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Tractor beams: A scientific fact
It is often noted that today’s science fiction is tomorrow’s science fact. For example, inspired researchers at NASA long been looking into warp drives, the fantastic engines that propelled ships in Star Trek at many times the speed of light. Similarly inspired, two physicists at the New York University have demonstrated a functional tractor beam, capable of moving mass at a distance without physical contact.
However, its capabilities are far from the marvelous contraptions of Star Wars, capable of pulling in an entire spaceship. The physicists’ beam moved 1.5-micron silica balls over the distance of some tens of microns, a far cry from tugging in tons of metal from kilometers away.
Using energy to move mass is not a novel idea. In fact, scientists have been using light to move microscopic particles for many years now. In many experiments conducted at extremely low temperatures, optical tweezers are used to hold atoms in place. Because light is an electromagnetic phenomenon, a beam of it can be used to generate an electric field to push objects. Optical tweezers take advantage of this fact by focusing a laser beam at microscopic scales, creating a strong electric field gradient. Atoms and molecules, which contain electric dipoles, are attracted or repulsed.
Optical tweezers, though, are not true tractor beams, as they can only push, but not pull. When optical tweezers attempt to pull, the momentum carried by the photons push the particles away. To overcome this limitation, the two physicists created a Bessel beam. Unlike a regular beam of light, Bessel beams do not diffract, or spread out, over distance. This allows for the intensity of the light to be manipulated in a much more precise way.
Using a small beam and a larger beam that interfered with the small beam, the researchers were able to create alternating areas of light and dark along the beam’s axis. The variation in intensity of the beams allowed the momentum of photons to be focused behind, not in front of, the particle. By changing the phase of the two beams of light, the particle trapped in the beam could be moved both forwards and backwards, creating a true tractor beam.
Not only is it a true tractor beam, the physicists’ creation can move two things in opposite directions at the same time. Bessel beams are self healing; if the beam is diffracted by an object, the beam reconstructs itself downstream as if nothing was in its way. Thus, the beam was still intact behind the first object, so more particles could be trapped. This was demonstrated as the physicists simultaneously pushed one ball and pulled another.
Scaling this tractor beam into macroscopic sizes will prove a considerable challenge. The physicists estimated the energy used to push and pull the 1.5-micron balls to be at around 17 mW. Scaling this up a few powers of magnitude for macroscopic sizes results in something in the range of kilowatts, making the laser beams so powerful that they vaporize whatever they target.
However, this tractor beam is still very promising for microscopic uses. The ability to push and pull will allow scientists studying cells, DNA and molecules to manipulate them with much higher precision. This tractor beam will no doubt become as useful as optical tweezers are today.