Search for Extra Dimensions
By: Professor Ho Jung Paik

The universe that we see and experience appears to have three spatial dimensions and one time dimension. But is this necessarily the case at the microscopic distances that we have not observed yet? In the past few years this has become a hot question among physicists.

String theory, which is the only self-consistent quantum theory of gravity so far, demands ten or eleven dimensions for the universe. The usual scenario is that the extra dimensions are curled up with very small radii (< 10 ^-30 cm) and therefore not visible. Recently, however, it has been pointed out (Arkani-Hamed, Dimopoulos, and Dvali, 1998) that the extra dimensions could be as large as 1 mm. Below this scale, gravity would propagate in higher dimensions, its force falling off faster than 1/r ². This could naturally explain why gravity is so weak compared to the other forces, the so-called hierarchy problem.

Astrophysical constraints may limit the large gravity-only extra dimensions to < 1 mm. To date the 1/r² law has not been tested directly at r < 100 µm. It is highly desirable to test Newton's law down to r less than or equal to 1 µm. At UM, we have designed a sensitive null test of the 1/r² law at sub-millimeter distances based on superconducting accelerometer technology developed in our laboratory. The principle of the null experiment is illustrated in Figure 1.

Figure 1. The Newtonian force due to an infinite plane slab of mass is constant on either side of the slab, independent of the position of the test mass.

In practice, we employ a tantalum disk of 15 cm diameter as the source mass. The test masses are also thin tantalum disks, which are located within 100 µm from the surfaces of the source mass. The differential acceleration between the two test masses is measured with a sensitive superconducting circuit.

A laboratory experiment, which will probe the extra dimensions down to several micrometers, is under preparation with NSF and NASA support. Eventually, we would like to fly an experiment based on the same principle in space. In zero-g, a much more sensitive superconducting accelerometer can be constructed using nearly free test masses. This will extend the search for extra dimensions to less than or equal to
1 µm. A side benefit of the space experiment will be a possible detection of the elusive particle, called the axion.

For more details, please visit the Gravitation Laboratory Website at www.physics.umd.edu/GRE