As many of you know, the Large Hadron Collider (LHC) turned on yesterday in Geneva. So maybe I should say a little about it, since this is the kind of physics I do for a living. The LHC is a very big deal for physics. It is likely to make the first major breakthrough in particle physics in over thirty years.
Since the LHC hasnt discovered anything yet, how can we know it will? Well, we cant know, of course. But there are strong reasons to expect it will see something important.
Some people say that the LHC will discover something called the Higgs particle . It almost surely will, but if that is all it discovers, it will be a huge disappointment and in fact a disaster for physics. It is (almost) certain that the Higgs particle is there. That in itself is no big deal. The big deal has to do with a deep puzzle connected to the Higgs, which I will now attempt to explain in a non-technical way.
The Higgs particle is also a Higgs wave ¯you all remember that quantum mechanics says that waves and particles are the same thing in a different guise. The Higgs wave is a wave in the Higgs field . All of space is permeated by this Higgs field¯just as all of space is permeated by electric fields, magnetic fields, and gravitational fields. Compared to the electric fields, magnetic fields, and gravitational fields all around us, however, the Higgs field is enormously strong. This Higgs field plays a very important role in the world: It accounts for the fact that most particles have mass. If you could turn off the Higgs field somehow, then most types of particles (including the good old electron, neutrinos, and quarks) would lose their mass. The world would be a vastly different place.
The deep puzzle, however, is that the Higgs field ought to be much, much more intense than it is. In fact, there are strong arguments that suggest that it ought to be about seventeen orders of magnitude (100,000,000,000,000,000) more intense than it is. That would make the electron seventeen orders of magnitude more massive than it is, and similarly for lots of other particles that we know and love. What makes us say that the Higgs field ought to be so large? The answer is that we know of various things that are generating a huge Higgs field, and thus there must be other as-yet-unseen things that are generating a nearly canceling Higgs field. And that is very mysterious.
The only really good idea for how something might cancel out the strong Higgs field that ought to be there is called supersymmetry . The idea of supersymmetry is that there is a new kind of matter that cancels the contributions of the ordinary matter. For example, if the electron produces a certain contribution to the Higgs field, there is a new kind of particle called the scalar electron (or selectron for short) that contributes the opposite amount, and so on. What people really are hoping to see at the LHC is evidence of these new kinds of matter predicted by supersymmetry. Until recently, most theorists probably thought that the chances were much greater than 50 percent that the supersymmetry solution of the Higgs problem is correct, and that evidence for it would be seen at the LHC. Some doubts are creeping in, however.
First of all, theories based on the supersymmetry idea are not without serious difficulties. But what has made the doubts increase in many physicists minds recently (including many top physicists) is the possibility that the Higgs puzzle may be explained anthropically rather than by supersymmetry. If we live in a multiverse , it is possible that, in different places in the multiverse, the Higgs field has different strengths. In most places it might have its natural strength. But in rare places it may happen to have the much smaller value that it has where we are. And¯it can be argued convincingly¯only in those rare places can there be life. We see a strangely small value of the Higgs field, because we are living in a highly atypical part of the multiverse, namely a part where the Higgs field has a value that allows life to exist.
For reasons of pure egotism, I will note that the paper that called attention to the possibility of an anthropic explanation of the Higgs puzzle was written by me and a few colleagues. It has become a fairly well-known paper in particle physics. If the LHC turns up no evidence in favor of supersymmetry or of other conventional explanations of the Higgs puzzle, it would strongly suggest that our anthropic explanation is correct. I still think that supersymmetry will probably be found at the LHC. But the anthropic dark horse is coming up strong on the outside track. And even if evidence of supersymmetry is found, the full explanation of the Higgs puzzle may well involve anthropic considerations that we talk about in our paper.
But whatever is seen or not seen by LHC, we will learn a great deal about a very basic question in physics: How do particles get their mass? And why do they have the masses they do? The answers will not come right away. It may be several years before LHC finds something and we know what it means. It is a very exciting time in this branch of fundamental physics.
Stephen M. Barr is a theoretical particle physicist at the Bartol Research Institute of the University of Delaware and the author of Modern Physics and Ancient Faith and A Student’s Guide to Natural Science .
The anthropic principle and the mass scale of the Standard Model , by V. Agrawal, S.M. Barr, J.F. Donoghue, and D. Seckel, Physical Review