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Gravity Probe B

Testing Einstein's Universe

Special & General Relativity Questions and Answers

What does Stephen Hawking currently think about time travel?

In the late 1980's, Hawking believed that time travel was impossible for the simple reason that we have not been overrun with thousands of time travelers from the future. General relativists, however, have been less certain about this impossibility, and in 1989 or so, Kip Thorne showed that if two 'cosmic strings' approached each other at nearly the speed of light, conditions favorable to the existence of 'closed time-like world lines' could arise in the ensuing spacetime fracus.

I have not kept up with Stephen Hawking's latest pronouncements, but I have heard frequent rumors that he feels that time is a lot less rigid than was previously thought. I am not entirely familiar with the details that form the basis for such a change of opinion. Certainly at the quantum level, time travel must be something of a commonplace; but only electrons and other quantum particles can really 'experience' it. Heisenberg's Uncertainty Principle conceals us from these possibilities just as surely as a black hole's event horizon hides us from this possibility at the other extreme of size.

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All answers are provided by Dr. Sten Odenwald (Raytheon STX) for the NASA Astronomy Cafe, part of the NASA Education and Public Outreach program.