Professor Dave here, let's take a look at the standard model. Thousands of years ago, we thought that the universe was made of earth, air, water and fire. Once science came along, we realized that these are not fundamental elements at all, and that everything on earth is made of atoms of different elements, like carbon, oxygen, hydrogen, and dozens more. Soon we realized that these atoms were also not the smallest things. They themselves are made of protons, neutrons, and electrons. The development of quantum theory brought along with it the discovery of completely new particles. New theories had to be developed to describe these particles and these theories ended up predicting even more particles. Modern-day experiments in particle accelerators use the concept of E = mc^2 to collide tiny particles together with incredible energy to see what other kinds of particles manifest out of the chaos. Fundamental particles are spotted and confirmed and the cycle continues. So many particles were hypothesized and confirmed in the 20th century that their ever-growing catalog was dubbed the particle zoo. But with so many particles it becomes very important to be able to define and categorize them, and the model that describes all of these particles is called the standard model of particle physics. Let's quickly go through the taxonomy of the particle zoo so that we can make sense of all these particles and how they relate to one another. All of the particles in the universe can be divided into two categories: fermions and bosons. Fermions are particles that make up matter, and bosons are particles that mediate force. Within the family of fermions there are two types of particles: quarks and leptons. Quarks make up the subatomic particles we are familiar with, the protons and neutrons found in every atom in the universe, and leptons are other massive particles that are not made of quarks, like electrons. On the other hand, in looking at quantum field theories we became familiar with particles that mediate force, which we call bosons. These include the photons that mediate the electromagnetic force, the W and Z bosons that mediate the weak nuclear force, the gluons that mediate the strong nuclear force, and the still hypothetical graviton, which ought to mediate gravity. So once again we've got fermions, the particles that don't mediate forces, and bosons, the ones that do. Fermions, as we said, can be split up into quarks and leptons. Leptons are stable by themselves but quarks are not, so quarks combine to make other particles called hadrons. Hadrons themselves can be split up into mesons and baryons, the latter being the nucleons we are familiar with. Protons and neutrons are examples of baryons. But there are many more given all the possible combinations of three quarks. So far that leaves us here. We can see the up, down, charm, strange, top, and bottom quarks. We see the leptons including the electron, muon, tau, and their respective neutrinos. We see the gauge bosons that mediate the three forces we have quantum field theories for, and the Higgs boson, the particle that bestows massive particles with their mass. The rest masses of these particles are often listed in mega electron volts, a unit of energy due to mass-energy equivalence. But wait, we're not through yet. Remember when we talked about virtual particles that can exist because of the Heisenberg uncertainty principle? These particles appear without cause simply due to their mere probability of existing, but in doing so they always obey symmetry, in that they arrive in pairs. One of these particles is considered regular matter and the other is called antimatter, so when an electron forms it is not without its antimatter counterpart, called the positron. These have the same mass but they are opposite in charge, and just as they are born out of energy, like experiments in particle accelerators, when they collide they annihilate back into pure energy, returning the energy that was borrowed by their existence. Protons and neutrons have antiprotons and antineutrons that are made of anti quarks. All the leptons have anti particles too, like antineutrinos. When we include antimatter, the particle zoo gets much bigger, because we can then include mesons, which are short-lived particles comprised of one quark and one anti quark. There are plenty more hypothetical particles, but we can pause here and review what we've learned. Once again, matter is made of fermions. Fermions are comprised of quarks, which come in six varieties, and leptons, of which there are also six. Quarks combine to make hadrons, which are either mesons or baryons, depending on the quark combination. Protons and neutrons are the baryons that make up atomic nuclei, which come together with electrons from the lepton family to form atoms, which then form molecules and all of the matter we see every day. On the other hand, we also have the force carrier particles called the bosons. These are the gluons that mediate the strong nuclear force, the W and Z bosons that account for the weak nuclear force, the photons that carry the electromagnetic force, and the as of now hypothetical gravitons that we believe mediate gravity. Quantum theory has been successful in showing that these fundamental forces are actually different manifestations of the same force. At very high temperatures, the electromagnetic force and the weak nuclear force become the same force, the electroweak force. At higher temperatures still, these combine with the strong nuclear force to become a single unified force. This force is described somewhat successfully by several models called grand unified theories, though there remains much work to be done in this area. That leaves just one more force to unify: gravity. So let's move forward and talk about that next. Thanks for watching, guys. Subscribe to my channel for more tutorials, support me on patreon so I can keep making content, and as always feel free to email me:
