“Beam me up, Scotty. There’s no intelligent life here,” a line long attributed to the 1960s Star Trek series might soon become a reality as a “tractor beam” may harness the energy of light to move physical objects in space. It was already known that light indeed pushes on objects, even while it is a weak nudge. In the field of optical manipulation, for example, optical tweezers use the tractor force of light to push objects of microscopic size, ranging from atoms to bacteria. The ability to pull with light would increase the precision and scope of such manipulation. And for future spaceflight, scientists proffer the idea of space ships winging through space with sails that capture light as if it were a cosmic wind.
In a paper published in April 2012, scientists propose that rather than towing future spaceships, tractor beams might be better used in biology and medicine. If you want to pull something towards you, you just reduce the pressure," says Mordechai Segev, a physicist at Technion–Israel Institute of Technology, who described the idea in an Optics Express paper. "You make a little bit of vacuum," he adds. The problem is that in sensitive medical applications, such as lung surgery, it is important not to change the pressure or introduce any new gases. "Here, the light will be the suction device," he says, "so the pressure would not change at all. It is just the light."
In the past, the thinking on tractor beams had focused on creating new gravitational fields to drag objects, heating air to create pressure differences, or inducing electric and magnetic charges in objects so that they move against the direction of an incoming laser beam.
In 1967, Russian physicist Victor Veselago first theorized the existence of something called negative radiation pressure in a paper about materials with an unusual property called negative refraction index. An index of refraction is a number that describes how light is bent when it goes into a lens or other medium. When Veselago published his paper, no one knew if this number could be negative in any material. Over the last twenty years, researchers proved that negative refraction can occur in specially made substances called metamaterials, which have led to limited invisibility cloaks and distortion-free "super" lenses. It is this phenomenon that the latest proposal seeks to exploit.
The mechanism of negative radiation pressure depends on two aspects of a light wave: its group and phase velocities. A light wave consists of groups of smaller waves, while the group velocity is the speed and direction of the overall wave group. As for phase velocity, it is the speed and direction of a point on one of the smaller constituent waves. The electromagnetic energy of the light wave goes in the direction of the group velocity. The wave's effect on a particle goes in the direction of the phase velocity. So, if these two velocities point in different directions, then negative radiation pressure can result.
So far, the use of metamaterials to move particles by using negative radiation pressure has been hampered since most of these materials are solid, while introducing a gap for particles would eliminate the negative radiation pressure. Additionally, all current metamaterials contain metals, which absorb electromagnetic energy, thereby annulling the pulling effect on particles.