Does energy exist?

Question

None of this is going to be rigorous or have a right answer, so please don't take it as though I'm demanding one. I'm just interested in everyone's thoughts.

It feels to me (and possibly you) that energy has a different ontological status than something like matter or light. If they are "first-order" it seems "second-order". While observing matter interacting with other matter, we realize that a certain mathematical quantity is conserved, which we call "energy". We then use it to great predictive effect. But it seems strange to then turn around and grant it the same kind of existence as matter -- it should be at a "higher level."

Of course, if you're asking whether something exists you're really asking "does it affect physical processes?" or "does it function as a cause of other physical processes?" My question is, does energy pass this test? If we observe a boulder falling from a height and crushing a village, at one level we could say that its potential energy was the cause, but at a lower level we could say that the pull of gravity on its individual molecules was the cause, and at a higher level we could say that the negligence of the boulder-watching crew was the cause. But we don't think of negligence "existing" as one of the primitive substances in the universe.

Can someone point me to a reference for prior writing or thinking on this?

Answer 1

There is a reason physics starts with kinematics, description of motion and evolution is what it is about and once you throw in the causes you get dynamics, which is everything.

It is not obvious that energy is secondary at all. And I dispute it.

How can you tell you have something? Because it interacts in certain ways (dynamics) or it has the capacity to do so. We don't actually have little labels, we just have interactions and capacities for interactions.

When two things interact the same way and have the same capacities we call them identical. We have to because there isn't anything else.

And the story of energy is the story of anything and everything. Information can only be stored by finding low energy states and putting things in those states by taking some of their energy and throwing it far away so it doesn't come back. That how nuclei form, how atoms forms, how molecules form, how planets and stars form and even how galaxies form.

Any thing that persists does so because it has lower energy than alternatives, so being a lower energy configuration is what makes something "a thing."

I wrote an answer to the linked question which points out that we trust energy more than we trust time or any supposedly obvious concept. And that the alternatives to energy are that things happen for no real reason (that they happen based on a non objective parameter called time instead of based on the phase space which has the real clocks)

If you look at the universe on a large scale you can even see energy as simply a way that spacetime curves and that this type of curvature is a thing that flows and evolves.

You could say that energy is nothing other than this type of curvature and that what the universe is a thing that evolves that type of curvature.

Really Einstein's equation can be taken as a definition of energy (get the stress-energy tensor from the Einstein tensor) and then physical content become that there are evolution laws for the stress-energy so they turn into dynamics for the Einstein tensor.