It's easy to take gravity for granted. We don't give it much thought unless someone drops an heirloom dinner plate or we peer over the railing of a high-rise balcony.
But scientists find gravity profoundly puzzling. It doesn't fit the otherwise consistent story of the nature of matter and its forces that physicists have uncovered, dubbed the "standard model." Without a clear understanding of how gravity fits into this picture - or, more likely, revises it - physicists say it would be extraordinarily difficult to explain the workings of the universe at its most fundamental level.
So far, gravity has thwarted physicists' hopes to show that nature's four basic forces are manifestations of one force that dominated the early universe.
The puzzles have grown sufficiently troubling - and the technology to measure gravity's effects has become so sensitive - that researchers are now spending hundreds of millions of dollars on experiments to probe this weakest of nature's basic forces. Gravity research is hot - encompassing everything from NASA's recently launched Gravity Probe B satellite and tabletop experiments in labs worldwide to gravity-wave detectors and a new generation of particle accelerators.
"It's an exciting time," says Eric Adelberger, a physicist at the University of Washington, noting the ferment among theorists to come up with fresh explanations for gravity.
To be sure, most people are clear enough on the concept of gravity to avoid walking under a piano as it's hoisted to an upstairs apartment. Aerospace engineers have a sufficient grasp of gravity to safely land robotic rovers on Mars or to use gravity as a fuel- saving "slingshot" to reach planets.
But each time scientists have taken a deeper look at gravity, they've uncovered new facets. Initially, these facets can appear to be merely subtle curiosities, but they can have profound technological implications. The Global Positioning Satellite system, for instance, would lose its widely touted accuracy by more than 10 kilometers (6.2 miles) a day if the system failed to adjust for effects Albert Einstein predicted in his theories of special and general relativity, says Clifford Will, physicist at Washington University in St. Louis.
General relativity showed that gravity was not really a force that two objects exert on each other, as Sir Isaac Newton described it. Instead gravity resulted from the presence of a mass, such as a planet, warping space and time around it, much as a bowling ball distorts the surface of a trampoline. Already in orbit around Earth, Gravity Probe B is preparing to measure this and related effects.
Among other things, this new view of mass distorting time as well as space implied that the farther you move a clock from Earth, the faster it will tick compared with an identical clock on the surface. Experiments later showed this to be true.
Yet for all the progress in understanding it, "gravity is the least well-known of the fundamental forces in physics," says Thomas Murphy, a physicist at the University of California in San Diego. These forces also include electromagnetism; the strong force, which binds particles in an atom's nucleus; and the weak force, which governs radioactive decay.
Among the conundrums: On small scales, why is gravity so weak compared with the other forces? Gravity will draw a needle to a table, yet even a small electromagnet will yank the pin back up and hold it against gravity's pull.
On large scales, the universe threw humanity's understanding of gravity for a loop when astronomers discovered that the universe's expansion rate was accelerating, not decelerating, as theories had suggested it should be doing. It's as though gravity suddenly became much weaker once the universe grew to a certain size.
Each of the other forces and the subatomic particles associated with them are described by quantum physics, yet gravity seems to have defied a similar description.
Many researchers hold that finding a quantum description of gravity is the only hope physicists have to show that these four basic forces are low-energy manifestations of one unified force that is thought to have prevailed at the earliest moments of the universe's birth.
Physicists have shown that electromagnetism and the weak force were a single force at one time. They say there is every reason to expect that evidence will show that this "electroweak" force and the strong force were once a single force. That evidence is expected to come from a new generation of particle accelerators, such as the Large Hadron Collider under construction at the European Organization for Nuclear Research (CERN) outside Geneva.
But gravity seems to operate by different rules. It's as though gravity "has seemingly nothing to do with everything else we know about physics," Dr. Murphy says. "There's this fundamental clash between quantum mechanics and gravity, and any naive attempts to unite the two end in a theoretical catastrophe."
Even some of general relativity's predicted effects, such as the existence of cosmic "gravity waves," have yet to be directly detected. These waves are held to ripple across space and time when two extremely massive objects, such as black holes, orbit one another. In principle, gravity waves might also open a window on the earliest moments of the universe, allowing astronomers to see further back in time than they can using radio waves, light, or any other form of electromagnetic radiation.
For many physicists, unifying gravity with the other forces of nature will require string theory. This idea posits that elementary particles are not pointlike, as they are widely held to be. Instead, they exist as one-dimensional strings. Add a dash of quantum mechanics, and interesting things begin to happen. Among them, researchers say: A hypothesized particle associated with gravity - the graviton - takes on the right properties without the shortcomings that gravitons in other quantum-based gravity theories encounter.
But for string theory to work, the universe needs 10 or 11 dimensions instead of the four that humans perceive. Where are the extra dimensions? Perhaps they are so compact they can't be seen.
As a whole, the idea is still a bit too vague to test it fully, says Steven Carlip, a physicist at the University of California at Davis. "Nobody understands string theory well enough to derive observational consequences," he says. But, he adds, the theory has inspired a range of simpler spin-off ideas that could be tested.
For example, Harvard University physicist Nima Arkani-Hamed and colleagues have argued that gravity's apparent weakness may be an illusion - that gravity is as strong as the other three forces, but it is the only force that easily moves from one dimension to the next. Thus, we see weak gravity only because it can "leak" into these other dimensions. This has led to experiments to see if at increasingly small distances - corresponding to the hypothesized size of the extra dimensions - gravity's properties change. Theories suggest these dimensions range in size from 10 microns (one-seventh the thickness of a human hair) to about 1 millimeter.
Dr. Adelberger and his colleagues have been checking to see if there's any change in the rate at which gravity weakens with distance at ever-smaller separations. So far, the Adelberger team has conducted experiments with a tiny, precisely machined pendulum that indicates nothing much changes, at least down to 150 microns. The next step is to push the experiment to smaller distances.
Murphy, on the other hand, is working on an experiment to bounce lasers off reflectors left on the moon by Apollo astronauts, to check for subtle changes in a basic physical "constant" known as the gravitational constant. String theory suggests that this and other figures humans have identified as "fixed" actually change over time scales approaching or exceeding the age of the observable universe.
Other evidence for gravity "leaks" may come from particle-physics experiments. Perversely, gravity is so weak that it would take a particle accelerator vastly more powerful than scientists could devise to reveal the graviton. But if gravity truly is a stronger force than it's perceived, the graviton might appear in a new generation of particle-physics experiments.