From the OCT/NOV 2005 issue of Seed:
Why do we only experience three dimensions of space—the familiar up-down, left-right, and forward-backward? Their existence is so deeply ingrained in our consciousness that most of us don’t even bother to ask. But for physicists, the mystery of why we experience just three dimensions, when we know there could be many more, presents an enormous challenge—so much so that at a conference at the turn of the millennium, it was singled out as one of the five most important unsolved problems in physics. Is it merely a cosmological accident, or is there a natural preference in the universe for three dimensions? My work on extra dimensions recently led me to a potential answer.
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String theory, which proposes that the fundamental units of matter are minuscule oscillating strings, consistently combines our theories of the very small and the very big—quantum mechanics and general relativity. But despite showing great promise, physicists have yet to determine the connection between string theory and our physical world. For example, string theory doesn’t manifestly describe a universe with three dimensions of space; it naturally suggests more: perhaps as many as nine or 10. Although it might be difficult for us to fathom so many additional dimensions, string theorists have no choice but to accept them and ask: "Where are these extra dimesions?," "Why haven’t we seen them?" and "Why are three large dimensions singled out in our universe?" Although physicists have thought about how to address the first two questions for some time, we have had only tentative ideas about the third.
Right now, a central and contentious issue in string theory (indeed, in all of physics) is which of the known properties of our universe, such as particle masses or the dark energy of the universe, can be predicted. Although it was initially advertised as a “theory of everything,” many physicists are now considering the possibility that we may not be able to calculate some of the physical properties of the universe—via string theory or any other means. These physicists accept the “anthropic principle,” which says that we live in our particular universe because it’s the only one that can support galaxies and, hence, life. Widespread acceptance of the anthropic principle would mark a revolution in physics because it gives up on predictability for some questions.
But before we dramatically abandon the goal of a deeper understanding of nature, physicists need to explore all the physical scenarios that string theory presents. These alternate scenarios, what string theorists call “the landscape,” are universes with distinct physical properties. The challenge for physicists, and the problem I tackle in my new work, is to find all possible qualitatively different universes—and to search for principles that determine which of these universes is most likely to exist. One route to resolving this question is to try to understand why we experience three spatial dimensions.
Physicists have considered the possibility of additional dimensions for a long time. In 1919, on the heels of Einstein’s theory of general relativity, which didn’t specify three dimensions, Theodor Kaluza suggested an extra dimension of space. Then, in 1926, Oskar Klein offered an answer to the question of why we wouldn’t see it—an answer that seemed to be the only possibility until the late 1990s. He proposed that an extra dimension could be rolled up into such a tiny size that it would have no visible effects. For example, if you imagine an extra dimension rolled up like a tube, the width of the “tube” could be so tiny that you’d never notice it. Any structure at this tiny size would be washed out, much as the atomic structure of this piece of paper is imperceptible.