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Sci-fi tractor beams materialize in real life

By TONY WU | December 6, 2012

In almost every science fiction movie, tractor beams are utilized to repel or attract objects. The seemingly magical force of the tractor beams are no longer works of fiction, according to the researchers at New York University.

A team led by David Grier, a physics professor at NYU, discovered a way to move particles using conveyor tractor beams. While the distances covered and the sizes of the objects are small, there is potential for improvement. Science fiction may not be fiction after all.

The underlying principle in the tractor beam is one of the most fundamental concepts in physics. The field of tractor beams is built upon Newton’s third law, which states that for every action, there is an equal and opposite reaction. By applying Newton’s third law to a beam, scientists can manipulate the motion of the particle, resulting in a movement toward the source of the beam instead of repelling the particle.

The light beam has to be scattered off of the particle, causing it to move. By changing the scattering, particles can be moved towards the source of the light beam. This method is common among spiral beams. However, the recent discovery is founded on an entirely different concept.

The conveyor tractor beams utilized by the researchers at NYU use the effect of radiation pressure, rather than the scattering of light beams. Every time light shines on an object, the beam exerts radiation pressure on the lighted item. Because radiation pressure is usually directed along the same direction as the light, it pushes the particle away from the light source.

To build a tractor beam based the principle of radiation pressure, the researchers have to construct a device that redirects a portion of the radiation pressure. The team led by Grier has successfully manipulated a part of the light beam, letting some of the pressure be redirected toward the central axis of the beam of light.

“[The resulting upstream motion] is the defining characteristic of a tractor beam,” Grier said.

This novel method has been up for discussion in the theory of optical forces, but has not been realized until Grier created the customized light beams through holograms.

While the tractor beams work as the theory predicted, the effective range of the beams is miniscule. Using current techniques, tractor beams can only operate in ranges of less than a millimeter. Even though theories allow for more powerful tractor beams, current technology does not permit the construction of stronger conveyor beams.

Furthermore, there are challenges in the field of optical forces. The holograms used to create the artificial light beams are not efficient. Too much energy is expended to create the light, making a more powerful tractor beam unrealistic. The loss in energy is the biggest obstacle in the creation of beams seen in the movies. Nonetheless, current results hold promises for future success.

“An optimized implementation base will enable us to greatly extend both the range and the strength of our tractor beams,” Grier said.

The discovery of tractor beams based on radiation pressure has many implications.

“[This technology] inspired us to delve into the fundamental theory of light-matter interactions [and] to understanding what forces and torques can be exerted by light,” Grier said.

Moreover, because the light beams can be calibrated to manipulate microscopic objects, scientists will be able to study and interact with particles that are currently too small to fully understand.

As tractor beams become more practical, they can be employed in various applications such as pollution monitoring, environmental sampling and grabbing materials from comet tails.

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