18 September 2008
In the eyes of a particle physicist, the universe's biggest mysteries are hidden within its tiniest parts. By analyzing the collisions of high-energy photons, these subatomic detectives seek to better understand the building blocks of matter — and to detect the particles that we can predict, but are not yet able to see. The aim is unimaginably grand: to bridge the chasm between general relativity — Einstein's geometry of space and time — and quantum mechanics, which describes physics at the subatomic level.
As of last week, the world's particle physicists have a new playground: the Large Hadron Collider (LHC). This 17-mile-long tunnel beneath the Swiss-French border is the most powerful particle accelerator ever constructed, designed to help answer some of the toughest questions in science. Activate posed some questions to Dr. Dan Hooper, an associate scientist at the Fermi National Accelerator Laboratory and the author of Nature's Blueprint, who celebrated what an exciting time it is to be a physicist.
AT: What do you expect the LHC to see, and what do you hope it might see?
DH: The one thing that I think is most likely to appear at the LHC is a hypothetical particle called the Higgs boson. The Higgs holds a very important place in how we think our universe works. In particular, it is because of this particle that other particles — like electrons, for example — have mass. Without the Higgs, these other particles would be massless and act more like radiation than matter, always traveling at the speed of light. We think that it is through interactions with Higgs bosons that these particles become massive and slow down. In this way, the existence of the Higgs transforms our world dramatically. If this particle does indeed exist, then the LHC should be able to see it.
So that is what I expect to see. What I would like to see is something completely unexpected — something that blows the collective minds of the physics community. With a machine as energetic and powerful as the LHC, the possibilities for discovery are practically endless. For example, one of the wilder possibilities that has been considered is that the LHC could discover extra dimensions of space, beyond the three we experience. If other dimensions do in fact exist, then it is possible that particles created in the LHC could travel through them, giving us a window into the higher dimensionality of space and time.
AT: Scientists at CERN have been working overtime to quash various doomsday scenarios. Are we playing with fire, or is it all just hot air?
DH: Nothing but hot air, I'm afraid. The Earth is constantly being bombarded by energetic particles from space called cosmic rays. The collisions that will take place at the LHC are just like the collisions between these cosmic rays and the Earth's atmosphere. The only difference is that the LHC collisions take place in an environment that we can more easily study. If the collisions at the LHC were able to create anything dangerous, the cosmic-ray collisions would have done so long, long ago. I don't know what kinds of particles — or black holes, or whatever — the LHC will create, but I do know that they will be completely harmless.
AT: Is the search for a so-called "Theory of Everything" nearing a breakthrough, or is it still at loose ends?
DH: It is never easy to predict these kinds of things, but I have seen a shift in recent years from string theory (the leading framework for a Theory of Everything) to more experimentally accessible kinds of physics — the kinds of physics that can be tested at the LHC, for example. I would be pretty surprised if 20 years from now we were much closer to a Theory of Everything than we are now.
AT: How would the existence of supersymmetry plug the holes in our current model of physics?
DH: The current model of quantum physics, called the Standard Model, suffers from a major sickness. When we use the theory to estimate how heavy the Higgs boson should be, we find that it should be more than a trillion times heavier than any particle we have ever observed. If it were so heavy, however, the whole Standard Model would fall apart and utterly fail to describe the world as we see it. We know that something has to stop the Higgs from becoming so heavy. This is where supersymmetry comes in.
In the late '20s, [theoretical physicist] Paul Dirac recognized that if the mathematics of quantum physics were to make any sense, then for every kind of particle, there must also exist a corresponding particle with an opposite electric charge and other opposing properties — i.e., antimatter. In 1932, antimatter was discovered, and Dirac was proven correct. Supersymmetry is similar to this relationship between matter and antimatter, but instead connects two classes of particles known as fermions and bosons.
Fermions are the particles we usually think of as matter, like electrons, quarks, and neutrinos. Bosons, on the other hand, are the particles that carry and communicate the forces of nature. Photons, for example, carry the electromagnetic force. Without photons, there would be no force of electromagnetism, and without bosons, there would be no forces at all. According to supersymmetry, for every kind of boson that exists, there must also exist a fermion with the same properties, and vice versa. Just as Dirac tied together matter and antimatter, supersymmetry ties together matter and force.
If supersymmetry is correct, then there are many varieties of particles that we have never observed. For example, the electron, which is a fermion, must have a bosonic counterpart called the selectron. In fact, every known particle must have a supersymmetric counterpart — i.e., a superpartner. If these superpartner particles exist, then the LHC is almost certain to discover many of them.
AT: In your book, you describe the Standard Model as "the single most successful theory that science has ever produced." Do you think that the future of physics is likely to feature such remarkable consensus?
DH: It is incredible how well the Standard Model has stood the test of time. As it has been tested with greater and greater precision, the predictions of the Standard Model have continued to match the measurements. No other theory has ever made so many accurate predictions so precisely.
That being said, the Standard Model is almost certain to be overthrown at the LHC. After we have learned everything that we can from the LHC, we may be left with a new kind of "standard model" that the whole community agrees upon and understands. On the other hand, we might find ourselves in a situation in which the data could be interpreted in different ways. We might also recognize that we are only looking at part of the puzzle and that other unknown aspects of nature will need to be discovered before another standard model can be built.