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 new, it's exciting, and who knows what it might be good for?" says Juan José Saénz, a theorist at the Autonomous University of Madrid, who was not involved any of the papers but whose own group was also working on the idea. "It's fascinating that four or five groups are at the same time coming out with the same idea."
Bizarre beam. A Bessel beam is made by overlapping ordinary light waves that travel at an angle (left). The entire beam propagates forward with an intensity profile (right) that remains constant along the length of the beam, and new calculations suggest it can act as a small-scale tractor beam.Credit: Egmason/Wikimedia Commons
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.
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 can also 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. 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 (see figure). 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.
From Science at aaas.org
Ah! Yes, the beginning of what the sci-fi writers dreamed up years ago.