Page 81 - A Brief History of Time - Stephen Hawking
P. 81

A Brief History of Time - Stephen Hawking... Chapter 10
   of a virtual particle/ antiparticle pair (say, the antiparticle) might fall into the black hole, leaving the other
   member without a partner with which to annihilate. The forsaken particle might fall into the hole as well, but it
   might also escape from the vicinity of the black hole. If so, to an observer at a distance it would appear to be a
   particle emitted by the black hole.

   One can, however, have a different but equivalent intuitive picture of the mechanism for emission from black
   holes. One can regard the member of the virtual pair that fell into the black hole (say, the antiparticle) as a
   particle traveling backward in time out of the hole. When it gets to the point at which the virtual
   particle/antiparticle pair appeared together, it is scattered by the gravitational field into a particle traveling
   forward in time and escaping from the black hole. If, instead, it were the particle member of the virtual pair that
   fell into the hole, one could regard it as an antiparticle traveling back in time and coming out of the black hole.
   Thus the radiation by black holes shows that quantum theory allows travel back in time on a microscopic scale
   and that such time travel can produce observable effects.

   One can therefore ask: does quantum theory allow time travel on a macroscopic scale, which people could
   use? At first sight, it seems it should. The Feynman sum over histories proposal is supposed to be over all
   histories. Thus it should include histories in which space-time is so warped that it is possible to travel into the
   past. Why then aren’t we in trouble with history? Suppose, for example, someone had gone back and given the
   Nazis the secret of the atom bomb?

   One would avoid these problems if what I call the chronology protection conjecture holds. This says that the
   laws of physics conspire to prevent macroscopic bodies from carrying information into the past. Like the cosmic
   censorship conjecture, it has not been proved but there are reasons to believe it is true.

   The reason to believe that chronology protection operates is that when space-time is warped enough to make
   travel into the past possible, virtual particles moving on closed loops in space-time can become real particles
   traveling forward in time at or below the speed of light. As these particles can go round the loop any number of
   times, they pass each point on their route many times. Thus their energy is counted over and over again and
   the energy density will become very large. This could give space-time a positive curvature that would not allow
   travel into the past. It is not yet clear whether these particles would cause positive or negative curvature or
   whether the curvature produced by some kinds of virtual particles might cancel that produced by other kinds.
   Thus the possibility of time travel remains open. But I’m not going to bet on it. My opponent might have the
   unfair advantage of knowing the future.




              PREVIOUS               NEXT


































     file:///C|/WINDOWS/Desktop/blahh/Stephen Hawking - A brief history of time/i.html (5 of 5) [2/20/2001 3:15:45 AM]
   76   77   78   79   80   81   82   83   84   85   86