So I'm back from working at a mysterious government site, and its time for me to blog about energy. Energy is one of the central topics of physics, it comes up in classical mechanics, E&M, statistical mechanics, quantum mechanics, relativity, and any other topic I'm neglecting to mention, but what is it?
Energy is the most misused term in physics, not by physists, but by pseudoscientists. Energy is not just any "non material" thing, there are also forces and fields (and force fields). Energy means something very specific, defined mathematically, but it can be thought of as being like a currency of motion. There are two types of energy: kinetic energy and potential energy. Kinetic energy is the simplest type, it is just proportional to the (square of the) speed of whatever object you are worrying about, in the money analogy it is the value of goods you can buy or sell. The actual money in the analogy is potential energy, you can turn it into kinetic energy, or kinetic energy can be turned into it. So what causes potential energy? Forces do, for example if you are at the top of a rollercoaster you have a lot of potential energy because you are feeling a force from gravity and if you fell you would accelerate. The moment you start going down the rollercoaster you are changing potential energy into kinetic energy "buying" motion with your "money". Then when you go up the next hill you slow down "selling" your motion.
So why is this a big deal? Well first of all total energy is always conserved (though some may be lost as heat, which is really kinetic energy of atoms or molecules bouncing around), this makes certain kinds of physics problems easy to solve. However this isn't that big of a deal, and for a for a while after the idea of energy was formalized it wasn't a big deal, but then some smart physists figured out that you could solve physics problems just using energy, without worrying about Newton's laws, and this made a lot of physics problems easier, and a lot of open problems were solved. Now when physics started branching out into and dealing with things that are outside of everyday experience it turned out that the only way to solve these problems was using energy. So for this reason all of quantum physics, as well as statistical mechanics (which deals with the behavior of large numbers of atoms or molecules), and relativistic mechanics are defined in terms of energy.
Now I don't know how that is evidence for the afterlife, but lots of people on the internet seem to think that it is...