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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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up vote 9 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
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@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
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If I was converted into gamma photons I would consider myself annihilated (although presumably only very briefly) –  Martin Beckett Feb 26 '13 at 17:02

Actually, no. The original Dirac's concept was the annihilation in the literal meaning of that word. According to that concept, anti-particle was a particle which has anti-energy, that is, negative energy. But, after discovery of positron, and when collision of it and electron was performed, and showed that the result are two gamma-photons (when slow electron and slow positron collide), then the corrections of Dirac's concept were made, in order to theoretically allow that case, too. And, the "true" annihilation, according to theory, still "does occur", but that is among "unobservable and undetectable" "virtual" particle and anti-particle pairs, that is, among particles which "spontaneously" emerge and "really annihilate", and are responsible for the, so called, "quantum fluctuations", and "vacuum energy".

So, the so called anti-matter (positrons, anti-quarks, ...) are actually matter, just the rarer kind (to use an analogy for descriptive purposes only: just like there are less left-handed men than there are right-handed men).

To conclude: using terms "anti-matter" and "annihilation" as they are used in physics is wrong. That has nothing to do with "sexier" wording, but with something else: you know, like when there is someone who has high reputation, and who claims something, and then it turns obviously out that he was wrong, and then him and his followers try to hide, to minimize that mistake, and say to others: "no, you do not understand, he used that word only figuratively, formally, because it is cumbersome to say 'sum zero of quantum numbers', because it is 'sexy',... ". So, they have hided the mistake in a very effective way: they kept the word, and when someone reasonably questions the justifiability of its use, they show him that there is the "deeper knowledge" behind that, and the one who "dared to doubt/criticize" retreats, being a bit ashamed, and/or even believing that he learned something "cool". And he will spread that "truth" further.

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Do you have a source for your assertion? It's pretty cynical... –  Floris Aug 18 at 20:35

protected by Qmechanic Aug 18 at 20:42

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