Undergrad What is the current perspective on quantum interpretation?

[andrew s 1905]

Most physicists accept that time dilation and length contraction follow from the relativity principle applied to the measurement of c, even though we have no constructive counterpart (causal mechanism) for that principle account. Indeed, this is the standard presentation of special relativity in the introductory physics textbooks. The mystery of quantum entanglement can be resolved in similar fashion via the relativity principle applied to the measurement of h. Would that principle account of quantum entanglement suffice here, or would you still require a constructive counterpart?

I have tried Google to follow this up but failed to find a way of phrasing it to get sensible hits. Would you kindly point me to where I might follow it up please. Thanks Andrew

 

[Tendex]

Oh, my god. I HATE Physics Forums discussions.

I agree that

1. In relativistic quantum field theory, the quantum state evolves according to local equations.
2. RQFT does not allow FTL signal transmission.

The issue is whether 1&2 imply that it is local. 1 by itself is not enough, because the smooth evolution does not include the measurement process by which a single outcome is selected out of a set of several possibilities. 2 is not enough, either, because although Local implies no FTL, no FTL does not imply local.

Again, if your goal is to make sure about Einstein relativity in quantum theories a definition of locality related to FTL is never going to help you because it can't make that distinction for quantum theories(you should be able to see this because all quantum theories share the Schrodinger evolution and the irreversible measurement evolution that you have pointed out several times), just like it did not distinguish classical electrodynamics from QM theories. So you are always going in circles, no wonder you hate discussions.
But it seems you are aware of this when you explain the issue about 1&2 in the quote above so I can't understand why you are not drawing conclusions from it regarding trying to find a definition of locality/nonlocality/FTL that helps any regarding compatibility with Einstein relativity for quantum theories if you know of its uselessness.

 

[Tendex]

Of course I also think for the same reasons that insisting on QFT locality to stress its being relativistically compatible is pretty useless. Using mathematical arguments referring to the differential operators used doesn't seem to help much either since QFT also uses nonlocal operators like Fourier transforms.

 

[martinbn]

Here is my take on locality in the usual two particle entangled situation. Consider the two events $A$ - Alice makes a measurement and $B$ - Bob does. Take a space-like hypersurface $H$ such that both events are to the future and let $S$ be the portion of it in the past null cone of event $A$. Then for locality, to predict anything about $A$ it should be enough to know information specified on $S$ alone. If on the other hand information on the $H$ that is not in $S$ makes it possible to make better predictions about $A$, then we have non-locality. In the Alice and Bob case we do have locality according to the above definition.

As said many times before, you can consider $H'$ so that $B$ belongs to it and $A$ is to the future. In other words Bob has already made the measurement and Alice is about to. Then it seems that knowing the result of Bob Alice can make a better prediction. Except that on this $H'$ the state is factorizable. Before $H'$ it may have been $|u,d\rangle - |d,u\rangle$, but on $H'$ it is one of the two summands. So given this information on $S'$ Alice can make the same prediction. Hence there is no non-locality. You can argue that this information is not available to Alice, but so what(?), lack of information is not a good reason to talk about non-locality.

I know that this is not going to change anyone's opinion (especially BMists), I am just expressing my view on the terminology. If you prefer you could say that QM is dynamically local and correlation non-local (or constrain non-local). But in my opinion there is no point in this unless you are trying to be unclear or you are a BMist and trying to push the BM agenda.

 

[Morbert]

Alice's measurement, like a classical measurement, only changed Alice's knowledge about Bob. It didn't change anything physical on Bob's end. But let's think about this. If after Alice's measurement, she knows that it is 100% certain that Bob will measure spin-down, and if Alice's measurement didn't change anything about Bob, then it means that before Alice's measurement, it must have already been true that Bob would measure spin-down (only Alice didn't know it). Well, we have to reject this, too, because it involves hidden variables (the value of Bob's measurement before he makes it).

If Alice is careful with her algebra and only includes propositions about spin-z, she can avoid hidden variables too. Upon observing e.g. spin-up, she can assume that her measurement revealed the pre-existing property spin-up of her particle, and that bob's particle has the pre-existing property spin-down which he will observe if he measures it. The logic based on these assumptions will be reliable.

 

[vanhees71]

The issue is whether QM obeys Einsteinian relativity, when one takes into account the measurement process. As I explained in a different post, quantum mechanics has two pieces: (1) the smooth deterministic evolution of the quantum state, and (2) the nondeterministic production of a measurement result. In QFT, it's pretty clear that (1) is relativistically invariant, but it's much less clear whether (2) is. According to some interpretations of QM, relativity is only approximately true, for macroscopic measurements, but fails at a microscopic level.

In any case, I don't agree with you that you need to assume Einsteinian or Galilean relativity in order to say what FTL means. It is enough that there is a single frame in which light travels at its characteristic speed. Then relative to that choice of a frame, you can ask whether or not there are FTL influences.

Of course QM cannot obey Einstein relativity, because it's a non-relativistic theory. It cannot be relativistically invariant to begin with.

There is no other known type of relativistic QT describing real-world phenomena then local relativistic QFTs (and the Standard Model). There's not the slightest hint that relativity is not true at the microscopic level.

In non-relativistic physics there's no problem to have relative velocities larger than the speed of light. There is simply no limiting speed in Newtonian physics, and there's also no causality constraint, because there's absolute time, and time is defined as an oriented 1D manifold, whose orientation defines which events can be causally connected, namely only such events can be the cause of another event, if the time of the former is less than the time of the latter.

 

[vanhees71]

Truly, I don't understand how what some people say about "collapse" isn't complete nonsense.

In an EPR experiment, with anti-correlated spin-1/2 particles, Alice and Bob agree to measure spin along the z-axis. Immediately before Alice's measurement she would say that there is a 50/50 chance that Bob will measure spin-up. Immediately after Alice's measurement, if she gets spin-up, the probability that Bob will measure spin-up changes to 0. How can you account for that sudden change?

When Alice has done her measurement she knows also what Bob will measure or has already measured. In no way can Bob know from her measurement than by just getting a message from Alice (which can only be transmitted by a signal that's not faster propagating than the speed of light). Without the exchange of information about the measurement results all what Alice and Bob get is a random result with probabilities 50% for the one or the other outcome. The 100% correlation due to entanglement (as well as the confirmation of the predicted probabilities for the measurement outcomes) can only be revealed when doing the experiment, and it doesn't depend in which temporal order Alice and Bob perform their measurements. Within local relativistic QFT there cannot be a causal effect of one measurement on the other if these measurements are space-like separated, and such an effect is not necessary to explain the correlations, which are a property of the state of the measured system prepared in the beginning before any measurement has been done on the particles.

 

[atyy]

When Alice has done her measurement she knows also what Bob will measure or has already measured. In no way can Bob know from her measurement than by just getting a message from Alice (which can only be transmitted by a signal that's not faster propagating than the speed of light). Without the exchange of information about the measurement results all what Alice and Bob get is a random result with probabilities 50% for the one or the other outcome. The 100% correlation due to entanglement (as well as the confirmation of the predicted probabilities for the measurement outcomes) can only be revealed when doing the experiment, and it doesn't depend in which temporal order Alice and Bob perform their measurements. Within local relativistic QFT there cannot be a causal effect of one measurement on the other if these measurements are space-like separated, and such an effect is not necessary to explain the correlations, which are a property of the state of the measured system prepared in the beginning before any measurement has been done on the particles.

Indeed, only the correlations exist ahead of time, but not the things that are correlated.
https://arxiv.org/abs/quant-ph/9801057v2

 

[Fra]

Immediately before Alice's measurement she would say that there is a 50/50 chance that Bob will measure spin-up. Immediately after Alice's measurement, if she gets spin-up, the probability that Bob will measure spin-up changes to 0.

This conclusion illustrates the "problem" of not explicitly acqknowleding that probabilities are conditional or JUST thinking of them as objective outcomes.

As Vanesh wrote, after Alices measurement, the probability as per Bob is still 50%. Obviously the way Bob (and possibly his measurement device) interacts with the incoming particle is independent of of Alices probability.

IMO, Qbism brings an insight that a probability has two sides.
1. It describes correlations or observations
2. It causally determines the action of the agent

/Fredrik

 

[RUTA]

I have tried Google to follow this up but failed to find a way of phrasing it to get sensible hits. Would you kindly point me to where I might follow it up please. Thanks Andrew

We're working on a more direct pedagogical version now, that's why I asked, but here are two longer versions:

Beyond Causal Explanation: Einstein’s Principle Not Reichenbach’s
https://www.mdpi.com/1099-4300/23/1/114/htm

Answering Mermin’s challenge with conservation per no preferred reference frame
https://www.nature.com/articles/s41598-020-72817-7

 

[Tendex]

I don't agree with you that you need to assume Einsteinian or Galilean relativity in order to say what FTL means. It is enough that there is a single frame in which light travels at its characteristic speed. Then relative to that choice of a frame, you can ask whether or not there are FTL influences.

Whether something is FTL doesn't depend on whether we're talking galilean or einsteinian relativity. It doesn't depend on relativity (galilean or einsteinian), since I'm only talking about a single frame. Relativity is about the relationship between two DIFFERENT frames. Within a single frame, relativity is not relevant.

Probably off-topic in a quantum subforum but I still don't know how anyone can say that talking about possible FTL influences doesn't have anything to do with relativity, but I would ask you how you are determining that light travels at its characteristic speed, using a one-way or a round-trip measure of lightspeed to campare it with the FTL influences.

 

[PeterDonis]

I would ask you how you are determining that light travels at its characteristic speed, using a one-way or a round-trip measure of lightspeed to campare it with the FTL influences

You don't have to know what speed light is traveling to test for FTL influences. You just have to emit a light signal at the same time you emit whatever supposed FTL influence you are testing, and see which one arrives first at the destination.

 

[Tendex]

You just have to emit a light signal at the same time you emit whatever supposed FTL influence you are testing, and see which one arrives first at the destination.

My point is that this procedure only makes sense in the non special relativistic scenario, since assuming SR the supposed FTL communication would arrive its destination before emiting it, so you cannot emit both a light signal and an FTL signal at the same time, it can only be done in Galilean relativity since there the speed of light is not a maximum and FTL doesn't have problems with causality , you have classical addition of velocities also at light speed.
End of the line for the subthread discussion, stevendaryl is assuming nrqm even if he is not aware and therefore can't solve anything about relativistic QFT from that standpoint.

 

[vanhees71]

Probably off-topic in a quantum subforum but I still don't know how anyone can say that talking about possible FTL influences doesn't have anything to do with relativity, but I would ask you how you are determining that light travels at its characteristic speed, using a one-way or a round-trip measure of lightspeed to campare it with the FTL influences.

Of course it all has to do with relativity. In Newtonian mechanics nothing prevents causal effects propagating ftl. It's even the usual way interactions are model, namely as actions at a distance.

 

[RUTA]

When Alice has done her measurement she knows also what Bob will measure or has already measured. In no way can Bob know from her measurement than by just getting a message from Alice (which can only be transmitted by a signal that's not faster propagating than the speed of light). Without the exchange of information about the measurement results all what Alice and Bob get is a random result with probabilities 50% for the one or the other outcome. The 100% correlation due to entanglement (as well as the confirmation of the predicted probabilities for the measurement outcomes) can only be revealed when doing the experiment, and it doesn't depend in which temporal order Alice and Bob perform their measurements. Within local relativistic QFT there cannot be a causal effect of one measurement on the other if these measurements are space-like separated, and such an effect is not necessary to explain the correlations, which are a property of the state of the measured system prepared in the beginning before any measurement has been done on the particles.

The quantum correlations are indeed realized just as predicted by non-relativistic QM (which equals QFT for low energy phenomena as in this case). So, the mystery here has nothing to do with QM vs. QFT vs. SR. All these theories are compatible with the observed results (more so than you might suspect, see this article for example). The mystery resides in the desire for a causal mechanism responsible for those correlations and that is assumed to function per the causal structure of M4 per "local realism." No one has that answer.

 

[Tendex]

The quantum correlations are indeed realized just as predicted by non-relativistic QM (which equals QFT for low energy phenomena as in this case). So, the mystery here has nothing to do with QM vs. QFT vs. SR. All these theories are compatible with the observed results (more so than you might suspect, see this article for example). The mystery resides in the desire for a causal mechanism responsible for those correlations and that is assumed to function per the causal structure of M4 per "local realism." No one has that answer.

But causality is a precondition for scientific predictions checked using measurements. What you are calling the mystery is just a departure from the classical physics understanding of how causation works probabistically.

 

[RUTA]

But causality is a precondition for scientific predictions checked using measurements. What you are calling the mystery is just a departure from the classical physics understanding of how causation works probabistically.

Supply the causal mechanism in accord with "local realism" and you will have solved the mystery. Simply stating that the formalism maps to the experimental outcomes fails to answer the mystery.

 

[PeterDonis]

assuming SR the supposed FTL communication would arrive its destination before emiting it

Only in some frames, not in others. In any case, SR in no way prohibits using spacelike curves, which is what "FTL" actually means in SR. It is perfectly possible to model objects that move "FTL" in SR using spacelike curves; Google "tachyons". That's not to say there are no issues involved in such models, but they do exist, so your claim that the procedure I described only makes sense in non-relativistic physics is wrong.

End of the line for the subthread discussion, stevendaryl is assuming nrqm even if he is not aware and therefore can't solve anything about relativistic QFT from that standpoint.

Your dogmatic declaration that no more can be said in this subthread is wholly premature. See above.

 

[Tendex]

Supply the causal mechanism in accord with "local realism" and you will have solved the mystery. Simply stating that the formalism maps to the experimental outcomes fails to answer the mystery.

But why in accord with "local realism"?, the correlations show it to be the wrong classical recipe.

 

[Tendex]

Only in some frames, not in others. In any case, SR in no way prohibits using spacelike curves, which is what "FTL" actually means in SR. It is perfectly possible to model objects that move "FTL" in SR using spacelike curves; Google "tachyons". That's not to say there are no issues involved in such models, but they do exist, so your claim that the procedure I described only makes sense in non-relativistic physics is wrong.

I'm obviously disregarding objects like "Tachyons" since they are not compatible with causation as understood in modern science. But I am not dogmatic at all so if you want to pursue that line I'll be delighted.

 

[PeterDonis]

I'm obviously disregarding objects like "Tachyons" since they are not compatible with causation as understood in modern science.

This is a very strong claim. Do you have any references to back it up? (And yes, there are plenty of peer-reviewed papers discussing tachyons.)

 

[Tendex]

This is a very strong claim. Do you have any references to back it up? (And yes, there are plenty of peer-reviewed papers discussing tachyons.)

You mean you are not aware of the issues of tachions regarding causality? You even mentioned them so I don't know how my mentioning them for the purposes of restricting my claim to particles different from tachyons is stronger than yours or requires references. Can you explain?

 

[Tendex]

Go ahead and perform mentally the experiment you proposed with photons and tachyons, dou you think it makes sense?

 

[PeterDonis]

You mean you are not aware of the issues of tachions regarding causality?

I mean that if you want to make claims about such things, you need to provide specific references to back up your claims.

Go ahead and perform mentally the experiment you proposed with photons and tachyons, dou you think it makes sense?

Sure. My experiment was one-way.

 

[vanhees71]

The quantum correlations are indeed realized just as predicted by non-relativistic QM (which equals QFT for low energy phenomena as in this case). So, the mystery here has nothing to do with QM vs. QFT vs. SR. All these theories are compatible with the observed results (more so than you might suspect, see this article for example). The mystery resides in the desire for a causal mechanism responsible for those correlations and that is assumed to function per the causal structure of M4 per "local realism." No one has that answer.

Photons for sure cannot be described with non-relativistic QM. There is no mystery at all. Only the philosophers have problems. Minimal interpreters have none ;-))).

 

[RUTA]

Photons for sure cannot be described with non-relativistic QM. There is no mystery at all. Only the philosophers have problems. Minimal interpreters have none ;-))).

Here is an experiment with photons in the Bell triplet state violating the CHSH inequality: D. Dehlinger and M.W. Mitchell, ``Entangled photons, nonlocality, and Bell inequalities in the undergraduate laboratory,'' American Journal of Physics 70(9), 903--910 (2002).

 

[vanhees71]

Yes, and it's in full accordance with QED! No mystery!

 

[RUTA]

But why in accord with "local realism"?, the correlations show it to be the wrong classical recipe.

Exactly, that's the mystery. So, the various interpretations propose various causal mechanisms that violate local realism in various ways to account for the correlations. Then the proponents of a given interpretation argue why they think the manner by which their interpretation violates local realism makes sense. So far, no one has produced an interpretation that has garnered consensus support. The most popular "interpretation" is no interpretation, i.e., Shut up and calculate (the Copenhagen interpretation per Mermin). See this poll: M. Schlosshauer, J. Kofler, and A. Zeilinger, ``A snapshot of foundational attitudes toward quantum mechanics,'' Studies in the History and Philosophy of Modern Physics 44, 222–230 (2013). \url{https://arxiv.org/abs/1301.1069}.

Thus, some foundationalists have taken to proposing a principle explanation rather than a constructive explanation of quantum entanglement, as Einstein did with special relativity (SR). In SR, we still don't have a constructive account of time dilation and length contraction, but no one cares anymore. Everyone long ago gave up on theories of the luminiferous aether and the textbooks simply show how everything in SR follows from the relativity principle applied to the measurement of c (with some additional mathematical assumptions like linearity). For example, Jeff Bub says the non-Boolean algebraic structure of Hilbert space is fundamental a la the non-Euclidean geometric structure of spacetime. But no principle account has garnered consensus support yet either. We've published a few papers in the past two years using the relativity principle applied to the measurement of Planck's constant h to derive the joint probabilities for the Bell states. That might have more success, since everyone is happy with SR and we're following the example of SR exactly. We'll see :-)

 

[RUTA]

Yes, and it's in full accordance with QED! No mystery!

As I keep saying, the formalism itself is the source of the mystery, not the resolution of the mystery. As we show in our papers, the conservation law at work for the Bell spin states when Bob and Alice are making different spin measurements holds only on average. It is explicitly violated on a trial-by-trial basis. What kind of causal mechanism is responsible for that kind of conservation? So, the formalism does not resolve the mystery, the fact that it works is the mystery.

 

[vanhees71]

I'd say we have understood a phenomenon if our theories agree with the observations. I don't know what else you mean by "understanding".

I don't understand what you are referring to when you say the conservation laws hold only on average. According to standard Q(F)T angular momentum is conserved, and it's conserved on an event-by-event basis. I'm not aware of any observation disproving this.