Collapse of the wave function under simultaneous measurement

[Philippe H]

TL;DR Summary

Supposing it could be done to perform a perfectly simultaneous measurement on 2 entangled particles, what would be the predicted correlation ?

Considering Bell’s theorem and the expected correlations between entangled particles or photons.
In a measurement setup e.g. Like Alain Aspect‘s with 2 entangled photons.
If we could make a setup that guarantees that the measurement on both photons is done at exactly the same moment, what correlation should we expect from those measurements?

 

[PeterDonis]

If we could make a setup that guarantees that the measurement on both photons is done at exactly the same moment

You can't, because "at exactly the same moment" has no invariant meaning.

Also, this doesn't matter, because the results of the experiment are independent of the order in which the measurements on the two photons are made.

 

[Nugatory]

If we could make a setup that guarantees that the measurement on both photons is done at exactly the same moment, what correlation should we expect from those measurements?

Every experiment in which the two measurements are spacelike separated (which is many of them) is a test of simultaneous measurement, as the measurements will be simultaneous in some inertial frame. (If you are not familiar with relativity of simultaneity and the notions of timelike and spacelike intervals you'll want to learn about these first - google for "spacetime interval" and "Einstein train simultaneity").

These experiments produce the expected correlations, which is completely unsurprising because the theory predicts that the order of the measrements does not matter.