extension
What does anti-matter do? model 4 ext a
We have seen that there is a neutrino and an anti-neutrino. In fact every particle has an anti-particle. There is an anti-proton, an anti-neutron and an anti-electron. The anti-electron was the first anti-particle to be detected and we call it the positron. It was discovered in 1932 by Carl Anderson. The positron has exactly the same mass as an electron but a positive charge instead of a negative one.
Beta plus decay
Positrons are given out by some radioactive atoms in a form of decay called beta plus decay. This is very similar to beta decay except that the fast moving particle is a positron rather than an electron. Also, a neutrino rather than an anti-neutrino is given out at the same time. In each case, the atom throws out one particle and one anti-particle.
Particles and anti-particles
Family Particle Antiparticle
lepton electron neutrino positron anti-
neutrino
quark up down anti-
up
anti-
down
Table 1
The electron and neutrino both have anti-particles. These two particles and their anti-particles make up the first generation of the lepton family of fundamental particles. The first generation of quarks also have corresponding anti-particles. There is an anti-up and an anti-down. All of the normal matter in the Universe is made of these first generation particles and their anti-particles.
What happens if particles and anti-particles meet?
Animation of electron postiron annihilation
An electron and a positron. Try rolling over the word disappear.
If a particle meets up with one of its anti-particles, they annihilate one another and release energy. Both particles disappear and all that is left is energy - usually in the form of gamma radiation. However, this only happens on a tiny scale. It is unlikely that there are whole atoms or molecules, let alone whole people, made of anti-matter. So the chances of your bumping disastrously into an anti-matter version of yourself are slight.
What does anti-matter mean?
As with many of the theories about these tiny particles, it is difficult to give a physical meaning to anti-matter. All we know is how it behaves and what it does, not what it is. However, Richard Feynman gave one description of a positron (anti-electron). He suggested it is like an electron (with negative energy) travelling backwards in time (all its behaviours agree with this description).

So, if there is an anti-matter version of you, at least you will not see it coming. This is because, according to Feynman, it is in the future coming back towards you. When you meet, you will not know a thing about it (although anyone standing nearby will not be so fortunate, as the collision between you and your anti-you will release more energy than an atomic bomb).

Energy, mass and Einstein
M4e
The story so far
Quarks and leptons are fundamental particles
THere are three generations of quarks and leptons
Every particle has a corresponding anti-particle
Particles and their anti-particles annihilate each other when they meet
Question M8

a) What is released in an annihilation of a particle with its anti-particle?

b) What is the name given to an anti-electron?

c) How did Richard Feynman describe what anti-matter does?

The story so far
  • There are two families of fundamental particle: quarks and leptons
  • Only quarks feel the strong nuclear force
  • Protons and neutrons are made of quarks
  • There is antimatter as well as matter
  • Electrons are fundamental particles - part of the lepton family
  • There are three generations of quarks and leptons