Quantum entanglement and simultaneity

[Freixas]

I know very little about quantum physics. I was looking up the definition of quantum entanglement and asked ChatGPT to explain it. Here is an interesting phrase in its answer: "Once the particles are entangled, measurements made on one of the particles will instantaneously affect the state of the other particle, regardless of the distance between them."

What I've learned from studying special relativity is to be suspect of any phrase using the words "simultaneous" or "instantly".

Cross-checking with Wikipedia, I see this: "According to some interpretations of quantum mechanics, the effect of one measurement occurs instantly." There's that word again. And I think I've heard it used often when talking about entanglement.

I asked ChatGPT for a clarification and it responded: "In other words, if the particles are entangled and at rest relative to an observer, then a measurement made on one particle will instantaneously affect the state of the other particle, regardless of the distance between them, as seen by that observer. However, if the particles are in motion relative to the observer, or if the observer is in a different reference frame, then the effect of the measurement may appear to be delayed or even reversed, depending on the relative velocities of the particles and the observer."

I'm checking because this sounds like nonsense and the use of the word "instantaneously" seems like an unnecessary complication. It seems sufficient to say that, once measured, the measured state of two entangled particles will be perfectly correlated.

I suspect "instantaneously" is added because a particle is considered to exist as a quantum wave function until a measurement collapses the function. For an entangled pair, collapsing the wave function for one particle seems to collapse it for both, and classical thinking leads to talking about the collapses occurring "at the same time". However, I believe the collapse of the wave function is not detectable; therefore, worrying about the simultaneity of the wave collapse becomes a question for philosophy, not physics.

Am I thinking about this properly? If it were possible to know when a wave function collapses without making a measurement, I think I would have a lot of follow-up questions for the Special and General Relativity forum.

 

[DrChinese]

Be aware that ChatGPT is not a suitable source to cite for PF.

As to helping out with the concept of simultaneity in collapse/decoherence (or whatever you choose to call it) of the wave function for entangled pairs:

1. As far as any theory can say, it is not dependent on distance and therefore appears to be instantaneous. Actual experiments place the value at a minimum of 10,000 c.

2. It is not clear that a measurement "here" changes anything "there", or vice versa.

3. Whatever does happen, if anything, it is an example of "quantum nonlocality". Please keep in mind that what this actually means is interpretation dependent. Many here vehemently deny there is action at a distance, and in fact deny that anything physical occurs around what is called "collapse". Hopefully any debate of that will not take place in this thread, as I am simply pointing out the wide range of viewpoints on this exact issue and am not asserting a position.

4. There is therefore plenty of room to take a philosophical perspective on any discussion. Regardless, relativity is not a factor in entanglement to any significant degree - even though the best quantum field theories are themselves relativisitic.

5. Time ordering is likewise not a factor in entanglement. And in fact, there are numerous experiments that raise significant questions about the role of time vis a vis entanglement. For example, it is possible to entangle a particle AFTER it has been observed; and in fact that can occur before its entangled partner is even created. This is done via "entanglement swapping", see this experiment for example:

https://arxiv.org/abs/1209.4191
Entanglement Between Photons that have Never Coexisted

 

[Freixas]

Be aware that ChatGPT is not a suitable source to cite for PF.

Of course--that's why I'm asking here. I'm always curious how far off-base ChatGPT will get with any science topic. In this case, I think it is parroting the common description that entangled wave collapse occurs simultaneously for two particles, regardless of their separation.

1. As far as any theory can say, it is not dependent on distance and therefore appears to be instantaneous. Actual experiments place the value at a minimum of 10,000 c.

Instantaneous is a term that can only be applied relative to an observer. Since the "it" in your sentence above refers to "the concept of simultaneity in collapse/decoherence", then I wonder how experiments have established that "[it] appears instantaneous".

4. There is therefore plenty of room to take a philosophical perspective on any discussion.

I usually separate philosophy from physics based on whether something is testable.

I was speculating that the wave collapse is not an observable event, therefore philosophy; a measurement is observable, therefore, physics.

5. Time ordering is likewise not a factor in entanglement.

Ok, you blew my mind (which generally happens about 2 minutes after I start looking into any quantum concepts). The paper is way beyond my pay grade.

Let me try again.

We have two entangled particles 5 light seconds apart. Each particle has a nearby observer. The two observers and particles have a relative velocity of 0 with respect to each other. The two observers have clocks that are are synchronized using a standard simultaneity convention.

Observer 1 measures his particle at time 0. Observer 2 measures her particle at time 0. Both observers' measurements, once compared, show a correlation. This seems clear.

We now change this so that the observers have a relative velocity of 0.866c. At time 0, they are co-located and synchronize their clocks to 0. Let's say at time 100, the first observer measures his particle. For the second observer to measure her particle at the same instant, we would have to decide whose frame of reference to use. If it were observer 1's frame, observer 2 would measure her particle at her clock time 50. If it were observer 2's frame, her measurement would be at time 200.

What I'm theorizing is that it doesn't matter. Observers 1 and 2 can make their measurements at any time and in any order relative to any observer. When the measurements are eventually compared, they will show that the particles are entangled.

Therefore, I conclude that any statement about the simultaneity of a wave collapse is philosophy, not physics. If one were able to detect a wave collapse, then I would have a lot of interesting follow-up questions for the General and Special relativity group.

Am I way off base here?

 

[PeterDonis]

Of course--that's why I'm asking here.

You should not be asking ChatGPT in the first place. "Hey, ChatGPT said this--is it right?" is not a valid question here at PF.

Thread closed.

 

[Nugatory]

According to some interpretations of quantum mechanics, the effect of one measurement occurs instantly." There's that word again. And I think I've heard it used often when talking about entanglement.

These descriptions are only used in the context of non-relativistic QM, where we are allowed to ignore relativistic complications like frame-dependent simultaneity.

The treatment of entanglement in relativistic quantum field theories is completely different and, as we would expect of a relativistic theory, avoids any notion of simultaneous global collapse of the wavefunction.