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The term "annihilate" literally means "turn into nothing". However, when a particle and antiparticle collide, they clearly do not turn into nothing; they simply transform into different particles.

Did the term originate at a time when physicists thought that matter + antimatter really did turn into nothing? Does anyone else find the term confusing and/or misleading? Can we introduce a better term?

(I find it especially irritating to hear physicists say that an electron and a positron annihilate into two photons... that's an oxymoron!)

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Presumably physicists do not care for etymology :p. –  alexarvanitakis Feb 26 '13 at 17:30
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1 Answer

up vote 7 down vote accepted

Elementary particles are characterized by quantum numbers, some of them esoteric, which have organized the known particles and resonances into specific multiplets of SU(3) or SU(2).

Different interactions conserve different quantum numbers, but the term "annihilation" is reserved for the annihilation of specific quantum numbers.

In the case of proton antiproton, baryon number is annihilated and becomes zero.

In the case of electron positron it is lepton number that is annihilated and becomes 0.

The energy released by these annihilations rearranges itself in different outgoing particles, conserving the quantum numbers of the given interactions.

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Indeed... I would consider "annihilation" to be those processes where all the quantum numbers that have opposite values for a particle and its antiparticle sum to zero (or multiply to one, for parity). So one could say it's the quantum numbers that are turning into nothing. –  David Z Feb 26 '13 at 16:09
Ah, I see; it's the quantum numbers of the particles that are annihilated. Well that's certainly more satisfying than my previous understanding of the term! –  Dmitry Brant Feb 26 '13 at 16:19
+1, but isn't it really just because "annihilation" sounds sexier than the alternatives? :) –  Mark Mitchison Feb 26 '13 at 16:48
@MarkMitchison which alternative would be in one word? "sum zero of quantum numbers" is cumbersome, though in german one could make it one word :) . –  anna v Feb 26 '13 at 16:50
@annav Well, quite. Perhaps we should all speak german to make ourselves sound sexier. Just kidding. –  Mark Mitchison Feb 26 '13 at 16:54
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