Neutrons and protons are collectively known as nucleons. Each nucleon consists of three quarks, with the quarks that form nuclei coming in two different types called up and down. Protons consist of two up quarks and one down quark, while neutrons consist of two down quarks and one up quark. These quarks are bound together, or constrained, into a small space by virtual particles called gluons, with each pair of quarks continually exchanging gluons with one another. As quarks are moved apart, the strength of the containment increases as more virtual particles appear and disappear between the quarks, pulling them back towards one another. Each gluon that is exchanged exists for only a tiny fraction of a second as it moves from one quark to the other, but this movement strongly binds the quarks together.
Each quark has several characteristics that scientists use to describe them. Some of these characteristics are colour, mass, spin and electric charge. An up quark has a 2/3 positive electric charge, while a down quark has a 1/3 negative electric charge. When they combine to form protons, the two 2/3 positive charges and the 1/3 negative charge add up to give a positive charge of 1. In a neutron, the 2/3 positive charge and two 1/3 negative charges combine to give an electric charge of 0. Colour charge is conceptually similar to electric charge, but comes in three types, red, green and blue, rather than the positives and negatives of electric charge. These colours should not be taken literally, they are simply a tool that physicists use to distinguish the three types of colour charge. When quarks interact by exchanging gluons, they also change their colour charge.
The strong nuclear force not only binds quarks together to form nucleons, it also binds protons and neutrons together to form the nuclei of atoms. Two particles with the same electric charge repel one another via the electromagnetic force, much as magnetic poles of the same type push against one another. Because all protons have a positive charge, the electromagnetic force acts to push them away from one another. The strong nuclear force is sufficiently powerful to prevent the positively charged protons being pushed apart by the electromagnetic force, and it does this by binding protons and neutrons together. It does this binding not with gluons but with another virtual particle called a meson. For reasons I won’t elaborate on, mesons are not defined as force carrier particles although they do play a key role in the way the strong nuclear force operates. Nonetheless, like gluons, mesons operate on only a very small scale that is slightly larger than a nucleon. Their binding ability is impressive. The binding strength of mesons is sufficient to keep multiple positively charged protons together within the nuclei of atoms, bound up with neutrally charged neutrons.
The weak nuclear force is weaker than the strong nuclear force, and much like the strong nuclear force it operates on a small scale. As with the other forces, there is a weak nuclear force field that pervades the universe, and the force is carried by force carrier particles called W and Z bosons. There are in fact two types of W boson, but we will not consider the details here.
When the weak nuclear force turns protons into neutrons and vice versa a particle called a beta particle is emitted. Beta particles are very high-energy particles and this makes them radioactive. The elements from which we are made, including hydrogen, oxygen, carbon, nitrogen and calcium, do not decay into other elements. They are stable, which is probably just as well, because if they did decay, we would quickly fall apart.
The first atomic nuclei to form in the universe were those of hydrogen and helium. These nuclei persisted in a hot plasma, along with unbound electrons, particles that have an electric charge of −1, until the universe had cooled sufficiently for the first atoms to form. At this point electromagnetism began to play a major role in the development of matter.
Electromagnetism is a hundred times weaker than the strong nuclear force, but it operates over much greater distances than either of the nuclear forces. Along with gravity, it is the force that we experience most in our daily lives. It is the force that makes compounds solid, liquid or gas, and it is the force that allows atoms to bind together in the form of molecules. Much as there are force fields throughout the universe for the strong and weak nuclear force, with gluons and W and Z bosons being the particles that carry the forces, there is also an electromagnetic force field. The particles that carry the electromagnetic force, and which allow protons and electrons to interact with one another, are called photons. These are also the particles from which light is made. Quarks exchange gluons when they interact, while electrons and protons exchange virtual photons when they interact. Virtual photons are popping in and out of existence as protons in atomic nuclei and electrons in orbitals around these nuclei interact.
The weakest force of all is gravity. It is 10 raised to the 38th power weaker than the strong nuclear force. Despite being so weak, when objects get large the pull of gravity is strong. The Earth is large enough to stop us floating off into space. Unlike the strong and weak nuclear forces, but in a similar manner to electromagnetism, gravity can operate over very large distances. Gravity and electromagnetism impact our daily lives more than the other two forces because we are very large compared to the scale at which the nuclear forces operate.
Up quarks, down quarks and electrons are not the only fundamental particles. My discussion so far has focused on only one of three groups of fundamental particles. Physicists refer to these groups as generations. I described the first generation, and that is the one all the matter we observe today is made from. I also omitted to mention a particle called the neutrino when describing the first generation. Neutrinos are extremely abundant, but they rarely interact with atomic nuclei or electrons. Perhaps somewhat obviously, you can only detect something if it interacts with something else, and not all particles are affected by all four forces. Neutrinos only interact via the weak force and gravity; they are unaffected by the strong force and electromagnetism. Every second, millions of neutrinos emitted from the sun pass through the Earth without interacting with any of the matter from which it is made. Nonetheless, a very tiny proportion of the neutrinos do interact with atomic nuclei. I did not discuss neutrinos above because they are not part of us, but they do need mentioning now to help appreciate how remarkable is the understanding of the universe that physicists and cosmologists have developed.
The first generation of particles contains four particles: up and down quarks, electrons and neutrinos (or, more accurately, electron neutrinos). Heavier versions of these particles make up the second generation, while even heavier counterparts constitute the third. As an example, the second generation of matter is made up of charm and strange quarks, the muon (rather than the electron) and the muon neutrino. ‘Charm’ and ‘strange’ are names rather than meaning that one of these particles is alluring and the other peculiar. The strange quark is heavier than the down quark, its sister particle in the first generation. Particles in the second and third generations of matter are not stable, and when they are formed at high energies they quickly decay to create the particles in the first generation.
Table of the Fundamental Particles
The four particles in each of the three generations combine to make a total of twelve fundamental particles that are the building blocks of matter. I have also described the four force carrier particles, the gluon, photon and W and Z bosons, which allow the other twelve particles to interact. Ignoring gravitons, which are hypothesized to exist but not observed, we are up to sixteen particles. There is one more particle to introduce, and it is called the Higgs boson, taking the total number of particles to seventeen. Without the Higgs boson, no particles would have mass, and stars, planets and you and I would not exist. The story of the discovery of the Higgs boson reveals just how well physicists understand the dynamics of the three generations of matter and the force carrier particles.