It is common that, when reading about the universe or about particle physics, one will come across a phrase that somehow refers to “matter and energy”, as though they are opposites, or partners, or two sides of a coin, or the two classes out of which everything is made. This comes up in many contexts. Sometimes one sees poetic language describing the Big Bang as the creation of all the “matter and energy” in the universe. One reads of “matter and anti-matter annihilating into `pure’ energy.” And of course two of the great mysteries of astronomy are “dark matter” and “dark energy”.
As a scientist and science writer, this phraseology makes me cringe a bit, not because it is deeply wrong, but because such loose talk is misleading to non-scientists. It doesn’t matter much for physicists; these poetic phrases are just referring to something sharply defined in the math or in experiments, and the ambiguous wording is shorthand for longer, unambiguous phrases. But it’s dreadfully confusing for the non-expert, because in each of these contexts a different definition for `matter’ is being used, and a different meaning — in some cases an archaic or even incorrect meaning of `energy’ — is employed. And each of these ways of speaking implies that either things are matter or they are energy — which is false. In reality, matter and energy don’t even belong to the same categories; it is like referring to apples and orangutans, or to heaven and earthworms, or to birds and beach balls.
