The Gravity of Small Objects and Distances

Does the force that keeps us on the home planet work differently at smaller scales?

By Tim Folger|Monday, October 24, 2005

Answers to some of the biggest questions in physics are hanging by a thread in a laboratory at the University of Washington in Seattle. The thread is made of tungsten and measures a bit over 30 inches long and less than one-thousandth of an inch thick. It is part of a tabletop instrument called a torsion pendulum, which can measure gravity’s strength across small distances with unprecedented accuracy.

Although the behavior of gravity acting on large bodies and over long distances is well understood, what gravity does between small objects at very small distances is uncharted territory. No one even knows if Newton’s laws (which state that the pull of gravity varies in proportion to the square of the distance between two objects) still hold at that level. Raman Sundrum, a theoretical physicist at Johns Hopkins University, is betting that they do not. If he is right, the Seattle experiment may explain strange goings-on halfway across the universe.

Seven years ago, cosmologists discovered something astonishing: The expansion of the universe is accelerating, perhaps driven by a repulsive force known as dark energy. The force seems to be a basic property of empty space. In some ways that makes sense because, according to quantum mechanics, empty space is not empty.

Rather, the vacuum is filled with fields and particles that constantly pop in and out of existence. The problem is that when physicists estimate how much energy is contained within those fields and particles, they come up with a number—called the cosmological constant—that is insanely large, 10120 times greater than what we observe. A cosmological constant of that magnitude would rapidly tear the universe apart. It is an embarrassing error, often described as the biggest mathematical mistake in the history of physics.

Sundrum suspects the error is telling us that we must change our thinking about gravity. He has proposed a strange, intriguing scenario in which gravity is transmitted by a particle, called a fat graviton, that could be as large as one two- hundreth of an inch wide—enormous compared with the particles that make up atoms. This fat graviton barely interacts with the matter and energy roiling through “empty” space, thereby eliminating the 10120 error in the size of the cosmological constant while leaving enough energy to account for the observed acceleration of the universe. Unlike many current physics speculations, Sundrum’s idea is testable. In his model, the fat graviton tends to skip over objects smaller than itself, so gravity should start to weaken over such short distances.

Blayne Heckel, Eric Adelberger, and their colleagues at the University of Washington are searching for this effect in their lab, where they are trying to measure the gravitational attraction between a pair of two-inch-wide molybdenum disks: one suspended on the tungsten thread, the other rotating freely on a bearing. They are right on the edge of the scale where Sundrum’s effect should kick in. Some preliminary results hint that gravity may show signs of weakening, but the researchers need to resolve systematic errors before they draw any conclusions. “It is very exciting,” says Sundrum. “But this is a really tough problem. A lot of people have had ideas, so I try to stay as cautious as I can. I’m definitely not jumping up and shouting eureka yet.”

Even if the findings do not support the fat graviton theory, they might place constraints on other speculative theories, such as Gia Dvali’s idea that gravity is weak compared with the other forces because most of it escapes into higher dimensions, outside the three we experience. Dvali, a physicist at New York University, was motivated by string theory, which holds that the universe may be an 11-dimensional structure—and a leaky one at that. “We realized that this leakage could cause cosmic acceleration, so maybe there is no dark energy,” Dvali says.

Dvali’s theory, like Sundrum’s, is testable. It predicts that the moon’s precession—the rate at which its axis of rotation shifts—is slightly smaller than what Einstein’s general relativity says it should be. Lunar ranging tests, which bounce lasers off reflectors left on the moon by Apollo astronauts, could settle the issue.

“The current accuracy is astonishing,” Dvali says. “We know the variations of the lunar orbit to about one centimeter. If we can bring that down to one millimeter, then our theory could be tested. Gravity is the biggest mystery. It’s the oldest force we know, but we still understand so little about it.”

Weight of a 150-pound person standing on . . .

A carbon atom: 10-35 pound

A dust speck: 10-27 pound

The Voyager1: 10-7 pound

A white dwarf: 40,000,000 pounds

Our galaxy's black hole: 70,000,000 pounds

A netron star: 20,000,000,000,000 pounds

The unexplainable lightness of being

Gravity’s force dominates massive objects—a planet, a star, or a black hole. For small objects, however, the pull of gravity is utterly swamped by the three other fundamental forces: the strong force (which holds atomic nuclei together), the weak force (which governs radioactive decay), and electromagnetism (which binds molecules together). The relative weakness of gravity, illustrated here by a person’s weight in different cosmic settings, is one of the great mysteries of physics. 

Nima Arkani-Hamed, a physicist at Harvard University, thinks the existence of other universes might explain the weakness of gravity and the nature of the “antigravity” force pushing our universe apart.

What is the connection between the accelerating expansion of the universe and the existence of other universes?
The observation of the accelerating universe has crystallized a crisis. It’s yet another parameter in our fundamental theory that appears to be finely tuned to just the right value. If it were a little bigger, the universe would be empty. But if the multiverse picture is correct, and there are all these different universes, then the value of the cosmological constant could just randomly vary from one universe to the next. We should not be surprised to find one universe with a cosmological constant that is not lethal to our existence.

Why do you refer to this as a crisis?
: Because it’s not a way that the vast majority of physicists wanted to go. It makes the task of figuring out the fundamental physics much harder if our universe is not directly connected to one set of fixed parameters. My feeling is that the new direction—picturing our universe as just a part of a multiverse—may very well be right, and all the controversy right now is because we’re in the middle of a difficult birthing process. It will open up new questions that we don’t even have the language to ask yet.

If many universes exist, does that mean that all physically possible universes exist?
It’s not that any universe you can dream of exists—there won’t be a universe with unicorns! There is some underlying theory of physics, perhaps string theory, and this theory produces a huge number of possible universes, but not every conceivable one. Still, these new ideas lead to a totally different sense of scale. The number of universes people talk about, just as a rough number, is 10500. That’s vastly larger than the number of atoms in our universe. So our significance in the multiverse would be very much less than the significance of one atom in our universe.

Will our belief in other universes always depend on circumstantial evidence, such as gravity tests, or could we find real proof?
It’s possible that we will figure out one day a more direct way to actually see the other universes. Many of the most important ideas in science were originally either rejected or met with a lot of resistance because we couldn’t see the objects involved. This happened with atoms, for example. There was the whole philosophical movement of logical positivism, which said that if you can’t see it, it doesn’t exist; now we’ve seen atoms with electron microscopes. The same thing could happen with other universes. It’s not in principle impossible to observe them. At least, we don’t know of any theorem that says it’s impossible. The situation isn’t like building a perpetual motion machine or going faster than light. In fact, I’m thinking right now about a simple, Earth-based experiment that might turn up evidence for little bubbles of the other universes.

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