# Relativistic velocity

1. Jan 25, 2006

### profilexis

I would have another question. If I travel at the speed of light (or 99.9999999999....% of the speed of light) in a spaceship and i begin to run from the back of the spaceship to the front in direction of the spaceship movement, will I be moving faster than the speed of light?

2. Jan 25, 2006

### arildno

Nope.
As measured from the Earth frame (in which the spacecraft has the velocity indicated), you'd be measured to have a velocity strictly below c.

3. Jan 25, 2006

### RandallB

You cannot just add velocities to get total velocity. Use:
w = (u + v)/ (1 + uv/c2)
If you define all the velocities in terms of light speed by dividing each one by “c” the formula is simpler and becomes:

w = (u + v)/ (1 + uv)

You’re more accustomed to living and working with speeds less than 0.0005
Where doubling that speed gives you
.001 / 1.00000025

You’ve just never needed the accuracy of dividing by such a small number at those small speeds. And just used the .001 part or (u + v).

But as one of the speeds becomes high, say above .25
Then using the whole formula and dividing becomes important.

4. Jan 25, 2006

### gyroverse

5. Jan 25, 2006

### krab

The .... means the 9's go on forever? That means you are traveling exactly the speed of light. This is impossible, unless, like a photon, you have zero mass.