It’s not nearly powerful enough to pull in the Millennium Falcon spaceship from “Star Wars,” but four groups of physicists have independently come up with the same basic idea for a real-life tractor beam. The laser beam the groups have dreamed up could only drag an object the size of a grain of salt or smaller, but experts say it could provide a new tool for manipulating tiny objects such as cells.
“It’s new, it’s exciting, and who knows what it might be good for?” said Juan Jose Saenz, a theorist at the Autonomous University of Madrid, who was not involved in any of the papers but whose own group also was working on the idea. “It’s fascinating that four or five groups are at the same time coming out with the same idea.”
It’s not news that light can move objects. Since the 1980s, scientists have pulled small objects around using so-called optical tweezers, which can grab hold of tiny objects such as cells and pull them around. And in the past half-decade, physicists have used the tiny force of light to set nanometer-scale beams and cantilevers aquiver — or to still their motion.
Physicists have identified a few basic ways that light can flex its muscle. Like a blast of water from a hose, a light beam exerts a tiny forward push when it hits something. Light also can pull an object using something called the gradient force.
This force comes about because the electric and magnetic fields in the light polarize the material in the object, and the polarized object can then reduce its energy by moving to where the light is most intense.
The tractor beam would work in a new way, making the manipulated object radiate more light forward along the beam than backward toward its source. The radiated light then acts like a reverse thruster, overcoming the forward push of the beam and driving the object back toward its source.
The details of the various groups’ proposals differ. And so far, none of the teams actually has created a tractor beam. So, can it really work? Probably, says Philip Marston of Washington State University at Pullman, who described a similar pull using a Bessel beam of sound waves.
“I think somebody will demonstrate a pulling force,” he said.
As for pulling a starship, don’t hold your breath. The dragged object must be smaller than the length scale over which the light waves remain orderly and coherent. So to feel the tug, the Millennium Falcon would have to be shrunk to less than a millimeter in length. And really, how much of Han Solo’s ego could you fit into such a tiny ship?
