What is the Big Round Thing?

by Clifford Johnson     Department: Physics & Chemistry

You may have seen Ziya interview Jim Gates on last week's show. If you did not, you can check it out here.

Somewhere in there, he mentioned the new experiment called the "Large Hadron Collider", or LHC, which several physicists are all excited about. It will turn on next year, we hope. (Clickable picture of the CERN site in Europe where it is located is to the right. Notice the giant circle. That's the LHC.) It will allow us to test several important ideas in physics, and so we're all sitting on the edge of our seats to find out what it will tell us. So what ideas will it be testing for us? Well, the first thing I should say is that although Kamala's introduction to the segment does imply that it will test string theory, that's a bit of a strong statement - it may give us vital clues in our ongoing construction of the theory, but string theory is simply not well enough developed yet to claim that we can test it with such an experiment yet. Surprises can happen, of course, but most people think that we can indeed hope for strong hints about whether we're on the right track with string or not, but we'll have to leave tests of string theory for future experimental and observational work, alongside better technical understanding of the theory. (I'll no doubt be saying more about that later on.)

So if not string theory, what will it test? Well, Jim said it. It is built to help us understand the origin of mass. This sounds like a grand statement, and it is, but it is often misunderstood what this means. What it does mean is that we want to learn the origins of the masses of the fundamental particles that make up the known (ordinary) mater in the universe. Recall that we're made of atoms, which are in turn made of electrons and a nucleus made of protons and neutrons. Protons and neutrons are made of quarks (the "up" and "down" quarks. There's a collection of other particles we know about too - four other types of quark, two cousins of the electron called the muon and the tau, and three types of neutrino. These particles all fit together in ways that we know very well (giving us all the matter we've directly observed in the universe. The remarkably well-tested theory that underlies all this structure is called the Standard Model (SM) of particle physics. What the LHC is designed to test is the mechanism by which these particles get their masses.

Get their masses? Well, the SM is built using certain tools (called "field theories") in which the particles start out with zero mass. They obtain their masses by interacting with another piece of physics, which we'd like to study more. It's simplest manifestation is as another particle, and the LHC is designed to study the precise details of this interaction, and the particle (the "Higgs" particle). The experiment is expected to examine the mechanisms which give the particles masses by creating high energy collisions that essentially allow physicists to examine the process (called "symmetry breaking") more directly that ever before. The results will either confirm the existence of the Higgs particle, or give us strong clues about what does the job if it's not the standard Higgs mechanism that is going on.

Now, before I go on, let me say that the above understanding of the origin of mass is not really much to do with the origin of the mass that you care about in everyday life. The stuff that makes you move the scales in the bathroom, or makes you want to get a lighter laptop for your bag, or that you use to hold down papers on your desk with the paperweight you got on vacation in some exotic locale. Don't be disappointed. It turns out that we understand that rather well already. It comes from binding energy - the strength in the forces that hold all those elementary particles together to make the everyday stuff we know. What I did not tell you about in our list above was another class of particles - the force carriers. These are the particles responsible for mediating the various forces we know about. The photon is the mediator of the electromagnetic force (which for example binds the electrons to the nucleus, and governs what goes on in chemistry), the two Ws and the Z particle govern the weak nuclear force, and the eight gluons govern the strong nuclear force, which holds the quarks together into protons and neutrons. It is the operation of these forces - again described nicely by the marvelous Standard Model of particle physics - that determines the masses of the complicated matter that we know about, and those masses are far greater than the masses of the elementary particles that make everything up. So to clear up the confusion: The LHC will tell us about the latter, not the former.

Ok, so what is this LHC experiment like? Well, it's only the single largest and most complicated machine every devised by human hands, involving the largest collaboration of scientists in the history of humanity. Well, to tell you about this giant circle (27km around) and all that goes into it will take another blog post, so instead I'll do some pointing. It turns out that there's been several articles written recently, such as a New Yorker article by Elizabeth Kolbert, and a New York Times article by Dennis Overbye. There are also two excellent radio pieces by David Kestenbaum on NPR that I recommend. You can find them here and here. I've done several related articles on Asymptotia, as well, and you can find them by searching on LHC. Of course, go to the website of CERN, the European laboratory where the LHC is located. Here's their "What is the LHC?" site, a good starting point.

So anyway, that's what physicists really mean when they say we are hoping to learn about the "origin of mass". Is mass the only thing we are hoping to learn about using the LHC? To be honest, no. We're hoping for surprises, and clues to what lies beyond that which we understand so well (the Standard Model). Some of the things might involve the "Supersymmetry" Jim Gates talked about in his interview (a symmetry that string theory likes a lot). The symmetry connects bosons ( such as the known force mediator particles: photons, gluons, etc) and fermions (such as the known matter particles: quarks, electrons, etc) and is regarded as rather appealing for several technical reasons. Another reason it is liked a lot, is because the many models of Supersymmetry give rather natural (from some perspectives at least) candidates for something else we're looking for - Dark matter. Just to remind you, over 80% of the matter in the universe is not of the sort I've been talking about above. It's made of stuff we have yet to identify and understand. This "Dark Matter" problem is one of the oldest and biggest problems in modern astrophysics and cosmology. It would be awfully nice if the LHC gave us some clues about what that stuff is. It's a pretty important clue to understanding our universe.

What else are we hoping for? Well, there's the exotic ideas - Maybe we might see physics signaling the existence of extra dimensions of space (another thing that string theories like a lot), and/or that the Planck scale (where quantum gravity is important) might be more easily accessible than previously thought. That would allow us to do all sorts of nifty new physics at the LHC involving things like microscopic quantum black holes and so forth. All somewhat speculative at this stage, but physicists will be keeping their eyes open just in case.

Of course, surprises would be nice. We'll take those too.

You'll have noticed that I have not talked about gravity at all (except a brief mention at the end in the more speculative stuff). Gravity's the fourth of the four forces, and it was largely left out of the above particle physics story, for good reason. I'll tell you more about that later.

-cvj

Tags: astrophysics, CERN, Large Hadron Collider, particle physics