Entanglement and action at a distance -- What coordinate system?

[Martian2020]

TL;DR Summary

Spooky action at a distance. Entangled particle state changes simultaneously from measurement. But I could not find specifics of that simultaneity. In what coordinate system per special relativity does it happen simultaneously?

Maybe my question is naive and due to my not deep enough knowledge of particle physics. I imagine we entangle two particles on Earth and then send one on spaceship going from Earth - two coordinate frames moving in relation to one another. Moments of simultaneity are different for them. When wikipedia and other articles state simultaneity (spooky action at a distance), they somehow do not state (at least I have not seen so) frame of reference for that event (measurement and simultaneity). What is that frame? Do we know for sure from experiment design?

 

[PeterDonis]

The results of measurements on entangled particles do not depend on whether they are simultaneous, or which one occurs first if they are not. So your question isn't really meaningful; "simultaneity" simply isn't a physical thing, and doesn't affect anything else.

 

[PeterDonis]

Entangled particle state changes simultaneously from measurement.

Whether this happens depends on which interpretation of QM you adopt. There is nothing in the basic math of QM that requires it.

 

[Martian2020]

The results of measurements on entangled particles do not depend on whether they are simultaneous, or which one occurs first if they are not. So your question isn't really meaningful; "simultaneity" simply isn't a physical thing, and doesn't affect anything else.

As I understood whole idea of action at a distance is that it happens faster than light and it "bothered" Einstein. From that point of view I see simultaneity is at heart of the issue and therefore frame of reference matters. I tried to read about experiments, but I did not find into about trying to move one of entangled parts at constant speed to another, to move measurement apparatus, etc... Maybe experiments are complex already as they are as "loopholes" were only recently closed.

 

[Martian2020]

The results of measurements on entangled particles do not depend on whether they are simultaneous, or which one occurs first if they are not. So your question isn't really meaningful; "simultaneity" simply isn't a physical thing, and doesn't affect anything else.

On a second thought I see your point, looks like there is no way to check that "simultaneity" in experiments. The results would just match...

 

[PeterDonis]

As I understood whole idea of action at a distance is that it happens faster than light

What happens to entangled particles upon measurement is not "action at a distance", at least not in the basic math of QM (it is, in a sense, in some interpretations, but discussion of QM interpretations belongs in the interpretations subforum, not this one). It cannot be used to send information and it is not any kind of "action" in the sense of a force or interaction that appears in the math.

I did not find into about trying to move one of entangled parts at constant speed to another, to move measurement apparatus, etc

That's because none of these things are expected to affect the results, so there is not much of an incentive to do them.

Maybe experiments are complex already

Yes, they are very complex, so experimenters have focused on the primary points that are expected to address open questions.

 

[Nugatory]

I tried to read about experiments, but I did not find into about trying to move one of entangled parts at constant speed to another, to move measurement apparatus, etc...

The Micius satellite?

But we don’t even need anything that elaborate. Because the Earth is a rotating ball any two points on its surface are always in relative motion. The farther apart they are the greater the difference in their speed, and entanglement effects have been observed across distances ranging from centimeters to hundreds of kilometers.

 

[Nugatory]

When wikipedia and other articles state simultaneity (spooky action at a distance), they somehow do not state (at least I have not seen so) frame of reference for that event (measurement and simultaneity). What is that frame? Do we know for sure from experiment design?

Does Wikipedia actually “state simultaneity”? If so, post the link to it and someone reading this thread will eventually fix it (this is one of the one the more interesting reasons why we generally ask posters to provide their sources).

In any case, simultaneity is irrelevant. Consider a typical experiment: we have a source that sends entangled particles in opposite directions and two detectors, one for each particle. Clearly the two detections won’t be simultaneous unless the distance from source to detector is exactly the same for both detectors, and in general it won’t be. So no matter what frame we use to attach timestamps to the detection events, we can arrange for the detection events to be simultaneous (that is, have the same timestamp) or for either one to happen first just by moving the detectors around.

If the source sends photons in opposite directions then the two detection events will necessarily be spacelike-separated and the converse will also be true: no matter how we position the detectors, there will be frames in which the detection events are simultaneous and frames in which either one happened first. This is the spooky action at a distance case.

 

[PeroK]

As I understood whole idea of action at a distance is that it happens faster than light and it "bothered" Einstein.

Popular science is obsessed with the Einstein-Bohr debates about QM. They often leave you with the impression that this is still a contemporary debate. If you want to learn about QM you should steer clear of popular science sources. In particular, your best bet is to ignore any source that uses the phrase:

Spooky action at a distance.

 

[DrChinese]

On a second thought I see your point, looks like there is no way to check that "simultaneity" in experiments. The results would just match...

Just to add to the excellent comments by PeterDonis, Nugatory and PeroK:

1. The term "simultaneous" is used not because it is actually simultaneous, because no one actually knows. However, as the time delta goes to zero, there is no observable change in the result. Quantum mechanics does not have a time term or element in calculating the expectation value.

2. Terminology: "Spooky action at a distance" has been replaced with "quantum nonlocality". What that is varies to some extent between quantum interpretations. There is a subforum for that, so I won't go farther on those. But generally speaking: a system of 2 entangled particles has spacio-temporal extent and follows entangled state statistics. They cannot be considered as a system of 2 independent particles. A system of 2 independent particles will follow product state statistics.