The Most Spherical Thing

Many atoms behave like tiny dipole magnets; they possess in effect a north and south pole separated by some small distance. In contrast no permanent electric dipole moment (EDM) has ever been measured for atoms. If such a moment could be found this would imply that although the net charge of the atom were zero, the charge would be slightly offset. That is, one could picture the atom as having a bit of positive and negative charge displaced by a small gap.

A new experiment at the University of Washington extends the search for a nonzero EDM in mercury atoms: the positive charge and negative charge in the atom can not be further apart than 2 x 10^-30 meters. How can such an exquisite measurement be made? The Washington group (Michael Romalis, 206-543-9586, romalis@phys.washington.edu) basically looks for a slight change in the precession of the Hg atoms which are subjected to a weak magnetic field and a strong electric field (see figure at Physics News Graphics).

They did not find such a precession change at the level of 0.4 nano-Hz. This corresponds to an energy-shift of less than 2.6 x 10^-43 Joule, the smallest that has ever been measured. Another way of describing the Washington result is to say that if the mercury atom were the size of the Earth, then their experimental limit on the atom's non-sphericity would correspond to a 0.001-angstrom bump at the North Pole. The results are so precise that they can be used to place constraints on various models of particle physics that predict new sources of CP violation to explain why the universe today contains only matter, despite the fact that equal amounts of matter and anti-matter were produced in the Big Bang. (Romalis et al., Physical Review Letters, 19 March; text available at Physics News Select.)