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