mac (01/21/83)
re: faster than light transmissions. Not everything that happens is a signal (or event). A wave guide can propagate waves faster than light, but cannot be used to communicate a signal that fast. Consider a planar wave front encountering a wire at an angle almost parallel to the front. This will appear to be a wave travelling down the wire faster than the speed of light. However, it can't be used to communicate from one end of the wire to the other. Such a phenomenon was recently observed in a gas jet from a quasar, travelling almost directly toward us. The jet appeared to move faster than light across the field of view. A point in these is that the signal at one end isn't a cause of the signal at the other, so modulating one end of the wire can't be detected at the other. Quantum mechanics seems to be more tangled with causality. In the Bell paradox causation appears to travel instantaneousy (faster than light). It's not clear, however, that this could be used to send signals. The event that is caused at the far end seems to be unobservable (a change in the state function, which is destroyed by observation). Remember the scene in "Forbidden Planet" where the space travellers remark something like "what we need is a good quantum mechanic!"? Anyone see any way to use this to communicate? I'd be interested in hearing them, but maybe you ought to file for a patent first. A whole 'nother question is the existance of tachyons, which can travel only faster than light. These really confuse the notion of causality.
jackson.PA@PARC-MAXC.ARPA (02/05/83)
Re;"Quantum mechanics seems to be more tangled with causality. In the Bell paradox causation appears to travel instantaneousy (faster than light). It's not clear, however, that this could be used to send signals. The event that is caused at the far end seems to be unobservable (a change in the state function, which is destroyed by observation)." Grumble...as I understand it the Bell Paradox involves a pair of particles that are constrained to occupy distinct states. When the particles are seperated and the state of one measured, the experimenter knows the state of the other particle, instantly without bothersome measurement. While this *seems* to imply FTL transmission of causality there is no way to signal in this fashion. For signalling to occur, the experimenter must be able to modulate the electron source (to send the recipient a spin up or spin down electron) which will happen at lightspeed or less. Then the electron has to travel from the electron source to the receiver, at lightspeed or less. Total time, assuming an instantainious modulator, is equal or greater than the signaling time for light. Of course, if the experimenter does not bother to modulate the source, and measures incoming electrons, then she knows instantly the state of the electrons at the receiver. No information is exchanged between the receiver and the experimenter though. In a sense, they each posess the same information, which traveled at lightspeed from the source. Stephen Jackson Xerox PARC (415) 494-4226