Matter and energy: false dichotomy

Often, reading articles about the Universe or about particle physics, one can come across a phrase that mentions "matter and energy" as if they are two opposites, or two partners, or two sides of the same coin, or two classes that make up everything else. It pops up in many contexts. Sometimes you can see how the Big Bang is poetically described as the moment when all the “matter and energy” of the Universe originated. It can be read that "matter and antimatter annihilate into pure energy." And of course, let us recall the two greatest mysteries of astronomy - “dark matter” and “dark energy”.

As a scientist and an expert writing on scientific topics, I am irritated by such terminology, not because it is absolutely wrong, but because such conversations mislead people who are not engaged in science. For physicists, it means little. These poetic epithets relate to what is clearly defined in mathematics and experiments, and ambiguous definitions simply express long precise phrases. But for those who are not experts, this is very confusing, since each context uses its own definition of matter, and its own meaning of the word “energy” - sometimes even archaic or simply wrong. And any of the methods implies that everything that exists can be either matter or energy - and this is not so. In fact, matter and energy in general belong to different categories - this is how to speak in one sentence about apples and orangutans, or about heaven and worms, or about birds and beach balls.

On this site, I will try to be more precise in order to help the reader avoid confusion arising from this way of expression.

In short

Since the article is long, I hope that it will be informative and enlighten everyone who loves to go into details. And here I will summarize everything that it says:

• Matter and energy do not belong to the same class of concepts and should not form a pair in the representation of a person.
• Matter is an ambiguous term. For her, there are several different definitions that are used both in scientific literature and in popular science. Each definition covers a particular subset of particles. That is, matter is always some substance, but which one depends on the context.
• Energy - the term unambiguous (at least in physics). But energy is not substance. This is what substance has.
• The term “dark energy” is confusing because it is not only energy. It is not a substance. Some substance may be partly responsible for its presence, but the details are unknown to us.
• Photons do not need to be called "energy" or "pure energy." All particles are disturbances of fields, and they have energy. Photons do not stand out in any way. Photons are matter, but energy is not.
• All the substance of the Universe consists of fields (the simplest components of the Universe) and their particles. This view dates back to 1973.

What is matter (and energy)?

To begin, we define the terms.

“Matter” is a terribly multi-valued term. There is no universal definition that does not depend on the context. In various places at least three definitions are used. Matter can call:

1. Atoms, the basic building blocks of what we perceive as “material” - table, air, stone, skin, orange juice - and those particles that make up the atoms themselves, including electrons, as well as protons and neutrons that make up the core an atom.
2. Elementary “material particles” of nature: electrons, muons, tau, three types of neutrinos, six types of quarks — all types of particles that do not transfer interactions (ie, except photons, gluons, gravitons, W and Z). Interestingly, the Higgs particle does not want to fit into this convenient division into particles of matter and particles of interactions, because this classification is initially a bit artificial.
3. Classes of particles that can be found in the Universe, and which on average move slower than light.

According to any of these definitions, electrons will refer to matter (although by the third definition they were not matter in the early periods of the history of the Universe, when it was much hotter). By the second definition, muons are also matter, like neutrinos, although no ordinary material consists of them. By the third definition, some neutrinos may or may not be matter, and dark matter will unambiguously be matter, even if it turns out that it consists of a new type of particles carrying interactions. I am very sorry that these definitions are so confusing, but if you want to know what “matter” means in various books and articles, you need to understand the different ways of using this term.

“Energy,” fortunately (in the sense in which physicists use it) is a well-defined concept with which all physicists agree. Unfortunately, this word has so many meanings in English [and Russian] that it is very easy to get confused when figuring out what physics means. I briefly described various forms of physical energy in an article on mass and energy . For now, suffice it to say that energy is not an object. Atom is an object, energy is not. Objects and groups of objects can have energy - this is a property of objects that describes their behavior and their relationships with each other. It is enough for us to know that particles moving by themselves through space can have two types of energy: mass energy (E = mc 2 , independent of particle movement) and energy of movement (this energy is zero for a particle at rest and becomes greater the faster it moves).

The annihilation of particles and antiparticles is not the transformation of matter into energy

Consider the notion of "matter and antimatter annihilating into pure energy." Simply put, this is not true for several reasons.

Just above, I gave you three different definitions of matter. Speaking about the annihilation of particles and antiparticles, the speaker can mean the first or second of them. I want to discuss the annihilation of an electron and an antielectron (or a positron), or the annihilation of a muon and an antimuon. Details of this process are described in the article on the annihilation of particles and antiparticles .

What is meant by "pure energy"? Most often, photons are described this way, and usually in the context of an electron and a positron (or another massive particle and antiparticle). But doing so is very bad. Energy is what photons possess, not what they are. I have a height and weight - but this does not mean that I am height and weight.

The term “pure energy” is a mixture of poetry, a brief description and rubbish. Since photons have no mass, they have no mass energy, therefore their energy is “pure energy of motion”. But this is not the same as saying: “A photon is 'pure energy'”, which is in strict physics, which is in the view of the amateur. Photons are particles, the same as electrons. Both are ripples of the corresponding fields, and both have energy. The annihilated electron and positron also have energy — as much energy as the photons they annihilate into, since the energy is conserved (the total amount of energy does not change upon annihilation).


Moreover, the process of converting a muon and an anti-muon into two photons occurs in exactly the same way and happens with almost the same probability as the process of transforming a muon and an anti-muon into an electron and positron, that is, the annihilation of matter and antimatter into another type of matter and antimatter. No matter what words are expressed, it is still impossible to say that matter and antimatter always annihilate into something that could even be approximated as “energy”; there are other possibilities.

Therefore, I do not use the expression “matter and energy” in my articles, referring to annihilation. I simply call the process by its name:

particle 1 + antiparticle 1 → particle 2 + antiparticle 2

With this terminology, it is clear why a muon and an anti-muon are annihilated in two photons, or an electron and a positron, or a neutrino and antineutrino in the same way. These are all processes of the same class. There is no need to create a non-existent classification that confuses the universality of the particle / antiparticle annihilation process.

In general, matter and energy are not all

Why do people sometimes talk about “matter and energy”, as if everything around is either matter or energy? I do not know in what context this expression was invented. The language reflects the story, and often responds slowly to new information. Part of the problem is that from 1900 to 1980 there were huge changes in the physical concepts associated with the world and with what it consists of. Now this process is almost stopped. Over the course of my career, he has been remarkably stable.

Our current understanding of the physical world has been shaped by a wide range of experiments and discoveries that occurred in the 1950s, 60s, and 70s. But previous ways of thinking and reasoning about particle physics did not die out until the 1980s and 90s, when I studied and was a young scientist. And no wonder - people who grew up with old ideas take a lot of time to rebuild to new ones, and some never get rebuilt. New vision takes time to take shape and deal with all the minor flaws.

Today, speaking of the world in the context of the modern point of view, we first need to talk about "fields and their particles." Fields - the main components of the world, according to the modern dominant paradigm. Fields seem to us more fundamental than particles, since there is no elementary particle without a field, and a field without a particle - it happens. But it turns out that each of the fields known to us has a particle known to us.

What is common in "fields and particles" with "matter and energy"? Not enough. You could call some fields and particles “matter,” but which of them are matter and which ones are not, depends on the definition of matter you use. But all fields and particles can have energy, and they are not energy.

Particles of matter and particles of interactions

In my articles, I divide the known particles into “particles of matter” and “particles of interactions”. I do not like it, because this separation is artificial. So far it works; particles of interactions and their antiparticles are associated with four interactions in nature known to us today, and particles of matter and their antiparticles are all the rest. In many cases, this separation is convenient. But at the Large Hadron Collider we can discover particles that do not fit into these categories, and even with the Higgs particle there are certain problems.

There is a completely different division that makes sense: what I call particles of matter are fermions, and what I call particles of interactions are bosons. But this may change after new discoveries.

In fact, it all comes down to the fact that all particles in nature are just particles, some of which serve as antiparticles to each other, and there is no unique way to divide them into classes. I use the words "matter" and "interaction" because it does not sound as abstract as "fermions" and "bosons" - but it is possible that I regret it, because we can detect particles that violate this separation.

Matter and energy in the universe

Another place where we meet these words is the history and properties of the cosmos as a whole. We read about matter, radiation, dark matter and dark energy. The use of these words by cosmologists differs from the expected - and in fact they have two or three different meanings depending on the context.

Matter and antimatter: the people talking about them mean the first definition of the three. They usually talk about the prevalence of matter over antimatter in the Universe - that the particles that make up ordinary matter (electrons, protons and neutrons) are much larger than their antiparticles.

Matter and radiation: this separation implies a third definition from the list. The universe has a temperature. At first it was very hot, and then gradually cooled, and now it is at a mark of 2.7 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 energy of the particle's motion will be k T, where k is the famous Boltzmann constant. In this sense, matter is any particle whose energy of mass mc 2 is greater than the average energy of motion kT. In such particles the speed will be much less than the speed of light. Radiation is any particle whose mass energy is small compared to kT, and, therefore, it moves at a speed close to that of light.

It turns out that in this context, what is and what is not matter depends on temperature, and therefore on time! In the early Universe, in which temperatures were trillions of degrees or more, the electron was what cosmologists would consider radiation. Today in the cold universe, the electron falls into the category of matter. In the modern universe, according to this definition, at least two of the three types of neutrinos are matter, and maybe all three. But in the early universe, all three neutrinos were radiation. Photons have always been and will be radiation, because they are massless.

What is dark matter? Based on the study of the motions of stars and other techniques, we can say that most of the mass of galaxies is something that does not glow, and the evidence that the known particles that behave in the usual way are not responsible for this, has been spent a lot forces. Many theories have been proposed to explain this effect, and many of them have been disproved (usually by observing the appearance and behavior of galaxies). Of the remaining theories, one of the leading says that dark matter consists of heavy particles of unknown type. But we know nothing more about them. Experiments can give us new information, although this is not guaranteed. I note: it may turn out that there is no point in talking about dark antiparticles, since dark matter particles, like photons or Z-particles, may turn out to be their own antiparticles.

And dark energy? Recently it was discovered that the Universe is expanding with acceleration, and not with deceleration, as it was in its earlier years. Presumably, what is called “dark energy” is responsible for this, but in reality it is not energy. As my colleague Sean Carroll likes to say, this is tension, not energy — a combination of pressure and energy density. Why is it called energy? Partly due to public relations. Dark energy sounds cool. Dark tension sounds strange, like any other more or less appropriate word. In a sense, it is a harmless thing. Scientists know what it is about, and the terminology does not cause problems from the technical side. Most of the public doesn’t care what it’s about, so we can say that there is no problem from the non-technical side either. But if you really want to figure it out, it is important to understand that dark energy is not a dark form of energy, but something more subtle. Moreover, like energy, dark energy is not an object or a set of objects, but a property that fields or combinations of space-time fields can possess. We still do not know what is responsible for the dark energy, the existence of which we judge by the accelerating Universe. And before we learn, it may take a long time.

By the way, do you know what astronomers mean by "metals"? Not what you think ...

After reading the article, you could get the impression that modern physicists are not particularly inventive, creatively or clearly dispense with the language. Obviously, this is not our forte. Big explosion? Black hole? The poets of the whole world will never forgive us for choosing such stupid names for such fantastic phenomena ...


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