EPR Experiment in QFT?

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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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Drakkith
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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.
 
  • #3
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?
 
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LarryS
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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.
 
  • #5
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"?
 
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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.
 
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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.
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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