matt@oddjob.UChicago.UUCP (Matt Crawford) (06/01/84)
Well if Ethan is off at the ITP in sunny Santa Barbara, the only possible vengence we can exact is to try to usurp his position... Terminolgy: a vector or direction is called "timelike" if it represents motion slower than light, "spacelike" if it represents motion faster than light, and "null" for the borderline case. The velocity of light in a vacuum is therefore a "null" vector. Equivalently, if two events in space-time are separated by a timelike or null vector, then an ordinary signal can get from the earlier to the later. Also, all observers will agree as to which event was the earlier. Why are black holes "black"? At any point inside a black hole all null vectors point inwards. That's geometry for you! The only directions that point outward are spacelike, along which only a hypothetical tachyon could travel. A particle outside a black hole does not need to receive gravitons from inside to feel the gravitational force. In the quantum-field view, the static gravitational field can be thought of as a coherent superposition of gravitons which are all outside the black hole. The time-travel objection to faster-than-light signalling runs along the following lines. If I have a machine that emits a beam of particles with some velocity v > c, then there can be observers in motion relative to me (but at ordinary speeds) who could receive my signal and respond with similar apparatus. In my frame of reference, their reply signal can seem to be moving backward in time and can be received by me before I send my initial signal. Any science fiction FTL scheme which wants to avoid time travel without contradicting relativity had better have its FTL motion take place relative to a special frame of reference such as the three degree background radiation. ___________________________________________________ Matt ARPA: crawford@anl-mcs.arpa Crawford UUCP: ihnp4!oddjob!matt