Please imagine that you are a graviton. I admit, I haven’t given you much to go on about what it would be like to be the carrier of the gravitational force. This is because I have no idea what it would be like, either. Gravitons, like photons, do not possess the property known as mass. They weigh nothing. Because of this, they travel at the speed of light. And according to Einstein’s other theory of relativity (the special one), anything traveling at the speed of light does not experience the sensation of the passage of time. I find this difficult to imagine, as I am imbued with the quality of mass, and thus ineligible, according to Einstein, to travel at the speed of light. But bear with me.
So you are this graviton moving along through the universe at the speed of light, except the universe, to you, is bigger than it seems to a nongravitational particle. Not wider or higher or longer, but fundamentally bigger. In addition to the three directions your fellow particles can move in, you can move in another direction, in the direction of the fourth dimension.
Now, there is some discussion about just how large this extra dimension has to be, and what it must look like. Here are a couple of options:
1) The extra dimension is small and round, though not nearly
as small as the truly minuscule dimensions associated with string theory. It can be up to about 1 millimeter long. One millimeter in particle physics is to us like the distance between here and the nearest quasar. It is magnificently large. For this reason, brane theory was originally called the theory of large extra dimensions (the theory, like our sphere earlier, is general, and can describe any number of extra dimensions you wish). Each extra dimension is curled up in a circle, so if you were a graviton, you would be able to move in this circular direction at every point in space, while still moving in a straight line through the three dimensions we all know.
2) The extra dimension is infinite. We are all very familiar with infinite dimensions. In fact, the regular dimensions we move around in every day are infinite. Theoretically, we could choose a direction—let’s say “up”—and move in that direction forever, never coming back to the same place, never reaching an end. When Lisa Randall and Raman Sundrum—she of Harvard, he of Johns Hopkins—first proposed the possibility of an infinite fourth dimension back in 1999, it was the first time anyone had taken seriously the possibility that an extra dimension doesn’t have to be tiny to be invisible. If only gravitons can escape into the extra dimension, then it could be perfectly straight and infinite and we’d never know. This proposal has the advantage of an extra dimension that looks just like the dimensions we know very well. Except that only gravity experiences it.
There’s a third option, but it’s fundamentally different from the two options above. Notice how those two require only one extra dimension. (Some variations of these theories feature two or three, but they only require one.) This is because gravity needs only enough extra space-time fabric to dissipate and grow weak. Whatever intricate particulate structure you wish to lay on top of this
But this third option is more of a combination of string and brane theory, a way to integrate the major ideas in one big package. In this scenario, there are two extra dimensions, both of them small and straight and finite, which lead us to a parallel universe.
Hear me out on this one. Remember how string theory required us to live
in a 10-dimensional universe? Well, imagine that instead of living in a four-
dimensional space (three plus one of time) and having the extra six dimensions curled up into balls so small we have no hope of ever directly observing them, there are two four-dimensional spaces: ours and a different one hanging out not too far away from ours. A two-dimensional surface connects these two branes (remember, a brane is the thing we are trapped on), which gives you
4 + 4 + 2 = 10 dimensions. This implies that there is another, mirror brane located as little as a millimeter or so away from us at all times, but which we can never reach, because we are not gravitons. This is the sort of thing that the string theorists come up with and think not “how manifestly ridiculous,” but—and I’m paraphrasing here—“wicked awesome!” It does make the math work, however, even if it breaks our brains.
What I have been describing is really a trip, to put it mildly. These theories ride the very edge of human understanding. No one yet knows if any of them are true, because no one has figured out how to conclusively test them against the firm foundation of the physical world. They are still too ethereal, too vague, too ill-defined. But these theories will ultimately become clearer both to the people who are devising them and to us. The search is on for a True theory of the universe. And the way things are going, that theory will describe a universe that we will completely understand yet still not be able to imagine.