Why Unitary Evolution? QM Justification Ideas

[physicsworks]

It has an explicit collapse postulate!

It does not. And not because Dirac uses the word "principle" instead of "postulate", or the word "jump" instead of "collapse". But this is indeed off-topic.

 

[A. Neumaier]

It does not. And not because Dirac uses the word "principle" instead of "postulate", or the word "jump" instead of "collapse".

You are mistaken. See https://www.physicsforums.com/threads/is-collapse-indispensable.854384/post-5359158

 

[physicsworks]

You are mistaken. See https://www.physicsforums.com/threads/is-collapse-indispensable.854384/post-5359158

Again, this is off-topic, but: thank you, I actually have read Dirac, both the original and the first Russian edition edited by Fock. Nowhere in the book did he elevate the notion of throwing out a part of the probability distribution describing a physical system to a grand status of a physical principle (using Dirac's language when writing the book). Therefore, not even esteemed Peres could find (not that he would or should have) even a small section in Dirac's book with the words "jump" and "principle" in its title (Dirac never uses the word "collapse" in his book). However, you will find an entire chapter on "The Principle of the Superposition" and separate sections on "Heisenberg's Principle of Uncertainty" and "The Action Principle".

 

[strangerep]

Again, this is off-topic, but: thank you, I actually have read Dirac, both the original and the first Russian edition edited by Fock. Nowhere in the book did he elevate the notion of throwing out a part of the probability distribution describing a physical system to a grand status of a physical principle (using Dirac's language when writing the book).

I have just now re-read the relevant section of Dirac's book (The Principles of Quantum Mechanics, 1982 ed). On p35 he writes:

We now make some assumptions for the physical interpretation of the theory. If the dynamical system is in an eigenstate of a real dynamical variable $\xi$, belonging to the eigenvalue $\xi'$, then a measurement of $\xi$ will certainly give as result the number $\xi'$. [...]

This is not exactly the usual collapse-like assumption, but Dirac goes on with: "some of the immediate consequences of the assumptions will be noted ..." (my emboldening). Among these "consequences of the assumptions" is the passage referenced by Peres and Terno, i.e., (p36):

When we measure a real dynamical variable $\xi$, the disturbance involved in the act of measurement causes a jump in the state of the dynamical system. 'From physical continuity, if we make a second measurement of the same dynamical variable $\xi$ immediately after the first, the result of the second measurement must be the same as
that of the first. Thus after the first measurement has been made, there is no indeterminacy in the result of the second. Hence, after the first measurement has been made, the system is in an eigenstate of the dynamical variable $\xi$) the eigenvalue it belongs to being equal to the result of the first measurement. This conclusion must still hold if the second measurement is not actually made. In this way we see that a measurement always causes the system to jump into an eigenstate of the dynamical variable that is being measured, the eigenvalue this eigenstate belongs to being equal to the result of the measurement.

It's pretty clear that Dirac infers a jump in the state of system (what in modern times would normally be called "collapse") as an "immediate consequence" of his assumption. Sure, it's not in a section title, afaict, but so what?