Word: one
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Dates: during 1990-1999
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...puzzle they tried to solve is utterly fundamental. In simply recognizing the problem, Einstein was so daringly far-sighted that only now has the rest of physics begun to catch up. A new generation of physicists has at last taken on the challenge of creating a complete theory--one capable of explaining, in Einstein's words, "every element of the physical reality." And judging from the progress they have made, the next century could usher in an intellectual revolution even more exciting than the one Einstein helped launch in the early 1900s...
...trouble is, neither Witten nor anyone else knows how many other pieces must fall into place before scientists succeed in solving this greatest of all puzzles. One major reason, observes Columbia University physicist Brian Greene, is that string theory developed backward. "In most theories, physicists first see an overarching idea and then put equations to it." In string theory, says Greene, "we're still trying to figure out the central nugget of truth...
Replacing particles with strings eliminated at least one problem that had bedeviled scientists trying to meld general relativity and quantum mechanics. This difficulty arose because space lacks smoothness below subatomic scales. When distances become unimaginably small, space bubbles and churns frenetically, an effect sometimes referred to as quantum foam. Pointlike particles, including the graviton, are likely to be tossed about by quantum foam, like Lilliputian boats to which ripples in the ocean loom as large waves. Strings, by contrast, are miniature ocean liners whose greater size lets them span many waves at once, making them impervious to such disturbances...
...other things, string theory requires the existence of up to seven dimensions in addition to the by now familiar four (height, width, length and time). It also requires the existence of an entirely new class of subatomic particles, known as supersymmetric particles, or "sparticles." Moreover, there isn't just one string theory but five. Although scientists could rule out none of them, it seemed impossible that all of them could be right...
...that had just three spatial dimensions plus time. As a result, he could use off-the-shelf mathematics to develop and solve his equations. M theorists can't: their science resides in an 11-dimensional world that is filled with weird objects called branes. Strings, in this nomenclature, are one-dimensional branes; membranes are two-dimensional branes. But there are also higher-dimensional branes that no one, including Witten, quite knows how to deal with. For these branes can fold and curl into any number of bewildering shapes...