Sonic Resonators Take the Quantum Out of Lasers

Lasers are usually thought of as bright beams of light that rely on quantum mechanics to work. But a new device that acts as a sort of laser for sound brings lasers back to basic physics.

An array of cell phone vibrators on an aluminum sheet can make a coherent humming in a mathematically similar way to how lasers make coherent light. The device could demonstrate that lasers are not as stuck in the quantum world as most physicists believe.

?Many of the things we normally associate with lasers are in fact present in a totally classical system,? said physicist Richard Weaver of the University of Illinois at Urbana-Champaign, coauthor of a study in the April 25 Physical Review E. ?It can be understood without any talk of quantum mechanics.?

Lasers used in everything from presentation pointers to military weapons all operate on the same basic principle. In a process called stimulated emission, atoms in the laser material (a ruby rod, in the first laser ever) absorb photons, which excite electrons. The excited electrons re-emit the original photon plus an identical photon that travels in sync with the first, making coherent light. Mirrors at either end of a tube bounce these matched photons back and forth, stimulating the emission of still more photons. When enough of the photons march in lockstep, the light abruptly switches from mostly-disordered to perfectly coherent laser light.

Physicists mostly think of lasing in quantum mechanical terms. For instance, lasing works because light is both a wave and a particle. It also works because photons happen to be particles called bosons, which can all clump together in the same energy state.

But Weaver thinks lasing doesn?t need to be so complicated.

?It?s not widely appreciated that most of the features of a laser are things that you can get in a totally classical system,? he said. ?Physicists have a way of explaining things using ? the language of quantum mechanics, which can be very confusing if you don?t need it.?

Weaver and graduate student David Mertens built a simple analog of a laser out of small mechanical motors, like the kind used to make cell phones vibrate. The researchers attached 16 motors to a thin aluminum plate, and set them buzzing by applying a voltage.

Left to their own devices, each motor had its own favorite vibrating frequency. But when they were linked through the aluminum plate, which Weaver says plays the role of the ruby rod, they all started buzzing at the same frequency. The cacophony of every motor for itself transitioned sharply to a harmonious low hum when the motors started buzzing in sync.

That?s analogous to the distinct transition between incoherent light and coherent laser light, Weaver said. What?s more, the motors amplified each other, buzzing with more energy together than the sum of what they would each produce on their own.

?A lot of the words you use to describe a laser ? stimulated emission, spontaneous coherence, single frequency, superradiance ? all these things show up in this very very classical mechanical system,? Weaver said. ?It doesn?t look at all like a laser, but it?s mathematically like it.?

It?s also mathematically similar to other natural and engineered systems that exhibit an eerie synchronization, like fireflies flashing or the footsteps of people walking over bridges. If these systems sync up by communicating via resonant waves, like the vibrations in the bridge, they could also be considered a kind of laser.

?You could probably say that the Millennium Bridge [in London] is like a laser, if you want to be a little provocative,? he said.

Weaver thinks his sound laser is mostly a ?mathematical curiosity,? and doesn?t expect it to have any practical applications.

?But, famous last words,? he said. When the light-based laser appeared on the scene in 1960, it was famously called ?a solution without a problem.? No one knew what it would be good for, either.

Image: 1) Laser inventor Theodore Maiman with the first-ever laser in 1960. Courtesy of HRL Laboratories, LLC ? Malibu, CA. 2) The array of motors that makes up the acoustic ?laser.? Courtesy of David Mertens.
Citation: ?Synchronization and stimulated emission in an array of mechanical phase oscillators on a resonant support. David Mertens and Richard Weaver. Physical Review E, April 25, 2011. DOI: 10.1103/PhysRevE.83.046221.

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Source: http://www.wired.com/wiredscience/2011/05/classical-sound-laser/

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