An obscure property of light puts a spin on astronomy
By George Musser | November 3, 2003
You’d think we’d have figured out light by now. Kids learn about prisms and lenses in elementary school, people wear Maxwell’s equations on T-shirts, and the quantum version of those equations is the most precise theory in science. Yet knotted up within the theory is a phenomenon that physicists are still unraveling: an unexplored property of light.
In addition to color (which depends on the wavelength of the electromagnetic wave) and polarization (the orientation of the wave), light beams can also possess orbital angular momentum (the shape of the wave fronts). Optics researchers discovered this property a decade ago, but for some reason this realization has failed to propagate much beyond a small community of specialists [see “Hands of Light,” Innovations, Scientific American, August]. It has barely been noticed even by those with the greatest need to exploit every conceivable aspect of light–namely, astronomers.
An astronomer has now taken it upon himself to spread the word. In the November 10 Astrophysical Journal, Cornell University emeritus professor Martin Harwit suggests that the orbital angular momentum of light could convey new information about celestial bodies–information unavailable by looking just at color and polarization. “The paper was mainly meant to be provocative,” he says. “People are flabbergasted that this should even be possible.”
In an ideal beam of light, produced by a laser or a distant star, the wave fronts are flat. On each slice through the beam, the wave is at the same phase in its oscillation cycle: crests line up with crests, troughs with troughs. But in a slightly more complicated beam, the phase changes with the angle around the beam’s axis. The 12 o’clock position on a slice might correspond to a crest, the 6 o’clock position to a trough [see illustrations]. If you connect the wave crests, they form a helix. The next most complicated possibility is a double helix, in which the phase changes twice as rapidly (with troughs at 3 o’clock and 9 o’clock); beyond that is a fusilli-like triple helix (2 o’clock, 6 o’clock and 10 o’clock), and so on.
Like polarized light, twisted light carries angular momentum: in lab experiments, it has set small plastic beads spinning. If you think of light in terms of particles (photons) rather than waves and neglect some quantum-mechanical caveats, it is as though the photons were zipping along a corkscrew path.
To create twisted light, physicists shine a laser through a helical lens or a special diffraction grating. Harwit argues that light could also be twisted by natural processes in the universe, such as lenslike density variations in interstellar gas or the warped spacetime around rotating black holes. Alien civilizations might transmit information by twisting light rather than using other encoding methods (as indeed physicists have proposed for terrestrial free-space communications). The most sensitive way to measure the twist would be a series of interferometers, as demonstrated last year by a team led by physicists Jonathan Leach and Miles Padgett of the University of Glasgow.
One peculiar aspect of twisted light could prove especially endearing to astronomers. Just as Earth’s North Pole sits in every time zone, the central axis of the beam contains waves of every phase. All those waves cancel one another out, leaving utter blackness. As a result, a lens focuses twisted light to a ring instead of a point. In 2001 physicist Grover Swartzlander of the University of Arizona proposed using this feature to look for extrasolar planets. Installed in a telescope, one of the special diffraction gratings would smear starlight into a ring, leaving a hole so dark that a nearby object millions or billions of times as faint could become visible. “It’s a completely original idea,” Padgett says. “When I first read the paper, I said, ‘Gosh, that’s a cute idea.'” Contemporaries of Newton probably thought it pretty cute that white light could be split into a rainbow of colors. Maybe one day twisted light will come to seem just as commonplace.
By George Musser | November 3, 2003