black holes

The Event Horizon Telescope released the first-ever image of the behemoth at the centre of Messier 87… but what questions still linger?

The first black hole image has been released, but what questions still linger?

On 10 April 2019, the world was introduced to the very first picture of a black hole. It was taken using a worldwide collection of radio telescopes, forming what is known as the Event Horizon Telescope (EHT), which focused its attention on the centre of a galaxy 55 million light years away,

Messier 87. This is a feat comparable to reading a newspaper that’s in New York while sitting down in a street in Paris – a job only made possible by combining the power of eight telescopes from around the world. The final image of the black hole has allowed astronomers the opportunity to combine computer models with direct observations, thus confirming or denying many pre-existing theories.

But this doesn’t mean that all the questions have been answered, including some pretty big ones.

For starters, how do black holes produce their enormous jets of hot, fast matter? Well, all supermassive black holes have the ability to chew up nearby matter, absorb most of it past their event horizons and spit the remainder out into space at near-light speed in blazing towers astrophysicists call ‘relativistic jets’. M87 is renowned for its

enormous jets, and while the EHT has imaged an impressively small object, the jets take up a much larger area. In order to learn more, linking up more EHT observations is critical in understanding these features further.

Another mystery is how general relativity and quantum mechanics fit together. Quantum gravity is the great unknown in physics. For about a century, physicists have worked using two different sets of rules: general relativity, which covers very big things like gravity, and quantum mechanics, which covers very small things. The problem is that these two rulebooks directly contradict one another. Quantum mechanics can't explain gravity, and relativity can't explain quantum behaviour. Physicists hope to link the two together in a grand unified theory, likely involving some sort of quantum gravity. The black hole image was as predicted from a general relativity perspective, so it offered no new physics that might close the gap. Still, it's not crazy that people hope for answers from this sort of observation, because the edge of a black hole's shadow brings relativistic forces into tiny, quantum-size spaces.

Another question remains whether Stephen Hawking's theories are as correct as Einstein’s. The physicist Stephen Hawking's greatest early career contribution to physics was the idea of ‘Hawking radiation’ – that black holes aren't actually black, but emit small amounts of radiation over time. The result was hugely important because it showed that once a black hole stops growing, it will start to very slowly shrink from the energy loss. But the EHT didn't confirm or deny this theory – not that anyone expected it to. Giant black holes like the one in M87 emit only minimal amounts of Hawking radiation compared to their overall size. While our most advanced instruments can now detect the bright light of event horizons, there's little chance that they will ever tease out the ultra-dim glow of a supermassive black hole's surface.

“Giant black holes like the one in M87 emit only minimal amounts of Hawking radiation compared to their overall size”