A Macroscopic Tractor Beam with Acoustic Waves

Among the many examples of how science fiction and fantasy have inspired real science, an interesting one is the “tractor beam,” first conceived by the chemical engineer and writer Edward E. Smith in the 1930s in his novel Spacehounds of IPC. Smith’s hypothetical device was a beam capable of pulling any macroscopic object from a distance. In the case of a light beam, however, this is counterintuitive, because we have known since Maxwell developed his electromagnetic theory that, owing to linear momentum, light exerts radiation pressure on matter in the direction of propagation. But at the microscopic level, an optical tractor beam has indeed been demonstrated: researchers have developed special optical beams that can pull nanoparticles opposite to the propagation direction [1]. Now, a step towards a device that works on macroscopic objects has been demonstrated by a group of researchers led by Michael MacDonald of the University of Dundee, Scotland, and Gabriel Spalding of Illinois Wesleyan University in Bloomington. They were able to scale the tractor beam up to handle centimeter-size objects, about 6 orders of magnitude larger than those manipulated by previous schemes, as reported in Physical Review Letters [2].