Laser builds mirror by pushing beads together

A focused beam of green light has transformed 150 plastic beads into a functional mirror. The feat is the first step toward an ambitious goal: deploying lasers in space to assemble a cloud of dustlike particles into a giant telescope mirror.

“I think it’s really cool,” says Michael Burns, a laser physicist at Harvard’s Rowland Institute. “It demonstrates something that had only been discussed before.”

Most of the fundamental physics behind the idea of building space mirrors with lasers is solid, Burns says. Light provides a subtle push when it bounces off matter. It can also trap particles illuminated within a laser beam, which allows scientists to isolate individual cells and even atoms. Finally, light scattering off a particle can serve as a bonding force, enabling multiple particles to self-assemble into organized structures. Exploiting these properties of light, astronomer Antoine Labeyrie proposed in 1979 that a pair of continuously firing lasers in space could steer billions of tiny particles into a tightly bound parabola and hold them in place, creating an enormous, lightweight telescope mirror.

Since 2005, physicist Tomasz Grzegorczyk of BAE Systems in Burlington, Mass., and colleagues have been analyzing the physics of the laser-and-particle interactions that would form this seemingly magical mirror. To build a rudimentary mirror, they placed a few hundred micrometer-sized plastic beads into a water-filled glass tank and shined a laser beam into the tank from below.

The laser pushed about 150 beads to the top of the tank against the glass and forced them together into a crystalline, reflective structure. To test their creation’s reflectivity, the researchers projected an image of the numeral 8 from a plastic ruler onto the mirror and used a camera to capture the reflected image. The mirror delivered a fuzzy but recognizable reflection, Grzegorczyk’s team reports January 13 in Physical Review Letters, despite the mirror’s relatively rough surface.

Obviously there are many steps to go before NASA commissions a laser-assembled space telescope. Grzegorczyk’s mirror is only about 40 micrometers across, and it relies on the surrounding water to absorb some of the laser’s heat. Enormous technological hurdles also remain, including the need for two powerful lasers that run continuously for years in space to hold the mirror together. “With current technology, this is still closer to science fiction,” Burns says.

Yet the potential performance of such a mirror in space is so extraordinary that Grzegorczyk says he can’t quit. In theory, a pair of lasers could construct and maintain a 35-meter mirror, larger than any telescope mirror in space or on Earth. It would have the same mass as a hamburger patty. For comparison, the 6.5-meter mirror on NASA’s James Webb Space Telescope, which is due to launch in 2018, has a mass of nearly 700 kilograms.

Because larger mirrors collect more light, a laser-constructed mirror connected to a camera potentially could image planets orbiting distant stars as well as galaxies at the edge of the visible universe. Plus, the mirror could heal itself: If space junk shattered a section, the lasers would nudge displaced particles back into position.

For now, Grzegorczyk wants to make small strides. He is looking to build a mirror that floats in water rather than nestling against the roof of the tank. “If all we have to wait for is the technology,” he says, “then this will eventually fly.”