Laboratory gravity tests
There is a surprisingly simple way to detect the higher dimensions predicted by string theory: Look for deviations in Newton’s law of gravity. Newton deduced that gravity falls off with the square of distance. Double your distance from Earth, for example, and its gravitational pull feels one-fourth as strong. Gravity spreads out through all of empty space, so its properties are sensitive to the number of dimensions it is spreading through. If the additional dimensions predicted by string theory exist, some gravity should leak away into those dimensions as well. We would observe this leakage as slight deviations from the inverse-square pattern that Newton described.
Newton’s theory has been tested with exquisite accuracy in our solar system and beyond. It is so precise we can tell a space probe like Cassini how to weave its way through the rings of Saturn, a billion miles away. But according to string theory, at small scales like a millimeter (1/25 of an inch), gravity might hop across higher dimensions and perhaps into other, parallel universes, growing diluted in the process.
Six years ago, physicist John Price and his colleagues at the University of Colorado at Boulder conducted the first experiment to detect a higher dimension via gravity. The team constructed an ingenious device consisting of two parallel tungsten reeds. One of the reeds vibrated 1,000 times per second, creating a small gravitational disturbance that ought to tug subtly on the other reed. The motion of the second strip should then indicate how gravity traveled between the two.
Price’s device was so responsive it could measure a disturbance a billionth the weight of a single grain of sand, but the researchers could find no deviation from Newton’s laws of gravity with the reeds separated by a distance of only 0.108 millimeter (1/250 of an inch). A half dozen other groups have developed tests to probe the behavior of gravity over similar distances. So far there is no sign of other universes. (Or perhaps the experiment just showed that there are no parallel universes in Colorado.)
Perhaps the additional dimensions would show up only on smaller scales—string theory is still somewhat vague about this prediction. Other experimentalists are therefore trying to test Newton’s law of gravity over distances as small as the size of an atom. Umar Mohideen of the University of California at Riverside is attempting to measure the attraction between a minuscule gold-coated polystyrene sphere and a gold-coated sapphire plate. The attraction is due not just to gravity but also to an esoteric quantum phenomenon called the Casimir effect, caused by the latent energy present even in empty space. Mohideen has started by trying to measure gravity over distances of a few hundred nanometers, a thousand times the diameter of an atom.
A team led by Ricardo Decca of Indiana University–Purdue University has developed an alternative approach that would cancel out the Casimir effect and thus measure the gravitational interaction directly. He has recently completed a nanoscale experiment that compares the attractive force between a gold-coated sphere and test samples of gold and germanium coated with a shared layer of gold. A comparison of the forces acting on the gold and on the germanium makes it possible to subtract the role of the Casimir effect and expose any previously unseen aspects of gravity, which could provide evidence of string theory’s extra dimensions. In the future Decca and his colleagues plan to run an analogous experiment using closely spaced plates made of nickel-58 and nickel-64, isotopic forms that have identical chemical properties but differ in mass by about 10 percent. To date, Decca’s group has yet to find any sign of higher dimensions, but improved versions of the tests will soon be under way.
Like the search for extra dimensions, the hunt for particles may not require city-size, multibillion-dollar accelerators. Astronomical studies show that about 23 percent of the mass and energy of the universe consists of dark matter, particles that emit no light and that barely interact with ordinary matter except through gravitational pull. This unseen material surrounds galaxies and typically weighs several times as much as the galaxy itself. No one knows what it is made of, but string theory predicts the abundant existence of sparticles that are invisible and massive—precisely the characteristics of dark matter.
Dark matter seems to permeate our own galaxy, the Milky Way. If it consists of sparticles, they should be everywhere. As Earth orbits through the Milky Way, our planet should move continually through an unseen wind of dark-matter particles that pass right through the planet and everything on it: your neighborhood, your living room, your body.
Several teams in Italy, France, the United Kingdom, Japan, and the United States are racing to capture dark-matter particles. Many of them rely on high-purity materials such as liquid xenon and germanium crystals, cooled to low temperatures and placed in deep mines to shield the devices from the continuous spray of ordinary particles that strike Earth’s atmosphere. Most of the time, passing dark-matter particles would fly right through the material without hitting anything and thus becoming detectable. (At the quantum level of scale, atoms overwhelmingly consist of empty space.) But on rare occasions a dark particle might collide with an atom. The sudden recoil of the atom’s nucleus would trigger a shower of electrically charged particles and atoms as well as light and heat, which can be picked up by a sensor.
This approach is simple in principle but tricky in practice because many other events can mimic a dark-matter particle. In 1999 a group at the University of Rome announced that they had found dark matter in their detector, but other teams questioned their result when they could not duplicate it. The Cryogenic Dark Matter Search, located within the Soudan Mine in Minnesota, is currently about 10 times as sensitive as the University of Rome detector was, and yet it sees no sign of the urgently sought particles.
Once particles of dark matter are identified in the laboratory, their properties can be analyzed and compared with the predictions of string theory. A leading candidate for dark matter is the neutralino, the sparticle partner of force-carrying bosons. String theory predicts that neutralinos may have been created and immediately annihilated in tremendous numbers right after the Big Bang. As the universe cooled, a slight departure from equilibrium caused more neutralinos to be created than destroyed, leaving an excess that persists today. The latest calculations indicate that neutralinos may be 10 times as plentiful as atoms. That abundance roughly matches the inferred quantity of dark matter in the universe.
Most physicists are confident that the particles we refer to as dark matter will be found, whether or not they are the specific particles predicted by string theory. But what if, contrary to all predictions, nobody ever manages to identify a dark-matter particle? For cosmologists and physicists alike, that would trigger an intellectual crisis. Yet string theory has another, even odder explanation to offer. Perhaps the dark stuff does not consist of unknown particles in our universe. Perhaps it consists of particles residing outside our universe—hovering just above us in a parallel dimension.
That may seem like an explanation from a science fiction novel (and it does in fact resemble the principle of invisibility set out in H. G. Wells’s The Invisible Man), but it emerges naturally in the higher-dimensional mathematics of string theory. Imagine for a moment that our universe is two dimensional, like a piece of paper. Now envision another, separate paper-sheet universe lying parallel to ours. We would be oblivious to that other universe even if it were just a fraction of an inch away. We would not be able to see it because there is no way of sensing or pointing to the higher-dimensional direction that leads to the other universe.
If another, three-dimensional universe were separated from us by a higher dimension, we similarly would not be able to see it directly even if it were right next to us. A few physicists, such as Joe Lykken of the Fermi National Laboratory and Lisa Randall of Harvard University, speculate that our situation in the real universe is just like that. Einstein’s general relativity predicts that gravity from matter in the other universe would leak into ours. We would thus feel the tug of matter that we cannot see—another possible explanation of dark matter. This unseen pull could be a sign of the higher-dimensional universe predicted by string theory.
Astronomers have noted that invisible matter seems to cluster around galaxies, forming a spherical halo stretching up to 10 times the diameter of the visible galaxy. Perhaps this occurs because huge clumps of shadow matter in a parallel universe pull on matter in our universe, causing our galaxies to form in mirror-image locations.
There are no convincing proposals of how to test this idea, but scientists could be forced to take it seriously if all the searches for dark matter located within our universe come up empty.