Physicists have found the law of nature which prevents time travel paradoxes, and
thereby permits time travel. It turns out to be the same law that makes sure
light travels in straight lines, and which underpins the most straightforward version
of quantum theory, developed half a century ago by Richard Feynman.
Relativists have been trying to come to terms with time travel for the past seven
years, since Kip Thorne and his colleagues at Caltech discovered -- much to
their surprise -- that there is nothing in the laws of physics (specifically,
the general theory of relativity) to forbid it. Among several different ways
in which the laws allow a time machine to exist, the one that has been
most intensively studied mathematically is the "wormhole". This is like a
tunnel through space and time, connecting different regions of
the Universe -- different spaces and different times.
The two "mouths" of the wormhole could be next to each other in
space, but separated in time, so that it could literally be used as a time tunnel.
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