# Antimatter annihilation.

## Main Question or Discussion Point

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.

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mgb_phys
Homework Helper
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.

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?

mgb_phys
Homework Helper
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 )

ZapperZ
Staff Emeritus
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.
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.

mgb_phys
Homework Helper
Ok, mere mortals can't but the universe can.

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