If you travel faster than the speed of light, can you go back in time?

> I was wondering if you could explain something that
> confuses me about time travel. I have often heard that
> in Einstein's theory of relativity the speed of light
> is a constant and that nothing can travel faster than
> the speed of light. But then I've also heard that if
> something could travel faster than the speed of light
> that it's possible to travel backwards in time. Could
> you shed some light on that theory (no pun intended)?

This last thing that you heard is not correct. If you look at the
mathematical equations for special relativity, if you wish to move faster
than the speed of light, then concepts like time, mass and length become
imaginary numbers (An imaginary number is a number that includes the
square root of -1). Nobody really knows what it would mean to have these
quantities be imaginary (imaginary numbers are perfectly acceptable
mathematically), but in order to travel backwards in time, these
quantities would have to be negative.

The reason that we say that nothing can go faster than the speed of light
is due to the fact that it takes energy to accelerate an object. As you
get closer and closer to the speed of light, it takes more and more energy
to accelerate. In fact, calculations (and experiments) show that it takes
an infinite amount of energy to accelerate an object to the speed of
light, and an infinite amount of energy does not exist in our observable
universe. The only way to move at the speed of light is to be created
moving at the speed of like (like a photon).

An interesting sidelight is that if you were created moving more quickly
than the speed of light, and ignoring the fact that time, length and mass
would be imaginary numbers, the speed of light would be impossibly slow!

So, alas, there is currently no theoretical way to move faster than the
speed of light, or even at the speed of light. [This is the main
difference between the light barrier and the "sound barrier" - the light
barrier is mathematical in nature, whereas the sound barrier was a phobia
- mathematics permitted things to move faster than the speed of sound, and
many things (like bullets and meteors) did so well before Chuck Yeager!]

Thanks for writing,
Kurtis Williams

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