# Relativity Question

1. Jan 31, 2010

### Void123

1. The problem statement, all variables and given/known data

A light pulse is emitted at a position $$x_{A}$$ (horizontally) and is received at position $$x_{B} = x_{A} + r$$. Considering that $$v = \beta c$$ for a moving reference frame, I must calculate the separation $$r'$$ between the point of emission and reception.

2. Relevant equations

$$x_{B} - x_{A} = \gamma(x_{B}' - x_{A}')$$

3. The attempt at a solution

I used the above equation, solving for $$x_{B}' - x_{A}'$$, but the answer provided by the book gives me an answer that differs from mine by a few operation signs. But I cannot think how they got it.

2. Jan 31, 2010

### vela

Staff Emeritus
Try writing down the spacetime coordinates in the rest frame for when the pulse is emitted and when it's received. Then use the Lorentz transformations to calculate the coordinates in the moving frame.

3. Jan 31, 2010

### Void123

This is precisely what I have written above:

$$l_{moving} = \frac{l_{rest}}{\gamma}$$

4. Feb 1, 2010

### vela

Staff Emeritus
No, it isn't. That's the formula for length contraction, which isn't applicable for this problem because the emission and reception occur at different times.

5. Feb 2, 2010

### Void123

Okay, for the time:

$$t_{2} = t_{1}'\sqrt{\frac{1 - \beta}{1 + \beta}}$$

Essentially, this is the doppler effect.

Can I rewrite the times in terms of $$l$$ and $$l'$$ to get the answer I want?