Imagine some kind of futuristic ray gun that could be aimed from a distance to immobilize just one person in a moving crowd. What if the marksman could then move the immobilized person to another location simply by aiming the ray gun somewhere else?

Researchers at AT&T Bell Laboratories have developed just such a device, although it works on the microscopic scale and traps individual cells, allowing biologists, for example, to separate one bacterium from a swarm under the microscope.

The device -- which uses a tiny, low-power, infrared laser beam -- can even be focused on a single organelle within a cell. A researcher could, for example, trap the cell's nucleus within the laser beam and move it to a new site within the cell to see if it makes any difference.

The device, called an optical trap, makes use of a phenomenon known as radiation pressure. The pressure of radiation from the sun, for example, pushes the gas and dust of a comet's head into a tail that always streams away from the sun. The phenomenon is used differently in an optical trap. A laser beam is focused so that the intensity is strongest at the beam's center and weakest around the periphery. The result is a complex set of forces that impinge on very small objects from all directions.

Once trapped, the object can be moved about by pointing the beam elsewhere or changing its focal point. The procedure appears not to damage cells, although the infrared rays do warm them up slightly.

The ability to manipulate cells and organelles is expected to give cell biologists an important new research tool. One interesting finding has already emerged.

The Bell Labs group trapped an organelle within a Paramecium, a one-celled organism common to pond floors, while the animal glided along a glass surface. As the animal moved, the immobilized organelle was, in effect, dragged to the creature's tail end. Then it suddenly snapped back to a position nearer the Paramecium's middle.

It looks as if the organelle was held in position by some kind of unseen but elastic suspension system. When the trap stretched the suspension system too far, the system yanked the organelle out of the trap's grip.

The infrared optical trap was developed by A. Ashkin, J.M. Dziedzic and T. Yamane of Bell Labs in Murray Hill and Holmdel, N.J., who published their report in the Dec. 31 issue of Nature magazine.