The Particles

Since the dawn of science, people have been wondering what the universe is made up of; what the most fundamental objects are in the universe. Well, the answer to this has changed over the years, and what you see here may or may not be the final answer, but it's the best answer we have right now.

The most fundamental particles we know about can be divided into three categories: quarks, leptons, and gauge bosons. Within the spectrum of these particles, there are several patterns that emerge. Historically, when patterns emerge in a collection of particles, this is an indication of the substructure of the particles. That is, it's an indication of what the particles that make up the particles are like. For example, some years ago, Mendeleyev arranged all the known elements according to their chemical properties into an array which we call the periodic table. The patterns he discovered are indicative of the fact that these elements are made up of protons, neutrons, and electrons. As another example, earlier this century physicists noticed patterns within a group of particles called "hadrons." Two physicists, Gell-Mann and Zweig, discovered that these patterns could be explained if hadrons are composed of more fundamental particles, now called quarks.

And so the patterns observed in the spectrum of quarks, leptons, and gauge bosons may be indications that these are made up of other particles. However, so far there's no evidence that this is the case, and no indication of what these smaller particles might be like. So these particles, as far as we now know, are made up of nothing smaller.

Matter

Anyway, here's a table of the quarks and leptons, which are sometimes referred to collectively as "matter."

quarks u u u c c c t t t
d d d s s s b b b
leptons e m t
ne nm nt

Okay, I guess some explanation of this table is required.

Names

The six quarks, abbreviated u, d, c, s, t, and b are called "up," "down," "charm," "strange," "top," and "bottom." The upper three leptons are the electron, the "muon," and the "tau" (rhymes with "cow"). The bottom three are called "neutrinos," and there's one corresponding to each of the upper three. (This correspondence is illustrated by the subscripts on each of the neutrinos.)

Electric Charge

You're familiar with the idea of electric charge. Electric charge is what makes balloons stick to the wall after you rub them against your hair, what makes the cellophane wrapper on a CD so hard to throw away, what makes doorknobs shock you in the winter, and so on. You're probably also familiar with the fact that electric charge comes in two kinds: positive and negative. Opposite charges attract each other, while like charges repel each other. Anyway, most of the particles above have electric charge. The only ones that don't are the neutrinos. This is important, because electric charge is the whole basis for electromagnetism. Neutrinos, because they have no electric charge, are completely unaffected by electromagnetic forces.

Color

The quarks have a property which is called color. This name has nothing to do with color in the usual sense, and is used only as an analogy. When physicists first postulated this property of quarks, they realized that there had to be three different types, just as there are three primary colors of light: red, blue, and green. And when you combine the three types, you must end up with something which is neutral with respect to that property, just as a mixture of red, green, and blue light leaves something colorless: white light. (We'll see how the analogy breaks down later.) Anyway, color plays the same role in the strong force that electric charge plays in the electromagnetic force. Leptons don't have color, and so they're unaffected by the strong force.

Mass

Mass, if you're not familiar with the idea, is similar to weight. There's a difference between mass and weight, but that's largely unimportant for our purposes. What is important about mass is its effect on the stability of particles.

Stability

Many of the particles listed in the table above are unstable. That is, they decay into other particles. For example, a muon will live for only a short time before it decays into an electron and two neutrinos. (Well, not exactly two neutrinos. See below.) Particles can only decay into lighter particles, never heavier ones. As an example, a muon is heavier than an electron, and a tau is heavier than a muon. So a muon can decay into an electron (and other things) but it can't decay to a tau. In the chart above, the masses tend to get larger as you move from lower left to upper right, so in general particles tend to decay from right to left, and top to bottom. (Neutrinos, for example, are massless as far as we can tell.* If this is true, then they're stable.)

Also, any quark that decays will always decay to at least one other quark, and an unstable lepton will always decay to at least one other lepton. This fact, along with the fact that particles must decay to lighter particles, and a couple of other constraints, is the reason that some particles, such as electrons, protons, and neutrons (inside nuclei) are stable (don't decay, or at least don't decay for a very long time).

Because most of the particles above are unstable, all of what we normally consider matter, rocks, trees, people, etc., are made up of just u and d quarks and electrons.

Anti-particles

Yes, anti-matter exists! It's not just a fantasy of the Star Trek writers (although what they do with anti-matter on Star Trek is pure fantasy... so far). Basically, each particle listed above has an anti-particle associated with it. This particle will have exactly the opposite properties, for any properties that have exact opposites, and will be exactly the same otherwise. For example, the tau particle has an anti-particle called the anti-tau. It has exactly the opposite electric charge. (The tau is negative, so the anti-tau is positive.) It has the same mass, since there's no such thing as negative mass. It's unstable, just like the tau, and decays after the same amount of time. It decays into the anti-particles of the particles that the tau decays into!

Similarly, a charm quark has an anti-particle, the charm anti-quark (or anti-charm). It has the opposite charge of a charm quark, the same mass, decays into the anti-particles of the particles the charm decays into, after the same amount of time. And it comes in three colors: anti-red, anti-blue, and anti-green. (What on earth does "anti-blue" look like? Well, as I said, the whole color analogy breaks down after a bit.)

Incidentally, you may recall I mentioned that a muon decays into an electron and two neutrinos. Well, that's not strictly true. A muon decays into an electron, a muon neutrino, and an electron anti-neutrino. And an anti-muon, therefore, decays into a positron (this is what an anti-electron is called), a muon anti-neutrino, and an electron neutrino.

Gauge Bosons

First of all, the name: gauge bosons. Bosons refers to a property of these particles called spin, with which you need not concern yourself. As for why the bosons are called "gauge" bosons, well, really for historical reasons, and not very good historical reasons at that. In other words, don't worry about why they're called gauge bosons. Just accept it. Anyway, the gauge bosons fall into four classes: Each of these is related to one of the four forces described on the previous page.

Electric Charge

All of these are electrically neutral except for the W particles. There are two W's: one positive and one negative.

Color

Only the gluons have color. In fact, each gluon carries a color and an anti-color, so that gluons come in bizarre colors such as "blue/anti-green" and so on. (We see the whole color analogy break down even further.)

Mass and Stability

The photon, gluons, and graviton are, as near as we can tell, massless. Therefore, these particles are also stable. That is, they don't decay into other particles. The W's and the Z are quite heavy by particle standards (over 80 times the mass of a proton!) and are unstable. They decay after a very short time into lighter particles. (There are actually a variety of combinations of lighter particles each of these can decay into.)

Anti-particles

If a photon had an anti-particle, its anti-particle would have no charge, no color, it would be massless, stable, and would interact with other particles in the same way as the photon does. In other words, it would be another photon! So in this sense, a photon can be thought of as its own anti-particle. the same can be said of the graviton and the Z particle. The anti-particle of the W+ is the W (and vice versa). The eight gluons include their own anti-particles in a similar manner. So while each of the gauge bosons has an anti-particle, these are included among those I've listed above, and are not separate particles.

Experimental Evidence

Almost all the particles listed above have been observed in experiments, either directly or, more often, by observing their decay products. The only exception to this is the graviton which is only conjecture (but very strong conjecture) at this point.


The Classical Picture of Forces
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Dave's Microcosmos


* Flash! Since the paragraph above was written, there appears to be compelling evidence that at least one of the neutrinos does have mass. Science doesn't sit still. Back