Proof of state collapse?

1. Aug 6, 2008

hdsncts

Proof of state "collapse?

Maybe I don't even have the terminology correct for this... I'm a quantum physics noob :)

My question is this: take the EPR experiment for example. What I've heard is that each electron in the pair is in an indefinite state of spin (either + or - with respect to a certain axis). Supposedly, when one is measured, it "collapses" into a definite state of spin (say + with respect to the Z axis).

What proof is there that the electron wasn't always definitely spinning +1/2 with respect to the Z-axis. Why do physicists conclude that it was indefinite until the spin was measured?

2. Aug 6, 2008

DrChinese

Re: Proof of state "collapse?

That was a possibility that was raised in the EPR paper. In fact, they considered any other perspective to be "unreasonable" when their paradox was presented in 1935. They did not have any experimental evidence for that view, but they guessed what the outcome might be. They assumed that the Heisenberg Uncertainty Principle (HUP) would be shown to have a limited scope, because QM was incomplete. EPR proved: IF QM was complete - i.e. that the HUP was not of limited scope - THEN the reality for one particle would be dependent on the reality for its distant partner particle. They considered this impossible, because they believed in "realism". Realism is the idea that a particle's attributes have definite values independent of the act of observation - the same as your question about having a definite spin of +1/2 with respect to the Z-axis. Or, to paraphrase Einstein, that the moon exists even when we are not watching it.

But that was long before Bell's Theorem. Bell (1965) showed that QM made some predictions which were incompatible with the EPR realism concept. This meant that an experiment could distinguish between these alternatives. In 1981, Aspect performed the experiment; the predictions of QM were upheld. This means that the realism assumption is now in doubt.

There is another possibility, and you may prefer this one: if there are non-local forces at work, then the experimental results could be explained. You could then keep realism.

3. Aug 7, 2008

hdsncts

Re: Proof of state "collapse?

I guess that makes sense. So you are saying that in the current context, then both locality and realism cannot be true. If non-local forces were at work, this would be described by hidden variables, am I correct? Man this stuff is so counter-intuitive for me... so hard to understand.

4. Aug 7, 2008

DrChinese

Re: Proof of state "collapse?

Yes, that is correct.

If it is confusing, consider this: what would Einstein have thought of Bell's Theorem? Einstein was a VERY strong advocate of both Locality and Realism. Assuming that he would have leaned towards respecting Locality (as the father of relativity, i.e. the fundamental importance of c as a limit), he would have been forced to ditch Realism. That would be a blow to the EPR paper, his last major referenced work.

No one really has any idea of how things are working at the physical level. If there are non-local forces at work, why don't we have any other indication of their presence? And non-realism is perhaps even more counter-intuitive.