EPR Experiments in QFT: Entangled Fermions & Time's Absoluteness

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Discussion Overview

The discussion revolves around the implications of entangled fermions in the context of EPR experiments and their interpretation within relativistic quantum mechanics (QFT). Participants explore the nature of instantaneous measurement outcomes, the concept of absolute time, and the reconciliation of quantum mechanics with special relativity (SR).

Discussion Character

  • Exploratory
  • Debate/contested
  • Technical explanation
  • Conceptual clarification

Main Points Raised

  • Some participants assert that measuring the spin direction of one entangled fermion instantaneously determines the spin direction of the other, suggesting a conflict with the notion of time in SR.
  • Others propose that there are multiple theories regarding the communication between entangled particles, including the possibility of hidden variables that could explain the observed correlations without necessitating faster-than-light (FTL) communication.
  • One participant questions the definition of 'instant' in the context of different reference frames, suggesting that time dilation effects must be considered when discussing simultaneity.
  • Another participant reiterates the idea that the EPR experiment implies absolute simultaneity, which contradicts SR, while expressing a lack of knowledge about QFT.
  • There is a mention of Einstein's skepticism regarding entanglement, referring to it as "spooky action at a distance," highlighting historical perspectives on the topic.
  • A participant argues that the occurrence of perfectly correlated events does not imply that time is absolute, drawing an analogy to spatial correlations.

Areas of Agreement / Disagreement

Participants express differing views on the implications of entanglement for the nature of time and simultaneity, with no consensus reached on how QFT reconciles these issues. Multiple competing interpretations and hypotheses are presented.

Contextual Notes

Some participants acknowledge limitations in their understanding of QFT, which may affect the depth of the discussion regarding its implications for the EPR experiment.

LarryS
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Consider 2 entangled fermions. If the spin direction of one particle is measured, then the spin direction of the other is instantly determined, regardless of the distance between them. This is the famous EPR-type experiment in non-relativistic QM. This implies that time is absolute, in contradiction to SR.

How does relativistic QM (QFT) explain measurements of entangled particles? Is the spin direction of the other particle still instantly determined?

As always, thanks in advance.
 
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I think it is like this, not sure though.

There are multiple theories on this. Currently the evidence points towards the particles "instantly" communicating to each other I believe. Measuring one causes it to assume a definitive state while its entangled partner assumes the other.

However, another version, not quite completely ruled out yet, says that there are possible variables we do not know about that can cause it to look like it does. These hidden variables would mean that the particles ARE in certain states when created or after interacting, and we just can't know about it until measurement. In this version you would know the other particles spin because it wouldn't be its partners and you don't need FTL to explain it.
 
Surely an 'instant' can only be defined and measured by observers in the same relative time frame? Wouldn't there need to be very accurate compensations made for any time dilation effects to establish when that 'instant' actually happened?
 
Lost in Space said:
Surely an 'instant' can only be defined and measured by observers in the same relative time frame? Wouldn't there need to be very accurate compensations made for any time dilation effects to establish when that 'instant' actually happened?

You're right. That is was SR tells us. But the EPR experiment, when it was first proposed by Einstein as a thought experiment implies that there is such a thing as absolute simultaneity between distant events, in contradiction to SR. I know very little about QFT, except that it reconciles the original non-relativistic QM with SR.

How does QFT view the original EPR experiement? That was basically my question.
 
referframe said:
You're right. That is was SR tells us. But the EPR experiment, when it was first proposed by Einstein as a thought experiment implies that there is such a thing as absolute simultaneity between distant events, in contradiction to SR. I know very little about QFT, except that it reconciles the original non-relativistic QM with SR.

How does QFT view the original EPR experiement? That was basically my question.

Wasn't Einstein inclined to be somewhat sceptical of the implications of entanglement, calling it, "Spooky action at a distance"?
 
referframe said:
I know very little about QFT, except that it reconciles the original non-relativistic QM with SR.

How does QFT view the original EPR experiement? That was basically my question.
If that is all you know about QFT, I think it is it is impossible to answer your question in a satisfying way.
 
referframe said:
Consider 2 entangled fermions. If the spin direction of one particle is measured, then the spin direction of the other is instantly determined, regardless of the distance between them. This is the famous EPR-type experiment in non-relativistic QM. This implies that time is absolute, in contradiction to SR.
If two (perfectly correlated) events happen at different positions at the same time, it does not mean that time is absolute.

To understand why, consider two (perfectly correlated) events that happen at different times at the same position (it's easy to find an example from everyday life). Does it mean that space is absolute? Of course not. By analogy, time does not need to be absolute in the case above.
 
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