Q & A: What is Energy?

Let me set the scene for you: I’m fresh off my Ph.D., teaching introductory physics at the University of Wisconsin. I’m trying to demonstrate how to turn potential energy into kinetic energy, and so I ask this simple question:

What is Energy?

And I get a stunned silence back from the room. One of those silences where 25 faces look back at you with eyes that say, “no, you’re the one teaching us; you need to answer that one!”

And I confess, this is one of those questions that’s looks like the easiest thing in the world, and yet there’s no good answer for it. Put simply, energy is possibly the most basic thing in the whole Universe. We know a whole lot about it:

  • we know that all mass and matter contains it,
  • we know how to quantify it,
  • we know how much is stored electrically, chemically, thermally, sonically, etc.,
  • we know how to convert it from one form to another,
  • we know how to use it to accomplish things (i.e., to do work),
  • we think it can never be created nor destroyed,
  • and we can generate, calculate, and measure its various forms.

So at the end of the day, we can identify it, make it, and do stuff with it, which makes it incredibly useful. But we don’t even know how to define it, except in terms of how we use it. In fact, the huge wikipedia article on energy doesn’t even have one sentence attempting to address this. The Department of Energy answers the question by telling you what energy can do, or what it allows us to do, but never tells you what it is. The Canadian government doesn’t do any better. In fact, this website gives the circular definitions:

A very good definition of energy is:
Energy is the ability to do work.
And a very good definition of work is:
Work is the transfer of energy.

All I can conclude from thinking about this is that energies, like velocities, don’t mean anything on their own; they only have meaning when we talk about it relative to something else. In the train on the left, if I ask the man in blue what his velocity is, he might say 75 miles per hour. Or he might say he’s at rest. It depends on whether he thinks I mean relative to the world outside the train or inside his train car.

Well, energy is only meaningful when we look at it relative to something else, too. If I give you a proton and ask you what its energy is, you can tell me how much electrical energy is in it and how much rest mass energy is in it. But you’re also telling me that’s relative to having no charges and no masses around. Does that mean that there is such a thing as “absolute zero energy” in the Universe? Well, there may be, but the best zero-point energy definition we have is not zero! Even completely empty space has energy in it; this seems to be the apparent dark energy that drives the accelerated expansion of the Universe.

Have any good thoughts on what energy is? Have a good definition of it? Leave a comment and share it with us!


  1. #1 michelle
    February 25, 2008

    You did a great job of summarizing my frustration with a few key ideas in Physics that are fundamental to our world…but defy adequate definition. Besides Energy I also have a difficult job with students’ understanding of Gravity.

  2. #2 ethan
    February 25, 2008

    Michelle, thanks for your feedback! It’s amazing how many things there are, as simple and common as “energy,” that we don’t even know how to define. Gravity’s a hard one, too; try asking this and see how many people’s brains you break: How can two objects that are millions of miles apart, and never touch one another, exert a force on each other? Imagine that, and you’ve just imagined gravity!

  3. #3 The Advantages mean
    December 18, 2008

    The disadvantages Advantages Advantages The are

  4. #4 Daniel
    June 4, 2011

    Something that is able to produce motion.

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