On March 14th, 2018, Stephen Hawking passed away at the age of 76. He was perhaps the most popular physicist in the world, having captivated the general public with his brilliance and his perseverance against the seemingly insurmountable obstacles that his neurodegenerative disease presented him. I first got to know Stephen Hawking through his book ‘The Universe in a Nutshell’, long before his guest appearances in The Simpsons and the Big Bang Theory. Having read the book from cover to cover however, I still hadn’t realized the exact contributions that Hawking made to theoretical physics. It was only several years later, during my undergraduate years in the Physics Department of the National and Kapodistrian University of Athens, Greece, that I first learned about the Hawking-Bekenstein radiation and its relevance to black holes.
The concept of an object that is so massive that the pull of its gravity does not allow anything, including photons, to escape first appeared as early as the 1780s but was properly formalised roughly 140 years later, in the 1910s, when Karl Schwarzschild and Arthur Eddington used Albert Einstein's theory of General Relativity to show that such objects do exist. Since then, black holes have assumed a central role in both theoretical physics and astronomy and astrophysics. Beyond science, black holes have exerted a fascination on science enthusiasts both due to their exotic properties as well as the fundamental break-down of physical laws at their centres - the infamous singularity.
Yet, in 1974, Stephen Hawking published in Nature his seminal paper titled 'Black hole explosions?' (http://rdcu.be/I2L1) - blatantly disregarding current Nature style rules to include a punctuation mark in his title! To understand Hawking’s inspiration and the origins of this paper, we must look at a paper published by Jacob Bekenstein a year earlier (https://journals.aps.org/prd/abstract/10.1103/PhysRevD.7.2333). Bekenstein posited that there is an analogy to be drawn between black hole physics and thermodynamics. As such, a black hole not only should possess entropy but also be characterized by a temperature.
Coming back to his 1974 paper, Hawking showed that if a black hole can be assigned a temperature then, similar to any other physical object, it must radiate away energy. The temperature of a black hole depends on its surface gravity, κ, following the relation (κ/2π) (ħ/2k), where ħ and k are the Planck and Boltzmann constants. Attacking this problem from a quantum theory point of view, Hawking argued that the energy of the gravitational field of the black hole can lead to the pair production of particles and anti-particles that, accumulated over the age of the Universe, 1017 seconds, can be significant - especially when compared to the relevant Planck time 10-43 seconds. As the black hole radiates away energy, its mass is reduced such that its lifetime would be 1071 (M☉/M)-3 s. As can be easily seen, for any black hole more massive than 1015 grams the process of evaporation through Hawking radiation is simply too slow given the Hubble time. However, tiny black holes created in the particle soup of the primordial Universe could in principle be in the right mass range to have evaporated sometime before a redshift of zero. Hawking indeed showed that such black holes would go out with a bang, releasing energy at a rate of 1029 erg/s during the last 0.1 seconds of their existence.
For the astronomical community, Hawking-Bekenstein radiation is perhaps the most relevant, if not the most relatable, piece of scientific research that Stephen Hawking produced in his prolific career. While astrophysical black holes (of a few solar masses or more) would produce negligible amounts of such radiation, the idea of something coming out of a black hole challenges scientific and layman's preconceptions of what a black hole really is. Closer to home, learning about Hawking radiation was an almost philosophical revelation for me, leading to a paradigm shift of how black a black hole actually was.
As with the vast majority of Hawking's body of work, Hawking-Bekenstein radiation finds itself on the interface between general relativity and quantum theory offering a bridge between the as of yet two irreconcilable theories. And while astronomers may not be able to look for observational evidence for this effect for the foreseeable future, experimental physicists have managed to create analogues of black holes that produce effects similar to the Hawking-Bekenstein radiation (http://rdcu.be/I2Ve). By creating an analogue black-hole horizon within a low density, very low temperature atomic Bose–Einstein condensate, Jeff Steinhauer showed the emergence of self-amplifying Hawking-Bekenstein radiation. This experimental configuration can reveal further information about both these extreme fluids as well as the black holes of which it is an analogue.
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