How To Build A Black Hole Laser

One of the more fascinating discoveries in theoretical physics in recent years is the formal mathematical analogy between the way spacetime and certain materials effect light. 

Physicists have used this idea to show how to build black holes using so-called metamaterials that steer light in exotic ways. Indeed, Chinese physicists built just such an artificial black hole back in 2009. 

The analogy can be taken even further. Not only is it possible to make black holes in this way but also their time-reversed equivalents–white holes. What’s more, both these white and black holes should produce Hawking radiation. This is the spontaneous emission of radiation at a hole’s point of no return, called its event horizon. 

This spontaneous emission of radiation is a curious and complex phenomenon. Nobody has observed Hawking radiation either from astrophysical black holes or laboratory-based ones (although there are one or two claims currently under debate). But few physicists doubt its existence. 

So if Hawking radiation really does exist, what could we do with it? Today, we get an answer from Daniele Faccio at Heriot-Watt University in Scotland and a few pals. These guys explain how to use this spontaneous emission of radiation to make a laser. 

Their idea is to create a black hole next to a white hole so that their event horizons are separated by just a few hundred micrometres and create a small cavity. Then they show that when light is fired into this cavity, it is reflected off the white hole horizon onto the black hole horizon, back to the white again and so on.

Faccio and co go on to show that during each reflection, Hawking radiation effectively adds to the beam, thereby amplifying it.  They say this additive process is logarithmic so a small seed of light ends up producing an intense beam of radiation.

Their real triumph, however, is in showing how such a device could be made in the lab. They point out that the refractive index of certain materials depends on the intensity of light inside them. So the light itself changes the refractive index. 

That means a very intense beam can create a huge gradient in the refractive index. This gradient can be so steep that it behaves like an event horizon. In fact, a single pulse can create black hole horizon at its leading edge and a white hole horizon at its trailing edge.

That’s exactly the condition these guys are looking for. They go on to say that it ought to be possible to do this in optical waveguides made of diamond. They’ve tested the idea numerically and say it works as expected.

Faccio and co are quick to point out that it is possible to grow diamond into more or less any shape so it ought to be possible to test this idea in the lab now. “This would therefore seem to indicate that this kind of novel amplication process could be observed in real settings,” they say.

That would be an extraordinary experiment– a black hole laser in a lab. Cool!

Ref: arxiv.org/abs/1209.4993: Optical Black Hole Lasers