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.