A question for all the physicists and physics enthusiasts on the Fedi: What is the best exposition you've seen of something approximating the Copenhagen Interpretation or a relatively orthodox view of quantum measurement? That's intentionally a bit vague (because people even seem to disagree a bit about what these words mean), but I'm definitely excluding things like MWI or Bohmian mechanics here.
Ever since I first read "The Many-Worlds Interpretation of Quantum Mechanics" by Dewitt, I have been a subscriber to the MWI. I was essentially following the maxim of Arthur Conan Doyle, "When you have eliminated the impossible whatever remains, however improbable, must be the truth;" The MWI was the only interpretation of QM that ever seemed internally consistent to me.
But I sometimes wonder if I just haven't seen a really good presentation of a more orthodox view. People who subscribe to something that might be called the orthodox or Copenhagen Interpretation often don't really seem to think the topic merits serious discussion, so perhaps that's why they don't make very convincing arguments. Is there a really good version of these ideas that I've been missing?
@internic For me (undergrad level), it's Allan Adam's intro course. He references various interpretations, but it's a nice compromise between "shut up and calculate" and "WTF is actually going on?". And yeah, personally I'm a fan of MWI, but recognise that it's currently untestable and just a philosophical angle.
https://www.youtube.com/watch?v=lZ3bPUKo5zc
@_thegeoff Does that course actually grapple with the "problem of measurement" (i.e. questions like under what specific circumstances state reduction occurs, etc.)? That would be surprising for what appears to be an intro course.
@internic Not in detail, that I recall. But it doesn't specify one particular interpretation, which given current evidence seems to be to only honest way to present it.
@_thegeoff I guess I'd say that generally in order to talk about measurement you sort of have to implicitly choose an interpretation (because it really affects how you describe things), even though the math then turns out to be the same (setting aside the really difficult cases like mesoscopic systems or the wavefunction of the Universe).
@_thegeoff Ah, good, this brings out that I need to clarify my question and thinking. So, in the MWI, I think you can in principle give a reasonable answer to question like "when is a measurement considered to have occurred" or "when does a measurement process become irreversible?"
In principle you can define a Hamiltonian for the system being measured and the measuring apparatus, describing their energetic structure, as well as an interaction Hamiltonian that describes the way in which they interact and how strongly. You can then use the Schrödinger equation to predict how that setup evolves in time.
Say that the system to be measured has a property, say the z-component of the magnetic moment m_z, and the measuring apparatus has some internal "pointer" variable u that records the value. The apparatus starts in some default pre-measurement state u=u_0, and then the Hamiltonian will cause the pointer variable u to evolve based on the value of m_z, u -> u_- for m_z = -1/2 and u -> u_+ for m_z = +1/2.
If the system starts in a superposition of m_z = -1/2 and m_z = +1/2 then the Schrödinger equation causes the system to evolve into an entangled state where the possibilities m_z = -1/2, u = u_- and m_z = 1/2, u = u_+ are in a superposition. This occurs over a finite time (given by the Hamiltonian), and once it has occurred another apparatus that measures the same system will no longer see a coherent superposition of the states m_z = +/- 1/2. The measurement becomes irreversible on a timescale given either by the Hamiltonian of the measuring apparatus (if it's sufficiently big) or on a timescale given by its coupling Hamiltonian to its environment (the latter being what then divides the Universe into branches).
Of course, for a macroscopic apparatus you can't actually do the exact math for any of this, and the complexity of the measuring apparatus and the shortness of the timescales may mean that you can't practically test this theory, but at least there is an answer in principle.
So what I'm looking for is a similarly detailed accounting of this measurement process using a more orthodox interpretation.
@internic But to do that we need to define, I dunno, stuff like the timescale of wave collapse (if it exists), and we don't know that, or even whether time is quantised....or what time is, and so on and so forth.
This is why I'm a MWI fan, it wipes out many of these complications at a stroke...but it's still just a nice idea.