Occult, mysterious and ad-hoc, energy should have no place in modern physics. Newton had no use for it. Motion, in the form of momentum, was enough for him.
Momentum keeps the universe moving. But what causes the planets to orbit the sun and the moons orbit their planets? What is the watch-spring of the universe?
Leibniz believed momentum was not enough. He wanted something to explain what made the ball roll down the hill; the arrow leap from the bow; the wind begin to blow on a still day. He wanted to find the force that gave life to motion.
So Leibniz created a new quantity – something that could move from the spring to the cog; from the bow to the arrow. He created double-entry accounting for the universe: energy.
The idea of energy developed from the idea that there is something in the universe that remains constant. At first it was simply a pendulum bob at the top of its swing transforming its energy into kinetic energy at the bottom, and then back into kinetic again as it ascended on the other side. It took years to make the equations work – to make the calculations add up: kinetic energy plus potential energy is constant. Different ways of calculating each energy had to be tried, tested and discarded until the right calculations were found.
Then chemical energy was calculated in a brewery in Manchester to answer a question. Which gives more energy for your money: a zinc battery or coal for an engine? Chemistry was made to fit into the energy idea.
And then heat. Without heat, where does all of the energy go? Why does the ball stop rolling? Why does the river slow down? Heat was the missing energy. It was possible to make heat add up: heat plus chemical plus kinetic plus potential is constant.
Electricity, light and sound followed: equations were found that allowed the sum to add up. Even nuclear energy could be made to behave according to the rule.
So energy is nothing more than an accounting trick. But then something magical happens. These made-up solutions to the problems of swings, planets, motors and atoms never stop working. The same equations apply from atoms to galaxies to us. Somehow Leibnitz intuition that there was something else is true.
By adjusting and fiddling with the definitions until they fit, we have uncovered the truth – there is something constant in the universe that keeps it going. But we still don’t know what it is.