The Holy Grail for many of today's theoretical physicists is a complete quantum mechanical theory of gravity--useful for understanding the behavior of black holes, big bangs, and whole universes. But bridging the gap between the smallest and largest constituents of reality will probably require a few totally new concepts (and shake our faith in some old ones). One researcher looking for these missing pieces is Raphael Bousso of Harvard University. The 31-year-old shared first prize in an international competition for young physicists last year for his work on the so-called holographic principle, which aims to reconcile quantum mechanics with black hole physics. His research has led him to think hard about string theory and cosmology, too.
Scientific American: String theory tells us that particles are these tiny loops or squiggles all tangled together. What has string theory done that we should we believe any of that?
Raphael Bousso: String theory offers a framework in which all forces are naturally, completely unified, including gravity. We really don't have anything else that achieves the same kind of unification. If you ask what string theory has done, for example, it describes in principle how the particles that make up gravity interact with each other. And it has explained in certain very special cases the entropy of black holes, which is a big problem in quantum gravity.
SA: In the past few years string theorists have started thinking about cosmology, in particular this dark energy that seems to be making galaxies spread apart faster over time. What's the fascination?
RB: We don't really know what this stuff is. There are a lot of different possibilities for types of matter that will act the way that this dark energy seems to be acting on our universe. From a theorist's standpoint they are very different things. An example of that distinction is whether that dark energy is really what we call a cosmological constant, in which case its density is fixed and will never change. Or if it is something that acts for a while like a cosmological constant, accelerating the universe, but eventually gets diluted. That's something that we currently can't distinguish very well with the experimental evidence, but it has an enormous effect on the large-scale structure of the universe and what the world will look like in the far future. It would be very nice to understand what kinds of dark energy are favored from a theoretical perspective. The biggest question is, why is it there at all? And we really have a hard time understanding that because it's so incredibly small yet it isn't zero. That poses a huge challenge for theory to explain.
SA: So far string theorists haven't had much luck tackling dark energy, correct?
RB: There is a tension there. What most people would agree on is that it's very likely that the explanation of the origin of the cosmological constant will come from a quantum theory of gravity. That wouldn't have to be string theory. But string theory is the best, most accurate and most powerful candidate theory that we have. Now the tension comes from the fact that it's been very difficult to find cosmological solutions, in particular solutions with positive dark energy, in string theory. [In general relativity, the cosmological constant could be positive and repulsive or negative and attractive.] While string theory's our best candidate for quantum gravity, we're a little disappointed by the fact that so far we've not managed to model universes with dark energy in string theory.
SA: What makes the problem difficult?
RB: There is a problem in universes with a positive cosmological constant. Now this problem doesn't occur for all types of dark energy, but for a certain class of universes that exhibit dark energy you cannot measure the [end] state [of particle interactions], because particles get causally separated. There's no one who can see them all. In that sense you cannot measure the result of any scattering [of particles]. Experiments in high-energy physics are best described by the formalism called the S-matrix, which gives you the probability, for all the different things that you put in, of all the different things that could come out. That formalism comes from the days when we thought that all of the world of physics is particle accelerators, where we're playing "God" [more literally, an observer at the edge of spacetime] sitting on the outside sending some particles in--say an electron and a positron or something--crashing them together with great force and looking at what comes out.