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I recently asked this question: How close does a particle-antiparticle pair need to be for annihilation to happen?

And that received a good answer. But there was a second part to my question that was not addressed, and so I'm posting it here for more direct attention.

When an annihilation event occurs, how fast does it occur, and can the release of energy somehow be moderated (in a similar manner in which a fission reaction is moderated)? Or is it all or nothing?

With charged pairs I would think the task of moderating the reaction might be difficult since the two particles would be strongly attracted to one another. But then for a neutral particle pair (e.g. neutron and anti-neutron) there might be hope?

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docscience
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2 Answers2

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Within current quantum field theory, it does not make sense to ask "how long" a particular process takes to occur. There is a certain probability that a particle and its antiparticle annihilate. But there is no concept of a "process of annihilation".

There's the in-state (particle and anti-particle) and the out-state (products of the annihilation, usually photons), which are assumed to lie in the infinite past and future where no interaction is possible (since there is no notion of "particles" in the interacting case), and there's the probability to get the out-state from the in-state. Quantum field theory offers no description of "how" the in-state is converted into the out-state, except that it's unitary time evolution. You can expand the amplitude in Feynman graphs and think of the individual graphs as possible "processes" producing the out-state, but this is not rigorously meaningful. In particular, you can't tell which one of those processes produced the out-state, so the notion that a specific one of them did is not meaningful.

ACuriousMind
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in a similar manner in which a fission reaction is moderated

I think you are confusing ideas here, and it is leading you to ask a somewhat nonsensical question. The individual nuclear fission events don't change at all in a moderated nuclear pile. What does change is

  • the odds of a neutron from one causing another and
  • how long it take for that neutron to find the next target and
  • how much energy that neutron has when it gets there.

That is, you moderate the chain reaction rather than doing anything to fission per se.

dmckee --- ex-moderator kitten
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