Re: Faster than light transmissions

Dear Dan!

First of all you have to realize one central fact that lies pretty much at the basis of known physical theory: It is impossible to transmit information with a velocity that is greater than that of light! This stems from Special Relativity, and as quantum theory is compatible with SR, no quantum effect can do what you imply in you question. If you use the MAD search engine with `faster than light' as the key you will get lots of references to previous questions in which this topic is discussed.

There is, however, one problem with quantum mechanics. We are not quite sure how to interpret certain aspects of its predictions, and that is where many misconceptions about information transport have their origin. This previous question and answer discusses one recent, quite prominent example. The recent experiments about quantum entanglement also triggered new discussions.

What I presume you are aiming at is actually the famous `EPR Paradox' of quantum mechanics. I won't get into any detail here, but I just want to tell you that it is not like you have described it. Suppose you have somehow produced an electron-positron pair, and somehow you know that the total spin of the two equals zero, then QM tells you that if you measure, for instance, the spin of the electron, then the spin of the positron will be determined unabiguously. That means if anybody measures the positron spin, you know the result in advance, regardless of the distance that you and the other experimenter might be apart. BUT: It is impossible to use this mechanism to transmit information. Knowing something that someone else will know a little bit later, with you two being far apart, is nothing special and does not need QM to be realized.

Now what is the special thing about QM? The way many people look at it, measuring the electron spin `does something' to the electron-positron pair as a whole. It removes the uncertainty about the spin measurement on the positron instantly! This is the central point: The first measurement has an immediate, undelayed impact on the second measurement, and that is , if you want to put it that way, the only faster-than-light interaction you can get with this setup. And it is, as I have mentioned before, not a means to transmit actual information faster than c.

Well, now I hope that I haven't added to your confusion even more. If so, feel free to ask more questions to the MSN!

Georg.