Time Travel: Forward into the past!

Max More (maxmore@primenet.com)
Fri, 10 Apr 1998 10:24:48 -0700

MOVING FORWARD INTO THE PAST. Many novelists and not
a few scientists have pondered the possibility of returning to a point
in one's past. In principle physics does allow time travel via closed
timelike curves (CTCs). Along such a warped spacetime trajectory
the traveler is always moving into the future (locally) but eventually
finds himself back where he started from. (Think of sailing around
the world; heading west you return from the east.) J. Richard Gott
and Li-Xin Li at Princeton have speculated on whether spacetime
could both allow travel into the past and insure a consistent
chronology (the traveler must remember shaking hands with his
older self as he sets off). They have determined that in part of space
no time travel would be allowable, but in another part (separated
from the first by a surface called a Cauchy horizon) a time machine
could be built, subject to this restraint: if you build a time machine
in the year 3000, you might be able to use it to go from 3002 to the
year 3000, but not back to the year 1998 because that would have
been before the machine was built. Of course even this limited sort
of time travel would be very difficult because you would need a lot
spacetime warp for it to work, and this could only be provided
through the agency of a black hole or a decaying cosmic string
loop. For example, to move even one microsecond back into the
past would require the presence of a space-warping mass equal to
one tenth the mass of the sun! And then there's no guarantee the
black hole wouldn't swallow you and the space around you. Then,
as Gott says, you'd be able to circle around and meet your earlier
self, but you wouldn't be able to escape to boast about it. (Physical
Review Letters, 6 April; as usual journalists can obtain a copy by
contacting AIP Public Information.)