LISA RANDALL: Particle physics has contributed to our understanding of many phenomena, ranging from the inner workings of the proton to the evolution of the observed universe. Nonetheless, fundamental questions remain unresolved, motivating speculations beyond what is already known. These mysteries include the perplexing masses of elementary particles; the nature of the dark matter and dark energy that constitute the bulk of the universe; and what predictions string theory, the best candidate for a theory incorporating both quantum mechanics and general relativity, makes about our observed world. Such questions (along with basic curiosity) have prompted my excursions into theories that might underlie currently established knowledge. Some of my most recent work has been on the physics of extra dimensions of space and has proved rewarding beyond expectation.

Particle physics addresses questions about the forces we understand—the electromagnetic force, the weak forces associated with nuclear decay, and the strong force that binds quarks together into protons and neutrons—but we still have to understand how gravity fits into the picture. String theory is the leading contender, but we don't yet know how string theory reproduces all the particles and physical laws we actually see. How do we go from this pristine, beautiful theory existing in ten dimensions to the world surrounding us, which has only four—three spatial dimensions plus time? What has become of string theory's superfluous particles and dimensions?

Sometimes a fruitful approach to the big, seemingly intractable problems is to ask questions whose possible answers will be subject to experimental test. These questions generally address physical laws and processes we've already seen. Any new insights will almost certainly have implications for even more fundamental questions. For example, we still don't know what gives rise to the masses of the fundamental particles—the quarks, leptons (the electron, for example), and electroweak gauge bosons—or why these masses are so much less than the mass associated with quantum gravity. The discrepancy is not small: The two mass scales are separated by sixteen orders of magnitude! Only theories that explain this huge ratio are likely candidates for theories underlying the standard model. We don't yet know what that theory is, but much of current particle physics research, including that involving extra dimensions of space, attempts to discover it. Such speculations will soon be explored at the Large Hadron Collider in Geneva, which will operate at the TeV energies relevant to particle physics. The results of experiments to be performed there should select among the various proposals for the underlying physical description in concrete and immediate ways. If the underlying theory turns out to be either supersymmetry or one of the extra dimension theories I will go on to describe, it will have deep and lasting implications for our conception of the universe.

Right now, I'm investigating the physics of the TeV scale. Particle physicists measure energy in units of electron volts. TeV means a trillion electron volts. This is a very high energy and challenges the limits of current technology, but it is low from the perspective of quantum gravity, whose consequences are likely to show up only at energies sixteen orders of magnitude higher. This energy scale is interesting because we know that the as-yet-undiscovered part of the theory associated with giving elementary particles their masses should be found there.

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