 We know from special relativity that mass can be converted to energy and vice-versa, and we know from quantum mechanics that light comes in energy packets called photons, where the energy of a photon is just planes constant times the frequency of the light. And what that means is that light can be transformed into matter and vice-versa. There are still lots of conservation laws that processes have to follow. For example, momentum still has to be conserved. An energy, even though it can be transformed to and from mass, the total energy still has to be conserved. What's more, the charge still has to be conserved. And in fact, we know about several other conservation laws as well. And so one of the more common ways that light can turn into mass and vice-versa is a very simple process called pair production. It's simple because what you have is you have a photon coming along, a little piece of light, and it turns into two particles. And one of those particles is an electron, and so it's going to go off at some momentum, and the other particle from the conservation of charge has to have the opposite charge, and it's going to therefore be a positron. And it's going to travel off with some momentum. And overall, the total momentum, the total energy, and the total charge has to be conserved. Let's look at the energy. So beforehand, we have the energy of one photon, which happens to have the Planck's constant times its frequency as its total energy, and we know from special relativity the energy of a charged particle. It depends on, so let's do the electron first, it depends on its mass. We'll call that M minus, and in fact, it also depends on momentum. So it'd be E equals MC squared if we had no momentum, but with momentum, it's going to be plus its momentum, squared and C squared in there. And because I've used that in my equation, I better put it on my diagram, so I remember which symbol I'm talking about, and I'm going to have a P plus, different momentum down there. And so the energy for my positron is going to be... Now I can write down the conservation of energy from here, E plus, and I guess this E, we might call it, we'll give it a subscript lambda to make it look a little bit like the energy of the photon. All right, and so we could solve that in various ways. And it turns out, because you need to have at least, even if you have absolutely zero momentum coming out of the end, so even if these are zero, you still have to have an awful lot of energy to make the mass of the particles. So you need a very high energy photon in order to make a pair of particles. And then any excess energy can go into the kinetic energy of the particles, and they can fly off with some momentum. It's common misconception that photons don't carry momentum, they do, and so that photon carries a bunch of momentum, and the momentum of the photon turns into the vector sum of the momentum of the two particles once they're created. And so it's just normal conservation momentum, the way you would do it with vectors. And the energy is, of course, conserved in the way we wrote down there. And all of this process can work backwards, so you can take a pair of particles, and they can come together to make a photon, and that's called pair annihilation.