Matt Strassler [April 12, 2012]
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.
On this website I try to be more precise, in order to help the reader avoid the confusions that arise from this way of speaking. Admittedly I’m only partly successful, as I’ll mention below.
This article is long, but I hope it is illuminating and informative for those of you who want details. Let me give you a summary of the lessons it contains:
- 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.
What’s the Matter (and the Energy)?
First, let’s define (or fail to define) our terms.
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.
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.
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).
Annihilation of Particles and Antiparticles Isn’t Matter Turning Into Energy
Let’s first examine the notion that “matter and anti-matter annihilate to pure energy.” This, simply put, isn’t true, for several reasons.
In the green paragraphs above, I gave you three different common definitions of “matter.” In the context of annihilation of particles and anti-particles, speakers may either be referring to the first definition or the second. Here I want to discuss the annihilation of electrons and anti-electrons (or “positrons”), or the annihilation of muons and anti-muons. I’ve described this in detail in an article on Particle/Anti-Particle Annihilation. You’ll need it to understand what I say next, so I’m going to assume that you have read it. Once you’ve done that, you’re ready to try to understand where the (false) notion that matter and antimatter annihilate into pure energy comes from.
What is meant by “pure energy”? This is almost always used in reference to photons, commonly in the context of an electron and a positron (or some other massive particle and anti-particle) annihilating to make two photons (recall the antiparticle of a photon is also a photon.) But it’s a terrible thing to do. Energy is something that photons have; it is not what photons are. [I have height and weight; that does not mean I am height and weight.]
The term “pure energy” is a mix of poetry, shorthand and garbage. Since photons have no mass, they have no mass-energy, and that means their energy is “purely motion-energy”. But that does not mean the same thing, either in physics or intuitively to the non-expert, as saying photons are “pure energy”. Photons are particles just as electrons are particles; they both are ripples in a corresponding field, and they both have energy. The electron and positron that annihilated had energy too — the same amount of energy as the photons to which they annihilate, in fact, since energy is conserved (i.e. the total amount does not change during the annihilation process.) (See Figure 3 of the particle/anti-particle annihilation article.
Moreover (see Figures 1 and 2 of the particle/anti-particle annihilation article), the process muon + anti-muon → two photons is on exactly the same footing and occurs with almost exactly the same probability as the process muon + anti-muon → electron + positron — which is matter and anti-matter annihilating into another type of matter and anti-matter. So no matter how you want to express this, it is certainly not true that matter and anti-matter always annihilate into anything you might even loosely call `energy'; there are other possibilities.
For these reasons I don’t use the “matter and energy” language on this website when speaking about annihilation. I just call this type of process what it is:
- particle 1 + anti-particle 1 → particle 2 + anti-particle 2
With this plain-spoken terminology it is clear why a muon and anti-muon annihilating to two photons, or to an electron and a positron, or to a neutrino and an anti-neutrino, are all on the same footing. They are all the same class of process. And we need not make distinctions that don’t really exist and that obscure the universality of particle/anti-particle annihilation.
Not Everything is Matter or Energy, By a Long Shot
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.
[String theorists might argue with what I've just said, pointing out that their great breakthroughs occurred during the 1980s and 1990s. That's true, but since string theory hasn't yet established itself as reality through experimental verification, one cannot say that it has yet been incorporated into our conception of the world.]
Our current conception of the physical world is shaped by a wide variety of experiments and discoveries that occurred during the 1950s, 1960s and 1970s. But previous ways of thinking and talking about particle physics partially stuck around even as late as the 1980s and 1990s, while I was being trained as a young scientist. This isn’t surprising; it takes a while for people who grew up with an older vision to come around to a new prevailing point of view, and some never do. And it also takes a while for a newer version to come into sharp focus, and for little niggling problems with it to be resolved.
Today, if one wants to talk about the world in the context of our modern viewpoint, one can speak first and foremost of the “fields and their particles.” It is the fields that are the basic ingredients of the world, in today’s widely dominant paradigm. We view fields as more fundamental than particles because you can’t have an elementary particle without a field, but you can have a field without any particles. [I still owe you a proper article about fields and particles; it's high on the list of needed contributions to this website.] However, it happens that every known field has a known particle, except possibly the Higgs field (whose particle is not yet certain to exist, though [as of the time of writing, spring 2012] there are significant experimental hints.)
What do “fields and particles” have to do with “matter and energy”? Not much. Some fields and particles are what you would call “matter”, but which ones are matter, and which ones aren’t, depends on which definition of “matter” you are using. Meanwhile, all fields and particles can have energy; but none of them are energy.
Matter Particles and Force Particles — Well…
On this website, I’ve divided the known particles up into “matter particles” and “force particles”. I wasn’t entirely happy doing this, because it’s a bit arbitrary. This division works for now; the force particles and their anti-particles are associated with the four forces of nature that we know so far, and the matter particles and their anti-particles are all of the others. And there are many situations in which this division is convenient. But at the Large Hadron Collider [LHC] we could easily discover particles that don’t fit into this categorization; even the Higgs particle poses a bit of a problem, because it arguably is in neither class.
There’s an alternate (but very different) division that makes sense: what I called matter particles all happen to be fermions, and what I called force particles all happen to be bosons. But this could change too with new discoveries.
What this really comes down to is that all the particles of nature are simply particles, some of which are each other’s anti-particles, and there isn’t a unique way to divide them up into classes . The reason I used “matter” and “force” is that this is a little less abstract-sounding than “fermions” and “bosons” — but I may come to regret my choice, because we might discover particles at the LHC, or elsewhere, that break this distinction down.
Matter and Energy in the Universe
Another place we encounter words of this type is in the history and properties of the cosmos as a whole. We read about matter, radiation, dark matter, and dark energy. The use of the words by cosmologists is quite different from what you might expect — and it actually involves two or three different meanings, and depends strongly on context.
Matter vs. Anti-Matter: when you hear people talk this way, they’re talking about the first definition within the green paragraphs above. They are typically referring to the imbalance of matter over anti-matter in our universe — the fact that the particles that make up ordinary material (electrons, protons and neutrons in particular) are much more abundant than any of their anti-particles.
Matter vs. Radiation: if you hear this distinction, you’re dealing with the third definition of `matter’. The universe has a temperature; it was very hot early on and has been gradually cooling, now at 2.7 Celsius-degrees above absolute zero. If you have a gas (or plasma) of particles at a given temperature T, and you measure the energies of these particles, you will find that the average motion-energy per particle is given by k T, where k is Boltzmann‘s famous constant. Now matter, in this context, is any particle whose mass-energy mc2 is large compared to this average motion energy kT; such particles will have velocity much slower than the speed of light. And radiation is any particle whose mass-energy is small compared to kT, and is consequently moving close to the speed of light.
Notice what this means. In this context, what is matter, and what is not, is temperature-dependent and therefore time-dependent! Early in the universe, when the temperature was trillions of degrees and even hotter, the electron was what cosmologists consider radiation. Today, with the universe much cooler, the electron is in the category of matter. In the present universe at least two of the three types of neutrinos are matter, and maybe all three, by this definition; but all the neutrinos were radiation early in the universe. Photons have always been and will always be radiation, since they are massless.
What is Dark Matter? We can tell from studying the motions of stars and other techniques that most of the mass of a galaxy comes from something that doesn’t shine, and lots of hard work has been done to prove that known particles behaving in ordinary ways cannot be responsible. To explain this effect, various speculations have been proposed, and many have been shown (through observation of how galaxies look and behave, typically) to be wrong. Of the survivors, one of the leading contenders is that dark matter is made from heavy particles of an unknown type. But we don’t know much more than that as yet. Experiments may soon bring us new insights, though this is not guaranteed. [Note also there may be not be any meaning to dark anti-matter; the particles of dark matter, like photons and Z particles, may well be their own anti-particles.]
And Dark Energy? It was recently discovered that the universe is expanding faster and faster, not slower and slower as was the case when it was younger. What is presumably responsible is called “dark energy”, but unfortunately, it’s actually not energy. As my colleague Sean Carroll is fond of saying, it is tension, not energy — a combination of pressure and energy density. So why do people call it “energy”? Part of it is public relations. Dark energy sounds cool; dark tension sounds weird, as does any other word you can think of that is vaguely appropriate. At some level this is harmless. Scientists know exactly what is being referred to, so this terminology causes no problem on the technical side; most of the public doesn’t care exactly what is being referred to, so arguably there’s no big problem on the non-technical side. But if you really want to know what’s going on, it’s important to know that dark-energy isn’t a dark form of energy, but something more subtle. Moreover, like energy, dark-energy isn’t an object or set of objects, but a property that fields, or combinations of fields, or space-time itself can have. We don’t yet know what is responsible for the dark-energy whose presence we infer from the accelerating universe. And it may be quite a while before we do.
By the way, do you know what an astronomer means by “metals”? It’s not what you think…
You might conclude from this article that modern physicists and their relatives have not been very inventive, creative, or careful with language. Apparently it’s not our collective strong suit. Big Bang? Black Hole? The world’s poets will never forgive us for choosing such dull names for such fantastic things….