25 Days of Particles: Day 5

Positron

Classification: lepton, fermion
Fundamental: yes
Family: anti-first
Mass: 0.51 MeV = 9.11e-31 kg
Interactions: Electromagnetic (charge +1), Weak, Gravity
Spin: 1/2

Having started with the most familiar particles, we're now going to branch into those slightly less well known. Well, maybe not so unknown to science fiction aficionados, and increasingly to those who've read a certain popular writer of thrillers with a historical bent.

In 1928, Paul Dirac published a paper introducing the Dirac equation, a mathematical way of describing particles that combined quantum mechanics, special relativity, and the then-new concept of particle spin. In particular, Dirac used his new equation to describe and explain some features of how electrons in atoms behave. This solution allowed two possible solutions, a positive energy/negative charge one and a negative energy/positive charge one.

The positive energy/negative charge solution was easily identified as the electron, but Dirac didn't know what to make of the other possible solution. In quantum mechanics, anything that can happen does with some probability, so Dirac couldn't just toss the possible solution as impossible because it looked funny. So there should be some particle running around with the same mass and spin as the electron, but with a positive charge. There was some thought that this positive particle was the proton, that had somehow managed to pull a trick not shown in the math and become 1000 times heavier than the electron, but that was discounted as impossible. If the proton and electron were really opposites like this, hydrogen atoms would self-destruct. In 1931, Dirac published a paper stating that a "anti-electron" should exist.



There were hints that such a particle existed before Dirac's work, but it wasn't until after he published that the hints were strongly pursued. One year later, Carl D. Anderson was observing cosmic rays, particles created when particles from the sun hit the upper atmosphere, and was bending them in magnetic fields. He saw particles that curved in the field just like electron, indicating they had the same charge and mass, but went in the opposite direction, indicating they had the opposite sign to their charge. He dubbed these particles the positrons, and won the Nobel prize in physics for discovering anti-matter in 1936.

The positron is the electron's twin, alike in almost every way. They share the same mass and spin and can interact in all the same ways, except that the positron is positively charged while the electron is negative. When a positron and an electron interact with each other, they annihilate, or turn into energy; their spins and charges cancel out, and all the mass, momentum, and energy they carry is converted into light, or pure energy. The reverse is also true. Pure energy can turn into electron-positron pairs. Normally the pairs don't last long, since the partners find each other quickly and annihilate.

This behavior can happen for all particles. Protons have anti-protons, neutrons have anti-neutrons (though those two guys are hard to tell apart), neutrinos have anti-neutrinos, and so on. Anywhere there is enough energy, pairs of particles and anti-particles can pop into existence and skip along their merry way until they decay or annihilate. Space is not nearly as empty as it seems. When particles physicists want some anti-particles for some reason, they typically produce them by smashing particles together (packing lots of energy into a small space), and then using magnetic and electric fields to make the matter particles go one way and the anti-matter ones a different way before they can find each other and annihilate. Storing them is a whole different issue, since like-charged particles don't like to be stored together, and these guys will annihilate if not kept isolated.

Anyway, we don't notice this particles hopping into and out of existence because the amount of energy involved is small. Take a positron and an electron that are just sitting around (no appreciable momentum) and annihilate them. Their combined mass of about 1 MeV gets converted into energy, probably in the form of light. In real world units, 1 MeV = 0.00000000000016 Joules. For comparison, a 60 Watt light-bulb spits out 60 Joules of light energy every second. A stick of dynamite can spit out about 2100000 Joules of energy, or that of 13,107,163,608,000,000,000 electron-positron pairs, which would be a whole lot more positrons than we can currently make or store.

So the electrons of the universe can continue on their work of holding molecules together and zipping through wires and heating my oven, safe from worry they will run into a positron any time soon, and I can get on with my holiday baking.