By Lisa Randall

Web edition: February 25, 2011
Print edition: March 12, 2011; Vol.179 #6 (p. 32)

Last month in Washington, D.C., at the annual meeting of the American Association for the Advancement of Science, theoretical physicist Lisa Randall of Harvard University spoke about her hopes for the Large Hadron Collider, the world’s most powerful particle accelerator. She sat down with Science News physical sciences writer Devin Powell after her February 19 talk to discuss what evidence the European collider, which is expected to operate at half power through 2012, might provide for her groundbreaking theories and for the Higgs mechanism, a process that would explain why particles have mass.

You’ve said that physics is entering a “new era.” What do you mean by that?
At the high energies of the LHC, you’re getting very precise, and you’re getting to, in some sense, simpler systems where you can see the more basic and more fundamental rules of physics going on.… Studying higher energies is the same as studying smaller scales. We have this target scale, the weak energy scale that the LHC is exploring — that is to say, the scale at which we know particles are somehow acquiring mass associated with the Higgs mechanism.

Explain the theory that you and Raman Sundrum developed to resolve the “hierarchy problem,” namely that gravity is much weaker than quantum physics would predict. 
The scenario we had in mind is that some stuff is stuck on an object called a brane, which exists in three dimensions, but there can be an extra dimension of space where gravity can be concentrated away from us. That would explain why gravity is so weak for us.… By an extra dimension, I really do mean another dimension beyond the three we’re familiar with: left-right, up-down and top-bottom. These extra dimensions are hidden somehow and part of the question is: Why are they hidden? They could be small or very warped.

What would you be most excited to see in the LHC’s detectors?
It would be extremely exciting if they saw evidence for our theory, which would consist of a particle that is called the Kaluza-Klein partner of the graviton. You’d see something that looks like a graviton, which communicates gravity, but it would really be from an extra dimension. 

If the LHC finds a Kaluza-Klein particle, what does that mean for the Higgs mechanism and string theory?
The Higgs could be there whether or not there are extra dimensions. If we found the Kaluza-Klein particle, it would be a nice target for string theory. When we first wrote down this theory, string theorists told us, “Oh, that’s very nice, but it doesn’t happen in string theory.” Actually they didn’t even say it was very nice. But a year later they found it in string theory. The energy of the Kaluza-Klein particle is much too low to prove or disprove string theory, but it gives you different ways to think about what the possibilities are in string theory. If this warped geometry exists, they’d have to say it’s part of whatever model comes out of string theory.

Could the LHC find a Kaluza-Klein particle before reaching full power?
We know roughly the energy of this thing. It could be that it’s an energy a little higher than the LHC. It could be that it’s the energy of the LHC. It probably won’t be the energies that they’re exploring now, and it probably won’t happen — if it happens — until they get to the higher energies.

What would the physics community be most surprised to see in the LHC data?
Something we haven’t thought of. If they don’t see anything, of course, there’s going to be a long period where we have to see: Are they not seeing anything because of experimental deficiencies, or are they not seeing anything because there’s really nothing there? We really do expect there to be something there playing the role of the Higgs boson … but one of the things you find as a theorist when you work out the details is that there could be a lot of stuff there that we just miss.

If you could design a machine to test your ideas, how would it look?

The SSC [the Superconducting Super Collider, an unfinished project in Texas that was canceled by the U.S. Congress in 1993] would have been a great machine. It would have had almost three times the energy of the LHC, and that would really cover a lot of what we’re looking for. Obviously, six times the energy would have been great too. The higher the energy, the more chances you have of seeing things at a small scale. I do think that at the SSC energy levels I would have felt much more confident. But the LHC is still a fantastic machine.… It’s the highest energy, highest luminosity, highest intensity, biggest machine in the world.