FROM J. RICHARD GOTT, COSMOLOGIST AND PROFESSOR OF ASTROPHYSICAL SCIENCES AT PRINCETON UNIVERSITY: In 1905, Einstein showed that moving clocks tick slowly, and this makes time travel to the future possible. The greatest time traveler to date is Sergei Krikalev, a cosmonaut who spent more than 803 days in low-Earth orbit, traveling at high speed, and thus has aged 1/48th of a second less than he would have if he had stayed home. When he returned to Earth, he found the Earth to be 1/48th of a second to the future of where he expected it to be. So he has time-traveled 1/48th of a second into the future.
If you were to fly out to the star Betelgeuse, 500 light-years away, at 99.995 percent of the speed of light and then return at the same speed, the Earth would be 1,000 years older when you got back, but you would only have aged 10 years.
What about time travel to the past? In 1915, Einstein developed his theory of general relativity, which explains gravity as a result of curved space-time. This theory has been tested many times. It predicted light bending around the sun, a phenomenon famously confirmed in a 1919 experiment. If space-time is sufficiently twisted, a time traveler can, while traveling toward the future all the time, circle back and visit an event in his own past. In much the same way, Magellan’s crew kept traveling westward and yet returned to Europe, having completely circled the globe.
In 1949, mathematician Kurt Gödel discovered a solution to Einstein’s equations of general relativity that allows time travel to the past. Gödel’s solution represents a rotating universe. Our universe is not rotating, so this solution does not apply to us; however, Gödel’s solution shows that space-times allowing time travel to the past are possible in principle, and, therefore, other time-travel solutions might exist as well.
In 1988, theoretical physicist Kip Thorne and his colleagues discovered a wormhole solution that allows time travel to the past, and in 1991, I discovered another solution involving two moving cosmic strings. Cosmic strings are thin threads of energy left over after the big bang, predicted in about half the theories of the early universe—we have not found them yet, but we are looking for them. The time machines proposed by Thorne and by me have the property that you can’t use the time machine to go back to a time before the time machine was created. If a time machine is created by twisting space-time in the year 3000, you might use it to go from 3002 back to 3001, but you can’t use it to go back to 2009 because that was before the time loop was created.
To understand whether such time machines can actually be constructed, we may need to understand quantum gravity—how gravity behaves at very small scales. That is one of the reasons the study of time-travel solutions is so interesting. Time-travel paradoxes—such as the danger of killing your grandmother—can be avoided simply by noting that space-time must be self-consistent: Time travelers don’t change the past; they were always part of it. Alternatively, using the many-worlds theory of quantum mechanics, University of Oxford physicist David Deutsch has proposed that if you change the past, it causes a new universe to branch off, such that you now have two histories (the original and the changed one), and paradoxes are likewise avoided.

J. Richard Gott appears with Kip Thorne, Seth Lloyd, Fred Alan Wolf, and Michio Kaku in "Is Time Travel Possible?" the 19th episode in the Closer to Truth: Cosmos, Consciousness, God TV series, hosted and created by Robert Lawrence Kuhn. The series airs Thursdays on the PBS HD network and many other PBS stations. Every Friday, participants will share their views on the previous day's episode.