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Question

When annihilation occurs what happens with the charge of the interacting particles?
Asked by: Victor M. Capistran

Answer

The short answer to your question is that the charge disappears. To see why this makes sense, let's look at the example of an electron and a positron (the antiparticle of the electron) annihilating into two photons (light).

Electron-Positron Annihilation

Before the interaction occurs, we have an electron with charge (-e) and a positron with charge (+e), so the net charge contained is (-e)+(+e) = 0. After the annihilation, we just have the two photons, which carry no charge, so again, the net charge is zero. So it looks like one charge of (-e) and one charge of (+e) disappeared, which seems a little strange! But the point is that the net charge didn't change, it was always zero. This is called conservation of electric charge. The total amount of electric charge is a constant. Nature is not concerned with the conservation of each individual electric charge, only the net total of all of the particles in the interaction.

This might seem a little confusing, but you can look at it like cold and hot air mixing. Initially you have a certain amount of cold air and a certain amount of hot air. After they mix, you have, usually, no hot air and no cold air, just warm air. One shouldn't stretch this analogy too far, but it might make charge disappearance a little more acceptable.

Finally, this doesn't just have to apply to electric charge. The same applies to any other conserved charge, such as color charge in the quarks.
Answered by: Andreas Birkedal-Hansen, M.S., Physics Graduate Student, UC Berkeley


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