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How do tractor beams work?
Category: Physics
Published: June 11, 2013
By: Christopher S. Baird, author of The Top 50 Science Questions with Surprising Answers and physics professor at West Texas A&M University
Up until recently, tractor beams (beams of light that tow objects) existed only in the world of science fiction. While large-scale tractor beams that can tow space ships are still machines of the future, microscopic tractor beams are here today. The idea at first seams to defy physics. Shoot light at an object and the light's momentum should push the object away from the light source according to the law of conservation of momentum; not pull it closer like a tractor dragging its cargo. But what if you managed to create a situation where upon striking the object, the light gains forward momentum instead of losing it. In that case, the law of conservation of momentum tells us that the object will lose momentum in the forward direction, even to the point of going backwards, towards the light source. So there is a way to make a tractor beam without violating any physics, if you can get the light to gain forward momentum upon interacting with the object. You need to the light to slingshot past the object instead of bouncing off of it. The most effective method to accomplish this feat at this point in time is to use solenoidal light beams. A solenoidal shape is like a wire wrapped in circles into a helix that forms a hollow cylinder. But in this case, there are no wires. The light itself, traveling through the air, is formed into this hollow, spiraling shape. San-Hyuk Lee of UC Berkeley and his collaborators have experimentally verified that the solenoidal light shape indeed draws objects – in their case, microscopic glass spheres – towards the light source. They have therefore successfully created a real tractor beam. You can think of the spiral-shaped light beam as a screw that twists the object up the beam. Only microscopic objects have been tractor-beamed so far because light carries so little momentum. In order to tow large objects, you would need very strong light beams. So strong, in fact, that damage of the object may become a problem. Research is ongoing.