- #36
vanesch
Staff Emeritus
Science Advisor
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chronon said:I think you can put it this way:
If the Bell inequalities are still violated when you have two experimenters choosing the detector settings at spacelike separation,
then either
(1) Your model must have something corresponding to FTL communication
or
(2) Your model must include the minds of the experimenters
I would agree with you if these two options were all there was to it. But it is not!
In (1), you still must explain me why the photodetector cannot be described by the unitary evolution of the schroedinger equation describing the photo detector processes. If the essential process is photo-emission (in a photomultiplier), then we perfectly know how that works, through unitary evolution.
So the "measurement problem" still stands unsolved: what physical processes are "measurements" (and don't follow the Schroedinger equation, but follow the Born rule and the projection postulate), and what physical processes are "interactions" following unitary evolution ? When is the emission of an electron by an impinging photon a measurement (and hence will collapse a wave function through a yet unknown FTL process), and when is it a physical process that could still in principle be used to do further QM with ?
And on top of that you have to invent an FTL transmission in the past in such a way that you cannot use it.
In (2), you can say that you know that already: it is the Born rule.
So in (2), you essentially solve the measurement process issue and you do not need to invent an FTL communication.
So the options are:
(1) Your model must have something corresponding to FTL communication AND you must STILL explain in what a measurement is physically different from an interaction.
(2) Your model of the minds of the experimenters is given by the Born rule.
I think that (2) is much closer to the actual formalism than (1). In fact, (1) expects a NEW theory, with new physics in it. Indeed, whenever the exact physical process responsable for the distinction between a measurement and an interaction is found, it will be possible to determine that experimentally (even if our current technology is maybe not yet up to it), because it will be IMPOSSIBLE in principle to obtain superpositions in that case. An application of the Born rule implies that you fix the basis in which you "measure", while unitary evolution let's you free to work in any basis.
I had a similar discussion in another thread here. Look at your photodetector, say, a PM. You will probably agree with me that it is the electron emission from the photocathode that is the "measurement process". All the rest is amplification.
So, you say, when a photon impinges on a photocathode, there is a relatively high probability that an electron is emitted. But what, in this process, is not unitary ? In what "photon basis" do we now apply the Born rule ? I think it is quite obvious that it is the standard "photon" basis of Fock space (there is no photon, or there is 1 photon, or there are 2 photons...). Does this then mean that in all interactions of light with a metal, we have to work in that basis and apply the Born rule ?
Hell no ! If that were the case, a metal surface wouldn't work as a mirror ! Indeed, to do so, you need to have a coherent light state (a superposition of Fock states) interact with the sea of electrons, in order for them to emit another coherent state which is the reflected beam. So now suddenly, the preferred basis is the basis of coherent states ?
You will then say: no, it is when a photon is "absorbed" that you apply the Born rule in the Fock basis. But isn't the mirror action an absorption and coherent re-emission of the coherent states by the sea of electrons then ?
Ah, you will say: it is when ENERGY is transferred between the EM field and the electrons that you have to apply the Born rule. But (ok, I failed to come up with the correct complete calculation) if that were true, stimulated emission couldn't amplify coherent states in a laser then !
And this is the case each time when you analyse a "measurement device". Each time you think you've found the pivotal process that "does the measurement" you can find situations where very similar interactions are necessarily described by unitary processes and superpositions have to remain so in order to be correct. So why, in some cases, do these processes "collapse" the wavefunction and send out their FTL signals, and not in other cases ?
So maybe there ARE indeed physical processes that collapse the wavefunction, and maybe there ARE then FTL messages sent out. But you agree with me that that is a whole lot of new physics to be added, so we're not talking about an *interpretation* of QM anymore. It is only in such a setting that (1) makes sense.
cheers,
Patrick.