Telling Stories about Energy (a lunchtime conversation between Brian and me)

I finally get around to blog about an interesting conversation that I had with Brian, over a week ago. We had lunch together, and a conversation about events that happened during the "Intuitive Quantum Physics" labs erupted and soon found its way to the topic energy.

We started talking about energy transfer in the case of light waves, and the Poynting vector. Pretty soon, our conversation went to more everyday things like kinetic and potential energy. I was wondering if we really need to introduce the concept "field" in order to be able to talk about potential energy, or if we can just say that an object has potential energy. This was the start of our conversation, which was then spread out over about two days.

Along the way we considered these stories describing physical scenarios (as well as others):

1. I had energy (in me) and when put in effort to lifting the ball, the energy left me and went to the ball.

2. The ball had energy when it was held up high in the air. When I let go, it hit the ground and the ball's energy went into the earth.

3. As the ball fell, it changed its energy from potential to kinetic.

The first two stories are similar. One kind of story we tell with energy is how it gets from one location (or object) to another location (or object). The first story relates energy to the everyday experience with effortful lifting, and the second story tells a story where the results of my efforts (the energy in the ball) went after the ball hit the ground. The results of my effort are now distributed across the earth. They got there through the process of the ball coming into contact with the surface of the earth.

The third story is very different. The third story is about potential changing to kinetic, and how that happens coincidently with the ball moving downward. This is not a story about energy going from one object to another. It is a story about energy changing within an object and that object changing location.

One question for these stories might be this: why does lifting cause energy to change objects and falling cause energy to change kind? Another question might be, do we need two kinds of stories? One attempt to unify the stories is introduce the concept of a "field". Lifting the ball makes energy go to the field; as the ball falls, energy goes from the field to the ball; and when the ball hits the earth, energy goes from the ball to the earth. Lifting, falling, and hitting are all processes that allow energy to go from one object to another. Now lifting and falling do the same thing.

We also touched upon other stories, such as energy going from the sun to the earth (via the process of radiation). We could ask "where was the energy when it was not in the sun and not in the earth?"  We might, for example, begin to fill in the gaps of the story by telling how energy went from the sun to the light, which is a new object in our story. We might then again ask about the process by which energy went from the sun to the light, and maybe we'd tell a process story of "electron" dropping energy levels. Now we are back to the potential energy issue again?

From this conversations we came away wondering many things:

We wondered if "asking where the energy is during a process" is essentially trying to fill in the gaps of an energy story. We wondered if this "filling in" requires some new object in the energy story.

We also wondered about the epistemic value of "objects fall" and the energy story from above? The story doesn't seem to explain anything about why objects fall.

We wondered if "energy moves from places where it is dense to places where it is sparse" is the explanatory base (or primitive) for energy narratives. The hot coffee is dense with energy and so the energy goes into the room where the energy is relatively sparse. This is another shot in the leg to the energy falling story.

We wondered if "energy" having some phenomenological correspondence is also a primitive explanatory idea. The coffee is hotter so it is denser in energy. The light is brighter when the energy flowing through it is denser. The spring is strained when it is compressed, and so has energy. Potential energy of the ball is weird, because the ball is not different (only its relationships to other objects is. In this sense, motion is also weird, but we won't get into that for now.

I hope that this collection of thoughts makes sense to you. It is a reconstruction of the conversation by Brian and me, and might not contain crucial steps that might be important for your understanding. So, if there is something missing, please ask away!

Also, I apologize for posting this with such a delay. I now that some people have been waiting for this post for quite some time...

Oh, and one more thing, I'd like to share and hear your opinions about something else that came up in our discussion. It's just a notion, not really a well thought-out and argued point:
We talk a lot about energy as a possession of objects (having substance-like qualities) and distinguish this "definition" from Feynman's. But "energy as a substance" together with "conservation" implies "accounting" (keeping track of something that happens to be conserved). So somehow the two definitions, Feynman's and the substance-like-thing one, are not that different!?