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

[sahashmi]

TL;DR

A recent paper was published by Tim Maudlin discussing a rift between QM and relativity

Link to paper: https://arxiv.org/abs/2503.20067

Thoughts?

 

[berkeman]

Thread is in Moderation for review by the Mentors.

 

[berkeman]

After a Mentor discussion the thread is approved provisionally.

 

[DaveC426913]

Maybe we could have a summary? Some aspect you want to talk about?

 

[Nugatory]

Maybe we could have a summary? Some aspect you want to talk about?

The abstract is a pretty good tl;dr:

It is commonly remarked that contemporary physics faces a challenge in reconciling quantum theory with Relativity, specifically General Relativity as a theory of gravity. But "challenge" is too mild a descriptor. Once one understands both what John Bell proved and what Einstein himself demanded of Relativity it becomes clear that the predictions of quantum theory, predictions that have been verified in the lab, are flatly incompatible with what Einstein wanted and built into General Relativity. There is not merely a tension but an incompatibility between the predictions of quantum theory and Relativity, and what has to give way is the Relativistic account of space-time structure and dynamics.​

However, I’d recommend reading the whole thing now that you know where Mauldin is going with his argument.

Do note that the incompatibility that Mauldin identifies is with “what Einstein himself demanded of Relativity”. That’s not quite the same thing as “Relativity”.

 

[Hornbein]

It seems to me the paper is all things I already knew. The conclusion is that Bell's theorem shows nonlocality while GR assumes locality so the theories are incompatible. It is presented quite clearly, which is nice. My only objection is to the author's predilection for italic font. While I enjoyed reading it and think it would be good reading for some people I confess that I got bored and skimmed through the second half.

 

[sahashmi]

The abstract is a pretty good tl;dr:

It is commonly remarked that contemporary physics faces a challenge in reconciling quantum theory with Relativity, specifically General Relativity as a theory of gravity. But "challenge" is too mild a descriptor. Once one understands both what John Bell proved and what Einstein himself demanded of Relativity it becomes clear that the predictions of quantum theory, predictions that have been verified in the lab, are flatly incompatible with what Einstein wanted and built into General Relativity. There is not merely a tension but an incompatibility between the predictions of quantum theory and Relativity, and what has to give way is the Relativistic account of space-time structure and dynamics.​

However, I’d recommend reading the whole thing now that you know where Mauldin is going with his argument.

Do note that the incompatibility that Mauldin identifies is with “what Einstein himself demanded of Relativity”. That’s not quite the same thing as “Relativity”.

Well he does explicitly say “There is not merely a tension but an incompatibility between the predictions of quantum theory and Relativity”, so it does seem as if he is saying relativity is incompatible with QM.

Note to other readers that he personally believes relativity is emergent and that we should work on a theory that has a preferred frame (and thus no relativistic space time) but in such a way where relativity is still emergent in the scales that we see. I’m not sure if this is the only way to tackle this issue though

 

[martinbn]

TL;DR Summary: A recent paper was published by Tim Maudlin discussing a rift between QM and relativity

Link to paper: https://arxiv.org/abs/2503.20067

Thoughts?

What about your thoughts?! Isn't this a bad way to start a topic for discussion. "Here is a paper, go."

 

[sahashmi]

What about your thoughts?! Isn't this a bad way to start a topic for discussion. "Here is a paper, go."

My thoughts echo the paper but I’ll summarize the pretty clear logical argument.

1. In the case of perfect correlations, Einstein determined, as per his EPR argument, that the only way to explain these correlations locally is through determinism. If each particle was evolving through local dynamics but was not determined, then over a series of trials, it makes no sense for each particle to always have the opposite spin on the same axis. So in some sense, Einstein ruled out local “non determinism”

2. Bell’s theorem and the experiments confirming it ruled out a local predetermined explanation. So, Bell ruled out local determinism

3. Since local determinism and local “non determinism” were both ruled out, non locality is unquestionably true, and physicists denying this are ultimately denying the results of experiments

 

[PeroK]

Perhaps someone should try to formulate a theory of quantum gravity?

 

[Fra]

TL;DR Summary: A recent paper was published by Tim Maudlin discussing a rift between QM and relativity

Link to paper: https://arxiv.org/abs/2503.20067

Thoughts?

My thoughts from reading it

1) I am guessing that Dr Chinese must love it!!??

2) I enjoy seeing his confidence in this, and it was nice summary of some historical turns

3) When I see his arguments, I think I understand more clearly why it's hard to see things from a different paradigm. It made me think, and i gave me some ideas.

/Fredrik

 

[javisot]

TL;DR Summary: A recent paper was published by Tim Maudlin discussing a rift between QM and relativity

Link to paper: https://arxiv.org/abs/2503.20067

Thoughts?

My first thought is "isn't QFT a theory that weakly reconciles GR and QM?" It is incorrect to say that GR and QM are not reconcilable, we could say that perhaps they are not completely reconcilable.

The last thing I thought is that the author's logic is built assuming that there is no theory without non-local influence capable of explaining all QM experimental data, an assumption that may be correct or incorrect.

It is easy to accept that non-local influence is necessary to explain all QM experimental data, but this does not seem appropriate if we want to reconcile GR and QM completely into an integrated theory.

 

[DrChinese]

My thoughts from reading it

1) I am guessing that Dr Chinese must love it!!??

But of course!

PS I am not Tim Maudlin hiding under an alias… :)

 

[PeterDonis]

My first thought is "isn't QFT a theory that weakly reconciles GR and QM?"

Not really. It's a theory that weakly reconciles special relativity and QM, by describing quantum phenomena in a way that's consistent with Lorentz invariance.

You can do QFT in a curved spacetime, or at least in some curved spacetimes (the asymptotically flat ones). But you're not really reconciling QM with GR, because there is no link between the spacetime curvature, as described by GR, and the matter-energy content, as described by the quantum fields. You have to specify the spacetime geometry by hand and hope that it's reasonably consistent with the matter content. And even that has limitations.

 

[sahashmi]

Note that the paper argues that QM is incompatible with special and general relativity, not just GR

 

[javisot]

Note that the paper argues that QM is incompatible with special and general relativity, not just GR

Have you read the paper? The paper deals with GR, not SR.

The author's words can cause confusion since he repeatedly uses "relativity" and "general relativity" to refer to the same thing.

 

[weirdoguy]

Note that the paper argues that QM is incompatible with special and general relativity, not just GR

Note that QFTs exist and are one of the most successfull theories to date, so arguing that QM may not be compatible with SR is, well, just crackpottery.

EDIT: Of course I'm not saying that non-relativistic QM is compatible with SR, since, well, that's obvious it's not, but there is also nothing to complain about, because it's non-relativistic

 

[Morbert]

Does the paper say anything novel or is it just reheated arguments Maudlin has made elsewhere?

 

[martinbn]

Does the paper say anything novel or is it just reheated arguments Maudlin has made elsewhere?

I didn't see anything new that has not been discussed here on PF.

 

[Lord Jestocost]

.... The last thing I thought is that the author's logic is built assuming that there is no theory without non-local influence capable of explaining all QM experimental data ....

When one takes for granted all assumptions of realism or classicality, all classical theories that allow for violations of the Bell inequality must be non-local.

 

[WernerQH]

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".

 

[martinbn]

His description of differential geometry in the beginning shows that he doesn't know any. Which of course is ok, one doesn't have to study every subfield of mathematics if one is interested in philosophy of science, but it implies that he cannot possibly understand general relativity beyond a very basic level. He also uses a few different notions of locality/nonlocality but pretends they are the same. So proving that relativity is wrong because it is local and experiments confirming violations of Bell's inequalities show nonlocality is very non-convincing. Also I am not convinced by the argument that locality in EPR implies that non measured variables have values. That is taken as obvious, but why is it? The only argument given is "how can it be otherwise".

 

[DrChinese]

1. 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.

2. 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".

OK, I'm only responding to these comments as they relate to EPR/Bell/Entanglement Experiments.

1. Without me trying to parse out or criticize your words, I just want to say: The combination of the polarizer and the detector cannot have any influence on the recorded outcomes. With PDC Bell tests, you have two of these apparati. If they contributed anything to speak of, you could not have the so-called perfect correlations with polarization/spin entanglement.

2. I can predict, with a narrow spacetime region, when and where a PDC photon will be detected. And I know similarly when/where it originated. So I would describe that as a quantum object moving through spacetime.

 

[javisot]

Let's assume this is true: nonlocal influence (in the Einsteinian sense) is necessary to fully explain the QM experimental data set. Let's deny the existence of QFT.

So we claim that GR and QM are completely irreconcilable but both theories are experimentally true, then there is no QFT, no string theory, no theory of quantum gravity, nothing.

And now?
Does anyone have a better plan?

 

[PeterDonis]

Let's deny the existence of QFT.

Why would we want to do that?

 

[javisot]

Why would we want to do that?

Obviously there is QFT, and ST and QG. I ask this for those who, like the author of the paper, claim that GR and QM are not compatible because one theory is local and the other is non-local, so are we stuck?

It seems contradictory to accept that nonlocality as the only explanation for quantum correlations and at the same time thought they could reconcile the two theories.

 

[A. Neumaier]

When one takes for granted all assumptions of realism or classicality, all classical theories that allow for violations of the Bell inequality must be non-local.

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-nonlocality and quantum relativistic locality are two compatible concepts.

 

[sahashmi]

Have you read the paper? The paper deals with GR, not SR.

The author's words can cause confusion since he repeatedly uses "relativity" and "general relativity" to refer to the same thing.

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:

 

[sahashmi]

Note that QFTs exist and are one of the most successfull theories to date, so arguing that QM may not be compatible with SR is, well, just crackpottery.

EDIT: Of course I'm not saying that non-relativistic QM is compatible with SR, since, well, that's obvious it's not, but there is also nothing to complain about, because it's non-relativistic

You can’t write down all the equations in QFT cleanly using only relativistic space time structure. So in that sense, no, it’s not “deeply” relativistic as Maudlin says in the video I just linked.

QFT also does not explain entanglement. It’s simply a workaround. There is no way to explain the bell type experiments with purely relativistic space time. If you think you can give a clear account of what is happening in those experiments step by step while only using relativistic space time, go ahead.

 

[sahashmi]

When one takes for granted all assumptions of realism or classicality, all classical theories that allow for violations of the Bell inequality must be non-local.

This distinction between “realistic” and “non realistic” is a misnomer. Science is about explaining what we observe. What we observe is “real”. No one who believes in “non realism” has ever managed to explain what that even means