Short of destroying a whole world with planet-breaking weapons, the most action-filled moments in science fiction come when opposing spacecraft clash. As phasers fire and missiles launch, ships frantically maneuver, attack, or spectacularly explode. But sometimes the aim is to capture a spaceship intact, or if she has superior speed, to grab and hold her while battering down her defenses. That is the space version of a fighting technique from the days of wooden sailing ships, which is to pull an enemy ship close and hold her with grappling hooks and ropes, then board her, or pound her into wreckage at point-blank range.
In space, this tactic needs a futuristic version of hooks and ropes – a tractor beam. Like the force of gravity in nature, a tractor beam pulls things toward its source; but in science fiction, it is stronger than ordinary gravity, and unlike gravity, it can be aimed. Tractor beams first showed up in the Buck Rogers stories and the “space operas” of Edward E. “Doc” Smith from the 1920s and 1930s. They still appear in Star Trek, the film District 9 (2009) and other contemporary science fiction.
Gravity may feel all too strong when you are jogging uphill, but technically it is the weakest of the universe’s four fundamental forces (in descending strength, the others are the strong nuclear, electromagnetic, and weak nuclear forces). Gravity’s pull takes on real power only when it arises from a massive object like a planet or a star. The mass of even the biggest spacecraft could not generate enough force to yank in another spacecraft. Going beyond this to create a powerful, directed artificial gravity needs a technology we do not yet have a clue about.
But there is another way to manipulate objects without touching them, using something you would not think has physical impact: light. Light’s intangibility might seem to imply that it cannot affect solid objects. By the weird rules of quantum mechanics, however, light is both an electromagnetic wave and a flock of particles – photons – that carry energy and momentum. As Isaac Newton worked out long ago, a change in momentum produces a force; and so, no differently from baseballs or billiard balls, when photons hit something and thus change their momentum, they exert a small force called radiation pressure.
On Earth, the radiation pressure from sunlight is 50 million times weaker than atmospheric pressure, but a laser can intensify the effect to a useful level. In the optical tweezers method, developed at Bell Laboratories in 1986, a focused laser beam suspends small biological objects like bacteria and DNA molecules in mid-air and can be used to manipulate them. On a larger scale, the Sun’s radiation pressure can drive a spacecraft, if the ship is outfitted with a big sail that can capture the push from many photons. In 2010, the IKAROS spacecraft launched by JAXA (Japan Aerospace Exploration Agency) deployed a sail with an area of 200 square meters and headed off toward the planet Venus, accelerated by sunlight.
These applications may not be all that surprising, because it is easy to picture tiny bullets of light pushing an object. But could light possibly attract rather than repel a body, and would that be of any earthly use? To NASA, the answer to both questions is “yes.” The agency envisions “unearthly” uses such as cleaning up the orbital debris ringing our planet and pulling an incoming space rock off course before it hits the Earth; but like those imaginary space battles, these efforts would require strong forces and will happen only in the future if at all.
However, NASA also wants to pull in smaller things like tiny extraterrestrial particles that can carry valuable information. Obtaining such samples is a rapidly growing part of space exploration. NASA’s Stardust space probe, launched in 1999, gathered microscopic dust particles from a comet called Wild 2, and returned them to Earth for analysis in 2006. In 2003, JAXA launched its Hayabusa space craft, which retrieved bits of a small asteroid and brought them back to Earth in 2010. And just this last November, NASA launched its Mars Science Lab with the Curiosity rover, which will scoop up samples of Martian soil and analyze them for signs of life processes.
Using light to attract and manipulate small samples in space or from planets and other bodies would complement and extend these kinds of missions, each of which takes considerable effort and expense. Light-based sample harvesting could maybe be done at a lower cost and could also allow continual monitoring, for instance, of a planetary atmosphere. According to recent research and some older results, there is reason to believe this is not just wishful thinking.
In two separate theoretical papers published last month, a group at the University of Central Florida, and another from the Technical University of Denmark and the National University of Singapore, showed how to make an object move backward along a light beam. The basic idea is deceptively simple; because the incoming photons carry momentum, if you can get some of them to bounce or scatter off the object in the same direction they are traveling – that is, in the area where the object would ordinarily cast a shadow – then to balance out the momentum going forward, the object has to move backward toward the light source. This would require a laser beam with a carefully designed pattern of varying brightness across its diameter, which is not easy to create, and so these ideas have yet to be experimentally tested. But NASA has enough faith in the approach that it started to fund research in optical tractor beams.
One other potential optical method goes back almost 50 years. In 1964, Victor Veselago, of Moscow’s Lebedev Physics Institute, theorized about an optical medium with a negative refractive index. In ordinary media like water or glass, the refractive index is a positive number that determines the speed of light in the medium and also how much a light ray bends or refracts when it enters another medium. Refraction is the reason that a stick partly inserted in water seems to break and bend upward at the water’s surface. But in a medium with a negative refractive index, the stick would display “backward” refraction and seem to bend down.
Negative refractive indices as Veselago envisioned them have been realized in carefully designed, artificially constructed media called metamaterials. In an especially intriguing breakthrough, these ideas also led to the creation of the world’s first true invisibility cloak, made in 2006 by David Smith and his group at Duke University.
Veselago predicted another “backward” result that amounts to a tractor effect, which is that a mirror embedded in a negative index material would be pulled rather than pushed by a light source. In 2009, Henri Lezec, of the U.S. National Institute of Standards and Technology, described how he and Kenneth Chau tested this theory. They fabricated a metamaterial with a negative refractive index in the form of a tiny nanoscale movable lever, and found that it was indeed pulled toward light from a laser. This would seem to be the first observation of light acting as a tractor beam according to Veselago’s theory, but the results do not fit the predicted behavior in detail. The experiment or the theory may be anomalous, and the experimental result remains under study.
If metamaterials display a true tractor effect that would be fascinating, but it would not be exactly what NASA needs to gather up space debris or interesting space dust, nor would it be useful in future space battles. To reel in a particular enemy ship, some unlucky crewmember would first have to don a spacesuit, venture out, and paint the target with negative refraction paint. If that is the case, we might just as well go back to grappling hooks, ropes, and spacesuit-wearing boarding parties armed with cutlasses.
Sidney Perkowitz is the Candler Professor of Physics Emeritus at Emory University. He can be reached for comments or questions through his website, http://www.sidneyperkowitz.net/.
Top image: Project Rho
Bottom image: S. Sukhov and A. Dogariu Phys. Rev. Lett. 107, 203602 (2011) Published November 10, 2011