26 APRIL 2007
Imagine what you could do if you could shine light through a wall. OK, maybe not much, unless you're a stage magician. But suppose you could shine light all the way through the Earth. Now you've got a communication system that would make WiFi seem like tin cans and a string.
(Note to young readers -- ask your grandfather about tin-can communication.)
Weirdly enough, experiments suggest that it might really be possible to shine light through walls, the Earth, and even through the sun. It all depends on the existence of a tiny hypothetical particle named after a laundry detergent.
Thirty years ago, physicists Robert Peccei and Helen Quinn proposed that such a particle could explain a technical glitch in the theory of the strong nuclear force. Frank Wilczek, later to become a Nobel laureate, named the particle the axion, after a detergent widely advertised in the 1960s by Arthur Godfrey (see an Axion ad here). (Arthur Godfrey? Ask your grandmother.)
An axion (the particle, not the soap) would probably be an ultralightweight particle, vastly less massive than even the electron, itself much lighter than other particles found in atoms.
Axions would also be difficult to detect. So far three decades of searching has been futile. Last year, though, an Italian experiment reported a peculiar effect that excited axion fans everywhere. The PVLAS collaboration, at the INFN Legarno National Laboratory in Italy, noticed a twist in the path of a laser beam passing through a magnetic field in a vacuum. (Technically, the magnetism rotated the beam's plane of polarization -- the alignment of the light waves.)
One explanation for this twist would be the axion. If they exist, axions can be converted into photons (particles of light) in a magnetic field. In the PVLAS experiment, it's possible that laser photons morphed into axions, and then the axions converted back into photons, just aligned in a slightly different plane.
If axions do exist, you really could shine light through a wall. Produce magnetic fields on both sides of the wall and then shine the laser at the wall. The magnetic field induces the laser photons to change into axions, which can travel through the wall. When the axions emerge on the other side of the wall, the magnetic field changes them back into photons.
So why stop with a wall? asks Daniel Stancil, of the department of electrical and computer engineering at Carnegie Mellon University in Pittsburgh.
"If the axion interpretation of the PVLAS experiment is correct, a new type of long-distance signaling and communication may be possible," he writes in a new paper. "In particular, it may be possible to construct a communication system that cannot be blocked -- even communicating directly through the planet from one side to the other."
Stancil is thinking big. Imagine a future company, say, Axion Telephone and Telegraph, that could send messages around (actually, through) the world with no need for satellites or cables. It would open up new markets for communicating to difficult locations, such as submarines near the ocean floor or mining stations deep underground. You could even communicate where cell phone reception is lousy, such as in western states at great distances from interstate highways. With enough power, axions could be used to communicate in space, Stancil suggests.
Not only that, the signals would be pretty secure, since laser beams are narrow and difficult to intercept.
Sure, there are few technical hurdles. And current knowledge suggests that the data rate would be rather slow, on the order of a few kilobits per second, which is worse than the Internet using dial-up.
A potentially more serious problem, though, is the chance that axions don't really exist. The PVLAS experiment is controversial, and other experiments get contradictory results.
Fortunately, a good new laboratory for testing the axion idea is on the horizon, namely the sun. Every once in a while the sun passes in front of a distant source of gamma rays (high-energy photons). The sun has a pretty impressive magnetic field, so some gamma photons entering the sun could be converted to axions, and then the axions could shift back into gamma rays on the way out, heading toward Earth.
No axions, and the sun blocks the flow of gamma rays to the Earth from an eclipsed source. But if axions exist, some of those gamma rays could be detected sailing on through, like light through a wall, physicists Malcolm Fairbairn, Timur Rashba and Sergey Troitsky suggest in a paper soon to be published in Physical Review Letters.
Current telescopes aren't sensitive enough to detect this effect, but the GLAST satellite, scheduled to be launched late this year, should be able to.
"By looking at the same source when it is eclipsed by the sun, GLAST . . . could observe the partial transparency [to gamma ray photons] predicted in this paper, confirming the interpretation of the PVLAS data as a new particle," the scientists wrote.
If axions are confirmed, all of physics would be put on a firmer footing. Axions could even be the dark matter lurking throughout the universe that astronomers have been hunting for.
But even if axions remain mythical, their story should remind everybody how powerful weird ideas can potentially be. If axions don't exist, something else very strange but so far undiscovered certainly does, with the power to perform magic tricks much more fascinating than merely shining light through walls.
E-mail: tsiegfried@nasw.org
