Special & General Relativity Questions and Answers
If we could travel faster than light, could we go back in time?
Yes, under some circumstances. Look at the very crude diagram below. Time increases from the bottom to the top, and space is the horizontal coordinate. We see two people John and Mary, although Mary could equally well be a mirror or a reflector of tachyon pulses.
. * . Photon . * . . * . . * -. Tachyon A . * - . .* - . . - . . - . . - . Tachyon B . - . . - . Tachyon C . - . . . John MaryJohn sends a signal to Mary using light, shown by the asterisks. If tachyons exist, John could also have sent Mary a Tachyon signal shown by the 'Tachyon A' line which alerted Mary to the fact that a light signal was about to arrive. Using Tachyon B, John could also have signaled to Mary at the same time he sent the light Photon, but Tachyon B would be received by Mary at a time well BEFORE John actually sent the light signal, even though he sent both of them simultaneously! This shows that with tachyons you can send signals into the past. Also, if Mary then receives Tachyon B, and immediately sent Tachyon C, John would receive Tachyon C BEFORE he sent Mary Tachyon B in the first place. This now shows the beginning of causality violation problems wherein you can inform yourself that you are going to do something long before you actually do it in the first place.
As tachyon signal A shows, it isn't necessary that faster than light travel always results in travel backwards in time, but as signals B and C show, there is allowance for this possibility.
For more about this, see Gerald Feinberg's article on Tachyons in the 1967 issue of Physics Reviews, vol. 159 page 1089. There was also a recent article in Scientific American during the last two years or so that talked about how in the quantum world, faster than light signaling can occur. Alas, it cannot be used by us macroscopic folks to send information!
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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.