Experiment Could Reveal "Extra Dimensions," Exotic Forces

The researchers have demonstrated an innovative way to measure a phenomenon known as the Casimir effect -- findings that also could have implications for the design of microscopic machines that contain tiny parts on the size scale of nanometers -- or billionths of a meter.

The scientists are taking their theoretical findings a step further by conducting an experiment to prove that the theory works, said Ephraim Fischbach, a professor of physics at Purdue University.

A paper that describes the theory for the experiment will appear in the Nov. 4 issue of Physical Review Letters, a journal published by the American Physical Society. The paper was written by Fischbach and Dennis E. Krause, a professor of physics at Wabash College, in Crawfordsville, Ind.

The Casimir effect, predicted in 1948 by Dutch physicist Hendrick Casimir, is a force that pushes together two plates of metal placed near each other in empty space -- or a vacuum. The closer the plates are to each other, the stronger the force.

What may be thought of as empty space is actually teeming with fleeting particles and electromagnetic fields. However, because the plates are so close to each other, many of the particles and fields cannot get between the plates. That means the space surrounding the plates contains more particles and energy than the space between the plates. The more energy-dense space surrounding the plates exerts a force on the metal, pushing the plates together.

The strength of the Casimir effect depends on the number of electrons in the metal out of which the plates are made. For that reason, the Purdue physicists will test the effect using plates made of isotopes of the same metal. Isotopes are elements that contain the same number of electrons but different numbers of neutrons in the atom's nucleus.

One portion of the experiment will use plates made out of nickel 58, an isotope of nickel that contains 28 protons and 30 neutrons in its nucleus. A second portion of the experiment will use plates made of nickel 64, which contains 28 protons and 36 neutrons.

Because the plates made of nickel 58 and 64 have the same number of electrons, the Casimir forces acting on both sets of plates will be nearly identical. That means any measurable difference in force between the two sets of plates must be attributed to some entirely new, as-yet undiscovered force acting on the respective nuclei.

Such knowledge could prove critical in the design of future devices containing tiny gears and motors that are measured in nanometers. Because these devices will contain moving parts placed extremely close to one another, they may be subjected to exotic forces that do not affect the parts inside large-scale machines.

"When you actually make little gears, for example, they may stick together in funny ways," Fischbach said. "You can't just make a microscopic version of your car's transmission and expect it to work. Suddenly, on such small size scales, when moving parts are very close to one another, a lot of funny things happen.

"In order to go from fundamental physics to applied nanotechnology, you really will have to understand the laws that govern what happens at a very small scale. This research helps to bridge the gap between very fundamental physics and really applied physics."

The discovery of new forces, could, in turn, provide evidence for the existence of additional dimensions beyond the three spatial dimensions of length, width and height.

"A new kind of gravity-like force would be the fingerprint of the fact that we may really live in a world that is more than three spatial dimensions," Fischbach said. "You wouldn't see this force over large distances, but you could see it over small distances."

However, scientists must first devise a way to confidently measure the Casimir force.

"Physicists know that the Casimir force exists," Fischbach said. "But we have to now understand it sufficiently well that we can say, 'I know when I line up the plates exactly like this, that I should see a certain force, which I can measure, and if I see something different, then there might be a new force on top of the Casimir force.'"

Because nickel 58 and 64 have the same number of electrons but different nuclei, any difference in forces observed between the two sets of plates could provide evidence that those nuclei were interacting with "extra dimensions" that exist side-by-side with the known three dimensions, Fischbach said.

Scientists have proven the existence of four fundamental forces of nature: gravity; electromagnetism; the strong force, which holds the nucleus of the atom together; and the weak force, which governs the energy production in stars and is responsible for some forms of radioactivity.

Researchers have theorized that the universe contains additional dimensions beyond the three spatial dimensions observed in the everyday world. Theory also has suggested that, of the four known fundamental forces of nature, all but one -- gravity -- are confined to three dimensions. This could help to explain why gravity is weaker than the other forces.

"In a sense, gravity gets dissipated by being spread out over more dimensions, and that's why gravity looks weak compared to the other forces," Fischbach said. "Gravity might sense and interact with these extra dimensions in such a way as to reveal their presence.

"The point is that gravity actually penetrates these other dimensions."

Previous research by Fischbach has suggested the existence of a so-called "fifth force" of nature. If other dimensions do exist, a gravity-like "fifth force" might be used to study and communicate with those dimensions, Fischbach said.

Fischbach and Krause have worked recently with Ron Reifenberger, a Purdue professor of physics, and Stephen W. Howell, a postdoctoral research associate in the Department of Physics. They are now collaborating with two experimentalists, Ricardo Decca, a professor of physics at Indiana University-Purdue University Indianapolis, and Daniel Lopez, a scientist who is a member of the Nanofabrication Research Lab at Lucent Technologies.

The experiment currently being designed by the team will use nanofabrication techniques to replace one of the plates in the above experiment with a tiny sphere. The remaining plate with the nickel coatings will be attached to a "microelectromechanical torsion oscillator," a setup that could be likened to a nanometer-scale version of a record player in which the record player's needle is the sphere. The device will record the force between the sphere and the plates, searching for a difference in the forces on the two nickel isotopes.

The research has been funded by the U.S. Department of Energy.