# Antimatter annihilation.

1. Dec 8, 2008

### Monitor16807

Hi, I don't think I understand why antimatter annihilates when it touches matter, should I picture has it as if it was a wave that canceled an other one?

Or is it something else entirely?

Thanks.

2. Dec 8, 2008

### mgb_phys

The best answer is probably because it can!
For two particles to be able to annihilate each other you have to be able to balance all the conserved quantities (charge, spin, momentum ) this is only possible if one of the particles is the anti-particle of the other.

3. Dec 8, 2008

### Monitor16807

Well while I'm here I might ask.

I read that 1 electron and 1 positron annihilate to make a gamma ray of 512 Mev, can you take a gamma rays of 512 Mev and make 1 electron and 1 positron?

4. Dec 8, 2008

### mgb_phys

You need to make a pair of gamma rays to balance the momentum - but yes you can collide two gamma rays and create an electron-positron pair.
ps. An electron is 511 KeV ( or 0.511MeV )

5. Dec 8, 2008

### ZapperZ

Staff Emeritus
Actually, gamma-gamma collision is not very likely yet (very small cross-section), and so far, we haven't produced any e-p pair that way.

Zz.

6. Dec 8, 2008

### mgb_phys

Ok, mere mortals can't but the universe can.

7. Dec 8, 2008

### Vanadium 50

Staff Emeritus
I'm not so sure that's true. There are what are called two-photon processes in e+e- collisions. What happens is that the electron and positron beams get near each other, and each beam feels an acceleration from the other, and radiates a photon: these photons can then interact and produce anything that couples to the photon: including an e+e- pairs. In a sense, you use the electrons to "carry the photons around" pre-collision.

So the process would be $$e^+ + e^- \rightarrow e^+ + e^- + \gamma + \gamma \rightarrow e^+ + e^- + e^+ + e^-$$.

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