Atoms can also be trapped by laser light. Light can exert forces on atoms and other neutral particles because it carries momentum. If an atom is bombarded with a beam of light of a particular frequency, it will continuously absorb and reemit photons, the quanta of light. As the atom absorbs photons, it will receive a barrage of momentum kicks in the direction that the light beam propagates. The kicks add up to produce a "scattering" force, which is proportional to the momentum of each photon and the number of photons that the atom scatters per second. Of course, for every photon the atom absorbs, it must emit one. But because the photons are released with no preferred direction, the changes in momentum caused by the emission average to zero. Absorption and emission have the net effect of pushing the atom in the direction that the light travels.
The magnitude of this scattering force is quite low. If an atom absorbs a single photon, its change in velocity is tiny compared with the average velocity of atoms in a gas at room temperature. (The change is on the order of one centimeter per second, the crawling speed of an ant, whereas an atom at room temperature moves at the speed of a supersonic jet.)
This scattering force was first detected in 1933 , when Otto R. Frisch used it to deflect a beam of sodium atoms. He prepared the atoms by vaporizing sodium in a container. To form the beam, he allowed the atoms to pass through a hole in the container and a series of slits. Once established, the beam was bombarded with light from a sodium lamp. Although, on average, each sodium atom absorbed only a single photon, Frisch was able to detect a slight deflection of the beam.
The scattering force that Frisch generated was far too weak to capture atoms. Decades later workers realized that the photon-scattering rate could be increased to more than 10 million photons per second, corresponding to a force 100,000 times greater than the pull of gravity by the earth. The first dramatic demonstration of the scattering force on atoms was made by two separate groups led by Phillips and John L. Hall at the National Bureau of Standards. In 1985 they stopped a beam of atoms and reduced the temperature of the atoms from roughly 3 00 kelvins (room temperature) to 0. 1 kelvin.
The power of the scattering force attainable with lasers gave researchers hope that they could not only stop atoms but trap them as well. But attempts to configure several laser beams so that they could collect and concentrate atoms in some region of space seemed doomed to failure. According to a principle known as the Optical Earnshaw Theorem, it is impossible to fashion a light trap out of any configuration of light beams if the scattering force is proportional to the light intensity. The problem is that the beams cannot be arranged to generate only inward directed forces. Any light that enters a trapping region must eventually escape and must therefore carry outward directed forces as well. Even if Luke Skywalker were a physicist, the (scattering) force would not always be with him.
Fortunately, an atomic trap can be based on another kind of force that light can exert on atoms. To understand this force, it is instructive to consider how small particles can be attracted to a positively charged object, such as a glass rod rubbed with cat's fur. The rod produces an electric field that polarizes the particle. Consequently, the average position of positive charges in the particle will be slightly farther away from the rod than the average position of the negative charges. This asymmetric distribution of charge is said to have a dipole moment. The attractive dipole force exerted by the electric field on the negative charges of the particle is stronger than the repulsive force on the positive charges. As a result, the particle is pulled toward the regions where the electric field is strongest. Notice that this force is analogous to the magnetic dipole force first used to trap neutrons and atoms. If the charge on the rod were negative, the electric field would induce a dipole moment of reversed polarity, and the particle would still be attracted to regions of high electric field.