Matter and Energy: A False Dichotomy
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.
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- Matter and Energy really aren’t in the same class and shouldn’t be paired in one’s mind.
- Matter, in fact, is an ambiguous term; there are several different definitions used in both scientific literature and in public discourse. Each definition selects a certain subset of the particles of nature, for different reasons. Consumer beware! Matter is always some kind of stuff, but which stuff depends on context.
- Energy is not ambiguous (not within physics, anyway). But energy is not itself stuff; it is something that all stuff has.
- The term Dark Energy confuses the issue, since it isn’t (just) energy after all. It also really isn’t stuff; certain kinds of stuff can be responsible for its presence, though we don’t know the details.
- Photons should not be called `energy’, or `pure energy’, or anything similar. All particles are ripples in fields and have energy; photons are not special in this regard. Photons are stuff; energy is not.
- The stuff of the universe is all made from fields (the basic ingredients of the universe) and their particles. At least this is the post-1973 viewpoint.
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The word Matter. “Matter” as a term is terribly ambiguous; there isn’t a universal definition that is context-independent. There are at least three possible definitions that are used in various places:
- “Matter” can refer to atoms, the basic building blocks of what we think of as “material”: tables, air, rocks, skin, orange juice — and by extension, to the particles out of which atoms are made, including electrons and the protons and neutrons that make up the nucleus of an atom.
- OR it can refer to what are sometimes called the elementary “matter particles” of nature: electrons, muons, taus, the three types of neutrinos, the six types of quarks — all of the types of particles which are not the force particles (the photon, gluons, graviton and the W and Z particles.) Read here about the known apparently-elementary particles of nature. [The Higgs particle, by the way, doesn’t neatly fit into the classification of particles as matter particles and force particles, which was somewhat artificial to start with; I have a whole section about this classification below.]
- OR it can refer to classes of particles that are found out there, in the wider universe, and that on average move much more slowly than the speed of light.
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With any of these definitions, electrons are matter (although with the third definition they were not matter very early in the universe’s history, when it was much hotter than it is today.) With the second definition, muons are matter too, and so are neutrinos, even though they aren’t constituents of ordinary material. With the third definition, some neutrinos may or may not be matter, and dark matter is definitely matter, even if it turns out to be made from a new type of force particle. I’m really sorry this is so confusing, but you’ve no choice but to be aware of these different usages if you want to know what “matter” means in different people’s books and articles.
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Now, what about the word Energy. Fortunately, energy (as physicists use it) is a well-defined concept that everyone in physics agrees on. Unfortunately, the word in English has so many meanings that it is very easy to become confused about what physicists mean by it. I’ve briefly describe the various forms of energy that arise in physics in more detail in an article on mass and energy. But for the moment, suffice it to say that energy is not itself an object. An atom is an object; energy is not. Energy is something which objects can have, and groups of objects can have — a property of objects that characterizes their behavior and their relationships to one another. [Though it should be noted that different observers will assign different amounts of energy to a given object — a tricky point that is illustrated carefully in the above-mentioned article on mass and energy.] And for this article, all we really need to know is that particles moving on their own through space can have two types of energy: mass-energy (i.e., E= mc2 type of energy, which does not depend on whether and how a particle moves) and motion-energy (energy that is zero if a particle is stationary and becomes larger as a particle moves faster).
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Why do people sometimes talk about “matter and energy” as though everything is either matter or energy? I don’t know the context in which this expression was invented. Maybe one of my readers knows? Language reflects history, and often reacts slowly to new information. Part of the problem is that enormous changes in physicists’ conception of the world and its ingredients occurred between 1900 and 1980. This has mostly stopped for now; it’s been remarkably stable throughout my career.
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