Hunting Hidden Dimensions

By Diana Steele, Science News

In many ways, black holes are science's answer to science fiction. As strange as anything from a novelist's imagination, black holes warp the fabric of spacetime and imprison light and matter in a gravitational death grip. Their bizarre properties make black holes ideal candidates for fictional villainy. But now black holes are up for a different role: heroes helping physicists assess the real-world existence of another science fiction favorite—hidden extra dimensions of space.

Astrophysical giants several times the mass of the sun and midget black holes smaller than a subatomic particle could provide glimpses of an extra-dimensional existence.

Out in space, astrophysicists are looking hard to see if large black holes are shrinking on a time scale that might be detected by modern telescopes. If so, it might mean the black holes are evaporating into extra dimensions.

In the laboratory, black holes far smaller than anything that could be seen with a microscope might be produced in Europe's Large Hadron Collider after it starts running again in November (SN: 7/19/08, p. 16). The detection of such a black hole, which would evaporate in a hail of subatomic particles in a tiny fraction of a second, would provide evidence that unseen dimensions of space exist.

What makes either of these ideas even plausible is a bold theory put forth just over 10 years ago that purports to explain the weakness of gravity by supposing that some of it is leaking out into extra dimensions.

Gravity feels strong to humans because it makes climbing hills hard. But one of the fundamental paradoxes about gravity is demonstrated by the fact that an ordinary refrigerator magnet can pick up a paperclip—counteracting the entire mass of the Earth pulling down on the clip.

Physicists call this the "hierarchy problem," referring to the fact that all the other forces of nature are more than 30 orders of magnitude stronger than gravity.

"It's hard to explain such a huge number from any mathematical postulate or any physical principle," says Greg Landsberg, a theoretical physicist at Brown University in Providence, R.I. "It's a bit of an embarrassment for our field, because what it really means is, we don't seem to understand gravity."

Measuring extra dimensions

Isaac Newton declared in the 17th century that gravity gets weaker by the square of the distance between two objects. If the moon were twice as far from Earth, it would feel one-quarter the gravity.

But in 1998, theoretical physicists Nima Arkani-Hamed, Savas Dimopoulos and Gia Dvali pointed out that gravity had never been measured below a distance of about a millimeter. Suppose, they suggested, that gravity differs from Newtonian expectations at distances smaller than a millimeter.

That could happen if there are extra dimensions of space that gravity leaks into. These hidden dimensions might be shaped, for example, like the circumference of a hose. From a distance, the hose looks like a one-dimensional line, but seen up close, it has a curled-up second dimension. Arkani-Hamed, Dimopoulos and Dvali—whose model is known as ADD, short for their names—suggest that there could be extra dimensions as large as a millimeter in diameter.

To read more of this article, click here. To read other feature articles by Science News staff reporters click here.