throopw@rtp47.UUCP (Wayne Throop) (06/20/85)
Article <2272@mordor.UUCP> mentioned some farily far-out ideas for space propulsion. In article <1684@amdahl.UUCP>, E. Michael Smith asks about Hawking, "vaccuum breakdown" and Mach's Principle, which were mentioned in the original article. Here is my contribution to the flood of articles that I assume will follow this query. Hawking is one of the current big guns in theoretical physics, and has published many articles about the physics of black holes, some of which have been popularized. Hawking radiation refers to an interesting theoretical effect he "discovered" which allows a black hole to radiate. It is related to "tunneling" out of a potential barrier too high to penetrate via classical physics, but one interpretation of it DOES actually have vaccuum "breaking down". In particular, quantum mechanics and the uncertainty principle raises some theoretical difficulties that are avoided by what is called "renormalization". Among other things, the mathematics of this process imply that particles appear and disappear spontaneously in vaccuum. Normally, what is created is a particle/anti-particle pair, and they normally self-destruct before they can be perceived (they persist a shorter time than the uncertainty involved in attempting to measure them). When this occurs near a black hole, one of the two particles (the anti-particle, say) can fall past the event horizon, and thus be unavailable for re-combination with a particle. This event is indistinguishable from the black hole emiting a particle. It is intuitively appealing (and even formally appealing, as I understand it) to think of this process as a "breakdown of vaccuum". A way to think of it is that the event horison of a black hole is "sharp enough" to slice vaccuum. (You real physicists out there note that I didn't say it was a *good* way of thinking about it.) Ernst Mach was an Austrian physicist who died in 1916. I have heard of "the Mach Principle" or "Mach's Principle" in science fiction stories, and it seems to have something to do with FTL travel. I don't know exactly what that is all about, but I suspect is is some pre-relativistic notion that isn't too relevant anymore. (I'd be delighted to find differently... anybody out there know any more about "Mach's Principle"?) Ernst had nothing to do with "mach number", which is speed measured in speed-of-sound units (a somewhat rubber measuring unit, it has always seemed to me). I think mach is a corruption of "mark", but I beg to be corrected if I'm wrong. -- Wayne Throop at Data General, RTP, NC <the-known-world>!mcnc!rti-sel!rtp47!throopw