# If distance and time can change then how can c be constant?

1. Sep 7, 2010

### ChristianKing

If distance and time can change then how can c be constant? I guess what i'm asking is how can someone prove that c=c' without relying on c in a solution such as setting the Lorentz space contraction over the Lorentz time dilation?

2. Sep 7, 2010

### mathman

Re: c=distance/time?

The Lorentz transformations cancel.

3. Sep 7, 2010

### JesseM

Re: c=distance/time?

If you're talking about the speed of light in a single direction (as opposed to measuring the two-way speed by sending a light beam away from a clock, having it bounce off a mirror and return to the clock, and using that time interval to divide the distance from the clock to the mirror and back), you can't derive it from length contraction and time dilation alone, you also have to take into account the relativity of simultaneity. I gave a numerical example of how it all works out in post #7 of this thread.

4. Sep 7, 2010

### ChristianKing

Re: c=distance/time?

thank you