For all his accomplishments, Einstein never lived to see his fondest dream come true. Our century’s best-known physicist spent most of his life searching for a comprehensive set of laws that would explain the behavior of nature on all levels, from quasar to quark. He had, in his twenties, already shown that space and time were intertwined. He then succeeded in showing how gravity is intimately related to the geometry of this curved space-time. But he failed when he tried to weave all aspects of nature--all its forces and fundamental rules--into one seamless cloth. The new science of quantum mechanics simply wouldn’t fit, no matter how hard he, or anyone else, tried.
Today physicists are still stuck in the same quagmire. Nature seems to play by two sets of rules, and they are incompatible. It’s as if physicists were being asked to go bowling with tiddlywinks or to jump-start a car with an eggbeater. The tools that work so well in one realm are totally inappropriate in another. Not only can’t they win at this game, they can’t even begin to play.
Einstein’s theory of gravity--also known as general relativity-- still describes the universe on its grandest scale with a power that continues to astound physicists. The structure and dynamics of stars, galaxies, black holes, the very shape and evolution of the universe--all are explored using the tools Einstein developed. Gravity, according to this theory, is not the result of invisible tendrils of attraction emanating from a mass, keeping planet to sun or boulder to Earth. Rather, gravity is the result of warps in space-time. Massive objects indent the flexible backdrop of space-time like boulders sitting on a rubber mat. The wells they create naturally attract and frequently capture nearby objects, just as potholes attract cars. The language of general relativity speaks of a gently curving space-time, a landscape of hills and basins, a continuous flow of smooth, connected forms. The alphabet of this language is geometry; its vocabulary consists of lines, angles, surfaces, curves.
Zoom in on matters subatomic, however, and the landscape suddenly changes. Einstein’s rules no longer apply. Atoms and nuclear particles buzz around like angry bees. Their energy and motion are served up in discrete bits, jumpy and blurred, their exact behavior and position forever uncertain. The words always and never, used so readily in describing the physics of our everyday world, are replaced with the terms usually and seldom. The language that describes this lumpy landscape is quantum mechanics. Keeping track of such a mad gambol of particles requires a vocabulary that deals with statistical relationships, the probabilities of events. Its alphabet is algebraic symbols and quantum numbers: 1, 1/2, 2.
Trying to do general relativity with the rules of quantum mechanics (or vice versa) would be like using the formula for the area of a circle to compute your chances of winning the lottery, or employing probability theory to measure the area of a house. Yet physicists find themselves in just such a position. They can’t proceed until they find a common vocabulary that will enable the quantum theorist to talk freely with the relativist, allowing the lumpy microcosm to join with the smooth macrocosm in an all-embracing theory of quantum gravity. In fact, given such strikingly different pictures of reality, it’s somewhat surprising that physics has been able to progress at all.
