# Relative speeds reaching c?

1. Apr 3, 2006

Relative speeds reaching c??

Just an idea....

Suppose i am travelling at the speed of slightly more than c/2 with respect to ground and there is a stationary plane mirror in which i can view myself approaching the mirror. Now won't my relative speed with respect to my image be more than c??
Since time dilation would be equal for me and my image..... so can i violate the law that speed of c cannot be attained??

Similarly if i am travelling at slightly more than c/2 and pass someone going in the opposite direction at the same speed as me.... again will i view him going at speed over c??

I know attaining speed of c/2 is highly improbable..... but if this is overlooked.... is this hypothesis possible??

2. Apr 3, 2006

### daveb

No one ever said that c cannot be obtained. They only said it cannot be obtained by things that have mass. Your image has no mass. It's no different than if you had a beacon of light that could be seen at 1 light second, then rotating whatever is emitting the beam at faster than 1 cycle per second. The light would appear to be travelling along the circumference of the 1 light second radius circle at a speed of pi*c. Google phase velocity and you'll see what I mean.

3. Apr 3, 2006

### Staff: Mentor

This is a variation of the idea of two ships moving away from a central point at .5C - do they add to 1C? The answer is no, they don't. Speed is something you measure between two points, not between 3. The center point could say that the ships have a "separation speed" of 1C, but that doesn't mean they measure their own speed relative to each other to be 1C - for that you still need the Lorentz equation.