I The Great Rift In Physics: The Tension Between Relativity and QM

[sahashmi]

Locality, causality, and determinism are firmly rooted in the classical (macroscopic) world picture. There is no reason to believe that these concepts must necessarily carry over to quantum theory. I quite like Maudlin's discussion of non-locality, but I strongly disagree with his take on relativity and determinism. Others have already mentioned QFT as a union of sorts of quantum theory with relativity. It strikes me as absurd to portray quantum theory as deterministic. We never predict the specific responses of particular detectors. What's possible in the real world are just statements after the fact about correlations between the responses of different detectors.

It is quite misleading to say that a detector oriented at, say, 90 degrees "measures" the polarisation of a photon. Just as well it might have been 30 degrees (with 25% probability) or circular polarization (with 50% probability). Polarization can be viewed as a property of the detector as much as that of a photon. It is usually inferred, attributed to a photon with hindsight, knowing the experimental setup (the "beam preparation"). What Bell's theorem shows is the impossibility of ascribing definite polarizations to photons in general.
My conclusion is that photons do not exist, at least not in the usual sense of quantum "objects" moving through space. In the Aspect et al. experiments excited calcium atoms lose a certain amount of energy, and a few nanoseconds later that energy is absorbed by atoms in the detectors. Quantum electrodynamics is silent about the whereabouts of that energy, other than that it is somewhere "in the field".

But wait, isn't it obvious that photons must exist? How else can we explain the correlations? Owing to our classical mindset (giving so much weight to causality), there is an extremely strong urge to explain correlations, rather than just describe them and accept them as a fact of Nature. Is quantum electrodynamics at all conceivable without photons? There is a historical precedent: for Maxwell electromagnetic waves were inconceivable without a medium carrying them (the ether). The existence of the ether was so obvious to him that he enquired about the possibility of determining the "motion of the solar system through the luminiferous ether" through accurate measurements of the eclipses of the Jovian satellites. Nowadays we know that Maxwell's equations make perfect sense without an ether, and we see the Michelson-Morley experiments as evidence against the ether. Perhaps future physicists will interpret the Bell-type experiments as evidence against the existence of photons. Not only quantum theory, but also experiments require an interpretation (which might be subject to change)!

To be clear, I do think that we can talk about photons in a meaningful way, but we have to be careful about the possible metaphysical connotations. One could say that the ether was not abolished, but replaced by something with higher symmetry that we now call vacuum. For me, the photon "propagator" merely expresses the non-local correlations between current fluctuations in what we perceive as "emission" and "absorption" events in the "source" and "detectors".

The interesting part about Maudlin’s paper is you don’t even need to believe in quantum theory.

Even if you had no idea what quantum theory is, whether photons exist or not exist, the fact of the matter is that there are correlations occurring that CANNOT be explained by local influences. And those observations occur in labs in the macro scale that none of us can deny. That’s the beauty of the experiment. So trying to escape this conclusion by resorting to “how the quantum world is different” just simply doesn’t work.

They must only be classically nonlocal. Note that Bell's theorem is not a theorem about quantum mechanics, but about classical local hidden variable theories. Thus the viaolation of the Bell inequalities only demonstrates that quantum mechanics is not a classical local hidden variable theory.

It does not demonstrate the incompatibility of special relativity and quantum mechanics. On the contrary, we know that special relativity and quantum mechanics are compatible since the local quantum field theory called QED is fully relativistic and fully consistent with the violation of Bell inequalities. After all, the relevant experiments were made with photons satisfying QED.

Thus classical Bell-locality and quantum relativistic locality are two compatible concepts.

Read above. This notion that you can escape the conclusion by talking about how the conclusions don’t apply to a “quantum world” simply doesn’t work

 

[PeroK]

No. He argues that any theory that has locality built into it and relativistic space time cannot explain the results. Thus, this includes special relativity. The paper is very clear on this.

In other avenues, he’s repeatedly argued that physicists are mistaken in thinking that QM is compatible with (even special) relativity

See 1:43:20 here:

Physics isn't philosophy. What is the definition of compatible? Unless you can show a contradiction in the mathematics, saying two things are incompatible is meaningless.

 

[weirdoguy]

That’s the beauty of the experiment.

Yes, and those say that QM and SR are compatible in form of QFT.

You can’t write down all the equations in QFT cleanly using only relativistic space time structure.

Which equations can't be written? Give us concrete example of such equation.

 

[sahashmi]

Physics isn't philosophy. What is the definition of compatible? Unless you can show a contradiction in the mathematics, saying two things are incompatible is meaningless.

I just gave you a metric. Write down all the equations in QFT using relativistic space time. Secondly, explain entanglement using QFT.

The correlations in entanglement are ultimately considered brute in QFT. There is no explanation for why certain particles that are locally independent of each other correlate. The point of Bell’s theorem is that you cannot explain these correlations without non local influences.

Note that we have never in scientific history observed strong non local correlations that did not end up having a cause. So when people say that non local causes violate relativity, that is correct.

But having no non local causes violates the Reichenbach's Common Cause Principle. This is the claim that if two events are correlated, then either there is a causal connection between the correlated events that is responsible for the correlation or there is a third event, a so called (Reichenbachian) common cause, which brings about the correlation. This principle has stood the test of time even longer than relativity.

 

[PeroK]

I just gave you a metric. Write down all the equations in QFT using relativistic space time. Secondly, explain entanglement using QFT.

The correlations in entanglement are ultimately considered brute in QFT. There is no explanation for why certain particles that are locally independent of each other correlate. The point of Bell’s theorem is that you cannot explain these correlations without non local influences.

Note that we have never in scientific history observed strong non local correlations that did not end up having a cause. So when people say that non local causes violate relativity, that is correct.

But having no non local causes violates the Reichenbach's Common Cause Principle. This is the claim that if two events are correlated, then either there is a causal connection between the correlated events that is responsible for the correlation or there is a third event, a so called (Reichenbachian) common cause, which brings about the correlation. This principle has stood the test of time even longer than relativity.

All this boils down to is saying that 20th century physics is wrong because it fails to satisfy pre 20th century philosophical beliefs.

 

[weirdoguy]

Write down all the equations in QFT using relativistic space time. Secondly, explain entanglement using QFT.

Have you even seen any QFT textbook? xD With all due respect, you don't know what you're talking about. Big time.

 

[mitchell porter]

I started on the paper and right away came upon what I thought was a stupid statement - that the Wheeler--de Witt equation, produced by naively combining GR and QM, is evidence of a "degenerating research program" because the resulting operator equation equals zero rather than being an ordinary Schrodinger equation. "Degenerating research program" is philosophy-of-science jargon for a research direction that is producing poor results and therefore going nowhere. I mean, give the poor equation a chance! How many years did it take Dirac and his peers to go from the Dirac equation to its quantum field interpretation?

Maudlin must know that Wheeler--de Witt is still commonly used in quantum gravity, I guess I'll see if he really does reject it based on some rigid first impression, or if he's just making some methodological point about changes to first principles...

edit: OK, in this paper he really does leave it there. But from remarks elsewhere, I find that his real problem with WdW is that it doesn't have obvious time evolution, leading to a "problem of time" and the suggestion in some quarters (e.g. Julian Barbour) that time is unreal. Well, I agree with him that time is real (more specifically, that change is real) - but it's not apriori clear how that works. The flow of time may be "local" or "internal" to individual physical systems... And besides, classical GR already led to many proclamations, including from Einstein himself, that time is in some sense unreal! You don't even need to bring in QM, to find GR challenging one's intuitions about time.

I found more from Maudlin on WdW, in a blog discussion hosted by Sabine Hossenfelder in 2016. Perhaps thanks to blog decay, Maudlin's comments show up as "Unknown" [as do many other posters], while his antagonists are two string theorists posting as "dark star" and "black hole guy". They are largely talking about WdW in the context of AdS/CFT, but many broader issues of quantum, and even classical, GR, come up.

 

[javisot]

No. He argues that any theory that has locality built into it and relativistic space time cannot explain the results. Thus, this includes special relativity. The paper is very clear on this.

Due to the generalized correspondence principle you can only mention GR and still include SR. The separation you mention doesn't make sense since SR is a subset of GR.

Regarding other things that both you and the author of the paper have said,I prefer not to comment..

 

[sahashmi]

All this boils down to is saying that 20th century physics is wrong because it fails to satisfy pre 20th century philosophical beliefs.

Well, when experiments seem to contradict relativity, simply asserting that it doesn’t is itself a philosophical belief. I’m not sure you understand that interpreting any experimental result involves a philosophical belief.

The Reichenbach principle is not just a belief. It’s a physical principle that has always been correct. The same applies to relativity. One has to go. You just choose to believe that the second remains intact. And that’s a belief not based in science as much as you think.

 

[sahashmi]

Due to the generalized correspondence principle you can only mention GR and still include SR. The separation you mention doesn't make sense since SR is a subset of GR.

My point was in response to people who seem to think QM is compatible with SR but not GR. Maudlin’s point is that it is compatible with neither. What you just said agrees with what I said. I’m not the one who made the separation

 

[PeroK]

Well, when experiments seem to contradict relativity, simply asserting that it doesn’t is itself a philosophical belief. I’m not sure you understand that interpreting any experimental result involves a philosophical belief.

The Reichenbach principle is not just a belief. It’s a physical principle that has always been correct. The same applies to relativity. One has to go. You just choose to believe that the second remains intact. And that’s a belief not based in science as much as you think.

There's a discussion on Bell's Theorem and QFT here:

https://physics.stackexchange.com/q...ith-or-even-predicted-by-quantum-field-theory

I choose to believe empirical science over the pure thought of philosophy.

That said, this is a science forum and philosophy is not discussed partly for the reason that there never be a reconciliation in these cases.

 

[PeroK]

My point was in response to people who seem to think QM is compatible with SR but not GR. Maudlin’s point is that it is compatible with neither. What you just said agrees with what I said. I’m not the one who made the separation

And from a physics point of view you are wrong and wasting everyone's time. Physics goes on whether you accept it or not. This forum is to discuss that physics. If you don't want to learn physics, then please leave and join a philosophy forum.

 

[Nugatory]

This principle has stood the test of time even longer than relativity.

Until quantum entanglement demonstrated its limitations….

And seriously, you are somewhat misrepresenting the problem.
Consider that the quantum description of nature does not conflict with anything in relativity: spacelike-separated measurements commute; entanglement experiments with spacelike-separated detectors produce the same outcomes no matter which one came “first”; it is impossible to detect any causal influence from outside of a past light cone so no conflict with relativistic causality, a solid “no signalling” theorem that shows QM disallows violation of relativistic causality…. QM is a theory about measurement results, and these results are not in conflict with relativity.
So what is the problem?
The problem is that, unlike QM, relativity is a theory of physical processes, and we have been unable to imagine any physical process that could lead to the measurement results predicted by QM and is consistent with relativity.

 

[sahashmi]

There's a discussion on Bell's Theorem and QFT here:

https://physics.stackexchange.com/q...ith-or-even-predicted-by-quantum-field-theory

I choose to believe empirical science over the pure thought of philosophy.

That said, this is a science forum and philosophy is not discussed partly for the reason that there never be a reconciliation in these cases.

You’re not understanding my point.

The empirical science agrees with non local theories that contradict relativity and any theories that don’t contradict it. That choice is based upon interpretation, not “empirical science.”

Your belief that the experiments do not contradict relativity is a philosophical belief on your part. So by your own standards, you should reserve judgment.

 

[sahashmi]

And from a physics point of view you are wrong and wasting everyone's time. Physics goes on whether you accept it or not. This forum is to discuss that physics. If you don't want to learn physics, then please leave and join a philosophy forum.

Again, your belief that the experiments do not contradict relativity is a belief, not science.

If you are intending to say that non local theories such as Bohmian mechanics or any other theories that may violate relativity are false, please show scientific evidence for this.

Until then, you are the one ironically espousing philosophy, and you are the one not talking about physics.

 

[sahashmi]

Until quantum entanglement demonstrated its limitations….

And seriously, you are somewhat misrepresenting the problem.
Consider that the quantum description of nature does not conflict with anything in relativity: spacelike-separated measurements commute; entanglement experiments with spacelike-separated detectors produce the same outcomes no matter which one came “first”; it is impossible to detect any causal influence from outside of a past light cone so no conflict with relativistic causality, a solid “no signalling” theorem that shows QM disallows violation of relativistic causality…. QM is a theory about measurement results, and these results are not in conflict with relativity.
So what is the problem?
The problem is that, unlike QM, relativity is a theory of physical processes, and we have been unable to imagine any physical process that could lead to the measurement results predicted by QM.

We are not unable to imagine it. Bohmian mechanics is a perfectly legitimate physical theory that imagines it. And who knows what else may come about.

You say that entanglement demonstrated the reichenbach common cause principle’s limitations. But that is merely a belief on your part. It could simply be evidence of relativity’s (and not the principle’s) limitations as well. This choice that you are making is not based on experiment

 

[sahashmi]

Have you even seen any QFT textbook? xD With all due respect, you don't know what you're talking about. Big time.

With all due respect, personal insults aren’t arguments. Please give me a clear step by step process by what leads to the correlations we see in bell like experiments using QFT, or admit that you instead don’t know what you’re talking about

 

[javisot]

With all due respect, personal insults aren’t arguments. Please give me a clear step by step process by what leads to the correlations we see in bell like experiments using QFT, or admit that you instead don’t know what you’re talking about

Since you're starting the discussions, at least look at the links they send you (the one from PeroK)

 

[sahashmi]

Since you're starting the discussions, at least look at the links they send you (the one from PeroK)

I did look at the link. The answer is correct that microcasuality (which is a locality condition) does not violate locality, but that is true for obvious reasons. QFT was built to be local at its very foundation.

This does not mean it explains entanglement. An explanation of entanglement should show why two entangled particles, the states of which are not predetermined, happen to correlate non locally once they’ve left the source at which they are created.

From scouring around this forum, it seems as if some physicists have the mistaken impression that the Bell experiments force you to choose between locality and realism. This is a false choice. Maudlin’s paper is written to highlight that.

This answer on the physics stack exchange also gives clear, logical reasons for why no local (realistic or “non realistic”) theory can explain the results of the experiments: https://physics.stackexchange.com/q...ne-explain-quantum-entanglement/828005#828005

 

[javisot]

My point was in response to people who seem to think QM is compatible with SR but not GR. Maudlin’s point is that it is compatible with neither. What you just said agrees with what I said. I’m not the one who made the separation

I reiterate, if Maudlin and you claim that GR and QM are incompatible it's not true, because QFT exists. (@Peter objected that it's more correct to say "SR is compatible with QM", but it's also true that SR is a subset of GR)

It's correct to say "SR and QM are compatible" and "GR and QM are partially compatible," unlike Maudlin and you, . I don't see the point in using Bell correlations to claim that.

 

[sahashmi]

I reiterate, if Maudlin and you claim that GR and QM are incompatible it's not true, because QFT exists. (@Peter objected that it's more correct to say "SR is compatible with QM", but it's also true that SR is a subset of GR)

It's correct to say "SR and QM are compatible" and "GR and QM are partially compatible," unlike Maudlin and you, . I don't see the point in using Bell correlations to claim that.

Entanglement is the most important phenomenon of QM, since it’s the phenomenon that departs most strongly from our day to day intuitions. As I already wrote, QFT does not explain entanglement. So its existence, although useful, does not imply that a local theory can explain the nonlocal correlations in entanglement.

Bell already showed that no local theory, including QFT, can. Bell was well aware of QFT. He is unfortunately heavily and widely misunderstood or misinterpreted. This is the point of Maudlin’s paper.

Since locality is built into relativity, and since no local theory can explain entanglement, no relativistic theory can explain entanglement. This is not a “speculation”. It’s what the experiments show.

 

[javisot]

Since locality is built into relativity, and since no local theory can explain entanglement, no relativistic theory can explain entanglement. This is not a “speculation”. It’s what the experiments show.

With GR you can conjecture er=epr, I understand that Maudlin and you consider that path incorrect?

 

[sahashmi]

With GR you can conjecture er=epr, I understand that Maudlin and you consider that path incorrect?

The ER=EPR approach would arguably be a non local hidden variable theory. That is only if entanglement can, on a fundamental level, be mechanistically explained by wormholes in spacetime

Whatever connection the particles would have would involve some sort of superluminal influence

 

[martinbn]

You use locality as if there is only one notion with that name!

 

[mitchell porter]

As I already wrote, QFT does not explain entanglement.

Hopefully you're aware that entanglement does exist in QFT?

 

[sahashmi]

Hopefully you're aware that entanglement does exist in QFT?

It exists but it is not explained. An explanation of why non local correlations occur involves showing why they occur instead of not. There is no reason given for this in any local theory. Non local theories do give reasons for this: the measurement of one particle can influence the other

 

[Fra]

I agree the distinction between descriptive value and explanatory value is important.

The focus of the paper was conceptual inconsistencies; not descriptive inconsistencies.

But it's hard not to think that the lack of progress, is not related to our lack of understanding, and I think this is what Maudlin thinks as well, even if I don't share his confidence in the claims.

What became more clear after reading Maudlins various argument is the strong constraints on reasoning that I think are implicit in the paradigm it self. And then I mean the system dynamics paradigm.

How can a description ever offer an explanation beyond the obvious timeless picture: initial state + law => future state.

To answer, why that particular law? why this statespace? We can finetune it to match observations, but what does that "explain"?

And in the case of entanglment, we are asking questions about parts of the systems interacting with other parts. Then we at minimum need to explain how a more complex system are formed, from combining smaller systems, and how the dynamical law of the joint system is like? and why! We can't "explain" this in the paradigm. The law is just "input".

I see a big problem in the reasoning about this, that has to do with our preconception of that "theory" must fit into the system dynamics paradigm. I think this is inhibiting. This disturbed me the most as reading his paper, and it's why I don't share the sub-conclusions of the paper, except that last scentence:

"What sort of space-time structure should be used in place of the space-timme of Special Relativity or the
spacetimemes of General Relativity is not at all clear, and should be a queston given the highest
level of attention by anyone seeking to “reconcile” quantum theory with Relativity"

No matter how much time passed since, respect to Einstein for not letting the conceptual disturbances pass, when most people let it go when theory after all makes good predictions and avoid headache we don't get paid for having.

/Fredrik

 

[sahashmi]

I agree the distinction between descriptive value and explanatory value is important.

The focus of the paper was conceptual inconsistencies; not descriptive inconsistencies.

But it's hard not to think that the lack of progress, is not related to our lack of understanding, and I think this is what Maudlin thinks as well, even if I don't share his confidence in the claims.

What became more clear after reading Maudlins various argument is the strong constraints on reasoning that I think are implicit in the paradigm it self. And then I mean the system dynamics paradigm.

How can a description ever offer an explanation beyond the obvious timeless picture: initial state + law => future state.

To answer, why that particular law? why this statespace? We can finetune it to match observations, but what does that "explain"?

And in the case of entanglment, we are asking questions about parts of the systems interacting with other parts. Then we at minimum need to explain how a more complex system are formed, from combining smaller systems, and how the dynamical law of the joint system is like? and why! We can't "explain" this in the paradigm. The law is just "input".

I see a big problem in the reasoning about this, that has to do with our preconception of that "theory" must fit into the system dynamics paradigm. I think this is inhibiting. This disturbed me the most as reading his paper, and it's why I don't share the sub-conclusions of the paper, except that last scentence:

"What sort of space-time structure should be used in place of the space-timme of Special Relativity or the
spacetimemes of General Relativity is not at all clear, and should be a queston given the highest
level of attention by anyone seeking to “reconcile” quantum theory with Relativity"

No matter how much time passed since, respect to Einstein for not letting the conceptual disturbances pass, when most people let it go when theory after all makes good predictions and avoid headache we don't get paid for having.

/Fredrik

Here’s the issue with this.

You may be right that “explanation” in science does often come down to initial state, law, and future state. But not every law is the same.

For example, a future law that tells you why the (+,-) result occurs instead of a (-,+) in the case of entanglement is clearly superior to a “law” that says one of them will occur (current QM). The structure remains the same: you have an initial state, law, and future state. But one is deterministic and the other is indeterministic. Determinism is superior in the sense that it provides more explanatory power and we should get as close to that as possible.

As scientists, we must peer further and further into phenomenons as much as possible. Until QM, we never came across anything that did not have a cause. And the fact that there are statistical laws instead of deterministic ones in QM shouldn’t be seen as evidence that millions of uncaused things are happening every second. Rather, it should be seen as a signal that we have more to explore

 

[Nugatory]

It exists but it is not explained.

This is true, but then again quantum mechanics doesn't explain anything. It is a theory about measurement results and doesn't pretend to explain why the results are what they are.

An explanation of why non local correlations occur involves showing why they occur instead of not. There is no reason given for this in any local theory. Non local theories do give reasons for this: the measurement of one particle can influence the other

And which non-local theories make that claim? You look at the observed correlations and assert that the correlations can only arise from one measurement influencing the other. But QM makes no such claim, it doesn't say anything except that its mathematical formalism accurately predicts measurement results.

 

[sahashmi]

This is true, but then again quantum mechanics doesn't explain anything. It is a theory about measurement results and doesn't pretend to explain why the results are what they are.
And which non-local theories make that claim? You look at the observed correlations and assert that the correlations can only arise from one measurement influencing the other. But QM makes no such claim, it doesn't say anything except that its mathematical formalism accurately predicts measurement results.

Sure but then it’s not a complete theory. One can still make inferences on there being nonlocal influences without having a complete theory though. For example, one could have made a reasonable inference that there was unknown physical mechanism that can be represented as a theory to explain lightning, before we actually discovered this.