One of the first lessons babies learn from Mother Nature is that objects dropped from a high chair fall to the ground. Yet physicists, despite all their higher learning, have yet to grasp the essential nature of gravity.
“Everyone thinks they know what gravity is,” said JPL physicist Paul Swanson. “But if you ask a physicist, it’s much more mysterious.”
As the late Nobel laureate Richard Feynman pointed out, understanding of the force that tethers the planets to the sun has not progressed very much since the days people thought angels pushed the planets around. “The only difference is the angels sit in a different direction and their wings push inward,” he said.
Recently, scientists have felt a special urgency to try to tame this unruly force because gravity seems to plop itself smack in the center of most major puzzles in physics and astronomy:
What is the nature of the alien “dark matter” that seems to make up most of the universe? Why does matter weigh anything? Why is gravity so radically different from all the other forces, such as electricity or nuclear forces?
Despite the elusiveness of answers, physicists have hardly given up. On the contrary, gravitational research these days is a hotbed of activity, with several experiments in the planning stages and dozens of theoretical schemes populating the pages of technical journals.
Experimenting on gravity won’t be easy. In fact, at a meeting of the American Physical Society this year, different researchers presented widely different measurements of the gravitational constant known as “g,” which indicates the strength of gravity’s force. “That’s certainly a puzzle,” said University of Washington physicist Christopher Stubbs.
One of the most puzzling aspects of gravity has been around since the days of Galileo. Legend has it that he dropped a cannon ball and a musket ball off the Leaning Tower of Pisa to show that they would fall at the same speed. Whether he actually dropped the balls or not, he did stumble upon the bizarre coincidence that heavy objects and light ones fall at the same rate.
The standard explanation is that while gravity pulls harder on massive objects, massive objects have more inertia and therefore resist gravity’s pull. The two forces--gravity and inertia--exactly balance each other.
The problem is that there is no reason this should be so. “There’s no reason why [gravity and inertia] should have anything to do with each other,” said Swanson.
One person who pondered this funny relationship between gravity and inertia was Albert Einstein. It was just too strange, he thought, to be mere coincidence. Instead, he speculated that gravity wasn’t a glue-like force after all, but rather more like a pothole in the pavement of space; anything that wanders near, falls in. The potholes are created by the weight of heavy objects, such as planets and stars, warping the fabric of space.
Einstein called this insight “the happiest moment of my life.” He came upon it in his usual way, by conducting a “thought experiment.” He imagined himself falling off a building. In such a situation, he realized, there is no such thing as gravity. Drop a briefcase while falling off a building, and it floats along with you, weightless. It makes no difference that the briefcase is much lighter than the person. Both are simply following invisible paths laid out for them by the curvature of space.
But this “equivalence principle,” as the balance between gravity and inertia is known, has never been tested at the extreme sensitivity required to see if it might break down--in the same way as a newspaper photo dissolves into dots if you view it with a strong enough microscope.
To remedy that situation, an experiment called Satellite Test of the Equivalence Principle (STEP) is planned for launch on a European space mission in the next decade. Under development at the Jet Propulsion Laboratory in Pasadena, the experiment will amount to a high-tech version of Galileo’s apocryphal Pisa experiment, using six super-cold cylinders orbiting in space.
The idea is that gravity will tug the cylinders toward Earth, while inertia will propel them out into space. If the exact balance between the two forces is disturbed, the detector will feel it--even if it’s off by only one-thousandth of the diameter of an atom.
Even if Einstein’s theory of gravity holds true at these extremes, it still poses numerous problems. Physicists, as well as lay people, find the concept of curved space a little hard to imagine. “With Einstein, you introduce the curvature of a thing you can’t see,” said UC Irvine physicist Gregory Benford, who is also a well-known science fiction writer. “That’s a little hard to believe.”
Fortunately, this hard-to-believe concept has some concrete implications: Starlight passing through a curved chunk of space should bend. And indeed, in 1919, astronomers found that it does.
But other consequences have not been tested. For example, an object sitting in a gravity well of its own creation should drag space around with it as it spins. “It [space] is like a fishnet lying on the ground,” Swanson said. “If you stand on it and then turn around slowly, you’ll drag the net with you.”
If Einstein is right, the spinning Earth should drag the space around it like like a bridal train. To find out whether it does, another satellite--Gravity Probe B--is under construction to fly in a NASA satellite in 1999.
Essentially, it involves a supersensitive, super-cold gyroscope that is pointed toward a guide star. As the gyroscope orbits the Earth, the dragging of space should drag the gyroscope’s axis as well.
Even if every gravity experiment now on the books answers every question it poses, none is likely to help solve the most outstanding puzzle: that gravity is a misfit with almost everything else physicists know about the matter and the forces that populate the universe. No matter how hard physicists try to make it conform, gravity refuses to make peace with the rest of nature. When they put together the equations that describe gravity, and those that describe everything else, they get pure gobbledygook.
It’s as if atoms and light are playing by the rules of chess and the planets and stars--ruled by gravity--are playing by Monopoly instructions. But somehow, because stars are made of matter and light, they all have to be playing the same game.
Worse, gravity cheats because it warps the playing board. It is both actor and stage.
To deal with these irreconcilable differences, physicists have come up with a wealth of potential explanations. One of the tamest involves strings of primordial energy vibrating in 10-dimensional space.
As for really strange theories, they are ample as well. Benford thinks there might be areas of negative gravitational energy in the universe, a kind of anti-gravity that would appear at the end of a wormhole between one galaxy and another. He calls them “subways to the stars” and thinks they might be the notorious “dark matter” that seems to pull invisibly on everything in the universe.
Caltech’s Roy Britton has proposed to explain the apparent pull of dark matter by the scattering of gravitons, or particles of gravity. Although his paper was published in the Proceedings of the National Academy of Sciences, “it is very unpopular,” he said. “Very few people even took it seriously enough to read it.”
Still, some physicists think it is only logical to try to explain dark matter by tinkering with gravity because dark matter is a purely gravitational problem. That is, it can only be seen by its gravitational pull on everything else. So maybe there isn’t any dark matter, these physicists speculate; instead, maybe there is something else weird that they don’t know about gravity.
Stubbs did his doctoral thesis on approaches to “funny gravity,” but failed to come up with any appealing solutions. “It’s hard to find a simple theory that does all the right things.”
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A Weighty Mystery
Gravity is a hot topic among physicists, whose goal is to better understand the still- mysterious force that pulls objects to the ground and anchors planets to the sun. Two experiments in space will study two aspects of Einstein’s theory of gravity.
THE RELATIVITY EXPERIMENT
According to Einstein’s theory of relativity, the pull of gravity is caused by the warping of the space- time “fabric” by massive objects. Einstein realized that our familiar three- dimensional space cannot be separated from the fourth dimension, which is time. Earth sits in an indentation in this four- dimensional space- time called a gravity well; an apple dropping from a tree- like any other object-falls into the well, while fast- moving objects such as the moon or satellites orbit inside it. Einstein’s theory also suggests that the spinning Earth should drag the space- time fabric around as it turns.
The mission of Gravity Probe B would be to detect the dragging effect. A hypersensitive gyroscope is pointed toward a guide start as the probe orbits Earth. The dragging of space should produce tiny changes in its orientation.
THE STEP EXPERIMENT FOR TESTING EQUIVALENCE
This is a Space Age equivalent of the experiment Galileo supposedly performed when he dropped two balls of different masses off the Tower of Pisa and saw that they fell at the same rate. The reason is that although gravity pulls harder on the larger mass, the larger mass has proportionately larger resistance to being pulled. This exact balance has puzzled physicists for generations and has never been tested at the extremes of sensitivity where theory suggests it might break down.
In the Satellite Test of the Equivalence Principle (STEP), the two balls are replaced by pairs of orbiting cylinders. As they circle the Earth, the inward pull of the planets gravity should always be exactly the same as the outward centrifugal force that tugs the cylinders outward. Therefore, they should stay exactly in line. If they don’t, as shown at right, it would indicate that at least one aspect of Einstein’s theory of relativity breaks down. Source: NASA, European Space Agency and Lockheed Missiles & Space Co., Inc.