# Opposite beams of photons or electrons

1. May 31, 2010

### motoroller

With two electrons emitted from a decaying atom in opposite directions, the speed can easily be greater than the speed of light between the two electrons (e.g. 0.7c in each direction). If two receivers then pick up the electrons, hasn't the information travelled between the two receivers at a speed greater than the speed of light? Similarly for two fibre optic beams, the central station can transmit to both receivers so that they receive the light at the same time, so that the information has travelled greater than the speed of light between the receiving stations?

2. May 31, 2010

### Passionflower

You are wrong about that.

3. May 31, 2010

### DrGreg

In relativity you can't just "add speeds", you use the formula

$$\frac{u+v}{1+uv/c^2}$$​

In your example, put u=v=0.7c.

4. May 31, 2010

### ZikZak

When a central station O sends a particle beam or a light beam to receiving stations A and B, then information has traveled from O to A and from O to B, but no information has traveled from A to B.

5. May 31, 2010

### haushofer

It can be a little subtle how to define "speeds". Sure, I can emmit a particle with v=0.9c in the x direction, and a particle with v=0.9c in the -x direction. Naively, you would say that the relative speed is now 1.8*c, because every second the distance between the two photons increases with 1.8*c meters. But that's from my point of view. Speeds are defined between two objects. So in this case you should define the speed between the 2 particles, and the formula is given by Dr.Greg.