Over the past 115 years, physicists have discovered that pretty much everything material, including rocks and rain, sun and sunshine, ocean waves and radio waves, can be described in terms of particles (and their corresponding fields.) Experiments have uncovered a large handful of types of particles that appear so far to be elementary (i.e., not made from yet more elementary things.) The full complexity of our daily world is constructed from just a few of these. The rest of the particles are evanescent, decaying away so quickly that we don’t encounter them in normal circumstances. But they may hold the keys to secrets of the universe that continue to elude us at the moment.
In this article you will find a brief overview of our present state of knowledge, as of August 2011, showing you the particles we know about and how they can be organized usefully into a few classes… a sort of periodic table of the particles, with a few twists. Along the way you’ll learn what the Higgs field does to the various particles, and the crucial role it plays in our universe.
Our present knowledge, along with our simplest conjecture for the workings of the Higgs particle and field, is summarized in a set of equations called “The Standard Model of Particle Physics”, or just “Standard Model” for short. The elementary particles of the Standard Model have somewhat wacky names (for historical reasons) and a very wide range of masses. In Figure 1 below, notice that
- I’ve drawn heavier particles at the top, the lighter particles at the bottom. (I do this because massless is as low as a particle can go, but particles can have an arbitrarily high mass; in short, there’s a hard floor below, but above, the sky’s the limit.)
- Instead of masses I’ve given the equivalent mass-energies (E = m c-squared) which is what particle physicists typically use. (Keeping track of energy, which is never lost or gained, is easier than keeping track of mass, which can change in some processes, such as decays.) The unit of a GeV is about the mass-energy of the lightest atom, hydrogen.
- I’ve indicated three classes of particles — charged leptons (blue disks), the neutrinos (black disks), and the quarks (red disks). (The quarks are typically sub-divided into two classes, up-type and down-type, which differ only in their electric charge.) The importance of this classification will become clear later.
- I have indicated three forces in boxes, along with their force carrier particles (see below). (Physicists call the forces “interactions”, and the force carriers “mediators”; I am using more intuitive language here.) I have not indicated a fourth force, that of gravity, to avoid clutter on the picture.
- The Higgs field (or something playing its role) is known to be on average non-zero in nature. I’ve represented this with a great green sea. (You can learn more about fields and particles, and the Higgs in particular, in the video clips from my public talk at the Secret Science Club.)
What are all these particles? [All of these particles have anti-particles, but to keep things short I won't mention them, except to point you to this page on anti-particles if you are interested.] Let me quickly review the structure of matter, pulling it apart until we get down to the right levels.
- Atoms, about a billion times smaller in radius than your head, are made from electrons and atomic nuclei.
- Atoms can absorb and emit particles of light, called photons. This occurs through the electromagnetic force, for which the photon is the carrier (which means that photons are always in action when the electromagnetic force is operating).
- Atomic nuclei are made from protons and neutrons, 100,000 times smaller in radius than an atom, and made from mostly up and down quarks (and anti-quarks) and gluons.
- The protons and neutrons are kept intact, and also kept within an atomic nucleus, by the strong nuclear force, carried by the 8 types of gluons.
- The sun shines, and some atomic nuclei decay, because of processes that convert quarks of one type to quarks of another type, while emitting electrons and neutrinos, particles that stream straight out of the center of the sun.
- These quark-conversion and neutrino-emitting processes are caused by the weak nuclear force, whose carriers are the W+, W- and Z0 particles.
- The last force we know about is gravity, carried presumably by the graviton. Because of gravity’s astonishing weakness, this is not an easy particle to discover.
Almost every aspect of our daily world is determined by these particles. But there are a few more. The electron, the neutrino-1, the up quark and the down quark are called a single “generation” of particles — generation being used here loosely in the sense in which it applies to a family tree [though the particles do not carry any familial relationship that the word might imply.] There are two heavier generations, each carrying a heavier copy of these four types of particles.
- The second generation consists of the muon, the neutrino-2, the charm quark and the strange quark.
- The third generation consists of the tau, the neutrino-3, the top quark and the bottom quark.
The generational structure divides these particles into horizontal strata. One can also divide them vertically into those classes I mentioned: people often speak of “the electron-type particles” or the “charged leptons” to refer to the electron, muon and tau, speak of “the neutrinos” in general, and divide the quarks into the “up-type quarks” (up, charm, top) and the “down-type quarks” (down, strange, bottom).
You may wonder why the neutrinos have boring names compared to the rest of the particles. We used to call them something else, but we learned a lot of new things about neutrinos in the past 15 years, and we’re still in the middle of that learning process. Maybe when things settle down we’ll get them new names.
We don’t know much about the Higgs particle yet (though we will soon) and in the meantime you can read about it (or them, or whatever is the true story) here.
Let’s take a closer look at the various masses. Not only is the range of masses huge, there isn’t an easily discernible pattern. Here are some comments about the particle masses, starting with the lightest ones:
- The photon and graviton are probably massless — they must be astonishingly lightweight in order to allow the observed intergalactic magnetic field and the immense structures found in the universe.
- The gluon is as massless as you can meaningfully define it to be — gluons spend their lives trapped inside hadrons such as protons, and you can’t easily measure how light they are.
- Theorists long debated whether neutrinos were massless or not. Experiments of the last decade or so nearly settle the issue (though the evidence is still indirect so small loopholes remain). Neutrino masses are very tiny, with the heaviest at least a billion times lighter than the lightest atom (hydrogen), and the lightest one possibly much lighter indeed.
- The masses of the other elementary particles are known. The electron is roughly 1800 times lighter than hydrogen, while the top quark has a mass almost 400,000 times heavier than the electron, only a few percent less than a single atom of gold. And the W particle and Z particles are about half the mass of the top quark.
- All of the known elementary particles that have substantial masses get them directly from their interaction with the Higgs field. (The neutrinos may get their masses in a more indirect way, but the Higgs field is essential for their masses too.) I have indicated this with green borders of varying thickness on the particle disks.
- We do not know the mass of the Higgs particle (or particles). The Large Hadron Collider should give us insights (and there are already hints).
I have sketched out the particles and forces in a different way in Fig. 2.
This figure shows which particles directly affect one another. In the figure I’ve drawn lines between all the types of particles that interact directly with one another. Here’s something interesting. Notice:
- None of what are often called the matter particles — charged leptons, neutrinos, or quarks — directly interact with one another.
- The matter particles only interact directly with the force carrier particles!
And this explains why the force carrier particles are so-named. When an electron in an atom interacts with an up quark inside the atomic nucleus, it does so indirectly. The electron interacts directly with photons, the quark interacts directly with photons, and the net (and rather complicated and initially counter-intuitive) indirect result is that the electron is pulled toward the quark and vice versa. Similarly, the force between two quarks is indirect, and stems from the direct interaction of quarks with gluons. All known forces between matter particles are indirect, and involve the mediation of force particles. When you push a door open, photons are at work.
The figure also captures a few other important features of the forces and the particle classes.
- All the particles of a given class are affected by the same forces; this is what defines them as a class, in fact. Neutrinos feel only the weak nuclear force; only quarks and gluons feel the strong nuclear force.
- Curved lines indicate that some of the force carriers interact directly with themselves or other force carriers. Note gluons interact with themselves, but the photon does not interact with itself (at least, not directly).
- There is a sense in which the Higgs particle is a force carrier too. But it’s a very special case. The stronger is the effect of the Higgs force on a particle, the larger is that particle’s mass when the Higgs field is not zero. (Note: The previous sentence is true for the known particles, but it may well be false for other as-yet-undiscovered particles.) I’ve indicated this tendency by showing the green sea as darker at the top of the page, indicating that it has a bigger effect on the heavier known particles. Similarly, the Higgs particle interacts with the heavyweight particles more strongly than with the lightweight ones.
This is an awfully strange-looking world, but like it or leave it: it’s ours. Although you can see some rough patterns, it is not exactly crisply organized. A lot of the disorganization turns out, in one way or another, to be associated with the Higgs field (or fields). You can read more about this association, and about what the world would be like if the Higgs field were on average zero, in this article.