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

In summary: But in general relativity, time and space are relative to the observer. So, if two events happen at the same time but at different positions according to one observer, they may happen at different times and the same position according to another observer. This means that the concept of absolute simultaneity is not valid in general relativity.In summary, the famous EPR-type experiment in non-relativistic QM suggests that time is absolute, which is in contradiction to the concept of relative time in special relativity. However, in general relativity, time and space are relative to the observer, so the concept of absolute simultaneity is not valid. The implications of entanglement are still being studied and there is no clear consensus on
  • #1
LarryS
Gold Member
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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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  • #2
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?
 
  • #4
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.
 
  • #5
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"?
 
  • #6
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.
 
  • #7
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.
 
Last edited:

1. What is an EPR experiment in QFT?

An EPR (Einstein-Podolsky-Rosen) experiment in quantum field theory (QFT) involves studying the entanglement of fermions, which are subatomic particles that follow the laws of quantum mechanics. This type of experiment is used to test the principles of quantum entanglement and to understand the behavior of particles at the smallest scales.

2. How are fermions entangled in EPR experiments?

Fermions are entangled in EPR experiments by being created in pairs with opposite spin states. This means that the two fermions are connected in a way that their spin states are dependent on each other, even when they are separated by a large distance. This entanglement allows scientists to study the properties of individual particles by observing the behavior of their entangled partners.

3. What does it mean for time to be absolute in EPR experiments?

In EPR experiments, time is considered to be absolute, meaning it is independent of the observer and the frame of reference. This is in contrast to the theory of relativity, where time is relative and can be affected by the observer's motion and the gravitational field. In EPR experiments, the absolute nature of time allows for the measurement of particle properties at a specific moment, regardless of the observer's perspective.

4. What insights do EPR experiments provide about quantum mechanics?

EPR experiments provide insights into the principles of quantum mechanics, such as entanglement and superposition. They also help scientists understand the behavior of particles at the smallest scales and test the predictions of quantum theory. These experiments have played a crucial role in the development of quantum technologies, such as quantum computing and cryptography.

5. How are EPR experiments relevant to real-world applications?

While EPR experiments may seem abstract, they have important real-world applications. The principles of quantum entanglement have been used in technologies such as quantum cryptography, which ensures secure communication by using the entanglement of particles. EPR experiments also play a crucial role in the development of quantum computing, which has the potential to revolutionize fields such as data encryption and drug discovery.

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