Exploring gravity, tides, and excited atoms.

It's hard to imagine how the force of gravity - normally associated with baseballs, planets, and galaxies- could possibly have a perceptible effect on the motion of electrons in an atom, where quantum mechanics and electrical forces reign. But given a sufficiently strong gravitational field and an excited hydrogen atom in which the electron spends most of its time at great distances from the atomic nucleus, such an interaction becomes possible.

A physicist has now established that, in principle, the gravitational field of a compact, dense object such as a neutron star is strong enough to influence loosely bound electrons in hydrogen atoms close to it. Fabrizio Pinto of Boise (Idaho) State University reports his calculations in the June 21 PHYSICAL REVIEW LETTERS.

The idea of studying how a gravita- tional field may influence the motion of

electrons in an atom and, hence, subtly change the characteristic wavelengths of light the atom may absorb or emit goes back more than a decade to the work of Leonard E. Parker of the University of Wisconsin-Milwaukee. He was interested in the possibility of using atomic spectra to measure strong gravitational fields.

Parker's calculations showed that for electrons tightly bound to atoms, only exotic black holes smaller than dust specks had sufficiently strong fields to influence electron energy. However, the situation looked a little more promising for excited atoms with loosely bound electrons.

Pinto carried this research further. His results reveal that electrons in freely falling, excited atoms close to the surface of a neutron star would experience gravitationally induced changes in energy large enough for a radiotelescope to detect.

Unfortunately. these excited atoms are also extremely fragile. Detection of a gravitational effect appears possible only at low temperatures and in the absence of significant magnetic fields. This rules out an environment such as the surface of a neutron star, which typically has a strong magnetic field and a high surface temperature. Pinto is now working to identify alternative situations where the gravitational effect may actually be observable.