The tubes of the Internet saw some extra traffic recently when NASA announced funding for a team of researchers to study tractor beams. That’s right, tractor beams, as in Star Trek and Star Wars (and countless other science fiction settings). The goal here isn’t to grab spacecraft (at least, not at first); instead, NASA wants to use the technology to collect particle samples for analysis on rovers and spacecraft.
This may sound like science fiction, but a few days after this announcement, a pair of papers appeared in Physical Review Letters, discussing the theory behind two approaches the NASA team plans to study (the papers’ authors appear to be unaffiliated with that team).
In general, shining light on something will cause it to move away from you as the photons are either absorbed or scattered—the conservation of momentum requires that the lost momentum is compensated by the object gaining some momentum in the photon’s original direction of movement. This is great if you want to push objects away, but how do you pull them toward you? By manipulating the light beams in different ways, you can force photons to scatter off the object in a way that causes motion in the opposite direction. The two papers present different methods of doing just this.
The first paper, from a team at the University of Central Florida, approaches this problem by using a light beam that consists of multiple plane-wave components with different directions. In practical terms, this means you focus a bunch of laser beams on the objects, ensuring that they hit from angles that differ from the direction you want the object to move. The beams can then be adjusted based on the size and shape of the specific object to optimize the resulting force.
They demonstrated this technique numerically, modeling it using a cluster of 160 tiny, randomly distributed spheres and 24 beams of light, all with the same angle away from the axis of motion (84°). Initially, the “tractor beam” actually applied a force that pushed the object away but, by adjusting the polarization and phase of the various light beams, they generated a force that pulled the spheres backwards, as desired.
The authors of this paper emphasize that, while their work focused on moving objects towards the source of the lasers (which would be important for sample collection, like the NASA study wants), you could use the same principle to move objects in any direction, providing a full range of translation and rotation movements—a true tractor beam.
The second paper, this time from people at the Technical University of Denmark, National University of Singapore, and Singapore Agency for Science, Technology and Research, tried a different approach using a single Bessel beam. This special type of light beam, named after the Bessel function that describes its amplitude distribution, doesn’t diffract and spread out as it propagates.
Focusing just any Bessel beam on an object won’t pull it backwards; it needs to be a nonparaxial Bessel beam. This means that the partial plane-wave components of the beam form large angles with the beam’s axis (which is also the axis of desired movement). Effectively, this is just like the approach of the first team, but all the different sources are contained in a single beam of light. As before, these large angles cause photons to scatter away from the source and the resulting momentum loss moves the object backwards. Unfortunately, creating a nonparaxial Bessel beam is easier said than done, and hasn’t yet been demonstrated experimentally.
The second group also talks about adjusting the properties of their tractor beam to attract certain types of particles while repelling others, which could be important when collecting samples for analysis. The first approach might be closer to an actual demonstration, however, since the underlying technology involves lasers and a sophisticated control system. Either way, these are some of the first, solid steps towards capturing Corellian corvettes and smuggling freighters.