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luni, 15 octombrie 2012

Can You Make a Laser from a Black Hole?

Is there any object more alluringly mysterious in the cosmos than black holes? How about their mirror image, white holes? Put the two together, and you just might have an intriguing concept for a unique kind of laser -- one that a team of Scottish physicists think they might actually be able to build in the lab, using analog versions of white holes and black holes.

A black hole can be visualized as a large funnel with a long throat. If you “cut” the throat and join it to a second black hole that has been flipped over (a white hole), you get an hourglass shape, with a thin filament connecting each end. Technically it's known as an Einstein-Rosen bridge (named for Albert Einstein and his collaborator, Nathan Rosen), an early theoretical incarnation of a wormhole.

Some physicists have become very adept at mimicking the signature behavior of black holes, at least mathematically. For instance, Ulf Leonhardt and the gang at the University of St. Andrew's in the United Kingdom have created analogue black holes using laser pulses and optical fibers.

William Unruh has worked with scientists at the University of British Columbia to create a water-wave version of an event horizon, as well as sonic analogues he calls "dumb holes." Here is Unruh's evocative analogy of "dumb holes":

    Imagine you are a blind fish, and are also a physicist, living in a river. At one place in the river, there is a particularly virulent waterfall, such that at some point in the waterfall, the velocity of the water over the waterfall exceeds the velocity of sound in the water.

    It is clear that if another fish, which has fallen over the falls, shouts after passing that point, that sound will never get out to someone on the other side of the river. The sound will still travel through the water with its same speed as always, but the flowing water will sweep that sound over the falls faster than that sound could hope to travel out.

    Furthermore, if a fish screams as it falls through that surface, the parts of the sound in that scream emitted at points closer and closer to that surface will take longer and longer to get out to a point far from the falls, because the net velocity of the sound will be smaller and smaller if the sound emitted is closer to that special surface. That last bit of sound emitted just before the fish goes through that surface will take an infinite time to get out...

Sounds like black hole behavior to me.

In 2009, Chinese physicists used so-called metamaterials to create black hole-like conditions that allowed microwaves in, but wouldn't let them out. And in 2010, scientists at the University of Milan created an analogue of a black hole that emitted Hawking radiation.

Hawking radiation is the result of virtual particle pairs popping out of the quantum vacuum near a black hole. Normally they would collide and annihilate into energy, but sometimes one of the pair gets sucked into the black hole, resulting in an apparent violation of energy conservation.

The mass of the black hole must decrease slightly (in the form of emitted radiation) as a result to counter this effect and ensure that energy is still conserved. Sure, nobody's technically observed Hawking radiation, but physicists are pretty sure it exists.

So, if black holes -- and, by extension, white holes -- can emit radiation, why couldn't their analogue counterparts be used as lasers? This was the thinking of Heriot-Watt University physicist Daniele Faccio and a few colleagues, including Leonhardt.

All lasers have a cavity with a lasing medium: either a crystal like ruby or garnet, or a gas or liquid. There are two mirrors on either end of the cavity, one of which is half-silvered, meaning that it will reflect some light and let some light through. (The light that passes through is the emitted laser light.)

The atoms or molecules of the lasing medium are “pumped” by applying intense flashes of light or electricity, so that more of them are at higher energy levels than at the ground state. Then a photon enters the laser cavity. If it strikes an excited atom, the atom drops back down to its ground state and emits a second photon of the same frequency, in the same direction as the bombarding photon.

Each of these may in turn strike other energized atoms, prompting the release of still more photons in the same frequency and direction. The end result is a sudden burst of light in a rapid chain reaction. It's called (don't snicker) "stimulated emission."

Leonhardt pondered the possibility of a black hole analogue laser back in 2008, using two pulses of laser light of different wavelengths to simulate an event horizon. (Sound waves work, too.) Both beams are fired through a lasing medium, but they travel through it at different speeds.

The first beam changes the refractive index of the material so that the second pulse must slow down, even though initially it was moving "faster" That means the second pulses will never overtake a black hole, slowing to a relative stop.

In other words, the light can't escape. It's an analogue event horizon. But remember that physicists have since demonstrated an analogue equivalent of Hawking radiation using such methods, too. This is where the laser potential comes in. If analogue black holes (and analogue white holes, which have also been created in the lab) lose mass via Hawking radiation, that means stimulated emission might be possible.

For this to work, you need two event horizons, side by side -- one for an analogue black hole, the other for the analogue white hole, with the connecting filament between them serving as the cavity (an analogue wormhole, if you will). Theoretically, the light pulses will bounce back and forth within the cavity and gradually gain energy from the associated Hawking radiation.

It all comes down to your choice of material for the lasing medium. Leonhardt proposed using a Bose-Einstein condensate, then injecting sonic (sound) waves into the medium. The Chinese physicists used metamaterials as a lasing medium, since they make excellent waveguides for light by changing the index of refraction and hence, the path light follows -- on a par with how gravity distorts spacetime.

That's the trick behind the potential for invisibility cloaks. The Chinese team managed to distort "space" so much that light shone into the metamaterial was unable to escape, just like the event horizon of a black hole.

Now Faccio, Leonhardt and pals think they can use metamaterials -- or even diamond grown into the appropriate waveguide shape -- to build a prototype black hole laser in the lab, using a very intense beam to produce the requisite extreme changes in the refractive index of the material.

Sure, it's a major challenge to build a working prototype of such a device in the lab. But the whole notion is pretty darned cool. Who knew conceptual analogues might have practical applications?

Images: (top) Illustration of hypothetical Einstein-Rosen bridge: a black hole and a white hole connected by a wormhole. Source: Wikimedia Commons. Credit: Allen McCloud. (bottom) Schematic for black hole laser in a lab. From arxiv paper by Daniele Faccio, Tal Arane, Marco Lamperti, and Ulf Leonhardt.

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