Steve Schear writes"
Why aren't the coding techniques commonly used in telecom and disk data encoding adequate to both synchonize and convey data?
Think of the classical case. I bake two fortune cookies, one with "FOO" written on the slip of paper inside, and the other reading "BAR." I then put them in a box and shake it for quite a while, until the final state has chaotic dependence upon initial conditions, and cannot be predicted. I then keep one fortune cookie, and mail the other one to Lucky Green in Tonga. Someday in the future, I open my cookie, and instantly know what Lucky will see when he opens his. In doing so, I have created a "instantaneous" correlation between two things separated by a vast distance, which were in an identical state of ambiguity prior to one of them being examined. I am sure we will agree that there was no genuine faster-then-light communication of information in this case. In quantum mechanics, pairs of observables may have the property that both of them may not be known precisely for a physical system. The Heisenberg Uncertainty principle states this for position and momentum. Similar relationships exist for energy and time, polarization or angular momentum measured with respect to different axes, and various other things. In addition, measuring a physical system for one such variable always changes its wavefunction into one for which the value of that variable is precisely specified, and the value of the other "non-commuting" variable is not. You can see this easily with three polarizing filters. If you shine a light through two of them at right angles to each other, it will be completely blocked. But it you insert third filter at a 45 degree angle, some light will get through. This is because light whose polarization is known to be vertical or horizontal is in a mixed state with respect to its polarization rotated 45 degrees. It is therefore tempting to think that perhaps the miracle of quantum mechanics could be employed in our fortune cookie experiment for the transmission of information. I generate many pairs of cookies, with random but identical polarization, keeping one of each pair for myself, and sending the other to Lucky. I then encode a stream of bits by measuring the polarization of 100 cookies, vertically if I wish to transmit a "0", and at a 45 degree angle if I choose to transmit a "1". I then know Lucky's corresponding cookie to be in an exact state with respect to one of these observables, and in a mixed state with respect to the other, and if Lucky measures the vertical polarization of his groups of cookies, there should be a correlation between his results and mine which can only be explained by non-local communication of my choice, on the fly, of which way I measured the polarization, to his apparatus. Now here we have good news and bad news. The good news is that when we do such an experiment, precisely enough to know for sure that there is a spacelike separation between the two measurement events, we do indeed see the correlation predicted by quantum mechanics. The bad news is that either end of the experiment, by itself, cannot see this correlation without knowing what results were obtained by the person at the other end. The correlation is between both ends. There is no experiment that can be done by either end alone which will turn out differently depending upon what the guy at the opposite end is doing. Thus, while such experiments involve non-local communication between two locations separated by a spacelike distance, such communication is obvious only to someone able to see what is going on in both places at once, but not to either isolated experimenter. Hence, the non-local collapse of quantum mechanical wavefunctions cannot be employed for the transmission of information. A similar argument applies to quantum teleporation, in which the value of some measurable variable is transferred from a dynamical system to one of two particles in correlated but unknown quantum states, causing the particle's twin to take on an identical value. Again, a person able to view both systems can see that non-local communication has taken place, but there is nothing either end can do by itself to learn what has transpired at the other end. This is because the correlation which proves non-local communication is present only in combined data from both ends of the experiment, but not in data from either end alone. -- Eric Michael Cordian 0+ O:.T:.O:. Mathematical Munitions Division "Do What Thou Wilt Shall Be The Whole Of The Law"