physics

Drawn to the Light

The age of tractor beams is here

by Matthew Hutson

“I’d like to teleport a cow like the Gary Larson cartoon,” physicist David Grier confesses. He may be wishing for the impossible, but he’s already accomplished what many thought couldn’t be done. His group at the Center for Soft Matter Research within NYU’s department of physics was the first to construct a working tractor beam—a ray of light that pulls objects toward the source. Grier and one of his graduate students, David Ruffner (GSAS ’16), reported their latest version last year in Physical Review Letters: a tractor beam they call an “optical conveyor.”

The beam starts out as a simple laser and bounces off a tiny TV projector, which reshapes the waves to form a hologram. Grier and Ruffner program the hologram to resemble a rod of light extending outward with stripes of brightness and darkness. Tiny particles—on the order of a few microns, or millionths of a meter—are polarized by the light and drawn to the bright spots. By smoothly changing the hologram, the bright spots move upstream and drag the target particles with them, much like a conveyor belt.

Unfortunately, the size of the objects the tractor beam can pick up is limited by the wavelength of the laser to about 10 microns. And so far, the beam reaches out only about 70 microns, but Grier thinks that they might get to a millimeter within a year. In principle, the range is unlimited, and NASA has provided funding toward the work.

“What’s nice about this [research] is that it doesn’t contradict anything that people have said over the entire 150 years of optics,” Grier says. “What it does say is that, in that theory, hidden, was a surprise.”

While Grier’s optical conveyor is still too weak to transport Captain Kirk, here are four ways that tractor beams may soon change our lives:

1. Environment
Anytime you want to sample particles from a safe distance, a tractor beam’s your tool. It would let you measure pollution from smokestacks or the properties of dust from volcanic vents or a nuclear meltdown. Shine a beam of light from a mile away, and the particles come to you.

2. Astronomy
NASA is particularly interested in collecting material from the tails of comets for study. The space agency has done this once before, but it required flying a craft directly into the tail, exposing it to flying debris. A tractor beam would stay clear, and it also lacks mechanical joints, which are prone to failure. One piece of grit in a joint can scuttle a billion-dollar mission.

3. Electronics
A tractor beam could precisely arrange delicate components on a chip. It could also pull tiny samples out of an assembly line for quality assurance, automatically and noninvasively.

4. Medicine
A cell, at about 10 microns, and its nucleus, at about 1 micron, are the perfect size for tractor beam manipulation. Scientists could probe poisonous or infectious samples through a sealed window. Doctors could perform in vitro fertilization without the danger of damaging an embryo through rough handling. Blood-typing could be reduced from a 10-minute procedure to a 10-second procedure, saving thousands of lives.