Gravity is Normal Down to the 100-nm Level

Gravity at the level of planets is well studied, and was known accurately even in Newton’s day. This is owing to the fact that the other physical forces, such as the strong and weak nuclear forces, don’t operate over such great distances, and electromagnetic forces between immense far-apart, electrically-neutral objects like planets are dilute. Gravity at shorter lengths, by contrast, is harder to measure, partly because all the other forces are in full play.

Furthermore, theories of particle interactions hypothesizing the existence of additional spatial dimensions suggest that the strength of gravity will depart from Newton’s famous inverse-square formulation. To test these propositions, various tabletop setups have been devised to probe gravity below the micron level. One previous experiment, conducted by Eric Adelberger’s group at the University of Washington, ruled out extra gravity components having a strength comparable to conventional gravity down to a size scale of about 100 microns (http://www.aip.org/pnu/2000/split/pnu483-1.htm).

A new experiment, carried out by a Indiana/Purdue/Lucent/Florida/Wabash collaboration examines a shorter distance scale---100 nm---but is able to rule out only corrections to gravity that are, in fact, a trillion times larger than gravity itself. Nevertheless, such measurements help to constrain the general pursuit of unified theories of particle physics, including explanations of gravity. The sort of “Yukawa” corrections being sought are analogous to the force proposed by Hideki Yukawa in the 1930s to explain how mesons transmit the nuclear force between nucleons and would come about because of transmission of the presumed force particles associated with the hypothetical extra dimensions.

The present measurements improve the exclusion of such corrections by a factor of ten. According to Ricardo Decca of Indiana University-Purdue University (rdecca@iupui.edu, 317-278-7123), the sensitivity of the apparatus should grow by a factor of a hundred over the next year. The size of the sample is smaller here than in many other tabletop gravity experiments.

The flea-sized torsional apparatus must operate with such concern for forces acting over small distances that one of the chief goals here is reducing the background produced by the Casimir force---a quantum effect in which two very close objects are drawn together because of the way they exclude vacuum fluctuations (that is, the spontaneous creation of pairs of virtual particles) from occurring in a slender volume of space---between a flat plane and sphere lying only 200 nm apart. (Decca et al., Physical Review Letters, 24 June 2005F