Quest to unlock universe's missing link

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.

Page: 1 | 2