Energy can never be created or destroyed. Heard that one before, aye? This is an example of a conservation law, the statement that some quantity will not change over time. In this case, that quantity is the total energy in the universe. Physics is full of conservation laws, conservation of energy, conservation of momentum, of angular momentum, of charge, the list goes on.
Conservation laws are powerful – often all you need to know about a process is what’s being conserved in order to make predictions. Many who did physics in school will remember calculating the final momenta of two objects that smashed into each other using nothing but conservation of momentum.
It’s tempting to think of conservation laws as fundamental rules in physics – conservation of energy seems like something that just is, just like Newton’s laws or the laws of thermodynamics. But actually, if you look hard enough, you’ll find that conservation laws are a product of some much deeper facets of reality. And uncovering the hidden insights that these conservation laws betray has been indispensable to modern physics, particularly particle physics.
Conservation laws are a product of the symmetries of nature. So when we use conservation laws to make predictions, we’re actually using these symmetries.