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What is Energy?

May 26, 2011

Lisa Crystal, a Ph.D. candidate in the Harvard University Department of the History of Science, has written an interesting book review in the April, 2011, issue of Physics Today.[1] The review is of Jennifer Coopersmith's book, "Energy, the Subtle Concept: The Discovery of Feynman's Blocks from Leibniz to Einstein."[2] The review and the book are interesting because they consider the question, "What is energy?"

First, a disclaimer. I don't have much time to read anything longer than articles a few pages in length; so I've read the book review, but I haven't yet read the book. On the basis of the review, I've put this book on my Father's Day list.

I'll proceed based on the idea that most scientific papers are written without their authors knowing all the facts. Some article on management that I remember reading states that it's easy to make a decision if everything is known, but a real leader can make decisions without knowing all the facts.

The principle of the conservation of energy is a triumph of physics. It's an idea that goes back to the time of Thales of Miletus, about 600 BC. Aristotle summarized Thales' philosophy in his Metaphysics:[3]
"... They believe that nothing is either generated or destroyed, since this kind of primary entity always persists."

Thales considered the "primary entity" to be water, but by making the correction to have either matter or energy as the stuff of the universe, we have a first statement of a conservation law.

Ignoring mass-energy equivalence for the moment, the conservation of mass is easy to see, since mass is easy to see. You can see, touch and hold most of its forms. It would be nice if energy were that simple.

The creators of the Transformers animated television series decided that the only way they could portray energy was by representing it as a physical object. Energon cubes were glowing cubes of stored energy used as food/fuel by robots. What's nice about these cubes is that they could harvest energy from any source, including falling water, without any intermediate generators.

Energy is not as simple as that portrayal. Energy exists as a symbol in many different mathematical equations, and it's known by many names.

Thermal energyMagnetic energy
Chemical energyElastic energy
Electric energySound energy
Radiant energyMechanical energy
Nuclear energyLuminous energy

At this point, you might describe energy by saying that you know it when you see it, and point to the list. A little while later, you find dark energy,[4] revise the list, and finally decide that you don't really understand energy at all.

In the introduction to her book, Coopersmith quotes Richard Feynman's description of energy conservation that's contained in his Lectures on Physics.[5] He imagines energy to be similar to the Transformer's Energon cubes. In this case, the cubes are wooden blocks that a child enjoys hiding from his mother. The ever vigilant mother is able to account for every block during the course of the day, sometimes by elaborate deductions and calculations, such as displacement of water in a bath tub.

Although he doesn't state it explicitly, Feynman's analogy identifies the mother with physicists, and the mischievous child with nature. What he does state is that, for energy, there are no blocks, but the system still seems to work. And what vigilance is needed to make it work! You need to keep track of potential and kinetic energies, and convert things like mechanical friction to thermal energy.

Sadi Carnot

Nicolas Léonard Sadi Carnot (1796-1832), who is often called the "Father of thermodynamics."

In his study of heat engines, Carnot discovered the necessity of the second law of thermodynamics, the law that involves entropy.

Carnot is pictured wearing the dress uniform of a student of the École Polytechnique.

Image via Wikimedia Commons)

Feynman, Nobel Physics Laureate and an icon of theoretical physics, had an understandable affection for the action principle from the time he was a secondary school student. Action is a mathematical function that has dimension of (energy x time), and what's interesting about action is that physical systems evolve in a way in which action is minimized.

Since action involves the product of these two fundamental physical quantities, energy and time, it's apparent that they're connected. It's the physics version of "time is money." Energy and time are conjugate variables, which gives rise to the well known expression of the Heisenberg uncertainty principle, ΔEΔt ≥ h, where h is Plank's constant. If ever there were a fundamental thing about nature that deserves more notice, it's action.[6]

One interesting aspect of Lisa Crystal's book review is her description of energy as existing simultaneously as a mathematical abstraction and as something that can be measured. Such a thema/antithema pairing was the basis of a venerable book by Gerald Holton, also of Harvard's Department of the History of Science, so the tradition continues.[7]

Ernesto E. Marinero of the Hitachi San Jose Research Center and Purdue University mentions energy in his candidacy statement for the office of General Councilor of the American Physical Society.
"Physics entered my life at the age of 11, when I was given as a school prize a book simply entitled Energy. To this date I remember the profound impact of the opening statement of the book 'All things in the Universe are a Manifestation of Energy'..."

Ernest Rutherford once said, "If you can't explain your physics to a barmaid, it is probably not very good physics." I'll trust that a corollary is true and wait for Penny to explain energy in The Big Bang Theory.


  1. Lisa Crystal, Reviewer, "Energy, the Subtle Concept: The Discovery of Feynman's Blocks from Leibniz to Einstein," by Jennifer Coopersmith, Physics Today, vol. 64, no. 4 (April, 2011), pp. 61-62.
  2. Jennifer Coopersmith, "Energy, the Subtle Concept: The Discovery of Feynman's Blocks from Leibniz to Einstein," Oxford University Press (August 13, 2010), 392 pages (via Amazon).
  3. Aristotle, "Metaphysics," from Aristotle in 23 Volumes, Vols.17-18, translated by Hugh Tredennick, Harvard University Press, Cambridge, MA; William Heinemann Ltd., London, 1933), Chapter 1, Section 983b; Greek text from Aristotle, "Aristotle's Metaphysics," ed. W.D. Ross, (Clarendon Press, Oxford, 1924). Aristotle, Metaphysics, 1.983b

  4. Paul Rincon, "New method 'confirms dark energy'," BBC News, May 19, 2011.
  5. Preview of chapter 1, Introduction: Feynman’s Blocks, Ref. 2, via Oxford University Press.
  6. In 1998, the American Physical Society had a contest for physics bumper stickers. I had a finalist entry, and another, runner-up entry, Physicists Do It with the Least Action.
  7. Gerald Holton, "Thematic Origins of Scientific Thought: Kepler to Einstein," (Harvard University Press, 1988), 510 pages (via Amazon).

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