Hawking Radiation

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Alright this will be my final post on black holes, I promise. Unless you want more, that is? Actually that’s a silly question, of course you’d like more. But unfortunately you’re not getting any, so make sure you savour this one. It’s going to be… astronomical… (I’ll see myself out).

From what we’ve seen so far, black holes appear to be immovable, ever-growing bodies that are permanent structures in the sky. Indeed this was believed to be true for a long time – if black holes were constantly absorbing matter but never excreting, surely they would last indefinitely? It turns out that black holes can in fact be destroyed. In 1974, the legend himself Stephen Hawking proposed a theoretical argument that black holes should be able to evaporate by emitting energy in the form of electromagnetic radiation, which has been named Hawking radiation.

The fabric of the universe is sometimes conceptualised as a ‘quantum foam’, in which the particle ‘bubbles’ constantly pop in and out of existence. These virtual particles are always produced in pairs – a normal matter version and a corresponding antimatter one. For most places in the universe, these pairs are constantly being produced and then rapidly disappear by annihilating each other (I know, sounds brutal right?), with energy and momentum always being conserved. Normally we can never observe these pairs, and only know of their existence through effects like the Casimir effect. Only in a region of incredibly strong gravitational forces would these two particles be able to be wrenched apart from each other before they mutually destroy themselves. I wonder where we might find gravitational forces strong enough…

Hawking theorised that if a pair were to be produced at the event horizon of a black hole, where one particle was created on either side of the horizon, one may get pulled into the black hole while the other escapes. This interpretation is a physical insight into the mechanism that drives this phenomenon, but when Hawking first proposed this idea, it was based on solid, pure mathematical physics. If lines and lines of rigorous mathematics is your cup of tea, do hunt down his original paper and have a read. The energy being dispelled from the black hole would in turn cause the black hole to shrink, due to the conservation of energy. If the black hole does not feed by other means, it will gradually wither away and vanish. However, for all of the black holes in our universe, disregarding the fact that they will certainly be gaining mass from the cosmic background microwave radiation, they lose mass at such a tiny rate that it would take a black hole the mass of our sun over a billion billion billion billion billion billion billion years to evaporate. Feel free to wait that out, but I still have things to do. The effect would have had a much larger impact on the smaller, short-lived black holes that existed in the primordial universe.

Scientists have never actually physically observed this effect since they do not have the technology to look at black holes to such a high degree of accuracy. We’re only just recently beginning to image black holes for the first time in history. Laboratory-scale experiments have been conducted, but results are still unverified and debatable. Therefore Hawking’s hypothesis still simply remains just that, a hypothesis, no matter how widely accepted it is.

I hope that, after covering the ins and outs of black holes in this series, you now don’t find them as daunting as you might originally have. When it comes to the end of the universe, the immortal black holes will fade away just like the rest of us… hopefully.

Harvey

 

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