Frontiers | Pages 186-187 | See Linked Version

Hidden Dimensions

The mind-stretching concept of more than three spatial dimensions is actually demanded by the mathematics of string theory. As illustrated here, these additional very small spatial dimensions may be contained within the familiar dimensions we commonly experience. If the drainage pipe at left were seen from a great distance away, it would look like a one-dimensional line. This horizontal dimension is emphasized by the blue arrow. At closer range a second dimension, width, becomes apparent (green arrow). Finally, as we approach even closer, we perceive the pipe's depth (curving arrows). We might say that the second and third dimensions were "curled up" inside the pipe. In similar fashion, additional dimensions might become visible if we could turn up the magnification on the fabric of space-time (below)

Each level at left represents an increasing magnification of the fabric of space-time. The grid lines represent the dimensions we ordinarily experience. To convey the idea of where the extra dimensions of string theory might hide, the top level shows how one additional dimension might be viewed as circles, akin to the loops in a very thick pile carpet. For simplicity the circles are shown spread out at the intersections of the grid lines, but in reality they would exist at every point. Two additional curled-up dimensions are represented above as a sphere and a torus, or doughnut.

Second, string theory predicts that six or more extra dimensions exist beyond the four dimensions of space and time with which we are familiar. If you have a hard time picturing the four-dimensional weave of space-time, you may throw up your hands at the concept of six more dimensions. But as a real-world analogy, imagine driving on a flat highway and seeing a large drainage pipe jutting out into a field next to the road. From far away the pipe looks like a one-dimensional line emerging from beneath the asphalt. As you approach, the pipe gains width; it appears two dimensional. Finally, if you stop your car and walk into the field, you see the pipe's complete shape. You can even climb inside it, a three-dimensional experience that only opens up when you touch the pipe instead of looking at it from afar. In a similar way, the six extra dimensions of space are curled up--"compactified" in the lexicon of physics--under ordinary conditions. We can't see them unless we get up close. Once again it's an issue of energy. Some physicists think the accelerators of today or the near future will open up these hidden dimensions. New particles might pop out, and gravity might behave very differently from the way it does on large scales. If we see such effects, we'll be one step closer to writing our grand recipe for the universe.

The Source of Big EXPLOSIONS

If extra dimensions do exist, they would have contributed to the fabric of space during the earliest moments of the Big Bang. But no matter how far out into the universe we look--that is, no matter how far back in time our telescopes can peer--we will never see those first incandescent moments directly. The cosmic microwave background blocks our view. The microwaves, which provide us with such solid evidence of a once-hot universe, date to a time when the universe was roughly 300,000 years old. At that point the entire cosmos was about as hot as the surface of our Sun, about 10,000 degrees Fahrenheit. The universe was opaque; light scattered back and forth within the plasma, just as photons ricochet within the Sun's body. When the temperature dropped below that level, the universe became transparent to light because the first hydrogen atoms formed, allowing photons to stream through without striking loose electrons. The first photons that traveled freely away from the hot smog make up the remnant microwaves that our telescopes see today.