Einstein’s monsters are broadcasting footage of the universe’s history – and there are ways we could get a clearer view, says Stephen Battersby
THE picture was seen by billions: a hazy ring, glowing orange-bright, surrounding a heart of darkness. The work of many minds over decades, it was above all a tribute to the brilliance of one. Yet as the world marvelled at the first ever direct image of a black hole – one of the cosmic monsters predicted by Albert Einstein’s theories – the researchers behind it found themselves confronted with a rather basic puzzle.
“After the result was published, we were all getting together and asking: what does this thing mean?” says radio astronomer Michael Johnson at Harvard University. They had been so wrapped up in turning their data into a picture that no one had really stepped back and tried to digest what it was telling them.
Over the past year, their quest to find answers has led them into a cosmic hall of mirrors, where the black hole’s gravity takes light from all directions, warps it and beams it to us as an infinitely recast image of the hole’s surroundings. The result is an epic movie of the history of the universe, as witnessed by a black hole, playing on a dramatically curved screen tens of billions of kilometres across.
From way back here in the cheap seats, about 55 million light years away, we will never be able to see the action’s full sweep, but we can catch glimpses. They could be enough to unlock the true history of giant black holes, put Einstein to the test like never before and maybe even lead to a deeper understanding of space and time.
Black holes are perhaps the most breathtaking prediction of Einstein’s general theory of relativity, the description of gravity he presented in 1915. No cosmological observation has been found to contradict its depiction of massive objects warping space and time around them. A black hole takes that idea to the extreme: it is a concentration of mass so great that space-time is warped to an infinite degree. Anything venturing too close is drawn across its event horizon, beyond which we can never see.
Although Einstein doubted that they actually existed, observations in recent decades have persuaded us that black holes are real. Small ones, just 10 or 20 times the mass of our sun, form when huge stars collapse at the end of their working lives. The gravitational waves detected by the LIGO collaboration in 2015 were ripples in spacetime caused by two such objects merging. These are dwarfed by supermassive black holes of millions to billions of solar masses that appear at the heart of almost every galaxy, including our own Milky Way. The image presented in 2019 was of M87, a giant elliptical galaxy in the Virgo cluster. It houses a beast of a supermassive black hole, with a mass probably 6.5 billion times that of the sun. The international Event Horizon Telescope team, which includes Johnson, used sophisticated signal processing to combine data from radio.
telescopes from around the world into one image of M87’s core. The resulting resolution matched that from a single radio dish the size of our planet. The darkness at the image’s centre is a shadow of the black hole; an image of the event horizon, magnified and distorted by the hole’s gravity. But what exactly is that surrounding glow? That was the question that initially no one could really answer.
To help decode the image, Johnson reached out to some more theory-minded researchers, including Alex Lupsasca, also at Harvard. “We had been colleagues side by side for many years,” says Lupsasca. “They were listening to us, but only with half an ear because they were busy doing their experiment.” “My role was finding the common language,” says Johnson. “We have black hole observers, black hole simulators, black hole theorists… It sounds so silly. But actually it is extremely difficult to communicate between these subfields; they are all very technical.