 So there's one other essential property that we haven't yet talked about that we've sort of alluded to when we explored how the positron was discovered back in the first video, and that is whether we are dealing with a particle or an anti-particle. In other words, whether we are dealing with matter or anti-matter. So the example that I just gave you, the positron, is the anti-particle of the electron. Now anti-particles have the same mass as their corresponding particles, but have opposite charge. There's one other key distinguishing characteristic of anti-matter. If an anti-particle of mass m collides with its corresponding particle, they can annihilate. That is, they can make each other disappear. In their place, they emit two photons each with an energy equal to m times the speed of light squared. E equals mc squared is Einstein's famous mass energy equivalence equation. We can write this process out for an electron-positron annihilation as e plus plus e minus goes to two photons. Each photon, in this case, has an energy of 511 kiloelectron volts. The mass of an electron or positron is equal to 511 kiloelectron volts divided by c squared. Basically, the electron-positron can't just annihilate, leaving nothing in their place. Instead, the energy equivalent to two of their masses is carried by those two photons. One important thing to remember is that all particles have anti-particles. So for all particles, there should be an anti-particle out there with the same mass and opposite charge that will cause some kind of annihilation process like this. Now one of the cool things is we can actually detect the gamma rays that come from one of these annihilation processes, and so we can use that to look for matter and anti-matter collisions.