spectral line. This uncertainty causes the features to blur and the lines, again, to broaden.

The use of this transverse atomic beam offers some help; the resulting line broadenings at least are generally less severe than those that arise from observing a simple hot gas, with its full-blown Doppler effect. But to study the lines in their full detail, one must contrive to get rid of the motion of the atoms. That means cooling them close to absolute zero. One cannot do this in a chamber, however; the atoms would condense on the cold walls and then would have different properties than those a physicist seeks to observe. Hence, one must chill the atoms as they fly through space. Lasers can do this, and the principal approaches rely on turning the Doppler effect from an adversary into an advantage.

LASER COOLING

The words ''laser cooling" sound like a contradiction in terms. We ordinarily think of the laser as a source of intense heat. Those of the Pentagon's "Star Wars" program, for instance, are to have sufficient energy to destroy a missile in flight. But when directed against atoms rather than missiles, a laser indeed can act to slow them down, which is the same as cooling them.

To do this, one begins by appreciating that the frequency of a spectral line represents a condition wherein atoms absorb photons of light particularly readily. Such photons, having that frequency, come from a laser tuned to the appropriate wavelength. Then, after absorbing such a photon, the atom rapidly reemits it, which puts the atom in a condition to absorb another one.

The absorbed photons all come from one direction, that of the laser beam. The reemitted photons, however, fly off in every direction. During both absorption and reemission, the atom feels a force sufficient to change its velocity; for sodium atoms this change amounts to 3 centimeters per second. The absorbed photons produce a cumulative effect, combining to slow the atoms. The reemitted photons, by contrast, lead to no more than small changes in each atom's path through space, because they produce no combined or collective effect. Here, then, is a powerful technique for slowing and cooling a beam of atoms.

Right at the outset, though, there are problems. The usual procedure tunes the laser to a frequency just below that of a spectral line and points the laser directly into the oncoming beam. Due to the Doppler shift, atoms in flight see a laser wavelength close to that of their spectral line and indeed absorb the photons quite readily. But as the atoms slow



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