Elegant experiments at CERN and other high energy accelerators in the 1960s and 1970s showed that the atomic nucleus and in particular protons and neutrons are not fundamental particles but they have substructure. Meanwhile, analogous experiments on electrons could not detect substructure. Electrons appear to be truly fundamental particles. These elegant experiments showed that protons and neutrons have subconstituents called quarks. These quarks are fractionally charged which is a very unusual situation for subatomic particles, never seen before. The size of these particles is incredibly small. The atomic nucleus is a few parts in 10,000 compared to the size of an atom. The quarks on a scale hundreds of millions of times smaller than the size of the atom itself. It took enormous energies to liberate evidence of these quarks and the quarks themselves cannot exist as free particles but hints of them were seen. In continuing experiments in Europe and the United States with accelerators in the 1970s and 1980s, a whole set of new particles were produced, high-energy particles that are composites of other subatomic fundamental particles. Mesons or mid range particles, baryons or heavy particles, and leptons or light particles like electrons. In the totality of these experiments, over a 100 perhaps a 150 different subatomic particles were created. When physicists tried to make sense of all these experiments and the resulting 100 or more subatomic particles, some patterns did begin to emerge. Over a period of time what's called the Standard Model of particle physics was produced, which is a schema or arrangement of many of the subatomic particles that shows which are more fundamental or important to the structure of matter than others. Eventually, three generations of particles were seen at the subatomic level. Quarks were seen in three generations. The up and down, which form protons and neutrons, strange and charm, and top and bottom. Those six were only found in the last decade. Going up in these three generations corresponded to higher and higher mass and also increasing instability. Analogously, there were three generations of light particles found, the classical electron, the muon which is a few 100 times more massive, and the tau particle which is thousands of times more massive, and corresponding to each of those three particles a particular type of neutrino. Alongside all these subatomic particles in their three generations or families, were four fundamental forces and the implied particles that carry those forces. The most familiar force of course is the electromagnetic force and its carrier particle is the photon, equally familiar. The two nuclear forces also have carrier particles. The first to be identified and isolated in 1970s was the carrier of the weak force in the nucleus, responsible for radioactivity. These were the W and Z particles found at CERN. Somewhat later, the carrier of the strong nuclear force which binds the atomic nucleus together were found, these are called gluons. These are the exchange particles between quarks that keep the nucleus together. The final particle is only hypothesized, it's a particle that might carry the gravity force called the graviton, and it has never yet been observed. There's one feature of the universe where the universe as a whole deviates from the situation in high-energy physics or lab physics. In the lab and in the equations of high-energy physics, it's equally possible to produce matter or antimatter from pure energy. In fact pure energy can easily create particle, antiparticle pairs in equal numbers. Given that the universe contains a lot of energy, we might imagine there should be antimatter in the universe as well as matter. Yet when astronomers look out with their telescopes or use other diagnostics, we do not see antimatter. We also don't see antimatter on the Earth. It can be created fleetingly from pure energy or from particle collisions but not exist stably. Given sufficient energy, it's possible to make particles and antiparticles in equal numbers, this happens routinely in the physics lab. Energy creating particle, antiparticle pairs is a routine occurrence, the antiparticles however do not last long. Anytime an antiparticle meets a particle, it disappears with the creation of annihilation radiation typically gamma rays. So normal antimatter cannot exist stably in the universe. If there were an antiparticle it would disappear very quickly in a tiny fraction of a second. But it does lead to a cosmic puzzle; if matter and antimatter exist on an equal footing in the laws of nature and physics which they do, then why is the universe overwhelmingly composed of matter? First of all, how do we know this? Astronomers have made careful observations and because even though interstellar space or intergalactic space is fairly empty, it's not a complete vacuum. We know that if there were regions of the universe where there were antimatter such as anti-stars or anti-galaxies, the interfaces between that antimatter and normal matter would lead to gamma radiation. We have had sensitive enough gamma ray telescopes in space to observe these gamma rays for decades. We're very confident that the universe does not contain sectors or regions, or individual anti-stars and anti-galaxies. So we accept that the universe is made of normal matter, and we're left with the puzzle of why the universe ended up so asymmetric, given that the fundamental physics is perfectly symmetric between matter and antimatter. As we'll see in the discussion of cosmology, the explanation for this asymmetry probably lies in the very early universe. When radiation was dominant and particles, and antiparticles were freely coming and going in terms of radiation, a tiny asymmetry in nature and the forces of nature produced the resultant residue of particles and all the antiparticles annihilated leaving us with a universe with far more photons in it than particles. The conventional way of thinking about the structure of the universe or the structure of matter is what we'll call a top-down view. We imagine the universe containing objects, the largest of which are galaxies, within those galaxies are stars or planets and those stars are made of atoms. Then we look for what those atoms contain and we find that the atoms contain subatomic nuclei made of particles and orbiting electrons which are subatomic particles. So we drill down from the highest scales to the smallest scales to try and understand nature. This is the standard reductionist or a top-down view of matter. To summarize substructure, physicists have taken normal atoms and using high-energy experiments and accelerators trying to divulge what is inside an atom. Beyond finding the atomic nucleus and the orbiting electrons, experiments in the 1960s and 70s show that protons and neutrons are not fundamental, but are made of fractionally charged quarks, which seem to be fundamental subatomic particles. But they also produced a wide array of subatomic particles some of which only lasted a very short period of time, and struggle to produce a pattern of all these subatomic particles. The particles exist in different families with different mass ranges and as yet there is no fundamental explanation as to why particles are arranged this way. However recently, with the discovery of the Higgs boson a vital piece of this puzzle, the Standard Model of particle physics was put into place, where we think we understand the mechanism by which protons, neutrons, electrons, and quarks, all subatomic particles get their mass.
