Our theory of very nearly everything: the particles

What is everything made of? Even asking this question may seem a bit naive, let alone expecting a sensible answer. But one of the big miracles of science is that we know the answer to a spectacular (albeit not quite perfect) level of detail.

Atoms and beyond

In school, we learn that ordinary matter is made of atoms. Atoms were originally considered the smallest building blocks of matter (the word atom means "indivisible") and the 100 or so different types of atoms are collected in the periodic table that hangs on the walls of chemistry classrooms in every corner of the world.

The periodic table of elements (Image from PubChem blog at the National Institutes of Health.)

But atoms are not the end of the story. You will remember that an atom consists of electrons orbiting a central nucleus made up of protons and neutrons. The electrons arrange themselves in shells around the nucleus and get stuck to the electrons in other atoms to form complicated arrangements; this is the basis of chemistry and of all the variety in the different kinds of matter we see around us. In fact, protons and neutrons are themselves composed of tiny constituents called up-quarks and down-quarks (a proton consists of two up-quarks and one down-quark, a neutron of one up-quark and two down-quarks).

A proton is made of two up quarks and one down quark. Image Arpad Horvath.

This is certainly worth pondering. All the matter in our everyday lives – the air, the oceans, rocks and metal, trees, ducks, human resource managers, our friends and enemies, every planet and every star – is made out of just three particles: the electron, the up-quark and the down-quark. All the differences between these various types of matter stem from how those particles are arranged together.

The particles of our universe

Is there anything in the Universe that is not made out of quarks and electrons? It may be difficult to think of such a thing immediately, since I eliminated most possibilities in the previous paragraph. But there is one familiar substance that eventually springs to mind: light.

It may seem strange to refer to light as a "substance", but modern physics has firmly established that light is in fact quite similar to matter. It moves at a finite speed (the famous number c, which is approximately 300 000 km/s), it is affected by gravity (as predicted by Einstein's theory of general relativity) and it even consists of tiny particles, called photons.

If even light is made of particles, it seems a fair guess that everything is made of particles. Apart from the four we know of so far, can we find any more? The answer is yes; since the birth of modern particle physics, we have discovered a whole slew of them in cosmic rays and in particle colliders. Fortunately, after much head-scratching, it has turned out that they are all different combinations composed from a small set of particles that are – as far as we know today – fundamental (ie, truly indivisible). There are seventeen of them, as shown in the table below.

The known fundamental particles (Image MissMJ)

First, we have three families (we call them generations) each consisting of four matter particles: two quarks and two, so-called, leptons. In the first family we find our by now familiar up-quark, down-quark and electron, as well as a fourth particle, the electron neutrino. This is an almost massless particle that is produced in huge quantities in the sun but mostly passes right through ordinary matter. The pattern of two quarks and two leptons is repeated twice more, so that there are twelve matter particles in total, grouped into three generations. Apart from being heavier, the particles in the latter two generations have exactly the same properties as those in the first. This is a rather strange state of affairs, but it seems to just be that way.

Next, there are four, so-called, gauge bosons, of which the photon is one. The gauge bosons are associated with three of the four fundamental forces of nature: the gluon corresponds to the strong nuclear force, the photon to the electromagnetic force, and the W and Z bosons to the weak nuclear force. (The fourth fundamental force is gravity – more on this in the followup articles.)

Finally, there is the Higgs boson, world-famous since its discovery at the Large Hadron Collider at CERN in 2012 . The Higgs boson is perhaps the strangest of the known fundamental particles (even stranger than the aptly named strange quark). If you followed its discovery, you may recognize the claim that particles gain mass through their interactions with the Higgs boson.

The particles shown in the table above, together with Einstein's theory of gravity, account for every observation ever made in physics, with only a small handful of exceptions (mostly in astronomy, you can read more in the final article). In particular, all of the things we encounter in our regular lives ultimately arise from these particles interacting with each other; the interactions individually are rather simple, but together adding up to all the complexity we observe, like a complex machinery where each component on its own behaves according to simple rules. In the next article I will tell you about these rules in a little more detail.

About the author

Elias Riedel Gårding grew up in Stockholm and chose physics instead of programming for his undergraduate degree because his secondary school physics class was frankly not very good, and he wanted to see what he was missing. He has always been interested in the most basic laws of nature – those of fundamental physics – but it wasn't until his master's degree in theoretical physics that he got to study them properly. He thinks quantum field theory, the basic paradigm of particle physics, deserves to be more widely known, hence this article series.