No relativistic Bohmian mechanics and superluminal causation

[pines-demon]

Even if I do not agree completely with him, I have followed some of the works of Tim Maudlin on explaining Bell's theorem which have been illuminating. He founded the John Bell Institute and he is what I would consider a Bell fundamentalist.

However today I found on YouTube two very weird statements coming from him about non-locality:

Minutes 1:49:55 and and 1:57:00.

The first claim is that he is convinced that Bohmian mechanics is right and that is special relativity that is wrong. The second claim is that causality is not bounded by locality.

The first claim is that you cannot construct a relativistic version of Bohmiam mechanics because relativity is incompatible with Bell's nonlocality. In his words:

It's not a bug it's a feature so what does this mean though that it has a problem can we it means I can't write down those [relativistic Bohmian] equations, so if I go and try and write down, say, the non-relativistic version which involves two equations, the Schrödinger equation which is already a little puzzling because it governs the wave function, which is non-local in space-time at all, and then the guidance equation which tells you, given the wave function, how the particles move, what their trajectories are. Now you can't write down the guidance equation without using absolute simultaneity in the in the non-relativistic theory. I mean the time order between distant experiments, and David [Bohm?] has a nice example of this in his book:

Allison Bob are doing experiments in separate labs, very far away from each other. In a local theory nothing that Alice does makes any difference to what Bob sees, nothing Bob does makes any difference to what Alice sees. That's sort of the intuition of locality. The one that Einstein had. If you put them far enough away, they're causally isolated from each other. In pilot wave theory they absolutely are not. In pilot wave theory, you first of all write down the equation with this absolute simultaneity and sometimes if Alice does something, that will make a difference to what Bob sees and it will depend on whether what she does, she does before Bob does his experiment, or after. So the absolute time order between these two distant experiments is physically important it determines whether Bob's experiment comes out this way, or that way.

The non-locality of the pilot wave theory is in your face, in the equations. It's manifestly non-local and that's why Einstein didn't like, because Einstein didn't like action-at-a-distance he didn't like the non-locality. Einstein didn't understand of course Bell's result, that you can't get away from it, so Einstein spent the rest of his life trying to find a theory that was local. Only after he died, did Bell prove you're out of luck you're not going to find a local theory so from my point of view the correct moral of that is sorry relativity, it looks like we need we need something like absolute simultaneity back again we need a foliation back again, for reasons Einstein didn't understand. So when you say there's a problem, yeah you can't write down that theory using only relativistic space-time structure but I don't think that's a problem with Bohmian mechanics, I think that's a consequence of the world itself being non-local and I know the world itself is non-local because I can do experiments that violate Bell's inequality. That's not interpretation that's go in the lab do his experiments here's a constraint on local theories, that constraint is violated conclusion the world is not local.

He later says that Tumulka did some relativistic Bohmian mechanics that is totally ad-hoc and not well-constructed in his view.

The second weird comment that Maudlin makes it's about superluminal causality:

So if you have a local object, which means something that can be pegged to regions of space-time, it has a trajectory, and you can ask does that trajectory ever go outside the light cones? does it go faster than light? There are theoretical particles called tachyons which would do that. Their trajectories would as it were be horizontal. There's no indication there's anything like that, there's no indication there is a locally definable thing that follows a trajectory and the trajectory goes outside the light comes.

But then you ask about causation. Causation doesn't require that, right? I mean if the theory tells me, if I press a button here, a light will go on there, and regularly it does, and furthermore I can press it at random, and I want to even signal to you, I send you a a message in Morse code by pressing this button and that light's going on and off, we might say, yeah there's a causal connection there. And that might be at space-like separation, that might be I press the button here and the light goes on outside the future light cone. That would be superluminal causation, every quantum theory has that.

That's what it is to say you can only get this phenomena with a non-local theory. The locality is causal locality. The Bell's locality condition is a causal condition, and what he says is causation as it were has to go faster-than-light but go faster than light is misleading because it's not like you can track something going from here to there, it's just this event cause that event at space-like separation so that you need.

Isn't this totally misrerpresenting special relativity? I know Maudlin is working on alternative space-times but claiming that we need back simultaneity is a stretch. Is it possible to construct such a theory and keep general relativity and QFT? I very skeptical of that. Relativistic QFT makes a lot of predictions and never has to invoke simultaneity.

The second thing is that superluminal causation is forbidden right? Isn't that the whole point of the no signaling theorems?

 

[PeterDonis]

The first claim is that he is convinced that Bohmian mechanics is right and that is special relativity that is wrong.

@Demystifier has taken a somewhat similar viewpoint in papers he has published--his take, as I understand it, is to construct a model in which the Lorentz invariance of relativity is only an emergent, approximate symmetry, and the underlying dynamics it emerges from is Bohmian mechanics or something like it.

 

[Demystifier]

@Demystifier has taken a somewhat similar viewpoint in papers he has published--his take, as I understand it, is to construct a model in which the Lorentz invariance of relativity is only an emergent, approximate symmetry, and the underlying dynamics it emerges from is Bohmian mechanics or something like it.

Yes. I would also like to add that there are many suggestions that Lorentz symmetry is emergent without assuming Bohmian mechanics or anything similar. Many such suggestions are based on condensed-matter type models. One interesting paper of that sort that I only recently became aware of is https://arxiv.org/abs/1102.0789v2 .

 

[pines-demon]

@Demystifier has taken a somewhat similar viewpoint in papers he has published--his take, as I understand it, is to construct a model in which the Lorentz invariance of relativity is only an emergent, approximate symmetry, and the underlying dynamics it emerges from is Bohmian mechanics or something like it.

Yes. I would also like to add that there are many suggestions that Lorentz symmetry is emergent without assuming Bohmian mechanics or anything similar. Many such suggestions are based on condensed-matter type models. One interesting paper of that sort that I only recently became aware of is https://arxiv.org/abs/1102.0789v2 .

Thank you for your responses. This is pretty informative. I think my skepticism stands from the fact that such a feat would immediately rewrite QFT and GR so any modification in that sense has to be very very subtle.

Any of you care to comment on the second part? Is superluminal causation even a thing? Or would that fall outside the Standard Model?

 

[Demystifier]

The second thing is that superluminal causation is forbidden right? Isn't that the whole point of the no signaling theorems?

No, the key is to distinguish causation from signaling. Signaling is anthropomorphic, causation is not. Causation is about what happens in nature, at the fundamental microscopic level. Signaling is about what macroscopic agents (like humans) can do in practice, without access to fine microscopic details of macroscopic systems. For example, even in classical physics, causation does not distinguish future from the past, while signaling does. In the same spirit, according to deterministic interpretations of QM like Bohmian mechanics, superluminal causation exists, but superluminal signaling is, in practice, impossible.

 

[Demystifier]

Thank you for your responses. This is pretty informative. I think my skepticism stands from the fact that such a feat would immediately rewrite QFT and GR so any modification in that sense has to be very very subtle.

It doesn't need to be so subtle, it's enough to assume that apparently continuous spacetime is actually discrete at very small distances, like the Planck distance.

Is superluminal causation even a thing? Or would that fall outside the Standard Model?

Yes and yes.

 

[pines-demon]

No, the key is to distinguish causation from signaling. Signaling is anthropomorphic, causation is not. Causation is about what happens in nature, at the fundamental microscopic level. Signaling is about what macroscopic agents (like humans) can do in practice, without access to fine microscopic details of macroscopic systems. For example, even in classical physics, causation does not distinguish future from the past, while signaling does. In the same spirit, according to deterministic interpretations of QM like Bohmian mechanics, superluminal causation exists, but superluminal signaling is, in practice, impossible.

Ok but the only evidence of superluminal causation would be the result of entanglement measurements? Or are we talking also about quantum erasers and delayed-choice experiments?

 

[pines-demon]

Yes and yes.

The second yes is teasing, does that mean that there are new phenomena that we could test?

 

[Demystifier]

The second yes is teasing, does that mean that there are new phenomena that we could test?

In principle yes. For example, a particle collider stronger than LHC might find signs of Lorentz invariance violation at very high energies.

 

[Demystifier]

Ok but the only evidence of superluminal causation would be the result of entanglement measurements? Or are we talking also about quantum erasers and delayed-choice experiments?

Quantum erasers and delayed choice experiments have nothing to do with superluminal causation, or at least not in the Bohm-type theories.

 

[pines-demon]

It doesn't need to be so subtle, it's enough to assume that apparently continuous spacetime is actually discrete at very small distances, like the Planck distance.

It has to be subtle in the sense that it would right away lead to corrections to general relativity so it has to be "flexible" enough.

 

[martinbn]

Isn't this totally misrerpresenting special relativity?

I think he is misrepresenting quantum theory. He says

...And that might be at space-like separation, that might be I press the button here and the light goes on outside the future light cone. That would be superluminal causation, every quantum theory has that.

Surely every quantum theory has the no signaling theorem that says exactly the opposite.

 

[martinbn]

@Demystifier has taken a somewhat similar viewpoint in papers he has published--his take, as I understand it, is to construct a model in which the Lorentz invariance of relativity is only an emergent, approximate symmetry, and the underlying dynamics it emerges from is Bohmian mechanics or something like it.

Is there an actual model that does that? The papers of have seen only say that may be there is a possibility for the existence of such model.

 

[Demystifier]

Is there an actual model that does that? The papers of have seen only say that may be there is a possibility for the existence of such model.

For an explicit model see e.g. https://arxiv.org/abs/hep-th/0209167

 

[martinbn]

For an explicit model see e.g. https://arxiv.org/abs/hep-th/0209167

Where does he show how relativity emerges?

 

[pines-demon]

I think he is misrepresenting quantum theory. He says

Surely every quantum theory has the no signaling theorem that says exactly the opposite.

If I understand what is going on here from @Demystifier 's response is that in Bohmian mechanics you need nonlocal interactions to explain entanglement, thus you need some weak version of superluminal causality in that interpretation.

Edit: but I am still bothered by Maudlin, I mean it is not the type of superluminal causality that allows to flick switches with space-like separations [unless you break the current models].

 

[Demystifier]

Where does he show how relativity emerges?

Sec. A.3 and references therein.

 

[martinbn]

Sec. A.3 and references therein.

No, that is just a comment about his pet theory of garvity.

 

[martinbn]

If I understand what is going on here from @Demystifier 's response is that in Bohmian mechanics you need nonlocal interactions to explain entanglement, thus you need some weak version of superluminal causality in that interpretation.

I've only seen some vague handwaving about this, and I remain unconvinced.

Edit: but I am still bothered by Maudlin, I mean it is not the type of superluminal causality that allows to flick switches with space-like separations [unless you break the current models].

I agree with you.

 

[pines-demon]

I've only seen some vague handwaving about this, and I remain unconvinced.

Sure, but do you agree that Bohmian mechanics is nonlocal? Then it can explain entanglement with a very restricted version of faster-than-light causation. Of course you can remain unconvinced of Bohmian mechanics altogether.

 

[martinbn]

Sure, but do you agree that Bohmian mechanics is nonlocal? Then it can explain entanglement with a very restricted version of faster-than-light causation. Of course you can remain unconvinced of Bohmian mechanics altogether.

Yes, but my impression is that Maudlin talks about a stronger version of action at a distance than this in the video. Also I have never seen the details of how BM handles entanglement so I am skeptical about that too.

 

[Demystifier]

Also I have never seen the details of how BM handles entanglement so I am skeptical about that too.

People have various objections against BM, but nobody ever argued that BM can't handle entanglement. Just the opposite, the way it handles entanglement is one of the main reasons why adherents of BM like it, because it makes the source of Bell nonlocality perfectly clear and explicit.

 

[martinbn]

People have various objections against BM, but nobody ever argued that BM can't handle entanglement. Just the opposite, the way it handles entanglement is one of the main reasons why adherents of BM like it, because it makes the source of Bell nonlocality perfectly clear and explicit.

I am not argueing that it cannot. I said that i haven't seen it. All i have seen lacks detail and involves some sort of "it is clear that it can be done" but no details.

 

[Demystifier]

I am not argueing that it cannot. I said that i haven't seen it. All i have seen lacks detail and involves some sort of "it is clear that it can be done" but no details.

Then you haven't seen much.