# Speed ?

1. Jan 12, 2010

### cragar

ok I know that nothing with mass can travel at c but i have to ask this question.
if I am in a rocket ship traveling at .5c and another rocket ship is traveling at .5c in the opposite direction then am I traveling at c relative to him . Probably Galilean transformations don’t work in this situation and I am probably missing something can you guys help me out.

2. Jan 12, 2010

### Eric McClean

Now I don't have its answer but I will tell you as I get an adequate amount of time.

3. Jan 12, 2010

### modulus

Well, I was reading in another thread that nothing actually has an absolute mass. It depends on which force you're considering (that is applied on that mass). for example, a nuetron's gravitational mass and nuclear mass may have slightly different values.

I think the same thing goes here. The guy in the other rocket ship will percieve you as a massless entity, but, somebody who is stationary wrt both of you, will not.

Just a fleeting thought...... I haven't verified this from anywhere.

4. Jan 12, 2010

### Staff: Mentor

You are correct that Galilean transformations don't work with such high speeds. You need to use relativistic velocity addition (derived from the Lorentz transformations):

$$V_{a/c} = \frac{V_{a/b} + V_{b/c}}{1 + (V_{a/b} V_{b/c})/c^2}$$

The speed of the other rocket with respect to you will be 0.8c, not c.

5. Jan 12, 2010

### cragar

6. Jan 13, 2010

### Zula110100100

Would a stationary observer see them coming together at a rate of c?

7. Jan 13, 2010

### Janus

Staff Emeritus
Yes. This is called the "closing velocity".