Alumnus strings together ideas to explain physics
Constructing a mind-bending picture of the universe in which all the matter around us is mirrored in an alternate set of particles at the very edge of space and time, he gave physicists tantalizing clues about how to develop their long-sought "theory of everything."
Maldacena, a professor of physics at Harvard is spending a year back in Princeton at the Institute for Advanced Study. On Oct. 15, he will offer a glimpse into that dual-view universe as part of the Graduate School centennial public lecture series. His talk, "Gravity, Black Holes and Strings," will introduce the audience to a resurgent area of physics called "string theory" and show how it holds out the promise of explaining everything from the workings of black holes to the origin of the universe.
String theory, while a great tool of simplification in physics, can be rather complicated for those outside the field. So here are a few concepts to review before Maldacena's talk.
Since Albert Einstein's day, there has been a nagging problem in the world of physics. The laws that govern things on very large scale -- the rules of gravity -- do not mesh with the laws that describe things on a very small scale -- the rules of particle physics. That is, Einstein's theory of general relativity, which describes gravity, appears to conflict with quantum mechanics, which describes all the other forces and particles.
Imagine, suggests Maldacena, an electron. Like the Earth, the electron exerts some gravitational pull, which increases as you approach it. But conventional physics describes the electron as an infinitely small point-like object. So, on paper, there is no limit to how close you could get to the electron, and calculations might show its gravitational pull soaring to infinity, which makes no sense. Physicists have complex mathematical tricks to "correct" for such problems with most particles, but the tricks fail when applied to gravitons, particles that carry the gravitational force.
Also, quantum mechanics tells us that we never know exactly where an electron is -- just where it is likely to be. "So if we can't say where it is precisely, we can't calculate its gravitational potential precisely," he says. Its gravitational field is smaller than the nebulous space where it is expected to exist.
String theory helps in a number of ways. It considers all the particles of physics to be string-like bands of a very small, but definite size. All the strings are the same, except they vibrate in different ways depending on what species of particle they are. So an electron is a tiny string that vibrates in a way unique to electrons. When physicists know how big the electron is, even if they still can't say exactly where it is, they can start to make calculations about its gravitational field.
Shortly after physicists first started hypothesizing about string-like particles in the 1970s, they noticed that one of the vibration modes corresponded nicely with the characteristics of the graviton. So not only does string theory eliminate some messy mathematics, it treats all the particles with a common set of rules.
String theory comes in particularly handy in describing black holes. Most of the time, scientists can ignore the gravitational effect of subatomic particles because the particles are too far apart to feel each other's gravity. That situation changes in a black hole, where particles are packed so tightly that they exert significant gravitational tug on each other.
One example of where this micro-gravity seems to be important is in the so-called information paradox. Light and matter that fall into a black hole appear to be lost -- no information contained in them could ever be retrieved. But laws of quantum mechanics insist that no information is ever lost; looking into a black hole, one should be able to retrieve information about what went into it, just as the color of a flame might describe the material that was burned in a fire.
Building on a series of advances by other string theorists, including Callan, Assistant Professor of Physics Steven Gubser and others, Maldacena's 1997 paper showed how all the material that goes into making a black hole, or any other object, could be represented as an array of particles distributed across an imaginary surface very distant from the object. This "boundary theory" allows physicists to continue to "see" the contents of a black hole and to perform calculations that describe quantum gravity.
In doing so, Maldacena showed how two completely different theories of physics -- one describing gravity and strings and the other describing all the other particles -- were really aspects of the same theory. "It is something of tremendous conceptual importance when you discover that one thing is really another in disguise," says Callan. Maldacena and other leading string theorists are now building on that 1997 revelation.
The real prize in all this is not just understanding common black holes, but deciphering the rules that governed the ultimate black hole, the cluster of matter that scientists say spawned our universe at the Big Bang.
"The origin of the universe is one of the main things that string theory wants to describe," says Maldacena. "But there is a lot more work to be done before we can do that."
In the meantime, one of the challenges is explaining what he is doing to people outside the field. "I am happy if people understand the background, if they get the flavor of the problem," he says. "I have given this talk once before, and I am going to work on it a little more to bring people along a little further."