When black holes aren’t black

At 21, Stephen Hawking was diagnosed with motor neuron disease and was handed a life expectancy of two years. Although death didn’t arrive in that timescale, the disease severely reduced his bodily control in subsequent years. Crucially for his research, however, his mind was left untouched. He did not let his disability stand in his way: “Look up at the stars and not down at your feet,” he once said.

His perseverance, coupled with his intellect, led him to carve out an illustrious career in the field of cosmology and one of the main contributions Hawking made to science was in relation to black holes. For 30 years, Hawking held one of the most prestigious academic posts in the world, the Lucasian Chair of Mathematics. This is a professorship at the University of Cambridge which was formerly held by Isaac Newton, Paul Dirac, and George Gabriel Stokes among others. To explain Hawking’s accomplishments, some context is required.

There’s no such thing as nothing

Anti-matter isn’t confined to the realms of science fiction. For every type of particle (e.g. an electron), there exists in opposition to this an anti-particle partner (e.g. a positron). When matter and anti-matter come into contact, they annihilate one another and cease to exist. This is occurring all the time in the vacuum of space: New particles and their anti-particle pair are constantly popping into existence and then immediately being destroyed by one other. In this way, there really is no such thing as perfectly empty space devoid of all particles, even in the vastness of outer space.

The effect of particle and anti-particle annihilation is governed by quantum mechanics. This is a theory which precisely describes the behaviour of stuff on the smallest scales, such as atoms. When describing the most enormous structures in the Universe, however, quantum mechanics breaks down. For objects like black holes and the orbits of the planets, we use Einstein’s theory of general relativity. Much of what we see in the Universe can be explained by one of these two theories, but the theories are mutually incompatible with one another. They both work in their own right but only on their own turf, and they don’t mix. One of the biggest questions in all of science is formulating a grand unified theory which agrees with both general relativity and quantum mechanics.

One of the first steps in what is an ongoing effort to find a grand unified theory was made by Hawking. He hypothesised that black holes emit radiation and to explain the effect, Hawking applied the small-scale theory of quantum mechanics to black holes.

First, a quick black hole recap: Matter exerts a gravitational pull on everything, and the more matter there is, the stronger the force of attraction. Black holes are sufficiently massive that nothing can escape their pull, not even light itself. The fact that there is no light coming from black holes is why they appear black.

I’ve said that when a particle and its anti-particle partner pop into existence, they hang around only briefly before destroying each other. However, Hawking showed that when this occurs at the precise edge of a black hole (known as the event horizon), it is possible that either the particle or anti-particle gets pulled into the black hole while the other particle (its partner) escapes. Then the pair of particles can no longer annihilate each other because one has been swallowed up by the black hole. From our vantage point, the black hole appears to emit radiation as these particles-without-a-partner are seen to be coming from the black hole.

In this way, Hawking showed that black holes are in fact glowing, and emit what is now called Hawking radiation. Black holes are black because nothing escapes their surface, but Hawking demonstrated that we should be able to see particles coming from black holes after all. Furthermore, as this radiation leaks out from black holes, it causes the black holes to evaporate over time. Black holes continue to shrink and eventually disappear. Just as the Colosseum in Rome has been eroding for the past 2,000 years, so too have the black holes of the Universe. While this process is extremely slow for the average black hole, it occurs much quicker for miniature black holes, which rapidly release radiation and explode out of existence (so the Colosseum analogy breaks down here).

Pairs of particles are constantly being created and then they destroy each other. Hawking radiation is observed when one of the particles is pulled into the black hole and its partner escapes. Source: Quora

This actually ties into one possible fate of the Universe. In 1 googol years (that’s 1 followed a hundred zeroes; it’s where the company Google got their name), everything will be pulled into one of the remaining supermassive black holes. These black holes will then evaporate via Hawking radiation, leaving the Universe nearly empty in what is called the dark era in cosmology. What happens after that is anybody’s guess, but physics as we know it goes out the window. To put this timeline into perspective, the Universe is just 13 billion years old now, and the last stars won’t cease to shine until the Universe is 100,000 billion years so this dark era really is a long way off, even by those standards.

From space dust we come to space dust we go

Hawking may have gained prominence in the scientific community for his work on black holes but his success extended well beyond academia. He inspired a generation to study science and ponder the Universe. In spite of his condition, he was a master of science communication. Hawking had to do a lot of mental arithmetic because writing was so comboresome but he was fully aware that this was not a prerequisite for appreciating science. “Equations are just the boring part of mathematics” he once said, and he had a talent for extracting the technically beautiful aspects of an idea and putting it plainly.

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Source: Rama Samkari

In 1988, he authored A Brief History of Time: From the Big Bang to Black Holes, using his scientific insight to translate complex ideas for a non-specialist audience. Since its release, the book has gone on to sell more than 10 million copies and has been translated into dozens of languages. He also featured in a range of popular media, including appearing on television shows such as The Simpsons.

Newton famously stated that “If I have seen further, it is by standing on the shoulders of giants,” referring to the fact that his discoveries — although groundbreaking — were built on the work of others. It is fitting then that Hawking held the same position as Newton at the University of Cambridge: Newton was the first person to formulate how gravity works and Hawking has done his bit in furthering our understanding of gravity too. Similarly, time marches on and the discoveries that Hawking has made will no doubt provide the bedrock for future theories for decades to come.