A Mirror Held Together by Lasers

22 January 2014 6:15 pm
Lightweight. About 150 tiny reflective beads are held together by laser light, forming this arrangement (inset) that can act as a mirror to see objects in space.

NASA, ESA, and E. Sabbi (STScI); (inset) T.M. Grzegroczyk et al., PRL (2014)

Lightweight. About 150 tiny reflective beads are held together by laser light, forming this arrangement (inset) that can act as a mirror to see objects in space.

Imagine a space telescope the size of a football field that weighs as much as a few slices of bread. Researchers have taken a step toward that goal by creating a small mirror out of tiny polystyrene particles, held together by lasers. Without any weight constraints, telescopes could be much more powerful than previously thought possible.

When it comes to space telescopes, bigger is better. Telescopes like the Hubble Space Telescope use large mirrors to gather light, and the bigger the mirror, the more light they can collect, enabling them to see the faintest and most distant galaxies. But big, heavy mirrors are costly to manufacture and launch into space. So scientists have been trying to figure out a way to go big without going heavy.

In 1970, physicist Arthur Ashkin of Bell Labs in Holmdel, New Jersey, realized that laser beams could hold tiny particles in place. Since then, scientists have used lasers to trap atoms, molecules, and other small particles. Using these so-called optical tweezers, for example, biologists have been able to probe viruses, cells, bacteria, and DNA.

Then in 1979, astronomer Antoine Labeyrie, now at the College of France in Paris, proposed that lasers could also be used to trap and corral a collection of particles to form a reflective surface, creating an extremely lightweight mirror for a space telescope. A 35-meter telescope, for example, would weigh only 100 grams. In contrast, Hubble's primary mirror is 2.4 meters wide but weighs 828 kilograms. Not only would the telescope be light, but it would also be able to fix itself if struck by flying meteorites. "The natural tendency of the particles would be to get back to its equilibrium state and reform the membrane," explains physicist Tomasz Grzegorczyk of BAE Systems in Burlington, Massachusetts. "This is an enormous advantage."

Reporting this month in Physical Review Letters, Grzegorczyk and colleagues at the Swiss Federal Institute of Technology in Lausanne say that they’ve used lasers to arrange about 150 beads that are 3 microns in diameter to produce a flat, reflective surface. In the experiment, the beads are contained in a water-filled glass cell. A laser beam shines under the beads, causing them to align themselves into a flat surface. To show that the surface was indeed a mirror, the researchers used it to reflect an image of the number eight made by shining light through a transparent ruler. They also calculated that a reflective surface made by shaping a flock of tiny particles into a parabola could focus an image just as a telescope mirror does.

There's still a lot of work to be done before such a telescope can be built, Grzegorczyk acknowledges. "It's one step toward it, but there remain enormous challenges," he says. For one, a real telescope probably would have to be at least about a million times larger, which would require powerful lasers that don't yet exist. Perhaps the biggest difficulty would be figuring out how to stabilize the particles in the vacuum of space, he says. In the experiment, the water in which the beads are immersed helps prevent the particles from wiggling around and causing the lattice formation to fall apart. Still, the remaining obstacles are mainly technological, Grzegorczyk says, and technology is constantly improving.

"The fact that they've succeeded in making a mirror is a fantastic achievement," says physicist Gian-Luca Lippi of the Nonlinear Institute of Nice in France, who was not involved in the work. While theoretical studies have previously suggested this was possible, he notes, this is the first experimental demonstration. 

But physicist Jene Golovchenko of Harvard University is more skeptical. Because the experiment relies on water inside a glass cell, it "completely ducks the really serious issues that require creative solutions for a space mirror," he says. The way in which the researchers confirm that the numeral eight is indeed reflected by the particles seems flawed, and the resulting data isn't quantified enough. The results lack measured intensities and a discussion of the spatial size and resolution of the images. "Hyping this up as a major advance I think is unjustified," Golovchenko says.

Physicist Michael Burns, also of Harvard, agrees that it's unclear how efficient the mirror is at reflecting light. But, he says, "they're showing that the principle is sound." The work makes a telescope just a little closer to reality, he adds. "You got to take steps in that direction, and this is a nice one."

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