explain this, please (instaneous effect)

[Mookow]

No sarcasm in the title, I just want either an explanation/clarification or a link to the experiment. Or of my friends was talking about an experiment by somebody named J.P. Bell (maybe J.B., anyway, Bell was definitely in there), where he took some atoms (I forget which element they were), combined them into a molecule, and then split that molecule. Then he seperated them, and flipped the spin on the electrons on one of the atoms, and the spin on the electrons one the other atom simultaneously flipped.

The above is probably a highly garbled version of what happened, and I know its not very clear (ie, not sure of the name of the experimenter, not sure of the material used, etc) in what details I stated. Anyhow, this is probably the best place to get my confusion cleared up (if, indeed, this even happened).

 

[CTho9305]

quantum entaglement?

 

[Sohcan]

It's J.S. Bell.

That's not really what happened. Quantum entanglement does not allow instantaneous transmission of data or energy, nor does it allow a state change on one particle to affect the state of its entangled pair....say you have a source that randomly produces an entangled pair of particles (say, an electron and a positron, entangled on charge...you don't know which is which), travelling in opposite directions. A single wave function describes the particles. If you were to observe one of the particles and find that it's an electron, you would know that the other particle is a positron...thus the wave function of the two particles instantaneously collapsed to its measured outcome. But in order to use that information about the other particle, you still have to send the result of the observation via classical means (at or below the speed of light) to the positron detector. This classical transmission of data is also necessary in quantum teleportation experiments, which despite the name doesn't actually teleport the original particle, but rather produces an exact replica (wave function) of the original particle at some distant location.

To quote one of my texts (Intro to Quantum Mechanics by David Griffiths, it has an excellent and easy-to-understand afterword on quantum entanglement and the nature of measurement):

"Well, let's consider Bell's experiment. Does the measurement of the electron influence the outcome of the positron measurement? Assuredly it does - otherwise we cannot account for the correlation of data. But does the measurement of the electron cause a particular outcome for the positron? Not in any ordinary sense of the word. There is no way the person monitoring the electron detector could use his measurement to send a signal to the person at the positron detector, since he does not control the outcome of his own measurement."

I hate to be brief on an esoteric subject (I have to go to work), but Bell's experiment was a generalization of the EPR/Bohm experiment that produced the Bell inequality: | P(a,b) - P(a,c) | <= 1 + P (b,c)

 

[Mookow]

Originally posted by: Sohcan

I hate to be brief on an esoteric subject (I have to go to work), but Bell's experiment was a generalization of the EPR/Bohm experiment that produced the Bell inequality: | P(a,b) - P(a,c) | <= 1 + P (b,c)

Well, when you get back from work, elaborate if you want to, I think its interesting

 

[RossGr]

I know that this analogy is not 100% correct but I think this is the sort of thing that is going on here.

I start with 2 balls 1 red and 1 green. with out looking I randomly select a ball and send it to you. Now when you open your box you have a key bit of infromation you need to answer the question, What color of ball was NOT sent to you. The other bit of information of course is that I started with a red and a green ball.

With that information you INSTANTLY know the color of my ball, which you have never seen and may be thousands of miles away.
While you are able to deduce information instantly, there was an essential component which had to travel at subluminal speed to enable your knowledge.

The actual quantum events of course is more involved.

 

[Sohcan]

Originally posted by: Mookow
Well, when you get back from work, elaborate if you want to, I think its interesting

I like RossGr's explanation...I'm not so hot at explaining the gory details without getting into the mathematics behind it, and something always gets lost in the translation. Plus I don't remember all the details since I haven't touched physics since I got my degree.

The EPR experiment involved the decay of a neutral pi meson into an electron and a positron, travelling in opposite directions; one is spin up, and the other is spin down to conserve momentum....you don't know which is which. If you observe one and find it's spin up, you know the other is spin down. Classically, one would say "duh", the one you observed was always spin up and the other was always spin down. But quantum mechanically there is a wave function describing the spin of the two particles (the wave function squared describes probability), and when you observe that one particle is spin up, you are "forcing" the wave function to collapse into spin down for the other particle...thus your observation "produced" the state for the other particle.

I think entanglement is really cool because it really shows the success of quantum mechanics. There have been countless experiments supporting quantum mechanics, as well as many practical applications in areas like high energy and solid state physics. But they don't prove the completeness or correctness of quantum mechanics...just like you can use classical mechanics to find the path of a thrown baseball, even though classical mechanics is incomplete. The entanglement experiments suggest that the probabilistic nature of quantum mechanics is correct, versus the other position that quantum mechanics is not complete and missing some local "hidden variable" that eliminates the uncertainty.

 

[killedradiostar]

ah, bell's theorem. so incredible.