Question: Why do the laws of physics change depending on the energy level?

Mark Jackson answered on 11 Mar 2014:

Suppose your monthly income was $1. What could you do with that? Nothing. Now suppose it was $10. Slightly more, but still not much. $100? Eh. $1,000? Now you could begin to eat simple meals and maybe share a very simple apartment. $10,000? You could eat pretty well and have a decent place to live, plus have some leisure money. $100,000? Eating very well, living in a very fancy place, and having plenty over for luxury. $1 million? You could buy ivory backscratchers to use during champagne parties on your private jet, and other things that ordinary people couldn’t conceive of. It’s pretty clear your lifestyle will change dramatically depending on your income level, because more income gives you more options on what to buy.

It’s very much the same with physics, but with money replaced with energy. Different processes “cost” different amounts of energy, and so the higher the energy available the most processes possible. The simplest example is making new elementary particles: Einstein discovered that particles of mass m cost E=mc^2 to make. This is why we couldn’t produce the Higgs Boson particle until recently: we did not have an accelerator capable of applying enough energy to produce them.

We only learn about physics by doing experiments, and so we are only able to test theories below the energy level possible in our experiment. We may understand “low energy” part of a theory very well, but have no idea about what happens at high energy.