I get email, Part 3

In an email from a few weeks ago, I told you about an email I got from a reader named Meir. He asked a series of about a dozen questions, none really appropriate to the “Ask a Physicist” column, mostly because they are quickies. But I liked them a lot, and I promised to get back to his really interesting list of questions. But so far, I’ve been a total deadbeat.

Today, I’m back on track. He asks:

6. (p. 208) I got one question of mine answered by your book. I always wondered if a neutron is the same as an anti-neutron, as they are both not charged. But now I understand (I hope) that they are not, as the are made of the anti of their quarks.

But you and every other book keep talking about a particle and its antiparticle annihilating themselves if they collide, giving up everything in a flash of energy. My question is, what happens if a particle collides with an anti-particle of a particle of another type. For instance, I know you have the tools to collide protons with protons, so overcome the electric repulsion. Then what happens when you collide a proton with a positron? Same thing as when you collide a proton with an electron (whatever that may be) but with an anti-electron? Or a proton with an anti-meson (what happens when you collide a proton with a meson?)

So first things first. There’s nothing special about something being an anti-particle. Photons, for example, are their own anti-particles, which means that “antimatter” collides with matter (say, the atoms in your eyeballs) all the time and don’t annihilate. You only get the annihilation process if you have an exact pair of particles and anti-particles. So a neutron will annihilate an anti-neutron, even though they don’t react much with each other until they get close.

So what happens if a proton hits a positron, for example? Nothing. They’ll just scatter off one another. There are other combinations, like a muon-positron scatter which might be a little more interesting, but we’re still not going to get the BOOM that comes from a particle and its exact mirror image.

Hope that helps, and I’ll answer another of these soon.

-Dave

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