B Quantum entanglement and special relativity

39
2
There are a pair of entangled particles moving in opposite directions. A measurement is done on particle A, the wavefunction collapses randomly, you observe either spin up or spin down, A does an action at a distance on B, particle B instantly collapses to the opposite spin state, a measurement of B reveals its now definite spin state. But in a frame of reference moving fast relative to the lab frame towards detector B, measurement B happened first. B made the random choice and collapsed the wavefunction, B performed the action at a distance on A, particle A collapsed to the opposite spin state, and detector A measured its now definite state.
According to special relativity, the time ordering of two events outside of each other's light cone is not uniquely defined. It is relative to your frame of reference.
I don't think any of these facts are in dispute, are they?
So if the collapse of the wavefunction is a real objective event, then which detector caused the collapse and which measured an already definite state? Which particle made the random choice and which obeyed its partner's instructions? A definite answer would pick out a privileged at-rest reference frame, which relativity says is impossible.
The only possible interpretation is that neither measurement collapsed the wavefunction, thus leading to the many-worlds interpretation of QM. Is my logic flawed in any way? Is there a way to defend the Copenhagen Interpretation?
 

Mentz114

Gold Member
5,405
280
This has been proposed and analysed often, usually in fewer words.

1. you don't need to asume a collapse - only assume before-measurement and after mearsurement-states.
2. you don't need instantaneous communication - only the fact that the pair will have the same(opposite spins) while entangled is required.
3. The quantum state of the entangled pair is shared by both the particles and says nothing about event ordering, so we cannot say which particle was projected first when the disentanglement happened.

No privileged frame is required but Lorentzian locality seems to be violated.
 
Last edited:

Nugatory

Mentor
12,335
4,816
There is something not to like about every interpretation, and you've hit on the thing not to like about collapse interpretations: the assumption that the wave function collapses at the same time everywhere interacts unpleasantly with special relativity.

However, there is no causality problem here, as the descriptions in both frames are equally valid and lead to the same outcome. Instead, we're stuck with non-locality, and that's a sticking point for other interpretations as well. You mention MWI... but how exactly does it turn out that both spacelike-separated observers end up in the same world without some appeal to non-locality?

Because the various interpretations are experimentally indistinguishable, there's no objective way of determining that one is better than another. Instead, we choose them based on our aesthetic preferences, and threads about them to degenerate into hopeless "Your baby is ugly, just like your favorite interpretation - my baby is cuter and my interpretation is saner" arguments. If that happens to this thread we'll have to close it - but we have plenty of older ones to review.
 
9,201
2,111
Another way of looking at it is correlations are precluded from the principle of locality that is used in Quantum Field Theory (the theory which ordinary QM is a limiting case) - called the cluster decomposition property:
https://www.physicsforums.com/threads/cluster-decomposition-in-qft.547574/

Interestingly ordinary QM is built on the Galilean Transformations which are inherently non-local. This is not usually emphasized but Ballentine - Quantum Mechanics: A Modern Development is an exception. It of course also applies to ordinary classical mechanics - again not usually emphasized - but Landau - Mechanics is an exception.

In a sense you can look on Bell and all that stuff as rather trivial. Its about correlation - correlations are precluded from locality so its a rather trivial result from that viewpoint. I emphasize that is just one way of looking at it - as Nugatory says there are quite a few others with pro's and cons to each.

Thanks
Bill
 

morrobay

Gold Member
686
93
Would you equate the principle of cluster decomposition for the "non local" correlations with Extended causality by @A Neumaier
 

PeroK

Science Advisor
Homework Helper
Insights Author
Gold Member
2018 Award
9,979
3,756
The only possible interpretation is that neither measurement collapsed the wavefunction, thus leading to the many-worlds interpretation of QM. Is my logic flawed in any way?
Your logic is flawed only in that you have jumped to a conclusion. There are other possible interpretations. The wave function can be considered a mathematical entity. Its "collapse" does not require any physical communication in spacetime. The issue is then about correlation (and nature somehow managing the correlation between measurements) not about communication between particles.

MWI requires the instantaneous creation of a pair (or infinite number) of parallel universes. That seems to me physically quite extreme in terms of how can that actually happen?
 
297
43
Your logic is flawed only in that you have jumped to a conclusion. There are other possible interpretations. The wave function can be considered a mathematical entity. Its "collapse" does not require any physical communication in spacetime. The issue is then about correlation (and nature somehow managing the correlation between measurements) not about communication between particles.

MWI requires the instantaneous creation of a pair (or infinite number) of parallel universes. That seems to me physically quite extreme in terms of how can that actually happen?
Yes. The Many Universes Interpretation is absurd. Not disprovable, just somewhat profligate with universes, a minor flaw in some people's eyes :) But Many Worlds is not Many Universes. The worlds are phenomenal - how the system appears to an observer who is part of the system. Many Worlds therefore only requires the existence of a single universe.
 
297
43
There are a pair of entangled particles moving in opposite directions. A measurement is done on particle A, the wavefunction collapses randomly, you observe either spin up or spin down, A does an action at a distance on B, particle B instantly collapses to the opposite spin state, a measurement of B reveals its now definite spin state. But in a frame of reference moving fast relative to the lab frame towards detector B, measurement B happened first. B made the random choice and collapsed the wavefunction, B performed the action at a distance on A, particle A collapsed to the opposite spin state, and detector A measured its now definite state.
According to special relativity, the time ordering of two events outside of each other's light cone is not uniquely defined. It is relative to your frame of reference.
I don't think any of these facts are in dispute, are they?
So if the collapse of the wavefunction is a real objective event, then which detector caused the collapse and which measured an already definite state? Which particle made the random choice and which obeyed its partner's instructions? A definite answer would pick out a privileged at-rest reference frame, which relativity says is impossible.
The only possible interpretation is that neither measurement collapsed the wavefunction, thus leading to the many-worlds interpretation of QM. Is my logic flawed in any way? Is there a way to defend the Copenhagen Interpretation?
No. Your logic is impeccable. If you assume locality, causality, and quantum mechanics, then wavefunction collapse by the detectors is inconsistent. If you prefer a non-quantum model, you have exactly the same problem, the definiteness of the detection events has to go in ordere to remain consistent with experiment. Having arrived at MWI it is then up to you whether to speculate that most of the "other worlds" somehow cease to exist. this is something I am thinking about raising in another thread.
 

Nugatory

Mentor
12,335
4,816
If that happens to this thread we'll have to close it - but we have plenty of older ones to review.
Yes. The Many Universes Interpretation is absurd. Not disprovable, just somewhat profligate with universes, a minor flaw in some people's eyes :) But Many Worlds is not Many Universes. The worlds are phenomenal - how the system appears to an observer who is part of the system. Many Worlds therefore only requires the existence of a single universe.
Closed.
 

Related Threads for: Quantum entanglement and special relativity

Replies
12
Views
1K
Replies
52
Views
6K
Replies
4
Views
1K
Replies
2
Views
630
Replies
61
Views
3K
Replies
1
Views
951
Replies
4
Views
562
Replies
107
Views
23K

Physics Forums Values

We Value Quality
• Topics based on mainstream science
• Proper English grammar and spelling
We Value Civility
• Positive and compassionate attitudes
• Patience while debating
We Value Productivity
• Disciplined to remain on-topic
• Recognition of own weaknesses
• Solo and co-op problem solving

Hot Threads

Top