. The difference in elapsed times that occurs in the twin paradox is a consistent and mathematically correct result of combining time with space in a metric of the form:

Δs² = Δx² + Δy² + Δz² − (cΔt)² or Δs² = (cΔt)² − Δx² − Δy² − Δz²

So if the proposals that underpin SR, QED, QCD etc are correct then the twin paradox must apply but you are obviously at liberty to maintain that this idea of a 4D universe is incorrect and to propose alternative theories. RobinH (talk) 10:15, 12 March 2008 (UTC)

[edit] I think there's a mistake in the pdf

In page 51 in the pdf (the energy section), it says

Which gives:

while it should be:

Which gives:

Thanks for spotting this. The PDF is getting increasingly out of step with the text. It is time for a new version. RobinH (talk) 08:26, 18 April 2008 (UTC)

[edit] "Undergraduate" reading level?

I see that one section says it is intended for readers at the "Undergraduate" level. As far as I can see, this level does not exist. The categories are red links, and it's not listed at Wikibooks:Reading Levels. —The preceding unsigned comment was added by Gawaxay (talk • contribs) 23:49, 20 March 2009 (UTC)

I haven't worked much on these reading levels, but this may precede the current reading level page. Please help us update this as you see fit. You may also be interested in {{prerequisite}}. --Swift (talk) 15:38, 21 March 2009 (UTC)

[edit] PDF Corrupt

When I download the pdf file and try to open it says "not a pdf or corrupt file". —The preceding unsigned comment was added by 124.109.20.84 (talk • contribs) 21:27, 17 August 2009 (UTC)

I have no problems viewing the it. Can you reproduce it on another machine? --Swift (talk) 07:40, 18 August 2009 (UTC)

OK. Got it thru another browser. ThanX!! —The preceding unsigned comment was added by 124.109.20.84 (talk • contribs) 11:47, 18 August 2009 (UTC)

You're welcome. Hope you like the book. --Swift (talk) 18:01, 18 August 2009 (UTC)

[edit] Introduction

Hi. First of all I thank to everyone who have contributed to this book. Second, I would like to bring to the attention of the editorial team something that I think should be changed. One of the introductory paragraphs reads:

Special Relativity begins by re-examining the basis of Newtonian Physics and demonstrating that the Newtonian treatment of relative motion is incorrect. As a result the whole of classical physics must be rebuilt to account for this error.

I think here incorrect is not a correct word, instead inaccurate would be better. We know that the Newtonian treatment of relative motion is correct but at low speed regime. The second sentence is also somehow odd. We know that the classic physics is out there such as the classical mechanics and the classical electrodynamics. The SP Relativity wasn't "rebuilt" of those theories but their generalization, or lets say, extension to high speed regimes. Gulmammad (talk) 23:03, 25 October 2009 (UTC)