Shadow world - dimensions in space

In a school of thought that teaches the existence of extra dimensions, Juan Maldacena may at first sound a little out of place. String theory is physicists' still-tentative strategy for reconciling Einstein's theory of gravitation with quantum physics. Its premise is that the subatomic particles that roam our three-dimensional world are really infinitesimally thin strings vibrating in nine dimensions. According to Maldacena, however, the key to understanding string theory is not to add more dimensions but to cut their number down. In his vision, the mathematical machinery of strings completely translates into a more ordinary quantum theory of particles, but one whose particles would live in a universe without gravity. Gravity would be replaced by forces similar to the nuclear forces that prevailed in the universe's first instants. And this would be a universe with fewer dimensions than the realm inhabited by strings. Just as a hologram creates the illusion of the third dimension by scattering light off a 2-D surface, gravity and the however many dimensions of space could be a higher-dimensional projection of a drama playing out in a flatter world. In physics parlance, the two theories would be dual to each other—two mathematically equivalent languages for describing the same reality. Physicists could study each phenomenon using whichever language that makes it easier to understand. Maldacena first presented his conjecture in November 1997, and it quickly became a leading theme in string theory research.

Ten years later, physicists still don't have proof of it, though many have tried and thousands of papers have been written. But hints have been accumulating, and recently experts have found "very strong evidence" that the conjecture is true, says Maldacena, now at the Institute for Advanced Study in Princeton, N.J. Meanwhile, the work by Maldacena and others has helped clarify a nagging paradox about black holes, gravity's most extreme phenomena, by translating the problem into ordinary quantum theory.