Tractor beam a new tool for manipulating tiny objects

It's not nearly powerful enough to pull in the Millennium Falcon, 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 drag an object the size of only a grain of salt or smaller, but experts say it could provide a new tool for manipulating tiny objects such as cells.

It's not news that light can move an object. 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.

The tractor beam would work in a new way. In this case, the light would pull an object toward the source of the beam even though the beam has the same intensity all along its length. The trick is to use a special type of laser beam. In an ordinary beam, each photon moves in the direction of the beam, so when a photon bounces directly back from an object, it imparts the largest possible push. However, physicists can generate a beam by overlapping light waves that make an angle relative to the desired direction. The overlapping waves produce a forward-moving beam known as a Bessel beam whose intensity remains constant along its length. But each photon is now moving at an angle relative to the beam. So when one bounces off an object, it exerts a smaller forward push.

Nevertheless, the beam is still pushing, and to overcome that push, physicists need to rely on another bit of physics. Again, the light will polarize the material in the object electrically and magnetically. The polarized object will then radiate and redirect the light. By adjusting the material properties of the object and the polarizations and synchronization of the individual light waves in the beam, physicists can make the object radiate more light forward along the beam than backward toward its source. The radiated light then acts like a reverse thruster, overcoming the already reduced forward push of the beam and driving the object back toward its source.

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?