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The standing waves we have just discussed are important for most electrons in normal objects surrounding us. Electrons do move but, generally, they are restrained to stay within atoms or metals because of electric forces, and so they kind of behave like they are trapped in boxes. We will continue our study of electrons in conductors and semiconductors next week. It is the basis for many important technologies.

We will conclude this week with a side story of what happens if we start moving electrons (or protons) at high speed (near speed of light).

This is what we do in particle accelerators where electrons or protons are moving a near light speed and then collide with each other or with fixed targets. The study of these high-energy collisions changed our view of the Universe and they have revealed a rich zoo of subatomic particles.

Combining Relativity and Quantum Mechanics

Because the electrons are small particles, we need quantum mechanics to describe them correctly. If they move at near light speed, we need special relativity. Combining the two theories together was a difficult task. When they succeeded, physicist discovered two things.

1. Every particle has an antiparticle with the exact same mass but opposite electric charge.
2. The number of particles can change. We can create particles out of pure energy.

Both of these theoretical discoveries (mostly made by Paul Dirac) have now been experimentally verified many times.

The anti-partner of the electron is called the positron. It has exactly the same mass but its electric charge is positive.

The proton also has an antiparticle called the antiproton. Again, the antiproton has the opposite charge as the proton (so antiproton is negative).

Some particles like, the photon, are their own antiparticle (in other words there is really no "antiphoton"). Essentially, this is because it is a neutral particle. The opposite of zero is still zero.

Things to remember about antiparticles:

1. They annihilate when they meet. An electron and a positron that meet will annihilate and transform all their energy into other particles, usually just photons (see Fig "Electron/Positron).
2. The world is made entirely of matter. It is still a big mystery why this is so but all the world we see is made of matter. If we want antimatter we need to artificially create it. And antimatter doesn't usually last very long. Not because it is unstable but rather because it easily collides with a matter particle to annihilate into.

External resources

• We have recently learned that antimatter is sometime produced naturally in big electric storms!