# Q & A: relative speed

Q:
I read in a book that even if you go 99% the speed of light while racing light, light would still go 100% faster than you. How is this possible, as then c would not be the speed of light in a vacum anymore.
- Socrates
SFS (Seoul Foreign school), South Korea
A:
Nice question. You're assuming that if the light is travelling at speed c away from you, as measured by you, and if you're going at speed 0.99c in the same direction away from some other guy, according to him, then the light must be going at 1.99c away from him, according to him. That is what common sense says (actually, this is what's called a "Galilean transformation", and it is useful and convenient, but only for speeds much less than the speed of light). If you try to write out why you believe it must be so, you will find that you make two assumptions:

1. That the time interval between two events (say light leaving a bulb and hitting a wall) is the same according to you and the other guy.
2. That the distances between things (say a bulb and a wall) are the same according to you and the other guy.

It turns out that both these assumptions are false. Einstein's Special Relativity gives a nice description of how these assumptions break down, and what the rules are for replacing them.

Mike W.

(published on 10/22/2007)