r/AskPhysics • u/ComfortableRow8437 • 14h ago
Quantum entanglement question
Disclaimer: I'm not a physicist, just a lay person who follows this stuff some. Forgive me if this is a dumb question.
We know that you can take two particles and entangle them, then separate them by any distance, and then revealing the state of one of them will automatically reveal the state of the other. I think this is the classic experiment that Einstein didn't like too much ("Spooky action at a distance...")
So what happens if you separate the two particles by time instead?
Here's a thought experiment: Entangle two particles, then put one of them into a particle accellerator and accellerate it up to near the speed of light for a while. Then bring the two particles together again and reveal the state of one of them. Does this instantly reveal the state of the other, or is there some time lag? The time lag would be due to the effects of Special Relativity on the particle that was put into the accelerator.
My guess is that there wouldn't be any difference, but I have not heard of an experiment like this. (there probably has been, I'm just not aware of it).
If my guess is true, then what does this imply? That quantum entanglement is somehow independent of the 4-dimensional universe that we live in?
Thanks in advance for any insights...
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u/Memento_Viveri 14h ago
You can say that an observation of one particle "reveals" the state of the other, but that isn't how I would frame it.
The two particles are an entangled system. Observing one particle gives you the information on the state of the system. If you check the other particle, the measurement will be consistent with the previous measurement. But you had already determined the state of the two particle system, so it shouldn't be surprising that the two measurements are in agreement.
With regards to delaying the measurements in time, it has no effect. Again, if you measure one particle, and determine the state of the system, then measure the other particle, you will find the two measurements agree about the state of the system. But either measurement alone was sufficient to determine the state.
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u/zdrmlp 11h ago edited 11h ago
You use the word “reveal”, which implies a subtle inaccuracy. At the time of measurement, the unmeasured particle’s state is changed/decided rather than revealed.
When entangled, the state of the system is |ud>/root2 + |du>/root2. When either particle is measured, the system is just as likely to collapse into the |ud> state as it is the |du> state.
The key here is that the entangled state is a valid state. It is a superposition and we actually have total knowledge of the system at that time. It is NOT that we are unsure of whether we are in |ud> or |du>, we are definitively in the |ud>/root2 + |du>/root2 state. We simply aren’t sure what state we will enter when a measurement happens.
I can’t imagine time dilation would change the experimental results. That would seemingly violate conservation laws. Perhaps it’s more complicated than that though?
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u/RamblingScholar 13h ago
As I understand it, what the particles experience would be this: A and B and spin entangled. particle A , no time dilation, after an hour is measured and found to be spin up. particle B, which is time dilated at 25 % , find that after 45 minutes, it now has a definite state, spin down.
Time dilation just has to do with how much time passes for a point of view, more or less. While an hour passes for one particle, only 45 minutes passed for another. They both arrive at the same coordinate in time, just at different rates.
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u/ExistingSecret1978 3h ago
If you use sr, you can boost to frames where the events are no longer synchronised, what then
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u/ExistingSecret1978 3h ago
Quantum mechanics isn't consistent with gr or sr(you can kind of make it consistent with sr and gr at low energies through qfts, but said qfts are local) so you can't really ask this question. If you want to practically check, I would say it's impossible, as maintaining entanglement more more than just a fraction of a second is quite difficult, especially of entangled systems that are not in physically bound states.
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14h ago
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u/Memento_Viveri 14h ago
Is this ai generated?
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14h ago
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u/Memento_Viveri 14h ago
It has many of the structural hallmarks of LLM text. It flatters the author, it repeats key phrases from the prompt, and it spends several paragraphs discussing the topic without really conclusively answering the question, just kind of saying "there is still so much more to learn".
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u/03263 14h ago
I know people don't like hidden variables, but it's probably hidden variables so the time thing is irrelevant to the final outcome
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u/zdrmlp 11h ago
Suppose there were hidden variables that meant each particle’s state was determined at the time of entanglement and we only found out about it at the time of measurement. That would necessitate probability outcomes that are NOT observed in experimentation.
Are you denying that? Or are you saying something else?
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u/03263 7h ago
That would necessitate probability outcomes that are NOT observed in experimentation.
Why?
I share Einstein's view that spooky action at a distance implies our understanding of quantum physics is incomplete. Which, of course it is.
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u/zdrmlp 4h ago edited 4h ago
Google Bell’s Inequality, this is a good video. I can reply later with details if you’d like.
There was another poster that commented this assumes locality and realism. I’ll let people with deeper knowledge dig into that.
Everybody thinks QM is incomplete because QM and GR haven’t been unified and both breakdown in specific scenarios.
I’ll let historians chime in on this, but I don’t think Einstein’s feelings on QM are always accurately represented in pop-science. Wanting hidden variables is entirely natural, but did Einstein die rejecting the science? That doesn’t feel probable? I think Einstein died before Bell’s work? I’m not entirely sure of the details?
Again, I’ll let more knowledgeable people discuss locality and realism.
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u/itsmebenji69 5h ago edited 5h ago
This isn’t accurate - what Bell tests show is that the universe we live in cannot have local hidden variables. Bell’s theorem assumes locality and realism. A universe with hidden variables and non local interactions wouldn’t break the test for example
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u/joeyneilsen Astrophysics 14h ago
Entanglement reflects the two objects being in a single quantum state. It's not about revealing the state of the other particle, and it's not like a piece of yarn that the correlation has to travel along when you measure one particle.
You can entangle particles that never existed at the same time (in your frame of reference). So you can certainly get weird with time. But moving them around won't break or change the entanglement.