# What is energy?

**xeni**#1

**Medievalist**#2

I like the idea and the (to abuse a word) *energy* of the presenter, but I can’t bring myself to like the way the question was answered.

energy = the ability to do work

work = the act of displacing something using force

force = any interaction that changes the motion of a mass

mass = a form of energy

conclusion: energy = everything

energy, work, force, mass… this all reminds me of Feynman’s wakalix.

I’m looking for a better answer. One that I can give my daughter, who would not like this one because she likes things more clearly cut. I taught her division, using pizzas and bags of pennies and apples, when she just wasn’t *getting* the math teacher’s jargon laden symbolic methods. She got it almost instantly - because she’s brilliant - and the only hard part was *un*teaching the mistaken impressions she had from trying to understand the schoolteacher.

I always liked using Zelazny’s phrase *“the force that through the circuit drives the current”* to describe energy, but it turns out that’s actually voltage. (I think I just topped out my unintelligibly geeky joke quota for the week.)

Maybe energy is mass in a format that’s been divorced from the direct influence of gravity? Hmm… maybe not. Still looking.

**AnthonyC**#3

Agreed, it’s unsatisfying, but I’m not entirely conceived you can do much better.

It’s like non-Euclidean geometry. For millennia we talked about points, lines, and planes without realizing that these are undefined terms. Or rather, that any “definition” consists only of the effects of the relations the axioms define among them. Change those relations, and the definitions change. Euclid’s fifth postulate (the parallel lines one) was considered ugly for millennia, but if turned out if you change it everything still makes sense, except all of a sudden you are describing “lines” which are the shortest paths on (positively or negatively) curved surfaces; the plane is no longer flat.

At root, what energy *is,* is a mathematical property (an operator) in the laws of physics. To paraphrase another Feynmann quote, it’s the thing that determines how fast (temporally) the phase of a quantum wavefunction changes. It’s also the thing that bends spacetime in relativity (more fundamental than mass, yes). It doesn’t have a definition, and if it did, it would be defined in terms of other undefined terms. But it *does* have meaning, in terms of the relationships and rules it obeys.

Not very helpful without calculus and algebra, but fairly accurate. As with wakalixes, you’re better off sticking to concrete examples for a while.