How can resolution of particle entanglement be detected, and why can't it be faster than light communication?

Question

Sorry for the length, but this is driving me crazy. And yes, there are other questions on this issue and I have reviewed them, but I cannot see the answer stated simply. What is different about my question is this conundrum - either the end of entanglement can be detected remotely (in which case, faster than light communication) or it cannot (in which case, how can we say entanglement is resolved at a distance?).

Specifically - and maybe someone can show we where if it has been answered - can we determine by looking at 1 particle that we knew was once entangled, whether its quantum state has been determined or not? I do not see that question answered. The references people all point to say "as soon as you determine the state of one particle, the state of the other is fixed." But that is different. Here I have Professor P with just one particle and I am asking, can he know whether that particle's quantum state has been collapsed or not? I assume the answer is that he cannot know, or else it is possible to have FTL communication.

But then the follow-up question is: If we cannot know if the state is determined, then how can we know when the state was determined? In other exchanges people say it is spooky that as soon as one particle's state is known, the other has to be a different state. But then, how do we know that the states were not determined the instant that the two particles were separated? Professor P's electron was always "up" and Scientist E's was always "down." No action at a distance.

And please don't down vote. I am very seriously willing to do research and look, and I have, and I have asked this question of others, but I have never found an answer. Please be kind and just say, "No, we cannot tell by looking at just 1 particle whether its state has been collapse or not through some distant action. But we know that it had not been collapsed until Scientist E looked at it because ... "

Let’s have scientist E on Earth, and professor P on Pluto, 3 light hours apart, each holding a particle of up or down spin that have been entangled. They synchronize their clocks to NY time. Scientist E says, “I will either peek at my particle at noon or not, and send you a radio signal saying what I did. You will get the radio signal at 3 pm.” The act of peeking or not peeking is information. It can be code for “I’ll peek if our team wins in the morning, and not peek otherwise.”

So per https://www.livescience.com/27920-quantum-action-faster-than-light.html, Professor P’s particle will have the entanglement resolved by 12:01 or sooner. If he can tell that the entanglement was resolved, he would know this information 3 hours before the radio signal arrives. My thinking is that if the entanglement is not resolved, the particle will have properties of both up and down spin. If the entanglement is resolved, the particle can only be one or the other.

Alternatively, if the idea is that any interaction with the particle will determine that it is up or down, so that Professor P cannot tell if Scientist E peeked, then I really don’t understand the big deal with entanglement. It would be like them picking at random either an ace of diamonds or king of diamond playing card. When Professor E looks and discovers he has the ace, then he knows Professor P has the king. Big deal.

Moreover, in that case, I don’t see how anyone can say when the entanglement is resolved. The particles might have always been up or down, just as the cards were always ace or king.

Just to repeat, can Professor P determine whether the entanglement is resolved at any point between noon and 3 pm? If yes, faster than light communication has been recorded. If no, then how can we possibly know that entanglement resolution happened faster than light?

Answer 1

You asked the questions, and then made a few comments too. So I will try to piece together a response.

1. There is no way to look at an individual particle and detect whether it is or was entangled prior to measurement. All any observation gives you is a random piece of information. I.e. up or down, when considering spin for example. Not a lot of information to be located in a random result. So your P scientist will only see an UP or a DOWN at 12:01, each very close to 50% of the time. I hope you can see that this cannot be used to encode any signal at all. And that is true regardless of whether the E scientist looked at his result or not.

2. Some interpretations want the result to be predetermined prior to measurement, even though that result is random. It is POSSIBLE in those interpretations to make that work. However, that leads immediately to some form of quantum nonlocality. According to Bell's Theorem (generally accepted and taught), predetermination requires some form of nonlocality. However, there is no consensus on what that mechanism might look like.

3. Keep in mind that there is no requirement that entangled particles have ever interacted in the past. They don't need to have ever ever existed in a common light cone to the past. So that kinda makes a mess of the "common" origin ideas and predetermination. See for example:

High-fidelity entanglement swapping with fully independent sources (2008) We have demonstrated high-fidelity entanglement swapping with time-synchronized independent sources. The swapped entanglement clearly violates a Bell-type inequality. These strong non-classical correlations between particles that do not share any common past are not only crucial for future quantum repeaters. They might also enable novel tests of quantum mechanics.

One of the authors of this paper won the 2022 Nobel for this and other work.

4. Experiments have been done in which the measurement settings are selected mid-flight, too late for any signal to be exchanged at light speed or less between the measureed particles. This is important in Bell tests using the CHSH inequality, because the Bell violations only clearly show up in specific relative settings between the 2 measurements (Earth, Pluto). So yes, your Ace/King example works when you look at some relative settings - those in which there are so-called perfect correlations. But those are actually special cases! In most cases, such as when one is measured at 0 degrees and the other is measured at 120 degrees: the statistics cannot be made to work out. That was the great discovery Bell made.

5. You can attempt to calculate something you can label the "speed of entanglement collapse". But there really is no such meaningful value, regardless of what interpretation you prefer. Any method you use can be shown to yield negative or infinite values depending on specific setup and assumptions you use. It is not clear what, if anything, actually collapses anyway.

Read some more on Bell, this will definitely help you to understand the true issues here. Also, note that an entangled system of 2 particles is described by a single non-separable wave function. It should really not be considered as 2 individual particles. For example: an entangled pair of photons is sometimes called a "biphoton" to make this point more clear.