The Small Picture

Now that we've covered all the particles in the microcosmos, and we've examined all the forces, it's time to step back and look at the big picture. Except that in Dave's Microcosmos, nothing is big. So I guess we'll look at the small picture.

The Standard Model

Setting gravity aside for the moment, the mathematical detail describing all this stuff is called "The Standard Model." The standard model consists of all the mathematics describing the interaction of gluons with quarks, and photons with charged particles, and W's and Z's with all the matter particles, and so on. We won't go into this, except to point out that there's one part of the standard model that I haven't yet addressed even on a qualitative level. That's the Higgs sector of the model.

The Higgs Particle(s)

A unique feature of the weak force, you may recall, is that the gauge bosons that mediate that force are very heavy, while the gauge bosons that mediate the other forces are massless. The question has to be asked: why are they heavy?* Well, a few mechanisms have been proposed for how the W's and Z acquire mass. The most popular of these is the Higgs mechanism. In the Higgs mechanism, several new particles, called Higgs particles, are introduced. These particles have the property that three of them are absorbed ("eaten," as physicists say) by the W+, W, and Z0. So what we refer to as the Z particle is actually part gauge boson, and part Higgs particle, and it's this absorption of the Higgs particle that gives the Z (and the W's) its mass.

Now there are lots of different combinations (an infinite number, actually) of Higgs particles that you can introduce into the standard model that will work. The simplest way to get the job done is to introduce four Higgs particles. Three of these are absorbed by the W's and Z, leaving one lying around for physicists to actually discover at some particle accelerator. If you've ever heard or read a physicist referring to "the Higgs particle," this is the one they're talking about. Now physicists prefer things to be as simple as possible (although you might never have guessed this if you've ever taken a physics class), so it's this configuration of Higgs particles that's been incorporated into the standard model.

But whether we like this collection of Higgs particles, or some larger collection, the fact remains that in any working Higgs model, there must be some Higgs particles that aren't eaten, and in principle these can be discovered. And so far, there's not a shred of experimental evidence for their existence. The Higgs mechanism and the Higgs particles are, at this point, purely theoretical conjecture. (This is why I haven't mentioned them until now.) I should point out, however, that this lack of evidence for the Higgs mechanism doesn't constitute evidence against it. It may simply be that we haven't yet built an accelerator powerful enough or a detector precise enough to discover a Higgs particle. (One of the motivations behind the design of the ill-fated SSC accelerator was to determine whether the Higgs mechanism is real or not.)

Unanswered Questions

But with or without the Higgs mechanism, few if any physicists believe that the standard model is the final word in subatomic physics. It describes what we know of the microcosmos spectacularly well, but it leaves important questions completely unanswered. Questions such as...

Where do all these seemingly random masses come from? Well, it may be a bit confusing to throw that question out there for you. So far, I haven't said much about the masses of all these particles. That's because the purpose of these pages is to enlighten, and listing the masses of all the matter particles provides no enlightenment whatsoever. The only pattern to the masses is that they tend to get larger as you move up and to the right in the table I gave you earlier. Other than that, they're pretty much random. So why is the mass of the electron .511 MeV/c2 while the mass of the c-quark is around 1500 MeV/c2. Nobody knows.

Why do the matter particles come in three nearly identical copies? Nobody knows why there are three generations of matter particles. In the past, observing patterns like this in the particle spectrum has indicated that the particles are made up of smaller particles. Maybe that's the case this time too, but so far there's no evidence for it.

Why are there four distinct forces that behave so differently from each other? The standard model contains three forces (omitting gravity), but you can create another model just like it containing seven forces or one force or any number you like, and you can vary the properties of these forces wildly. Somehow, nature has settled on three forces, with the properties I've described earlier. Again, nobody knows why.

The standard model can accommodate all these masses, and as many generations as you like, and three of the four forces we observe, but it doesn't explain why we find what we find. And for this reason, physicists are eager to find out what lies beyond the standard model.

Beyond the Standard Model

There are a lot of theories of what comes next. I won't go into detail on any of them here, but just mention what they are. I should point out that there's no experimental evidence for any of these yet. The standard model can accommodate all the experimental results that we have so far. The first evidence against the standard model may be just around the corner, or it may be waiting for faster accelerators and better detectors.

Compositeness
It may be that the particles that we know about now are composed of smaller particles. History is certainly on the side of this opinion, as we've found this to be true so many times in the past. Atoms are made of electrons and nuclei. Nuclei are made up of protons and neutrons. Protons and neutrons are made of quarks. What are quarks made of?

Grand Unified Theories (GUTs)
The idea behind grand unification is to take several forces and combine them into one force, sort of in the way that electricity and magnetism are combined into electromagnetism. In a grand unified theory of the three usual forces, there is only one force, mediated by many gauge bosons which include gluons, the W's and Z, and the photon. It's only when the universe is at a very low temperature, as it is today, that these mediating particles take on very different characteristics, leading to the appearance of three separate forces.

Supersymmetry
A supersymmetric theory is a theory in which every particle I've mentioned so far has a companion particle with similar properties, but with a different spin (a property I mentioned earlier, but if you forgot, don't worry about it) and a much heavier mass. Supersymmetry first became popular as a way to work around certain mathematical problems associated with grand unified theories. Now, I think, its most important role is as an important feature of superstring theory.

Superstrings
In superstring theory, the fundamental objects that make up the universe are not point-like (sizeless) particles, but are very short strings. (Very very short.) What makes superstring theory so attractive is that it appears to be the best shot at a quantum theory of gravity. This means that not only might a grand unified theory based on superstrings be possible, but maybe even a Theory of Everything (TOE). That is, a theory incorporating all four forces.

Technicolor
Technicolor is an alternative to the Higgs mechanism for providing masses to the W's and Z. In a technicolor theory, the Higgs particles are replaced by similar particles that are composite. The particles that make up these particles are held together by a "technicolor" force that is similar to the "color," or strong force.

Or...
There are plenty of other theories about what comes after the standard model. I've just picked out a few here, and not necessarily the most popular or most likely ones. Or, it may be that what comes after the standard model is something that nobody has anticipated. When the new physics comes, it may be a complete surprise.

So we wait.



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*Okay, I suppose you could ask the opposite question: why are the others massless? Well, the reason you ask the other question and not this one, is buried deep in the mathematics of the theory. The people that really know a lot about quantum field theory (that's what the math is called) have discovered that the darn things should be massless. So we don't ask why some of them are massless. That's already known. We ask why the W's and Z aren't. Back