So you’re reading the title and saying, “Wait, what? A black-hole laser? Doesn’t a black-hole suck in all the light around it?” Well, according to a theory proposed by Stephen Hawking in 1974, not only do black-holes suck in light, they also emit a faint radiation, now termed as ‘Hawking radiation’. Hawking predicted the existence of this radiation based on certain quantum effects that occur near the ‘event horizon’, a theoretical boundary in spacetime that defines the extent of influence of a gravitational field (in this case a black-hole). Within the boundaries specified by this event horizon, all matter and radiation is under the influence of the black-hole’s gravitational pull and cannot escape. Now, due to quantum vacuum fluctuations in the black-hole, a temporary change in the amount of energy at a point close to the event horizon takes place, causing the formation of a particle-antiparticle pair. One element in this pair is sucked back into the black hole, while the other escapes. When the particles under consideration are photons, it appears that a photon is spontaneously released from the black-hole – this should show up as the mysterious Hawking radiation, if observed in practice.
Recently, researchers from Heriot-Watt University in Edinburgh, UK came up with a method to test this theory in the laboratory. An artificial event horizon (representing a black-hole) was created using high-intensity pulses of infrared laser light focused on a piece of glass. The large intensity of these pulses caused a temporary boost of the refractive index of the glass medium; this boost was large enough to cause the light travelling through it to slow down. Since the light was in the form of a pulse, this resulted in a point of high refractive index that moved with the light pulse, which acted as the horizon. Any photons that entered the glass behind this high intensity light pulse were unable to cross the point of high refractive index; relative to the pulse, these photons were stationary. Using this as a model for a black-hole, the Scottish researchers were able to detect mysterious ‘extra’ photons that seem to have come from nowhere: the elusive Hawking radiation.
Having tested Hawking’s hypothesis and receiving positive results, the team at Heriot-Watt University has decided to go a step further and design a ‘black-hole laser’. A conventional laser works by bouncing monochromatic light back and forth between two mirrors and passing it through a gain medium in the process, stimulating its atoms to release photons of the same frequency to contribute to the beam. A black-hole laser works using a similar principle. Instead of using two mirrors, a black-hole and a white-hole are used. A white-hole is the ‘reverse’ of a black-hole; it allows radiation to come close but does not allow it in.
A black-hole and white-hole are artificially simulated using a diamond. Two light pulses are sent through the diamond in quick succession. This is equivalent to having a black-hole with a white-hole inside it. Light trying to enter the white-hole is already within the black-hole. By the definition of these structures, the light can neither enter the white-hole nor escape the black-hole, causing it to bounce back and forth between the respective horizons. If Hawking radiation does indeed exist, this light would get amplified by said radiation, forming a laser that would be hopefully easily detectable.
This discovery could result in future applications such as low-energy terahertz scanners. While these phenomena being explored in the lab are exciting in themselves, they probably do not come too close to modeling black-hole behavior. Hawking radiation remains a mystery – for now.
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