By Don Lincoln, Ph.D., Fermi National Accelerator Laboratory (Fermilab)
A theory of everything requires two crucial components. First, it requires that we identify a fundamental building block, which is to say a building block that can’t be further subdivided. And then we require a fundamental force which will hold the building blocks together and tell them how to interact. But our current understanding of the universe shows us that there is long way to go.
Building Blocks: Atoms and Smaller Blocks
It’s possible that you think that atoms can satisfy the criterion of being the fundamental building block. After all, atoms are the fundamental building block of chemistry. However, you know that atoms are made of smaller building blocks: protons, neutrons, and electrons. Protons have a positive charge, while electrons have a negative charge. In fact, electrons have the exact same amount of charge as protons, except the opposite sign. Neutrons are electrically neutral.
Since the proton and electron charge are the same but opposite, and further, nothing is known to have smaller charge, it’s possible to define that amount of charge as a unit of charge, like a mile or a meter or a second. If you do that, you could say that the proton has a +1 charge, the electron a ‘–1’, and the neutron is 0. Three fundamental building blocks with a nice and simple structure.
What you may not know is that protons and neutrons aren’t the final story. In 1964, the American physicist Murray Gell-Mann proposed a model for the particles that make up protons and neutrons. He called them quarks; and proposed 3. They were ‘up’, ‘down’, and ‘strange’.
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Quarks and Leptons
It turns out that quarks had even smaller charge than protons and neutrons. The up quark had a charge of 2/3 that of the proton, while the down quark and strange quark had a charge of 1/3 of that of the electron. So there was 1 positively charged quark and 2 negative ones. Physicists had discovered many particles in the years before the quark model was proposed, and quarks explained every one of them.
We went from the proton, neutron, and electron to an up quark, a down quark, and still the electron. Either way, there were 3 fundamental building blocks. It’s not quite as good as a single fundamental building block, but it’s still pretty economical. Take those 3 particles and you could make anything you see anywhere in the universe.
However, physicists have discovered more quarks and more copies of the particle like the electron. The class of electron-like particles are called leptons, by the way.
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The 12 Particles and 3 Generations
We now know of not 3 but 6 kinds of quarks. They are called up, down, and strange, of course, but also charm, top, and bottom. Up, charm, and top are the ones with +2/3 electrical charge, while the down, strange, and bottom quarks have –1/3 charge. There are also 6 leptons.
Three of them have –1 electrical charge, called the electron, the muon, and the tau. There are also 3 neutral particles, called neutrinos. The neutrinos are each associated with a charged lepton, so the neutrinos are called the electron neutrino, the muon neutrino, and the tau neutrino.
We can also clump these 12 particles into families, or what physicists call generations. Generation 1 is the up and down quark, the electron, and electron neutrino. Generation 2 is the charm and strange quark, the muon and muon neutrino. Finally, generation 3 is the top and bottom quark, the tau and tau neutrino.
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So Many Particles, So Little Cause
You’ll note that all 3 generations seem to be carbon copies of one another, although they do have different masses. And we only need generation 1 to explain ordinary matter. So that might be our first mystery that a theory of everything needs to explain: Why 3 generations?
To the best of our current knowledge, these 12 building blocks seem to be fundamental, which means they have nothing inside them. Now, that doesn’t mean that they don’t have something inside them; in fact, it is highly likely that they aren’t the end of the road. In the same way that protons and neutrons were once thought to be fundamental and then turned out to be composed of quarks, quarks, and leptons will be shown to be made of smaller particles still.
So, we’ve talked about the fundamental building blocks of the cosmos, but we haven’t spoken of the forces that hold them together. In order to build a theory of everything, you need both.
The Fundamental Forces
We know currently of 4 forces that we can’t explain as being derived from a more basic force. The first force is gravity, which holds us firmly on the Earth and guides the stars and planets as they march their stately paths through the cosmos. The second force is electromagnetism, which is the basis of most of our modern technology but is also the root cause of chemistry.
The third and fourth forces are a little more arcane. They are the strong and weak nuclear forces. The strong nuclear force holds protons and neutrons together inside the center of atoms, while the weak nuclear force is responsible for some kinds of radioactivity. We don’t encounter the nuclear forces in our day-to-day lives, but they aren’t new to scientists. We’ve known about them for basically a century.
Our goal of a theory of everything was to be able to explain everything using a single fundamental building block and a single fundamental force. But we’ve fallen short of our goal in terms of the building blocks. We wanted 1, and we have 12. And the same is true of the forces. We wanted 1, and we have 4. So that means we currently have a long way to go to achieve our goal.
Common Questions About Fundamental Particles and Fundamental Forces
The first subatomic particles to be identified were protons, neutrons, and electrons. Protons have a positive charge, while electrons have a negative charge. In fact, electrons have the exact same amount of charge as protons, except the opposite sign. Neutrons are electrically neutral.
The 12 currently known subatomic particles are organized into families or what physicists call generations. Generation 1 has the up and down quark, the electron and electron neutrino. Generation 2 has the charm and strange quark, the muon and muon neutrino. Finally, generation 3 has the top and bottom quark, the tau and tau neutrino.
The first force is gravity, which holds us firmly on the Earth and guides the stars and planets through the cosmos. The second force is electromagnetism, which is the basis of most of our modern technology. The third and fourth forces are the strong and weak nuclear forces. The strong nuclear force holds protons and neutrons together inside the center of atoms, while the weak nuclear force is responsible for some kinds of radioactivity.