The Theory of Everything
Talking with Ramzi Khuri About String Theory
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Drawing credit: A. Hobart, Chandra X-Ray Center, Nasa |
Your work is in a highly theoretical branch of physics. To the layperson, what is it all about?
The kind of work I do is in pursuit of what is called a unified field theory or a "theory of everything." This has been the Holy Grail of theoretical physics--finding this theory. To the best of our knowledge, there are four fundamental forces of nature. There are the three quantum mechanical forces: the strong nuclear binding force, the weak nuclear radiative force, and electromagnetism. And those three are well described by quantum mechanics. The fourth force, gravitation, is well described by Einstein's theory of general relativity. Now, the problem is that these two theories, quantum mechanics and general relativity, don't mesh--they're not mutually consistent. It's a big problem, because at the beginning of time--at the Big Bang--both theories would be valid, so we are trying to find a way of combining them, so that we understand not only the fundamental interactions of nature but also the origin and fate of the universe.
And you think the answer has to do with strings?Problems of a mathematical sort have plagued this attempt [to find a unified field theory] for quite a while. Sometime in the 1980s it was realized that instead of using point-like particles as the fundamental building blocks, you could use strings. And then certain problems you have in reconciling quantum mechanics with gravitation can possibly be circumvented. (A string is a one dimensional object that can, theoretically, vibrate into multiple dimensions. It is such a small string that we don't observe it as a string--we see it as a point. However, if we could see to 10-34 cm, then we would see that it is actually a wiggling, vibrating string.) In 1984, the so-called "first superstring revolution" was launched, bringing to everyone's attention the idea that the problem of reconciling quantum mechanics and gravitation might be solved by strings. However, a subsequent problem arose in string theory: there were too many different versions of it. We had no way of knowing which string theory would be correct. So, interest in string theory waned for a while, but then in the early to mid '90s there was the "second superstring revolution," in which my work was involved, when it was shown that certain different string theories are connected, that they are probably unified in a way.
How does string theory relate to black holes?A black hole is interesting, both theoretically and experimentally. It's a great test of Einstein's theory of relativity and of string theory. Black holes are the endpoint of gravitational collapse. (That's when a body collapses under its own gravity, and it has to have enough mass to do that.) A black hole has quantum mechanical features, but it also is a gravitational object. So, string theory includes gravitation, and it mathematically predicts black holes. One of the big successes of the "second superstring revolution" was--and this is a bit technical--that it showed that the counting of quantum states in a black hole agrees with the classical entropy disorder parameter of a black hole (i.e. it's surface area). Suffice it to say, any theory that would purport to combine gravity with quantum mechanics has to satisfy this result, and it does seem to be the case for string theory.
How was this discovered?Very mathematically. Just pen on paper, solving complicated equations. Often in solving equations, you follow your intuition. You guess. With any kind of mathematical or scientific system, once you get used to it, your intuitive understanding gets better and better without you realizing it. The more you do it, the more likely you'll just get a solution and interpret it as a black hole or an expanding universe. But I have to emphasize that none of string theory has been proven correct. This is still all speculation. We're still waiting for experiments, which are, so far, too costly. [Khuri notes that some particle-accelerator experiments to test string theory are tentatively planned for 2007 in Geneva.] A theory can be nice and elegant and mathematical, but if it isn't testable, if there isn't an experiment that can either verify it or rule it out, then it's not much use as a scientific theory.
What is it like working on such a speculative level?It's a bit strange. It can be frustrating, because you know you can work your whole life on something like this, and then everything you've done can turn out to be wrong. But a lot of people who have worked on great theories, and ultimately had their theories proved wrong, have made great advances nevertheless--like Niels Bohr, with his model for the atom, or Newton, whose theory of gravitation was ultimately disproved by Einstein's relativity. Sometimes the progress you make, by making a conceptual advance--even if the theory you are working on is not correct--can be helpful. If no evidence is seen for string theory, in a way that would be interesting, because then we would have to start from scratch and think again of how to understand these phenomena.
