Quantum entanglement and special relativity

In summary, the conversation discusses the concept of entangled particles and the measurement and collapse of their wavefunctions. It also touches on the issue of time ordering and the difficulty in determining which particle caused the collapse and which one was measured. The various interpretations of quantum mechanics, including the many-worlds interpretation, are also mentioned. The conversation highlights the challenges of understanding and interpreting quantum mechanics and how different perspectives can lead to different conclusions.
  • #1
kochanskij
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4
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?
 
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  • #2
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.
 
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  • #3
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.
 
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  • #4
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
 
  • #5
Would you equate the principle of cluster decomposition for the "non local" correlations with Extended causality by @A Neumaier
 
  • #6
kochanskij said:
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?
 
  • #7
PeroK said:
Your logic is flawed only in that you have jumped to a conclusion. There are other possible interpretations

All equally bad, in their own way.
 
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  • #8
PeroK said:
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.
 
  • #9
kochanskij said:
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.
 
  • #10
Nugatory said:
If that happens to this thread we'll have to close it - but we have plenty of older ones to review.
Derek P said:
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.
 

1. What is quantum entanglement?

Quantum entanglement is a phenomenon in quantum mechanics where two or more particles become connected in such a way that the state of one particle is dependent on the state of the other, even if they are separated by large distances.

2. How does quantum entanglement relate to special relativity?

Quantum entanglement is believed to violate the principle of locality in special relativity, which states that information cannot be transferred faster than the speed of light. This means that there is no known mechanism for how entangled particles can communicate instantaneously over large distances, which challenges our understanding of space and time.

3. Can quantum entanglement be used for faster-than-light communication?

No, quantum entanglement cannot be used for faster-than-light communication. While the state of one particle can be instantly affected by the state of the other, no information can be transferred in this process. Therefore, quantum entanglement cannot be used to send messages or signals faster than the speed of light.

4. How is quantum entanglement important in quantum computing?

Quantum entanglement is an essential component of quantum computing as it allows for the creation of quantum bits (qubits), which can exist in multiple states simultaneously. This enables quantum computers to perform certain calculations much faster and more efficiently than classical computers, making them potentially revolutionary for certain applications.

5. Is quantum entanglement a proven phenomenon?

Yes, quantum entanglement has been demonstrated through numerous experiments and is considered a proven phenomenon in quantum mechanics. However, there is still much debate and ongoing research on the underlying mechanisms and implications of entanglement in relation to special relativity and other fundamental principles of physics.

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