Relay-Version: version B 2.10 5/3/83; site utzoo.UUCP Path: utzoo!watmath!clyde!burl!ulysses!mhuxr!mhuxt!houxm!mtuxo!mtunh!mtung!mtunf!ariel!vax135!cornell!uw-beaver!tektronix!hplabs!sri-unix!AI.Mayank@MCC.ARPA From: AI.Mayank@MCC.ARPA Newsgroups: net.physics Subject: Re: Faster than light. Message-ID: <372@sri-arpa.ARPA> Date: Tue, 9-Jul-85 22:12:44 EDT Article-I.D.: sri-arpa.372 Posted: Tue Jul 9 22:12:44 1985 Date-Received: Sat, 13-Jul-85 09:36:58 EDT Lines: 69 From: Mayank Prakash>> The wave function is more than just a computational device - it is the actual >> probability amplitude, whose mod-squared gives the probability density of >> seeing a photon at a given point. The wave function collapse is the stronger >I certainly agree that the wavefunction is a probability amplitude which >mathematically propagates according to a set of equations of motion. It >is not however real physical entity like for instance an electric field. >It is just a probability amplitude that you square to get the probability >of various results. People get unhappy with the idea of it "collapsing >instantanously" because they think of it as a real physical object. This >causes them to think that someting is moving faster than light when it >"collapses"...... cup of sugar? Why is the electric field any more real? Isn't it just something in my mind as well? Can you pass me a cup of electric field please? If you do, I will give you two cups of wave function. How's that for a bargain? The point is, whether *real* or not (whatever that means in this context), the probability of finding a photon actually changed as a result of the measurement. If I understand you correctly, then your position is based on a misunderstanding of QM. For, let us imagine two situations as follows -(1) I am shooting bullets from an aperture in random directions. That is, the direction of the next bullet is unpredictable, all you know in advance is the probability of the bullet going in any given direction, i.e., its probability distribution. You place detectors all around to detect each bullet. Let us assume that we cannot see the bullets before they are detected by the detectors. Now, I fire my next bullet. Before it reaches a detector, you cannot tell me which direction it is travelling in, except the probability o f its actually going in any direction. As soon as it is detected, however, the whole probability distribution collapses to a point, and you now know exactly where the bullet is. Why is there no problem here? Because the whole probability distribution was used by us simply to express our ignorance of the bullet's direction. It did not represent the bullet in any sense - the bullet was moving in a fixed direction, even if we did not know that direction. Its detection simply fills the gap in our knowledge, and in this sense the probability distribution is something in our minds, and nothing really moved when it collapsed. In particular, no information was transferred by any one in the process. Now let us look at situation (2) I am preparing photons in identical states, and firing them from an aperture. Again, you know the statea I am preparing them in, and hence the wave function of each photon in advance. This wave function gives you the probability of finding the photon in any given direction. As before, you have detectors placed around the aperture to detect these photons. Now, I shoot the next photon. Before it is detected by a detector, all you can tell me is the probability that it will be found to be in a given direction. Why is there a problem here? Because it is not actually moving in any given direction *before* the detection takes place. In the classical case, the bullets were actually being in different states each time, and our ignorance of the state of each bullet forced us to resort the probability description. In the quantum case, we are shooting photon in identical states, and it is not our ignorance of its state that forces us to use probabilitiesm but because that is all there is. The wave function is as complete a description of the state o fthe photon as anyone has (including the photon), and therefore, when the photon decides to be found in one detector, it has changed its state across all space. In other words, it is not true that the photon was actually moving in given direction, and it got observed there etc., as we did in the previous case. THe photon was living a spread out existence, with a probability of being detected anywhere being given by its wave function, and before the measurement was made, it was not movving in any given direction. that is the root off the problem - how does it change its spread out out state to a localised one *instantaneousy*? And yes, don't forget the cup of electric field. - mayank. -------