Doctor Who visited Star Trek, and the crossover showed up in our reality as a macroscopic acoustic tractor beam. The newly-developed acoustic tractor beam can pull objects six orders of magnitude — that's a million times! — larger than any previous experiments.

Sonic Tractor Beam

The concept of manipulating matter through the careful application of fields first showed up in science fiction in Edward E. Smith's Spacehounds of IPC, a novel by the author/chemical engineer serialized in Amazing Science starting in 1931. Now, tractor beams have propagated to just about every corner of space-based science fiction. While some previous success has been achieved for tiny nanoparticles, a collaboration between researchers in Scotland and Illinois has managed to use tractor beams to manipulate relatively enormous centimeter-sized objects.

Getting the limitations out of the way, this tractor beam uses acoustic waves, and as every nit-picking scifi-fan knows, sound does not propagate in space. Even more inconveniently, shape matters: the tractor beam only works on objects that have a not-flat backside. But, for objects in the atmosphere that have junk in trunk, this beam can exert enough pressure to move it at will! ...as long as it only takes a few millinewtons of force to haul it around. It really is as though the Doctor watched a bit too much Star Trek and tinkered with his sonic screwdriver just enough to add a handy new mini-tractor beam to his multitool.

Sonic Tractor Beam

a. Send waves dead-on and they bounce right back at you b. Send waves at an angle, and they produce forward scattering. c. Get enough forward scattering, and you have a tractor beam! Image credit: APS/Alan Stonebraker

Disclaimers done, it's time to get to the science! This contraption works by using a nonpariaxial beam of ultrasonic waves. Ultrasonic just means above our hearing, so we don't need to start adding whomping sound effects to tractor beams in movies. The nonparaxial bit is trickier to explain.

Imagine a plane wave heading straight in, a nice set of parallel sheets, then scattering off a surface. Whatever new waves bounce off are going to all be equal or smaller than the original wave — the energy disperses. That'd be scenario A in the figure above. The beam would just push things away — interesting when contemplating repulser technology, but utterly useless for tractor beams.

Next consider a Bessel beam: light where the wave propagates as a cone-shape, not a flat sheet. Each individual wave vector has large angles with respect to the direction of motion, so if the wave hits something and scatters, some of those scattered waves actually constructively interfere to produce strong forward scattering. That's scenario B in the figure: using nonparaxial beams to manipulate the object.

The nonparaxial beam restriction is where this once again diverges from classic Trek-style tractor beams. The acoustic tractor beam only works by manipulating an object with a beam approaching at an angle. Sure, this could be a cone-shaped source wave from a single fixed-point location, to me it evokes yet another franchise to bring into this real-life crossover: Ghostbusters.

Sonic Tractor Beam

Think about it: the Ghostbusters can't go yanking ghosts around with the beam from just one proton pack. Instead, they need to spread out beam-source locations, flip 'em on, catch the object (ghost) in the intersection, and drag it down to anywhere they please (typically, a ghost trap). This tractor beam setup requires low-angle incidence on the object, so instead of a complicated cone-shaped Bessel wave, trap the object in the intersection of multiple angled linear waves. Apparently physics refuses to pick just one fandom and is cramming as many as possible into one new device.

Back to the nitty-gritty details of this thing:

Sonic Tractor Beam

Depending on the target object's shape, size, and interaction with the wave field, the scattering is going to be different. The end result can be anything from increasing negative radiation pressure (same as for plane waves) to decreasing positive radiation pressure: pushing things away or pulling them in.

So, what works best to haul in using an acoustic tractor beam? A hollow isosceles triangular prism. The researchers suspended the prism above the array, bombarded it with 550 kHz acoustic waves, then measured the pressure field.

a. Experimental setup. B & C. Normalized pressure maps. D & E. Measured pressure maps. Image credit: APS/Alan Stonebraker

Pointed at a hollow, light object with a nice, pointy backside, the tractor beam worked perfectly, producing a pressure differential in front of the object, pulling it in just like a good scif-fi tractor beam should. Had the triangle been flipped around to point into the beam and with a flat backside, the scattering would have been far less beneficial and the tractor beam rendered inoperable. Borg Cubes are safe between hiding in space, having an uncooperatively flat backside, and occupying substantially more than a few tens of cubic centimeters volume.

Why does it only work for sound, not light? The linear momentum of light is a it more complicated, with reflection, refraction, and absorption all playing a role, while sounds waves are pretty much just reflected or absorbed.

For now, the utility of this tractor beam is mostly limited to medical applications (it's like a tiny pair of tweezers magnetized for flesh!), but physics is constantly coming up with new tricks. One day the Borg will need to be wary of our technological prowess!

Read the full article in Physical Review Letters #112 [paywalled or open access], or the accompanying analysis of the paper [open access]. You may also like my analysis of (some of) the physics in Doctor Who in Physics Today. Grumbling that this wasn't about Star Wars tractor beams? Get your real Star Wars science here.