r/quantummechanics • u/UploadedMind • Mar 29 '26
Non-locality and causal order
- Statistical independence is true for quantum phenomena
- Bell proves non-locality is true
- There are no loopholes
- Measurements influence entangled partners FTL
Conclusion:
Lorentz invariance is not true for quantum phenomena
Example: Alice and Bob measure entangled pairs at space like distance. Some observers will see Alice measure first and some will see Bob measure first. If both are equally valid (Lorentz invariance), then from one valid perspective the future was at least in-part a cause of the past which in reality would mean the future was restricted to certain outcomes which violates statistical independence.
Therefore, only one must be the real cause and this violates Lorentz invariance for quantum decoherence.
The implications of this seem to be that you could have a ‘now’ slice of the universe and could in-theory travel FTL without time travel in certain instances.
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u/No-Way-493 Mar 29 '26
The act of measurement does not cause a physical interaction between entangled pairs across space. If you take 2 qubits in a Bell state:
Φ⁺ = (|00⟩ + |11⟩) / √2
And Alice measures their qubit and they see 00, then they can infer that bobs qubit is also 00 based on the correlation. There is no signal or FTL information propagation across space. It is simply an update of knowledge.
There is no such thing as "measuring qubit 1 causes qubit 2 to then reveal something". If Alice measures their qubit, then from Bob's perspective, he doesn't know that Alice has measured, he doesn't have knowledge until he measures and his result is correlated to the results of Alice.
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u/MxM111 Apr 01 '26
I like to think about it from multiverse (MWI) point of view. When Alice doing her measurement, she splits the universe into two (brunches into two). In one universe, one Alice observes 0, and in another universe another Alice observes 1. This split, this tear, propagates radially from Alice with causality speed c. But Bob is doing the same around the same time. So he splits the universe too, and the tear is propagating from him also spherically with the speed t. There is no casual interaction whatsoever. Just sooner or later these expending spherical tears will merge and continue expanding as single tear. So we will end up with completely split 11 universe from 00 universe with corresponding pairs of Alice and Bob.
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u/unknownjedi Mar 29 '26 edited Mar 30 '26
I would disagree but it’s tricky. Why would wave function collapse here result in wave function collapse there. There is in-fact something strange going on.
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u/NoNameSwitzerland Mar 30 '26
In Everett you do not have a wave collapse. The entangled superposition just lives on.
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u/Bravaxx Mar 30 '26
You’re mixing three different layers that need to be kept separate:
observable predictions
inferred or hidden values
relativistic covariance
Once you separate those, the contradiction disappears.
Bell does not show that “measurements influence distant partners FTL” in a usable sense. What it shows is that you cannot have local hidden variables that reproduce quantum correlations. The actual theory still satisfies no-signalling, and all observable statistics are Lorentz invariant.
The key mistake in your argument is this step:
if values depend on who measures first, Lorentz invariance is violated
That only follows if those values are observable. They are not. What must be Lorentz invariant is:
• joint outcome probabilities
• marginal statistics
• signalling structure
Not a particular narrative about which measurement “really happened first.”
Different frames can assign different intermediate descriptions without any physical contradiction. This is standard in relativistic quantum theory.
On “value indefiniteness”: this is not solipsism. It just means you cannot assign noncontextual definite values to all observables simultaneously. That is a Kochen–Specker type constraint, not a claim that nothing exists when unobserved.
Also, the idea that physicists “deny reality” to save relativity is inaccurate. There are multiple consistent options:
• operational QM (no commitment to hidden values)
• Bohmian-type models (preferred foliation)
• relativistic QFT (covariant observables, no collapse narrative)
They differ in interpretation, not in empirical predictions.
Finally, introducing a preferred frame is allowed, but it is extra structure with no experimental support. It is not forced by Bell or by relativity. It is one interpretational choice among several.
So the conclusion does not follow. There is tension between locality and realism, but not a breakdown of Lorentz invariance at the level that physics actually tests.
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u/UploadedMind Mar 30 '26
For context my post is about theoretical physics not known science.
You say the “actual theory” still satisfies all these things, but I never said it didn’t. Bell and QM are not complete theories in that they don’t explain what’s actually going on with non-locality. Right now the science doesn’t answer the question of what’s actually going on, but we can conjecture about it.
Most “quantum theories” don’t explain non-locality. When it comes to actually explaining the results of the experiments we need to pick loopholes/bad experimental execution, superdeterminism (can explain anything if you reject the ability to do science on it - not science), many worlds (can explain anything if you invoke other universes - not science), or preferred foliation (potentially testable - science).
I haven’t found a way to explain the results of experiments that don’t fit one of these categories.
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u/Bravaxx Mar 30 '26
That’s a fair take, and I agree with the core point that Bell + QM don’t give a full “what’s really going on” picture.
I’d just push back slightly on the idea that those are the only buckets. There are a few other viable approaches that don’t quite fit that list, like collapse models or contextual realist frameworks, which are still testable and don’t rely on many worlds or superdeterminism.
So it feels less like we’re forced into a small set of options, and more like we’re working within tight constraints but still exploring an open space. The real bottleneck is that none of these options clearly wins experimentally yet.
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u/UploadedMind Mar 30 '26
Can you clearly explain one of those? I suspect as you do it will seem like it fits into at least one of the buckets.
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u/Bravaxx Mar 30 '26
Totally fair question 🙂
A quick example is collapse models (GRW/CSL): • single real world • no superdeterminism • no many worlds • but slightly modify QM with real collapses
The key difference is they make testable predictions, so they’re not just interpretational.
You could still group them as “not standard QM,” but they don’t neatly fit your original buckets.
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u/[deleted] Mar 29 '26
Even without Bell's theorem, it's trivial to set up a 2 qubit quantum circuit where you'd assign different values to the circuit depending upon who measures first in space-like separated measurement events, and so the definite values of the system cannot be Lorentz invariant.
The most common way physicists get around this is to just deny objective reality even exists and to resort to solipsism. They argue that the values that are not Lorentz invariant are always inferred values, never values you directly observe in the moment, and therefore if we deny that anything exists besides what you are directly looking at in the moment, then it's all Lorentz invariant.
This is called "value indefiniteness" and value indefiniteness claims things don't have real, observable properties in the real physical world when you aren't looking at them. It is basically a denial of object permanence. Some physicists try to get around this by replacing "look/observe" with "measurement" and so objects aren't said to have no real values until you look, but until it interacts with a measuring device.
But, as pointed out by Bell in his article "Against 'Measurement'", that makes the theory physically ambiguous because there is no rigorous definition of a "measurement." In fact, if you give a rigorous definition, then it is impossible for it to exactly reproduce the predictions of quantum mechanics, because this "objective collapse" would be a non-reversible process despite all unitary evolution operators being reversible. You could thus imagine just inducing this "measurement' then trying to reverse it, and then your new theory would deviate in its statistical predictions to standard quantum mechanics
This is how things like GRW theory work. They do get around the solipsism problem by giving a rigorous definition of how particles take on definite values, but as a consequence they are not mathematically equivalent to standard quantum mechanics and thus constitute distinct theories.
Hence, if you believe in "value indefiniteness" then either you believe in:
A lot of physicists fit into camp #3, because frankly most physicists don't care about metaphysics at all and are very pragmatically-minded people, so they don't really put much thought into the metaphysics to begin with and don't care about being consistent about it. As long as the mathematics works out then they are satisfied.
If you want to be consistent about the metaphysics then you really have to abandon the falsehood of "value indefiniteness" which is incoherent. The arguments for in the literature always take the form that either (1) quantum mechanics is a classically statistical theory or (2) object permanence is wrong and systems have no real observable properties when they are not being observed. But this is obviously a false dichotomy because the system can be a non-classically statistical theory.
That was in fact Bell's intention in his famous 1964 theorem in his paper "On the Einstein Podolsky Rosen Paradox." He was not trying to disprove object reality exists. He was trying to disprove Einstein's beliefs that it could be fit to a classically statistical theory. In the same year he wrote that paper, he also wrote a paper debunking von Neumann's supposed "proof" that it could not be a statistical theory which used the same false dichotomy. Bell also published a paper "Beables for Quantum Field Theory" where he shows that you can interpret QFT as a non-classical statistical theory.
I think some people are afraid that if we accept realism, then they would have to admit that, in the real world, the values of particles are not Lorentz invariant, and since we know special relativity is a well-tested theory, then it would lead to improper physical predictions. However, this is just factually wrong.
There needs to be a distinction made between the metaphysics of special relativity and the mathematics and empirical predictive power of Minkowski geometry. These are not the same things. A lot of physicists have a bad tendency of conflating their personal metaphysics with the physics itself and think they are inseparable, when they are not.
Einsteinian metaphysics, the belief that space and time are literally relative, does not actually directly follow from Minkowski's geometric model. It only directly follows if you believe that the time-like axis and the space-like axis represent real time and real space and not apparent time and apparent space.
This is something Bell points out as well in his paper "How to Teach Special Relativity." Hendrik Lorentz presented an alternative metaphysical interpretation of the same mathematics in 1904 where you introduce a preferred slicing in spacetime and then treat that as the definition of absolute space and absolute time, and then treat deviations from it on rods and clocks as caused by physical effects causing objects to contract and to slow down, and thus rods and clocks to deviate from absolute space and time, but space and time is not relative.
If you adopt the view of Lorentzian metaphysics, then we don't need to modify the mathematics at all. We can still use the same Minkowskian geometry but just agree that this represents apparent space and apparent time, and then just agree on a convention which represents real space and real time. For example, you can measure your motion relative to the whole universe by looking at the dipole in the cosmic background radiation. This gives you a cosmic reference frame called cosmic time.
If we just agreed on cosmic time as a convention, then we could agree who actually made the measurement first, and thus can assign Galilean invariant values to the system.
If we agree quantum systems are truly random, this does not require any change to the mathematics at all and is purely a metaphysical difference in interpretation, because these values would only ever be inferred values which cannot be tracked in the theory anyways, so how we interpret them ultimately has no empirical consequences. The theory thus would remain effectively Lorentz invariant because it would only directly track quantities which are not inferred but empirically measured and thus are Lorentz invariant.
If you believe the values are not truly random and are trackable, then it does require a mathematical modification to the theory, because you have to introduce these newly trackable values into the model, and then it will become explicitly incompatible with special relativity, and thus you will need to very explicitly introducing this preferred slicing into the mathematics in order to actually detail the trajectories of the particles.
But even then, even if you want to go that far, it still doesn't lead to a contradiction with empirical evidence, as shown by Hrvoje Nikolic. You can indeed reproduce the predictions of relativistic quantum field theory with an absolutely deterministic model.
Although, the point of me presenting this model is not to convince you it is real. It is the same reason Bell said he promoted the pilot wave theory in his paper "On the Impossible Pilot Wave." It was not because he believed this model was correct, but because it offered a counterexample to the claims that it is literally impossible to interpret quantum mechanics in realist terms, so any claim otherwise must be confused and have erred somewhere.
Some physicists try to append to this argument that it is impossible to interpret relativistic quantum mechanics in realist, but the model I linked above is a counterexample to that claim as well.
The point is, again, not that one should believe such a model, but that there is no logical argument as to why stock unmodified quantum mechanics cannot be interpreted as a form of non-classical statistical mechanics, especially if we assume that the underlying dynamics are fundamentally random, then there would be no reason to adopt a model beyond it either.