# Is Simultaneity still alive in QM?

• I
Gold Member

## Main Question or Discussion Point

In Classical Mechanics, according to SR, the concept of simultaneity is dead, a meaningless concept. But in QM, entanglement implies that some limited form of simultaneity exists. If we have two particles correlated due to entanglement, a measurement of one particle immediately gives us the probabilities of measurement outcomes for the other particle. “Immediately” implies some kind of simultaneity. (I realize that the above scenario does not imply faster-than-light transfer of information).

Is simultaneity still alive in QM?

## Answers and Replies

Related Quantum Physics News on Phys.org
PeroK
Homework Helper
Gold Member
In Classical Mechanics, according to SR, the concept of simultaneity is dead, a meaningless concept. But in QM, entanglement implies that some limited form of simultaneity exists. If we have two particles correlated due to entanglement, a measurement of one particle immediately gives us the probabilities of measurement outcomes for the other particle. “Immediately” implies some kind of simultaneity. (I realize that the above scenario does not imply faster-than-light transfer of information).

Is simultaneity still alive in QM?

Well, if I post a pair of shoes: one shoe is posted to you and the other shoe is posted to someone else. Then, when you open your package you "immediately" know what's in the other package. Not sure how that contradicts SR.

Also, there's a big difference between "simultaneity is frame dependent" and "simultaneity is a meaningless concept", which sounds like pop-science nonsense, if you don't mind my saying so.

PeterDonis
Mentor
2019 Award
in QM, entanglement implies that some limited form of simultaneity exists
No, it doesn't. What it implies (or rather requires) is that if you make measurements at spacelike separated events, they must commute--the results cannot depend on the order in which the measurements are made (since the order is frame-dependent).

a measurement of one particle immediately gives us the probabilities of measurement outcomes for the other particle
No, it doesn't, because the information about the result of the measurement of the first particle only tells you the probabilities of measurement outcomes for the second particle if you also know what measurement is being made on the second particle. And the information about what measurement is made on the second particle travels no faster than light. (This is another way of stating why information can't travel faster than light in measurements on entangled particles.)

atyy
No and yes.

No: Considered operationally, QM does not permit faster than light communication

Yes: The formalism of QM (wave function collapse) requires the specification of a notion of simultaneity. The wave functions of different reference frames are not related to each other by unitary transformation.

vanhees71
Gold Member
2019 Award
In Classical Mechanics, according to SR, the concept of simultaneity is dead, a meaningless concept. But in QM, entanglement implies that some limited form of simultaneity exists. If we have two particles correlated due to entanglement, a measurement of one particle immediately gives us the probabilities of measurement outcomes for the other particle. “Immediately” implies some kind of simultaneity. (I realize that the above scenario does not imply faster-than-light transfer of information).

Is simultaneity still alive in QM?

The answer is relativistic local QFT, which underlies the Stanard Model of elementary particle physics. You have local interactions only but as any quantum theory it also of course describes the (observed!) long-distance correlations of entanglement. There's no contradiction between locality of interactions and microcausality (leading the linked-cluster principle to hold) and long-distance correlations described by entanglement (which often is misinterpreted as non-locality of interactions, most prominently by Einstein, but this is only due to the misleading collapse assumption of some flavors of the Copenhagen interpretation).

Of course quantum states observed from the point of view of observers in different inertial frames are related by unitary transformations. The very construction of relativistic QFT starts with finding the unitary representations of the proper orthochronous Poincare group (see Weinberg, QT of Fields vol. 1).

Gold Member
No, it doesn't, because the information about the result of the measurement of the first particle only tells you the probabilities of measurement outcomes for the second particle if you also know what measurement is being made on the second particle. And the information about what measurement is made on the second particle travels no faster than light. (This is another way of stating why information can't travel faster than light in measurements on entangled particles.)
What if Alice and Bob agree ahead of time what type of measurements to perform? They both agree to measure the spin of their spin one-half particles in their respective "z" directions. Alice and Bob's frames would be kept parallel with their "z" directions the same.

phinds
Gold Member
2019 Award
What if Alice and Bob agree ahead of time what type of measurements to perform? They both agree to measure the spin of their spin one-half particles in their respective "z" directions. Alice and Bob's frames would be kept parallel with their "z" directions the same.
I think you are talking about the SETUP of the measurement and Peter was talking about the RESULTS of the measurement, so you are not getting what he is saying. That's my story and I'm sticking with it.

PeterDonis
Mentor
2019 Award
What if Alice and Bob agree ahead of time what type of measurements to perform?
Then there is no need for any information about the measurement settings to travel anywhere, since it was predetermined in advance.

Grinkle
Gold Member
What if Alice and Bob agree ahead of time what type of measurements to perform?
Human agreement to do something does not count as an observation that the agreement has been followed in any context- social, QM, SR, whatever.

Gold Member
Human agreement to do something does not count as an observation that the agreement has been followed in any context- social, QM, SR, whatever.
I was not referring to some kind of hazy social agreement. I was adding a new requirement to the SETUP phase of the experiment. During the SETUP phase, Alice would send out a laser beam of light in HER "z" direction. Bob would then locate the beam and align his frame so that the beam is pointing in HIS "z" direction. How is that not precise?

PeroK
Homework Helper
Gold Member
I was not referring to some kind of hazy social agreement. I was adding a new requirement to the SETUP phase of the experiment. During the SETUP phase, Alice would send out a laser beam of light in HER "z" direction. Bob would then locate the beam and align his frame so that the beam is pointing in HIS "z" direction. How is that not precise?
Can you explain what this achieves or what difference it makes? In general, in order to compare results effectively, the experiments would need common directions.

Gold Member
Can you explain what this achieves or what difference it makes? In general, in order to compare results effectively, the experiments would need common directions.
Agreed. My reply can be traced back to PeterDonis's point that (paraphrasing) one cannot call it simultaneity if Alice does not know what kind of measurement Bob is going to perform. That would be information that could travel only at light-speed. I am saying what if that information is conveyed one time, at light speed, during the SETUP phase of the entanglement experiment?

PeroK
Homework Helper
Gold Member
Agreed. My reply can be traced back to PeterDonis's point that (paraphrasing) one cannot call it simultaneity if Alice does not know what kind of measurement Bob is going to perform. That would be information that could travel only at light-speed. I am saying what if that information is conveyed one time, at light speed, during the SETUP phase of the entanglement experiment?
Yes, but so what?

PeroK
Homework Helper
Gold Member
@referframe the issue with entanglement is that the results of the (initial) measurements are always correlated. Regardless of what is measured and when. There is always a frame where the measurements are simultaneous; and a frame where A's measurement precedes B's; and a frame where B's measurement precedes A's.

The measurements do not affect each other. But, they are correlated.

Gold Member
@referframe the issue with entanglement is that the results of the (initial) measurements are always correlated. Regardless of what is measured and when. There is always a frame where the measurements are simultaneous; and a frame where A's measurement precedes B's; and a frame where B's measurement precedes A's.

The measurements do not affect each other. But, they are correlated.
I think I understand what you and PeterDonis are saying. My original post was whether or not QM (entanglement) has it's own special version of the concept of simultaneity. It seems like what you and PeterDonis are saying is that since a quantum measurement (the movement of a needle on a meter, the appearance of a spot on a CRT, etc.) is essentially a classical event, it's timing is subject only to classical physics, namely SR.

PeroK
Homework Helper
Gold Member
I think I understand what you and PeterDonis are saying. My original post was whether or not QM (entanglement) has it's own special version of the concept of simultaneity. It seems like what you and PeterDonis are saying is that since a quantum measurement (the movement of a needle on a meter, the appearance of a spot on a CRT, etc.) is essentially a classical event, it's timing is subject only to classical physics, namely SR.
I'm afraid I don't understand your point. Two events are simultaneous if they happen at the same time. That begs the question of how you are measuring time, or how you define the time coordinate. Nevertheless, that is the concept of simultaneity.

zonde
Gold Member
In order to speak about experimental consequences of simultaneity you would need certain phenomena that is not predicted by QM but is not explicitly forbidden in QM. Say if there is possibility that entangled pair of particles spontaneously disentangle by remotely entangling with third and fourth system you might set up experiment that detects preferred reference frame. It would be like that: you produce entangled pair of particles A and B, then at some time $t_{disentangle}$ you change that state of the systems C and D so that there increases probability that A and B will disentangle and remotely entangle with C and D instead. Then by making measurement of A and B at different relative times to $t_{disentangle}$ (and later comparing their results for presence of entanglement) you can detect when this $t_{disentangle}$ happens locally for A and B.

PeterDonis
Mentor
2019 Award
at some time $t_{disentangle}$
Time in what frame?

PeterDonis
Mentor
2019 Award
Say if there is possibility that entangled pair of particles spontaneously disentangle by remotely entangling with third and fourth system
I'm not aware of any such "remote entanglement". For two systems to be entangled, they have to interact, and interaction is local.

stevendaryl
Staff Emeritus
I'm not aware of any such "remote entanglement". For two systems to be entangled, they have to interact, and interaction is local.
No, I don't think that's true. I believe you can have the following situation:
1. Alice produces a pair of entangled particles. She keeps one and sends the other to Charlie.
2. Bob produces a pair of entangled particles. He keeps one and sends the other to Charlie.
3. Charlie measures the total spin of his two particles, and finds that it is spin-zero.
I believe that it is possible for Charlie's measurement of the two particles to force Alice's and Bob's particles to be entangled, even though they never came into contact. I think @DrChinese would know.

atyy
No, I don't think that's true. I believe you can have the following situation:
1. Alice produces a pair of entangled particles. She keeps one and sends the other to Charlie.
2. Bob produces a pair of entangled particles. He keeps one and sends the other to Charlie.
3. Charlie measures the total spin of his two particles, and finds that it is spin-zero.
I believe that it is possible for Charlie's measurement of the two particles to force Alice's and Bob's particles to be entangled, even though they never came into contact. I think @DrChinese would know.
Is this what you were thinking about?

https://arxiv.org/abs/quant-ph/0409093
Long distance entanglement swapping with photons from separated sources
H. de Riedmatten, I. Marcikic, J.A.W. van Houwelingen, W. Tittel, H. Zbinden, N. Gisin
(Submitted on 15 Sep 2004)
We report the first experimental realization of entanglement swapping over large distances in optical fibers. Two photons separated by more than two km of optical fibers are entangled, although they never directly interacted. We use two pairs of time-bin entangled qubits created in spatially separated sources and carried by photons at telecommunication wavelengths. A partial Bell state measurement is performed with one photon from each pair which projects the two remaining photons, formerly independent onto an entangled state. A visibility high enough to violate a Bell inequality is reported, after both photons have each travelled through 1.1 km of optical fiber.

Gold Member
No, I don't think that's true. I believe you can have the following situation:
1. Alice produces a pair of entangled particles. She keeps one and sends the other to Charlie.
2. Bob produces a pair of entangled particles. He keeps one and sends the other to Charlie.
3. Charlie measures the total spin of his two particles, and finds that it is spin-zero.
I believe that it is possible for Charlie's measurement of the two particles to force Alice's and Bob's particles to be entangled, even though they never came into contact. I think @DrChinese would know.
Isn't that called entanglement swapping?

DrChinese
Gold Member
No, I don't think that's true. I believe you can have the following situation:
1. Alice produces a pair of entangled particles. She keeps one and sends the other to Charlie.
2. Bob produces a pair of entangled particles. He keeps one and sends the other to Charlie.
3. Charlie measures the total spin of his two particles, and finds that it is spin-zero.
I believe that it is possible for Charlie's measurement of the two particles to force Alice's and Bob's particles to be entangled, even though they never came into contact. I think @DrChinese would know.
Yes. With entanglement swapping, the final entangled pair of particles has never interacted. Nor have they existed local to each other. And in fact they can be manipulated so that they never even co-existed, and yet are fully entangled and capable of exhibiting "perfect correlations". The below reference directly relates to this thread. Namely: entanglement does not imply there is something called "simultaneous" in QM. Does A cause B? Or does B cause A? Almost every variation on entanglement swapping implies that there is no preferred temporal ordering, causal direction, etc.

https://arxiv.org/abs/1209.4191

"The role of the timing and order of quantum measurements is not just a fundamental question of quantum mechanics, but also a puzzling one. Any part of a quantum system that has finished evolving, can be measured immediately or saved for later, without affecting the final results, regardless of the continued evolution of the rest of the system. In addition, the non-locality of quantum mechanics, as manifested by entanglement, does not apply only to particles with spatial separation, but also with temporal separation. Here we demonstrate these principles by generating and fully characterizing an entangled pair of photons that never coexisted. Using entanglement swapping between two temporally separated photon pairs we entangle one photon from the first pair with another photon from the second pair. The first photon was detected even before the other was created. The observed quantum correlations manifest the non-locality of quantum mechanics in spacetime."

zonde
Gold Member
I'm not aware of any such "remote entanglement". For two systems to be entangled, they have to interact, and interaction is local.
Of course it would be naive to expect any sigh of any such "spontaneous remote entanglement". But it would be less naive to look for signs of spontaneous remote disentanglement. However if photon beam is moving in some medium even in case of hypothetical disentanglement it could be that entanglement is transferred to medium locally. So the photon beam would have to move in vacuum and show signs of disentanglement (reduced visibility without some known explanation) in order to perceive it as indirect indication of "spontaneous remote entanglement".