Simple QM question

[Kriminal99]

Someone tell me why this scenario is different from QM with respect to its determinism...

I run into a government building with two time bombs set to explode at the same time. I run into a person in the building and attach one bomb such that he cannot remove it. He runs off and we both explode at the same time.

Or instead, I run out of some engineering lab with two gyroscopes. I give one to my accomplice and we both escape on skateboards and exhibit correlated movement patterns...

[cwes99_03]

Wow, Krim. Maybe provide some more info and a better stated question. I didn't see a question there, but maybe someone with an actual QM background understands.

[Kriminal99]

The question is, how is it known that entangled particles have not swapped something physical but unobservable (by us using current technology) that would cause their measurements to be correlated, or just have something in common that would cause their measurements to be correlated?

Or just another question why is it believed there are no local hidden variables in QM?

[Erasmus00]

Originally Posted by Kriminal99

Or just another question why is it believed there are no local hidden variables in QM?

In 1965, Bell wrote a seminal paper. In it, he analyzed the Einstein Podolsky Rosen paradox (a paradox regarding entangled particles). In this paper, he demonstrated that if there is some hidden variable then certain inequalities arise regarding the correlations. (I believe that paper is titled "On the E-P-R Paradox" you are welcome to read it). I beleive Griffith's intro quantum text also deals with Bell in an appendix.

These inequalities disagree with quantum mechanics. Hence, either quantum mechanics is wrong, or local hidden variables are wrong. Quantum mechanics has been experimentally validated time and time again.
-Will

[Kriminal99]

I was under the impression that the condradiction was either there are more than only local hidden variables, or quantum mechanics being wrong. Not that there were not any local hidden variables.

Meaning there were non local hidden variables.

But what I was asking is, why are local hidden variables not capable of explaining the correlation? I don't understand how it is impossible for measurements on two particles seperated by some amount of space to be 100% correlated through purely determinisitic means.

[sebbysteiny]

The question you are posing is,by enlarge, still at the cutting edge of science.

However, the basics are still capable of being understood by people like you and me.

Every particle is a wave and thus can be described as a wavefunction. A wavefunction contains all the terms that have been measured. However, due to Hysenburg's uncertainty principal, some measurements cannot be known when others are known, eg momentum and position. So what happens to the position of, say, an electron when you measure it's momentum exactly? It becomes a superposition of numerous position states, with each one having a probability. Thus, the electron efectively acts and behaves as if it's position wer blurred. It does not behave like it has a position, it's just we don't know it yet, it behaves as if it has all possible possitions.

What does this mean? An electron wave with a measured momentum, could go in one of two slits. Each slit is this a position state. When the electron passes through the slits, and you measure the electron position at a screen, you get a diffraction pattern as if every electron went through both slits at the same time and interferred with itself. However, if you try to measure which slit the electron went through, the act of making the measurement changes the wavefunction and thus the properties. The electron now behaves as if it went through one slit only so it doesn't interfear with itself and you get no diffracion pattern.

Young's slits is the most vivid example I can think of where a wavefunction acts not as if the unmeasured state is unknown, but as if the unmeasured state is a combination of all the possible states.

Entanglement is essentially the same. The electrons, before they are measured, have become coupled together. They act as if they are a combination of the two possible states, and not as if they are either one or the other but we don't know which. Thus, when you measure one of the entagled particles, you force it's quantum state and it's properties thus change. At that exact moment, the quantum state of the entangled particle also changes state and therefore it's properties.

Exchanging some quasi-particle between entangled particles simply does not explain why the behavior of one particle actually physically changes suddenly simply because the other particle in a different place has been measured.

[Erasmus00]

Originally Posted by Kriminal99

I don't understand how it is impossible...

I referenced the paper and Bell so that you could do your own research, and do some learning on your own. The internet is full of useful information.

Since you are unwilling, here is Bell's paper.

http://www.drchinese.com/David/Bell_Compact.pdf

Note that Bell himself favors a non-local hidden variables interpretation of quantum mechanics, attributed to Bohm.

However, this approach is unsatisfactory because such communication between remote pairs must be instantaneous, and as such would violate causality (according to relativity). Hence, any non-local variables cannot be thought of as causal.
-Will

[Kriminal99]

Originally Posted by sebbysteiny

Exchanging some quasi-particle between entangled particles simply does not explain why the behavior of one particle actually physically changes suddenly simply because the other particle in a different place has been measured.

How would you know if the other particle's behavior has changed without also measuring it even if indirectly?

Also with the slit experiment what methods are being used to measure the momentum and the position?

@ Ersas People who post simply pointing to something else or referring to other people's opinions don't understand the subject sufficiently to consider what they say valid in my opinion. If they did they would just answer the question directly. If you are going to bother to post, do something other than make some "other people claim x" post.

[Erasmus00]

Originally Posted by Kriminal99

People who post simply pointing to something else or referring to other people's opinions don't understand the subject sufficiently to consider what they say valid in my opinion.

I'm of the opinion that it isn't my job to educate people too lazy to spend fifteen seconds googling. You have a world of information at your fingertips.

If you want to have a serious discussion about Bell's inequalities, then by all means, lets. But it isn't my job to type out the mathematical proofs and arguments you can find faster then I can type. I've linked to the paper above. Read it.

If you want to learn, put some effort in.
-Will

[KickAssClown]

I've been struggling with this particular phenomena recently, as it is vital to my particle model. I thinking I have an Idea as to what is happening, Though I am not absolutely sure.

I think it may have to do with a particle swap, as you suggest, though I am unsure how to model that in such a way as to not violate Casuality, Relativity, and a few others.