One of the joys of attending a general physics conference like Physics@FOM is that I get to attend the sort of talks that just never seem to turn up in a universe near me. So when I saw that there was a session on subatomic physics packed with talks from cosmologists and other people of that stripe, I knew that I had to attend.

The idea of extra dimensions is, of course, nothing new in physics. But we know that these dimensions have to be small. It's not just the fact that we only perceive four dimensions directly; forces like gravity and electromagnetism see their strength fall off as the square of distance, so they would change strength at rates faster than the square if there were other dimensions around. The role of these extra dimensions, though, is thought to be quite important. There is a huge disparity between the strength of the electroweak force and gravity, one that needs to resolved before unification of the forces can take place. (There's also the matter of the masses of the different particles, which we can measure but not explain.)

At some scale (called the Planck scale) the forces have to link up, which is where the extra dimension can play a role. The idea, as I understand it, is that, by curving an extra dimension, the strength of gravity is reduced. My guess is that energy from the gravitational field is used to maintain the curvature, thereby reducing the apparent strength of gravity observed by objects in the other three spatial dimensions. So, warp enough dimensions and the strength of gravity seen in the remaining dimensions falls off a lot, but its native strength is comparable to the electroweak force—everyone is ready to unify.

Another important consideration is the observation of different particle masses. Adding an extra dimension that has a limited extent acts to confine the wave-like nature of the particles. In doing so, the waves have to form discrete sets or forms that have increasing amounts of energy. In normal space, we perceive the average of this, so different particle masses correspond to the diffent types of waves supported by the extra dimension.

We need a way to terminate an extra dimension and give it enough curvature to bring gravity down to size. A researcher named Damien George considered a single extra dimension that was very small (30 Planck lengths, to be precise), terminated at each end by a singularity. This requires that there are two (so far unobserved) fields permeating the universe. If I understood it correctly, the fields vary on a scale similar to that of the dimension—in the dimensions we perceive, they would be rather flat and hard to discern. These fields have to have a particular form to prevent the singularities from attracting one another and collapsing the extra dimension.

In their natural form, these singularities don't look like black holes: they have no event horizon. So, a particle can enter the singularity and, at some random time later, exit heading in some random direction. In other words, the laws of physics get thoroughly mugged by singularities without event horizons. George presented a good and a bad solution to this: the bad solution is to simply add in an event horizon by hand. But that seems rather arbitrary, so he proposes that the dimension is not terminated by just any singularity, but one of a very particular form, where time gets stretched out as you get closer to the singularity. The effect is that it takes infinite time for a particle to reach the singularity.

Like many extra dimensional theories, testing is going to be a challenge. The LHC should find sufficient numbers of new particles to eliminate whole classes of extra dimensional theories—of which this work falls into one particular class. So we probably won't be able to say if George is correct anytime soon, but we will be able to say if he is definitely wrong in the near future.