 In this module we're going to talk about the standard model. To put it simply, the standard model of particle physics summarizes everything we know about matter, what it's made of, how different types of matter interact, and in some cases what particles might exist that we have not yet discovered. To begin learning about the standard model, we're going to go back in time to the early 1930s, when we thought we knew a lot about the basic building blocks of matter. We knew about protons and electrons, Polly had proposed the existence of a neutrino, and the first evidence that would lead to the discovery of the neutron had already been gathered. For a while some physicists even thought that was the extent of it, that we knew all we needed to know about what the universe was made of, and that the only work that was left to do was to further refine our current understanding. At the same time we had begun investigating a rather mysterious source of radiation that seemed to fall from the sky, something we now call cosmic rays. Victor has got credit for discovering these in 1912, earning himself a Nobel Prize in 1936, but for a long time scientists didn't actually know what cosmic rays were made of and where they were coming from. In trying to answer this question, particle physics and what would eventually become the standard model was born. The first particle that was discovered with the use of cosmic rays was the positron, the electrons antiparticle. Carl D. Anderson gets credit for this discovery, which he made thanks to some clever enhancements he added to a cloud chamber. A cloud chamber is essentially a primitive particle detector. It consists of a clear sealed vessel, a glass bottle, for example, filled with a super saturated vapor of water or alcohol. Whenever a charged particle passes through the detector's body, it ionizes the molecules in the air within the chamber. The vapor then gets attracted to that ion trail and forms droplets along the particle's path. You can then take a picture of the particle's trail essentially through the detector. Carl Anderson added a couple of features to his cloud chamber that allowed him to figure out that the particle that he was identifying was actually something new. It wasn't something we already knew about. The first thing he did was he surrounded his chamber with an electromagnet, so charged particles would follow a curved trajectory within his chamber. He also added a sheet of lead to his chamber. Because charged particles slow down or lose energy as they pass through lead, he could then tell what direction his particles were traveling. This is the picture he took, which he published in 1933. Based on the trajectory he saw, the particle that created this track was positively charged, perhaps consistent with a proton. But a proton was a lot heavier and wouldn't have traveled nearly as far within the detector before stopping. The track, he argued, was consistent with something like a positively charged electron. Carl D. Anderson would later use a similar method to discover the miwan, one of hundreds of particles that would be found within the coming decade. Most of these early discoveries did involve the use of cosmic rays, and the reason for that was simple. Cosmic rays, as they would find out, are generally high energy protons or heavier nuclei that are originating from outside of our atmosphere. They fall into our atmosphere. They interact with particles there, producing subsequent reactions and therefore showers of particles. So they were basically a convenient way to look at some of the products of these high energy reactions in our atmosphere. They gave us a good idea of some of the more exotic particles that we had not yet discovered on Earth. Later on, we would produce particle accelerators to do some of these experiments to have better control, to look for particles that were not so easy to see with cosmic rays. So we will spend some time talking about that, in particular some of the most recent developments in a later video in this module. For right now, we're going to leave you with the brief history that we've talked about, and we're going to turn to some of the fundamental properties of these particles. The reason we're going to start doing that is because these fundamental properties are going to tell us a lot about those particles, how they're going to interact, how they're going to behave in physical systems, and this is all going to feed in directly into the standard model.