Annihilation

Why do particle-antiparticle annihilate when they meet? Why is there a tremendous amount of energy?
- allan (age 18)

Well, they don’t always. Sometimes they will just fly past each other. Sometimes they do something even more exciting. The antiparticle of the electron is called the "positron" because it is positively charged. If a positron and an electron meet up and aren’t going too fast, they will attract each other electrostatically, and some fraction of the time form a little "atom": an electron and a positron orbiting each other. This "atom", called "positronium", doesn’t last too long, and the electron and positron will eventually annihilate and make photons.

Some particles interact only very weakly, or not at all with each other. For example, photons are their own antiparticles, but they usually just sail past each other because they only interact with electrically charged particles, and photons are not charged (but they can do the inverse reaction of the above -- two photons can collide and make an electron-positron pair!).

In general, any particle that’s charged can annihilate with its antiparticle and can produce photons, in a process that’s guaranteed to happen because the particle interacts with photons. Other things may happen too, especially if the particle and antiparticle are hurled at each other at high energies (see other answers on this site about that).

What you call a "tremendous" amount of energy really depends on your point of view. Einstein tells us E=mc**2 -- if the original particles no longer exist after the annihilation, something has to take up the energy. Often it’s photons, but sometimes it’s other particles which can be produced. The speed of light c is a big number, 300 million meters per second. Square that and you get a big conversion factor between mass and energy. Physicists often say that the energy was there all along, it was just in the form of the rest mass of the incoming particles.

Tom

(published on 10/22/2007)