This Quantum World/Feynman route

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The Feynman route to Schrödinger[edit]

The probabilities of the possible outcomes of measurements performed at a time  are determined by the Schrödinger wave function . The wave function is determined via the Schrödinger equation by What determines  ? Why, the outcome of a measurement performed at — what else? Actual measurement outcomes determine the probabilities of possible measurement outcomes.

Two rules[edit]

In this chapter we develop the quantum-mechanical probability algorithm from two fundamental rules. To begin with, two definitions:

  • Alternatives are possible sequences of measurement outcomes.
  • With each alternative is associated a complex number called amplitude.

Suppose that you want to calculate the probability of a possible outcome of a measurement given the actual outcome of an earlier measurement. Here is what you have to do:

  • Choose any sequence of measurements that may be made in the meantime.
  • Assign an amplitude to each alternative.
  • Apply either of the following rules:

Rule A: If the intermediate measurements are made (or if it is possible to infer from other measurements what their outcomes would have been if they had been made), first square the absolute values of the amplitudes of the alternatives and then add the results.
Rule B: If the intermediate measurements are not made (and if it is not possible to infer from other measurements what their outcomes would have been), first add the amplitudes of the alternatives and then square the absolute value of the result.


In subsequent sections we will explore the consequences of these rules for a variety of setups, and we will think about their origin — their raison d'être. Here we shall use Rule B to determine the interpretation of given Born's probabilistic interpretation of .

In the so-called "continuum normalization", the unphysical limit of a particle with a sharp momentum is associated with the wave function

Hence we may write

is the amplitude for the outcome of an infinitely precise momentum measurement. is the amplitude for the outcome  of an infinitely precise position measurement performed (at time t) subsequent to an infinitely precise momentum measurement with outcome And is the amplitude for obtaining  by an infinitely precise position measurement performed at time 

The preceding equation therefore tells us that the amplitude for finding  at  is the product of

  1. the amplitude for the outcome and
  2. the amplitude for the outcome (at time ) subsequent to a momentum measurement with outcome 

summed over all values of

Under the conditions stipulated by Rule A, we would have instead that the probability for finding at is the product of

  1. the probability for the outcome and
  2. the probability for the outcome (at time ) subsequent to a momentum measurement with outcome 

summed over all values of

The latter is what we expect on the basis of standard probability theory. But if this holds under the conditions stipulated by Rule A, then the same holds with "amplitude" substituted from "probability" under the conditions stipulated by Rule B. Hence, given that and are amplitudes for obtaining the outcome in an infinitely precise position measurement, is the amplitude for obtaining the outcome in an infinitely precise momentum measurement.

Notes:

  1. Since Rule B stipulates that the momentum measurement is not actually made, we need not worry about the impossibility of making an infinitely precise momentum measurement.
  2. If we refer to as "the probability of obtaining the outcome " what we mean is that integrated over any interval or subset of the real line is the probability of finding our particle in this interval or subset.