# Electrons at the speed of light?

1. May 30, 2006

### rcgldr

If linear accelerator were to accelerate an electron very close to the speed of light towards a positron source, (or if the speed in a circular type accelerator with both electrons and positrons circling in opposite directions) could the electron reach the speed of light before collision with the positron? If so, does the electron turn into energy before collision?

2. May 30, 2006

### neutrino

No particle (other than a select few) can reach the speed of light. This includes the electron. They have been acceleated to 0.99c but, not c.

3. May 30, 2006

### rcgldr

Still my question is, in this case, a very high speed collision between an electron and positron, does the conversion of mass to energy take place at the time of collision or does it occur just before?

4. May 30, 2006

### Bob3141592

What exactly do you mean by a "collision"? And what do you mean by "just before"? Isn't a free electron essentially a point source, meaning it has zero radius when looked at clasically? That would make the probability of a contact collision with it's antiparticle zero. So it must be some interaction with a more extended field, or else we must have its spatial dimension quantized above some lower limit. Similarly for "just before". At .999c, the electron covers a fair bit of distance in a very little time, and there's not much chance to give the positron advance notice of the electron's imminent arrival.

It seems that at the level of detail you're asking about, the answer might be in how long it takes for the conversion to occur. How long does it take a photon to be emitted when a bound electron in an atom transitions between energy levels? I suspect that's how long it would take for the mass-energy conversion to occur in the annihilation, but that's entirely conjecture on my part. I really have no idea if that's on the right track.