The Reason for Mass: Theory Confirmed!

Physicists ask some very odd questions. It's sort of a longstanding tradition; many of the really interesting discoveries in physical science have been the result of some strange question whose answer seems obvious to most people at the time. The real answer, however, is usually even stranger than the question. The question of what it might be like to travel at the speed of light, for instance, led Einstein to his theories of relativity, which to this day leave most of us with a puzzled feeling as we try to twist our brains in knots in an effort to comprehend it.

They also have a fascination with chopping things up into little tiny bits. We can blame this on the Greeks. Democritus, a fascinating individual, can be credited with the original atomic theory of matter, although it was not until the seventeenth century that this theory began to be accepted and elaborated upon based on observed evidence. Atoms (and their children, molecules) were pretty easy to understand; they were little tiny pieces of known materials. When we chopped up atoms, we got protons, neutrons, and electrons - a little odder, but fairly easily visualized as little spheres creating miniature planetary-system-like objects. The electrons made plenty of sense; they explain electric currents, predict the composition of certain kinds of compounds, and explain certain features of the periodic table. The protons and neutrons fit in with our measurements of atomic masses. So far, so good.

And then it got weird. The masses didn't quite line up; there was some extra mass that couldn't be accounted for using only the masses of the protons and neutrons, and the bigger the atom, the bigger the discrepancy. Some atoms didn't stick together as well as others; they occasionally spat out bits of mass in various ways, in a process known as radioactive decay. Scientists started trying to figure out what made atomic nuclei stick together at all (given that their electromagnetic repulsion ought to make them fly apart), and came up with something called the nuclear "strong force." This implied that there was a whole bunch of energy bound up holding these nuclei together, and that this energy had mass. This was the energy released and the mass lost in radioactive decay and nuclear fission. It was apparently "carried" by a sort of invisible particle called a "gluon." Seriously. Gluon.

And that's where everything came unglued. It turns out the force that holds nuclei together is only a residual effect of the force that holds protons and neutrons themselves together - you see, protons and neutrons are made up of even tinier particles called quarks. And quarks combine in all kinds of different ways besides the well-known protons and neutrons; in fact, there were a bunch of different kinds of quarks besides the ordinary ones in protons and neutrons. There are six different "flavours" of quarks: up, down, top, bottom, strange, and charmed (I told you, physicists are weird!) which combine to make particles. These composite particles were dubbed "bosons." The electron, by the way, is an entirely different sort of animal; it's elementary all by itself, not made up of quarks, and has a whole family of other vaguely-electron-like particles called "leptons." And even worse, all these particles had antiparticles. By the time the current working theory of particle physics, called the Standard Model, was fully-formed, we had a veritable zoo of exotic particles with peculiar names, odd properties, and different masses and behaviors.

This is where physicists started asking the question: Why do these things have the masses they do? The answer seems obvious; stuff has mass because it's made of matter, which has mass because mass is our unit for measuring how much matter we've got. Right? The "why" question seems like circular reasoning. But it's not. The bosons are mostly not exactly made of matter. They're a little bit of matter - the quarks contribute some mass - held together by a lot of energy.

So the "Why do particles have mass?" question is really a two-part problem. The first involves figuring out why quarks themselves have mass; a mechanism for this was postulated in 1963-1964 by several physicists independently, though the theory was eventually named for its last discoverer, a British physicist named Peter Higgs. Essentially, it involves the existence of yet another particle, this one responsible for creating a field which endows other particles with mass as they move through it. This particle has yet to be detected, apparently because it's really big by particle standards (paradoxically, being big makes particles harder to find); it's one of the main things the Large Hadron Collider at CERN in Switzerland is looking for. You may have seen this on the news recently, since it just came on line this fall.

The second part, however, accounts for the vast majority of the mass of bosons: the energy involved in "gluing" them together. And that is the real subject of today's entry. It appears that the theory was correct - the energy of the interactions between quarks and gluons is enough to account for 95% of the mass of protons and neutrons! (remember that we've already established that energy is mass). It gets a lot weirder than I've described here, what with the gluons popping in and out of existence and changing the colour of quarks. I'm not going to get into quark colours; flavours are enough for one day. But check the article out at Discover Magazine! This is really a big deal; we've made a real, significant advancement in particle physics, and it didn't even involve expanding the particle zoo!