Today, magnetism is properly known as electromagnetism, one of the universe’s four fundamental physical forces. A fundamental force is one that simply exists. It is a never-ending characteristic. If you compare it to mathematics, it’s conceptually akin to a prime number—like 3 or 13—that can’t be divided into any combination of whole numbers except itself and 1. A fundamental force can’t be reduced into a more basic force; it simply is.
In theory, there are an infinite number of prime numbers. But in the universe today, there are only four fundamental physical forces—at least that we know of: gravity, strong nuclear interactions, weak nuclear interactions, and electromagnetism. (Caveat: scientists continue to look for a mysterious fifth force and make occasional, highly contested claims that they have found it. Stay tuned.) Each of these forces is intrinsic to the workings of the universe, indispensable, inescapable. They were born along with the universe, the sun, stars, moon, and skies.
Gravity is the force that made Isaac Newton’s apple fall to the ground and that keeps you from falling off the face of the Earth as it spins. It governs bulk matter and attracts but doesn’t repel. It is the weakest of the forces but stretches to infinite space. The nuclear interactions govern the insides of atoms but nothing larger. Strong nuclear interactions hold the cores of atoms together. Weak ones (called weak because their sphere of influence is even smaller than strong nuclear interactions) allow atoms to fall apart and metamorphose into other types of atoms. That makes the weak nuclear force the ultimate alchemist. It is responsible for radioactive decay. The energy of our sun, which makes Earth the warm, livable place it is, is the result of both types of nuclear forces. As you read this, the weak interaction is allowing hydrogen protons to shed enough energy to become heavy hydrogen (deuterium) and then the strong interaction allows the atoms that result to fuse together into helium atoms.
So what is electromagnetism? It is the force that holds matter together. Apart from gravity, which holds us down on Earth, everything we see around us is due to magnetic and electric forces, explained the American theoretical physicist Sean Carroll. It is the basis of the structure of the atom, holding electrons in place and allowing atoms to link up into molecules. But where did the structure of the atom come from? From the birth of the universe itself.
So, Big Bang, about 13.7 billion years ago. The universe is created. What makes up the universe and everything in it? Is it atoms and the elements they form? To quantum field theorists, the answer can be stripped back to something more fundamental than atoms. To them, the universe is fashioned of fields: a field for each of the fundamental forces and thirteen other fields governing matter. A field is simply a mathematical way of talking about fluidlike substances that are spread out everywhere throughout the universe and have a value everywhere in the world. They ripple and sway. It’s a difficult concept. In his famous physics lectures to undergraduates at the California Institute of Technology, the late American physicist Richard Feynman said he had never been able to develop a mental image of the electromagnetic field: “How do I imagine the electric and magnetic field? What do I actually see? What are the demands of scientific imagination? Is it any different from trying to imagine that the room is full of invisible angels? No, it is not like imagining invisible angels. It requires a much higher degree of imagination to understand the electromagnetic field than to understand invisible angels.”
Some portions of the electromagnetic field can be discerned. A wave of light is a bump in the electromagnetic field that travels through space. A particle, on the other hand, exists in only one location and nowhere else. But, like light, a particle is still a facet of a field, a little wave tied up into a bundle of energy. And particles make up atoms, or the stuff we can see and feel. The most basic particles, for our purposes here, are electrons and two kinds of quarks: up and down. Each of them has its own field. If you were to think about it in biological terms, they are like the base pairs of DNA that are the foundation of every living thing on Earth. The magic of the universe is that, conceptually, any of these quarks could be exchanged for any other quark. The same goes for electrons. They and their fields are the building blocks of all matter, including you.
The inevitable implication of this, to a theoretical physicist, is that what we observe is only a portion of what is there. What we normally think of as empty space is filled with this powerful electromagnetic force field that gives matter its concreteness, as well as the other forces and fields. To physicists, this is humdrum reality.
By the time the universe is a few millionths of a second old, it has cooled down enough for quarks to join together to create protons and neutrons, the bits that will eventually form the cores of atoms. (The word “atom” comes from the Greek meaning “indivisible.” Wrong, as it turns out.) Electrons don’t join up to make anything bigger; they remain solo. These particles aren’t forming atoms at this point; the universe is still too hot. They’re just bits.
At about the 100-second mark in the life of this new universe, things have cooled down enough for some protons and neutrons to link up and make the heavy centers, or nuclei, of helium atoms—two protons, two neutrons. Give it another 380,000 years and now it’s cool enough that some of those simple nuclei have got electrons in the space around them. The electrons are negative. The protons are positive. They are responding to the maxims of the electromagnetic field: Opposite charges attract and like charges repel. So the negative electrons are drawn to the positive protons. That attraction keeps the electrons inhabiting the space around the nucleus. Neutrons, as the name suggests, are neutral. Why are protons positive and electrons negative and neutrons neutral? No one has satisfactorily explained that; they seem simply to have been born with those differences and we happened to endow them with that nomenclature. Why do opposite charges attract? Again, it just seems to be part of how the fields showed up.
Most of the atom’s weight is in its center, in the protons and neutrons that are the nucleus. The electrons are lightweights, usually in motion. Some chemists like to say that if the whole atom were the size of a baseball stadium, the nucleus would be about the size of a baseball in the middle. That means most of an atom is what the early theorists of atomic structure used to think of as empty space. Today we know that it is filled with invisible fields. Because atoms create matter, that also means that most matter, not just space, is invisible fields. That includes the matter that makes up your body. I sometimes imagine what it must have felt like for the scientist who figured that out. I imagine him looking at his hand with renewed intensity, trying to peer through it.
It’s the arrangement of these three main components of an atom—electrons, protons, neutrons—that determines which type of atom is which. If you can wade through a few more points here, you’ll get to one of the ideas that lies at the heart of magnetism.
The number of protons is key. That number determines which element it is. In other words, an element’s very identity is controlled by the number of protons in its nucleus. So is its order in the periodic table of elements, because the periodic table is arranged by ascending atomic number, from hydrogen on up.
When the number of protons changes—for instance, during radioactive decay or nuclear fusion—then the name of the atom changes as well. So hydrogen is hydrogen because it has only one proton in its nucleus. When immense heat forces a hydrogen nucleus to fuse with another hydrogen nucleus, the atom that emerges has two protons, and therefore it is helium. As goes the number of protons in a nucleus, so goes the name of the element.
By contrast, the number of neutrons and electrons in an atom can shift around without changing the atom’s name. So carbon, for example, the sixth element on the periodic table, will always have six protons. But sometimes, in nature, it has different numbers of neutrons. Those variations are called isotopes. Too many neutrons, and an atom becomes radioactive and unstable and wants to metamorphose into a different, more stable element.
It’s the electrons, which inhabit the space around the nucleus, that provide one of the secrets to the puzzle of electromagnetism. Just over a century ago, when electrons were discovered, scientists imagined them as little planets moving in a fixed track, or orbit, around a home star, or nucleus, just as the Earth does around the sun. They even used that imagery in the names they gave things, like orbits.
Today, they say instead that electrons move in orbitals, which are mathematical expressions of where electrons probably are. To me, it sounds like gobbledygook. But it just means that electrons are not in a track but somewhere in a pretty well-defined three-dimensional cloud around the nucleus. Probably. You can’t point to a spot and say that’s exactly where an electron is right now. And it’s not necessarily in an orbit. Orbitals have many theoretical shapes, some spherical, others complicated three-dimensional figure eights or dumbbells, others far more complex.
The basic, extremely counterintuitive, but absolutely critical idea is that the electron and other particles operate as both a fluidlike component of a field and as a single physical particle at the same time. They are components of the fields that form the universe. For example, when electrons jump from one orbital to another, they are acting as individual physical entities. But when they are incapable of being in a single identifiable place at any time, they are acting like a wave or a field. To understand electromagnetism, we have to live with the complexity of this.
There’s a little more. While the planetary language to describe electron behavior is now obsolete, it is still helpful as a mental image. So is the fact that the orbitals are arranged in groups of concentric rings or layers or shells around the nucleus. That simplification makes them a little easier to visualize. A central point is that the farther away from the nucleus the electron is, the more energy it has and the more apt it is to be able to be persuaded to move away from an atom’s influence.
The idiosyncrasies of these electrons in orbitals provide one way to create a magnetic field. With some exceptions, every electron in the universe is held in one of these orbitals or is in the process of moving into one. But one of the unbreakable rules of the universe is that each orbital has room for only two electrons—a pair—and the electrons in a pair must spin in opposite directions. Confusingly, here the metaphor to describe direction comes from watchmaking: If one in the pair spins clockwise, then the other must spin counterclockwise. The point is that one movement must offset the other in order to reach balance. In addition, each orbital is contained within a shell, or grouping, that can hold a fixed number of pairs of electrons.
Electrons have strong preferences when it comes to inhabiting their orbitals. They fill them up in highly regimented ways. In fact, they have a rigid code of conduct that can be broken only in exceptional circumstances, filling up one shell of orbitals before moving on to another. Going back to the image of the baseball stadium, it’s a little like filling up seats at ground level close to the diamond first, and then, if there are enough electrons, moving higher up and farther away, section by section, row by row.
Electrons strongly prefer not to be in pairs. They’d rather have a solo slot in an orbital. Pairing is a last resort. But they’ll do it before spending the energy to move to a higher shell, or level in the stadium, where they could have a slot all to themselves.
A rather unselfconscious university chemistry teacher, perhaps remembering a beer-soaked baseball game from his youth, once described it this way: Imagine you have six young men who urgently need to urinate, and only three urinals. Each of the first three men in the line will naturally take a separate urinal. At that point, each of the others will ask to share one of the urinals. They don’t show up two to a urinal when there are empty ones. They prefer to have one to themselves. And when even the second spot at each urinal is taken, the knee-clenching next men in line are forced to go upstairs to another bathroom.
But while each orbital has an even number of slots—two—each atom does not have to have an even number of electrons. That means sometimes electrons have a slot in the orbital all to themselves. They are unpaired. The French call them “celibate.”
This is where magnetism comes in. When a material is made up of atoms with one or more unpaired spinning electrons, the atom itself creates a tiny magnetic field. But in some unusual substances, a majority of those solo electrons can be made to spin in the same direction, lining up, magnifying the field in a larger material, making a sphere of influence greater than themselves. In most substances, that field is weak and passing and can be measured only by sensitive instruments. In some, the fields within a substance organize themselves in such a way as to cancel each other out instead of amplifying each other. But a few atoms can retain a strong magnetic field. The most common are iron, cobalt, and nickel. The iron atom has four unpaired electrons in its outermost rank of filled orbitals. Cobalt has three and nickel, two. When those elements combine with others to make such materials as magnetite or terracotta or basalt, the magnetic field within the material can last a very long time.
Because unpaired electrons spinning in the same direction create a magnetic field, it makes sense that a magnetic field itself flows in a direction. It does. As with the orbitals, scientists use everyday planetary imagery to describe this phenomenon. They say that a magnet has a north and south pole, where the field is strongest, and that the field travels from north to south.
Magnetic fields moving in the same direction repel each other and those moving in the opposite direction attract. It’s the same idea as the positive and negative charges of the protons and electrons within the atom’s structure: Opposite charges attract, like charges repel. So when you try to stick a south-facing magnet to a north-facing one, they click together, making the field bigger, magnifying it. But try to put two souths or two norths together and the magnets push each other away. They resolutely refuse to join. This is the fundamental push and pull of the magnet. It is their strong, invisible fields that are doing the pushing and pulling, the same fields that the universe is made of.
Along with direction, a magnet also has a strength or intensity. As you can imagine, if you have a single atom, the field is pretty weak, no matter how many unpaired spinning electrons there are. Gathering a lot of atoms with unpaired spinning electrons together makes a stronger, or more intense, field. So a larger magnet is stronger than a small one. And joining two magnets together, like the south- and north-facing ones we just talked about, creates a more powerful magnet. It makes sense. You’ve got more of those unpaired spinning electrons all pulling in the same direction.
To fully describe a magnet, or the field it creates, it follows that you need to be able to talk about both direction and intensity. Mathematicians call something made up of two components a vector. We use the language of vectors when we talk about velocity, which is direction as well as speed. So, the car’s velocity is 100 kilometers an hour to the northeast. That’s different from saying that the car is traveling northeast (just direction), or saying that the car is traveling 100 kilometers an hour (just speed).
The big picture is that if the universe had been created without the fundamental force of electromagnetic interactions, it would be a profoundly, unimaginably different place, right down to the structure of every atom.