 We've learned a lot about the various conservation laws and constituents of matter that make up the standard model, but we have yet to put these tools into practice, like we might in a real experiment. In this video, what we're going to do is spend some time learning about what reactions and conservation laws that we already know can teach us about what reactions we might observe in a real experiment. We'll start this lesson by learning about a few new rules. These new rules are what physicists call symmetries. Symmetries are properties of an object that don't change when you change the object in a certain way. One good example, the rotation of a sphere about an axis passing through the sphere's center. The rotation transforms the sphere, but does not change how the sphere looks before and after the rotation. We can generally expect reactions to follow these symmetries as long as all conservation laws are obeyed. The first symmetry we're going to talk about is time reversal symmetry. Time reversal symmetry means we can reverse the time arrow in our reaction diagram and get a valid alternative reaction. This one can sometimes be tricky. Energy may or may not be conserved when you simply reverse the arrow, meaning that you might need to add energy to a system for a particular reaction to happen. The second is charge reversal symmetry. Charge reversal symmetry means we can replace all particles with their anti-particles, or vice versa, in any reaction equation. As it turns out, you can also move any particle from one side of a reaction equation to another if you exchange it with its anti-particle after the move. This specific case of charge reversal symmetry is called a crossing symmetry. There are some examples of these symmetries and processes of nuclear beta decay. For example, beta minus decay, which is a neutron goes to a proton plus an electron plus an anti-electron neutrino, or beta plus decay, where a proton goes to a neutron plus a positron plus an electron neutrino. Remember, beta plus decay is not the same as free proton decay. This process, or more specifically, the quark change that underlies this process, only happens when a proton is in a bound state within a nucleus. Take the beta minus decay reaction as an example. You can move the electron from the right hand side of the reaction expression to the left hand side and replace with its anti-particle. Next, we can move the electron anti-neutrino from the right hand side to the left hand side and replace it with its anti-particle. Finally, you can reverse the arrow, or apply time symmetry. Then you get proton goes to a neutron plus the positron plus the electron neutrino. This is just the equation for nuclear beta plus decay. Depending on the example below, some other possible reactions can be found by just applying both charge conjugation and time reversal symmetry. For example, the anti-neutron goes to anti-proton plus a positron plus an electron neutrino, or an anti-proton goes to an anti-neutron plus an electron plus an anti-electron neutrino. Both these reactions are also possible to detect, as long as energy and momentum are conserved. Let's have another look at beta plus decay. What happens if we move the electron neutrino to the left hand side? Well we get proton plus anti-electron neutrino goes to a neutron plus a positron. Turns out this reaction is what led Clyde Cowan Jr. and Frederick Reigns to experimentally verify the neutrino's existence in the 1950s. This led to a Nobel Prize for Reigns in 1995. When they first postulated this as a method for detecting a neutrino, no one had seen evidence that this reaction was possible. They used the crossing symmetry, just as we've done here, to guess that it would work. They then used what they knew about how the neutron and proton would react with particles in the body of their detector to figure out the unique signature for this reaction. Lucky for them, it all worked. And here are the papers that are relevant. So just to review what we've learned about these symmetries, both the time reversal and charge conjugation symmetries can operate at the same time. So you can get some very complex steps leading from one reaction to another. You do need to remember that these symmetries only apply when energy and momentum is conserved in a reaction. So that's something you always have to keep in mind and check if you're going to take what you know about one reaction and see what it might suggest about something new. In all of these reactions, the fundamental forces are at play. Next time we'll learn more about how the forces that act between particles actually depend on intermediaries known as virtual bosons.