Alain Aspect's Experiment Explained: Beginner's Guide

[tim1608]

Hi

I have just watched a fascinating documentary on YouTube called "Atomic Physics and Reality" which describes an experiment conducted by Alain Aspect regarding two entangled photons traveling in opposite directions towards polarised filters oriented at different angles.

I would be very grateful if someone could help me with something regarding Alain Aspect's experiment. I don't understand how the experiment can tell that the orientation of the polarisation of the photons is only decided upon passing through the filters and not upon emission from the source. Would it not be the case that either way would produce the same results because the orientation of the two entangled photons would be the same as each other in both situations? What am I missing here?

To be honest, even though I understand a little bit about polarisation works, I don't think I understand enough to know how it effects Alain Aspect's experiment. I would therefore be very grateful if someone could give me a ground-up, beginner's explanation of how Alain Aspect's experiment works and how the timing of orientation decision effects the results.

Thank you very much.

Kind regards

Tim

 

[Nugatory]

Google for "bell's theorem" and pay particular attention to http://www.drchinese.com/Bells_Theorem.htm as DrChinese is a science advisor here.

 

[DrChinese]

Just in case you had not read it, here is one of the key Aspect experiments:

http://www.drchinese.com/David/EPR_Bell_Aspect.htm
See 3.

And here is another experiment, also very influential, which may help.

http://arxiv.org/abs/quant-ph/9810080

What you are really asking (I think) is why experiments of this type rule out predetermined polarizations for the measured photons. There are a number of reasons that this is so, I will make a few comments which may help:

a. Bell's Theorem points the way for the realization that the settings chosen for observing polarizations are a factor in the outcome probabilities. These settings can be chosen by computer after the entangled photons are created and are on their way to the polarizers. That leads you to the conclusion that they cannot be predetermined since the time ordering is wrong.

b. But you will need to understand a little about Bell to see that. It is important to realize that not all angle settings show the Bell issue. For example, if both polarizers are set at the same angle, you get results that are consistent with your idea about predetermined polarizations. So those angles are *not* the ones used in a Bell Inequality.

c. There are interpretations of Quantum Mechanics which support the idea of predetermined polarizations. However, such interpretations are non-local and there is "communication" between observers' polarizer settings to account for the results. Bohmian Mechanics is the usual one cited for this perspective.

d. And if that isn't enough to convince you: there are experiments in which photons are entangled that have NO contact with each other at any time. In fact, they may have never even co-existed. In fact, they can even be entangled AFTER they are detected. None of these scenarios would be consistent with the idea of predetermined polarizations or some kind of "common cause". I can provide references for this when you are ready.