Faster than Sound and Backwards in Time: A Science Surprise

If you're a sci-fi fan you probably have heard about Tachyons, particles that move faster than light. No experiment has demonstrated the things exist, but the possibility isn't denied by the theory of Relativity. Not only doesn't the theory deny their existence, you can even use it to examine their properties. That's led to a number of sci-fi plots; because the theory says you can use Tachyons to send signals into the past. Maybe some of those super rich guys have used Tachyons to tell their younger selves to make the right investments. One of the best stories using Tachyons is by Greg Benford. In his novel "Timescape", written in the 1970's, physicists from the 1990's send warnings back about environmental disasters. Since we're still around I guess it worked.

But why should particles moving faster than light travel back in time? Well, we can get some idea of how this might occur by looking at what happens when an airplane is traveling supersonically, that is, faster than the speed of sound. To see how events in time can be seen to be reversed, we need to forget about communicating with radio and other electromagnetic waves. Instead for our situation only sound waves may be used for communication.

Following physicist Brian Greene's use of the Simpsons in his brilliant book, "The Fabric of the Cosmos: Space, Time, and the Texture of Reality", I'll borrow Bart and Lisa Simpson for this demonstration. Bart is racing faster than the speed of sound, say twice as fast or at Mach 2, toward Lisa on his jet skate board. Looking at his digital watch he counts the number of seconds it will take him to reach Lisa. In his loudest and highest pitched voice he says, one, two, three ... ten. But he's traveling faster than sound! This means that as soon as he shouts "one", he leaves the sound behind. Now he shouts two, but again he speeds past his own shout. Finally he reaches Lisa and stops, quickly asking her if she has heard him. As soon as he says this they both hear the word ten (almost, I'll get to that in a moment) followed by nine, eight ... one, that is they hear the message in reverse order. So Lisa perceives Bart's message in reverse time order. It's worse than that, just give it a little thought to see why.

Not only will the order of numbers be backwards, the sound of each number will also be reversed. Lisa will have to do a bit of work to decode the message as it will not sound much like English. So if you're expecting to get news from your older self on how to beat the stock market, you had better get to work building a Tachyon decoder!