While in high school studying physics I became curious about what this thing called energy was. I decided to ask the teacher to give me a good explanation, and I don’t recall being very satisfied with his answer. In college, I asked my Modern Physics professor the same thing and he failed similarly. The answers that irritated me were the circular definitions (Energy is the ability to do work. Work is a particular kind of change in energy. Grr…) and the ones that used specific examples to illustrate the concept: batteries, windmills, explosions, fire, heat.
What I want is a context-free explanation of energy that does not default into MathLand. I want an explanation of what this all-important quantity is. Unfortunately, this looks to be more of a philosophical question than one of scientific inquiry. Energy is the glue that binds different physical contexts and situations. It’s the universal currency in systems. It’s a state. Energy is mass. Mass is energy. Mass dents spacetime. Spacetime requires energy to bend. Is energy a quantity that measures how deep spacetime potholes are? The law of conservation of energy and mass says you can’t create energy without losing mass and vice versa—unless you create it and then quickly destroy it fast enough so that the uncertainty principle for energy and time is satisfied. Particles have energies associated with their motions. They exchange quanta of energy as photons. Black holes radiate energy. Energy is light. Light is energy. Heat is dirty energy. The second law of thermodynamics says that the best you can do is break even—meaning that eventually all energy will tend towards becoming heat energy. Things in motion have energy. Substances inherently have energy due to their structure of chemical bonds. What are bonds but local minima in of the chemical potential energy. Hell, even the vacuum has an energy.
I can ramble some more, but it won’t get me anywhere. You’d think that for such an important concept that some physicist out there would have tried to think more about what a unifying physical quantity like energy means. A lot of the terms in physics we just make up for bookkeeping and reference and to separate physical attributes from the more general phenomena governing the evolution of those attributes. A good example of this is the transition from weight to mass.
A long time ago, we remained on the ground—where gravity is effectively a constant force. We measured how much stuff we had by putting it on an apparatus called a scale and it measured the weight of our object. Well, it turns out that the weight it not a good unit to use for measuring how much stuff you have, because it changes based on how far away from the earth’s center you are. But you can then factor out the effects of the earth on your measurement of weight and what you are left with is an intrinsic attribute of the material: its mass.
This kind of thing goes on all the time. The thing I keep thinking is that perhaps what the different subfields of physics have done is to factor out virtually every single physical attribute from almost every measurement leaving behind values to describe the phenomena independent on the given system. This is what we call energy.
If my feeling is correct it would explain why defining energy in general terms is so difficult: any direct link to reality has been stripped from the quantity by its very nature.
