Energy is generally misunderstood. Our schoolroom textbooks tell us that energy is the capacity to do work, and work is the transfer of energy. They go round in circles without getting to the heart of it, and some children grow into adults with no clear concept of what energy is.
Let’s start by saying that energy is the property of a thing. It is not a thing in its own right. To illustrate this, I can talk about a red bus, or a red red ruby. All these things have the property that we call red. A thing can be red, but you cannot remove this red and hold it in your hand. You can remove the paint or the dye and hold that in the palm of your hand, but you are still holding a thing that is red. You cannot remove the red from the dye to hold the red in the palm of your hand. Even when you imagine red, the image in your mind’s eye is a thing. You always need a thing to be red. There is no such thing as “raw red”. In similar vein there is no such thing as “raw energy”.
Another illustration is money. You can spend money just like you can expend energy. But the money doesn’t disappear, just as the energy doesn’t disappear. Somebody else now has your money, just as some other thing now has your energy. Think about an old house, nestled in the countryside. It’s picturesque, worth a lot of money, and it’s built out of cob. Way back when, some guy put some energy into shifting earth and straw to make the walls of this house. He did the same thing with the wood, which grew out of the earth because the trees put energy into shifting water and CO2. The guy made money out of that house. What they were paid for was the energy he put into it, through the work he did moving stuff. That’s why money and energy are similar. They get things moving, they get work done. One makes the world go round, and the other one makes the world go round too.
But money isn’t what energy is, and nor is motion. You need mass and motion before you can talk about energy. Consider a 10 kilogram cannonball, in space, travelling at 1000 metres per second. Not that you can see it.
We talk about how much kinetic energy this cannonball has. We talk about KE = ½ MV2 and we do the maths and get five million Joules. But what has the cannonball really got? Its mass seems real enough, I hefted it into my spaceship this morning before I took off. And its motion seems real enough too, because one false move and it’ll be smashing through my viewscreen taking my head off. To find out more I take a spacewalk to place a thousand sheets of cardboard in the path of my cannonball. Each sheet of cardboard exerts a small braking force, slowing the cannonball to an eventual halt. This takes two seconds. We know that the cannonball will punch through more cardboard in the first second than in the second second, because it’s slowing down. So we deduce that a cannonball travelling at 1000m/s has more than twice the kinetic energy of one travelling at 500m/s. We can do the arithmetic for each second, then slice the seconds up finer and finer, and we end up realising that the ½V2 is the integral of all the velocities between V and 0. But what we don’t realise is that kinetic energy is a way of describing the stopping distance for a given force applied to a given mass moving at a given velocity. You can flip it around to think about force times distance to get something moving. Or you can think in terms of damage. But basically that cannonball has “got” kinetic energy like it has “got” stopping distance.
It’s similar with momentum. That’s a different way of looking at the mass and the motion, based on force and time instead of distance or damage. We look back to our cannonball and cardboard, and we know by definition that in the first second the same amount of time elapsed as in the second second. So we realise that a cannonball travelling at 1000m/s has twice the momentum of one travelling at 500m/s. But what we don’t realise is that momentum is a way of describing the stopping time for a given force applied to a given mass moving at a given velocity. A cannonball has “got” momentum like it has “got” stopping time.
But wait a minute. I didn’t fire the cannonball at 1000 metres a second. I dropped it off at a handy spot out near a GPS satellite, then zipped off in my spaceship in a big loop.
It’s me doing 1000m/s, not the cannonball. The cannonball is just sitting there in space. It hasn’t got any kinetic energy at all. I’ve got it. But I don’t feel supercharged with five million Joules of energy coursing though my veins. So where is it? Where’s the kinetic energy gone? It isn’t anywhere really, because all that cannonball has got, is its mass, and its motion. And that motion is relative to me. Kinetic energy is not a thing. It’s just a relative property.
There are other forms of energy. There’s the potential energy of mass and gravity, potential energy in springs, there’s electrical energy related to current and voltage, there’s chemical energy related to electron bonds, nuclear energy, all sorts.
They all deserve more explanation than I can give in this short essay. So I have to take a shortcut: how do you make something move? Easy. Hit it with something else that moves. And how did you make that something else move? Where did it all start? I pitch you a cannonball, you whack it with a baseball bat, and it tumbles away at one metre per second. You made that cannonball move. Now, where did the energy come from to make it move? From your muscles: “The release of ADP and inorganic phosphate causes the myosin head to turn, causing a ratchet movement. Myosin is now bound to actin in the strong binding state. This will pull the Z-bands towards each other. It also shortens the sarcomere...”
It gets complicated, and I want to make it simple. Maybe too simple. Sorry. But basically some plant somewhere caught the momentum of a sunbeam and stored that kinetic energy by making starch. Think of starch as a chemical spring. You eat the spring. You release it in your muscles, and boing and whack, you shift that cannonball. The kinetic energy of the cannonball came from a sunbeam that came from the sun. And the sun gets its energy from nuclear fusion. Squeeze a couple of hydrogen atoms together and you make helium. But when you do, ping, something breaks, and things spring out between your fingers, things like photons.
4 1H + 2 e --> 4He + 2 neutrinos + 6 photons
What is a photon? Particle physics comes with mental baggage that says it’s a speck, a point, a particle. But it’s more like the slink in a slinky spring. Only the slinky spring here is all of space, with its permittivity and permeability. A photon is a like a ripple on an electromagnetic oceanic “field” between the stars. A boat on the ocean can ride the ripple and the ripple passes on by. But tie that boat to the sea bed with a rubber rope, and you can capture the energy of the ripple, and save it in starch, or coal, or oil. You can use it to build your house, and whack that cannonball.
It all started at the beginning of the universe. Visualize yourself in a dark cylindrical room. The walls are banded and helical. You feel a tremble, and you realise with horror that the room is the biggest baddest spring you’ve ever seen. It’s the “prime mover”, and it is exerting an incredible pressure, but is bound by thick steel cables called “symmetry”. The cables are under impossible tension, and you can hear ping ping ping as individual cable wires snap. Symmetry is about to break, and you know your prime mover will disintegrate into a fireball of nuclear and electromagnetic springs that will go bouncing out to fill the night and make the world what it is.
It’s all analogy of course. Analogies are based on the things we experience, and these are not the things of the subatomic world. So analogies can be dangerous, like too much butter. But it’s enough to give you a grasp of what energy is. Enough to tell you that energy is to do with tension, which is negative pressure, which is the same as stress. Stress is force per unit area, and energy is force times distance, so energy is pressure times volume. So here’s the beginnings of a new definition:
Energy is the capacity to do work, and is also a volume of negative tension…
You know you can’t hold tension in the palm of your hand, and a volume of it doesn’t help. That’s why you can’t hold energy in the palm of your hand. There is no such thing as “pure energy”, just as there is no such thing as “pure pressure”. Things aren’t made out of energy. Things have energy. And how much they have, is relative to how much other things have. Yes, I know mass and motion doesn’t sound like volume and pressure. But to explain more, I’ll have to explain other things. Things like time and space, and mass and gravity.
Energy is a characteristic of change rather than a property of matter.