A Real Tractor Beam

In this weeks’ news, Weizmann Institute scientists and researchers in Australia have invented a sort of tractor beam. In essence, a tractor beam is a wave that propagates outwards but pulls objects toward its point of origin, rather than pushing them out. Like the science fiction versions, such “beams” might be designed in the future to manipulate solid objects in their wake. The waves the researchers created, rather than moving through the interstellar ether, propagate on the surface of water. Instead of latching on to Klingon spaceships, the scientists think the wave-induced surface flows could be used to clean up floating pollution, though some imagine that with a little tinkering, it could even be scaled up to latch onto a sea-going vessel and pull it in.

Interestingly enough, the basic idea is based on the 19th century physics of George Stokes.

Here is an image from their arXive paper Tractor beam on the water surface, showing  wave fields and surface flows produced by a cylindrical wave maker:

White arrows show the direction of wave propagation; red arrows show the direction of the surface flow and the ball motion

a: Lower acceleration shows nearly planar waves propagating away from the wave maker (bottom). b: These waves produce a strong outward jet in the direction of propagation and the return flow toward the edges of the cylinder.

c: As the acceleration is increased (by 30% over a), the modualtion instability destroys the wave planarity generating the 3-D wave field. d: This 3-D wave field generates a strong inward jet toward the center of the cylinder and the outward return flows away from the cylinder edges.

e: Consecutive positions of a pinpong ball on the water's surface perturbed by a linear wave (as in a). f: Motion of the pingpong ball in the inward jet (c,d); frame interval is 3.3 sec.