Black Holes are one of the most wonderful and confusing objects in the known universe, but now we may be one step closer to understanding them.

Since Einstein’s equations predicted them, physicists have strived for an explanation. We know now that most are formed when a very massive star, at least three times the mass of our Sun, collapses in on itself due to an overwhelming gravitational attraction in a relatively small amount of space. The gravitational field becomes so strong that nothing can escape its grasp – including light. This, however, raised a huge problem; you can’t see something that doesn’t emit or reflect light. And so, for years without physical evidence, Black Holes were considered oddities of mathematics and of no real physical relevance. Later observations, however, showed that they do exist after all.

Professor Stephen Hawking is one of the big names when it comes to Black Holes, having dedicated the most part of his life to understanding of them. One of his biggest ideas was Hawking Radiation.

Quantum Mechanics also allows for the spontaneous creation of particles in empty space…

Hawking Radiation is essentially a consequence of Quantum Mechanics, which postulates that you can never be quite sure of where a quantum particle is: you can only calculate the probability that the particle can be found in some particular region of space. Quantum Mechanics also allows for the spontaneous creation of particles in empty space. A particle can be created along with its direct antiparticle at which point they almost instantly annihilate each other and produce energy.

Then came Hawking’s biggest insight. With a Black Hole, you have an Event Horizon, a point of no return: if something were to fall past this point, there is no way to get back out again because it would require that this something would have to travel faster than the speed of light. So Hawking considered the case where two particles are created exactly on the Event Horizon. If this happened, due to the conservation of momentum, the particles would have to travel in opposite directions to each other. That is, one particle would fall past the Event Horizon, but one would escape out into space. Overall, this means that the Black Hole will lose energy over time and ‘evaporate’.

…an exhilarating step into a more knowledgeable future…

But only recently has this theory been experimentally tested. Daniele Faccio (Heriot-Watt University, Edinburgh) and Francesco Belgiorno (University of Milan) set up an experiment to simulate the effect of a horizon and try to observe this radiation. This worked by using the fact that light slows down in other materials, with how much it slows down proportional to the refractive index of the material. They used a laser to focus ultrashort pulses of infrared light with incredibly high intensities at glass, having the effect of raising the glass’s refractive index. This point of very high refractive index moves slowly along and acts in a very similar way to an event horizon in a Black Hole, stopping particles of light being emitted from in front of it. Detectors then search for any created photons.

The experiment was a success; the predictions of Hawking Radiation agreed very well with the data they received; however, many are sceptical, due to the created horizon and a real one having different properties, like the fact that a Black Hole’s horizon is created by the gravity and so may not behave in the same way. Nonetheless, this could well be an exhilarating step into a more knowledgeable future.


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