In April of 2017, an international network of telescopes turned its attention to a galaxy some 55 million light years away from home. At the core of Messier 87 lies a black hole 6.5 billion times more massive than our Sun—the first to ever be photographed, and “the strongest evidence that we have to date” that these bodies exist.
“We have seen what we thought was unseeable,” the director of the Event Horizon Telescope Project, Shep Doeleman, told reporters at a press conference in Washington. The image is blurry but unmistakable. Photons burn orange around a black centre, apparently curving under intense gravity. “What you’re seeing here is the last photon orbit. What you’re seeing is evidence of an event horizon.”
It’s the result of over a decade of work, and more than a hundred years’ worth of theories, models, and observations. As put by the director of the National Science Foundation, France Córdova, before the long-awaited announcement, “We have been studying black holes for so long that sometimes it is easy to forget that none of us have ever actually seen one.”
The Point of No Return
The general theory of relativity proposes that there is a boundary at which the gravitational pull of a massive object becomes so great that escape is impossible. In theory this massive object, packed into such a comparatively small space, has the ability to warp space and time. Light bends, time slows, and the laws of physics break down.
It begins with the collapse of a large star. The object increases in mass but decreases in radius. Get too close to this object, and the singularity at its centre becomes your future. There’s nothing in space or time that can prevent what happens next.
Until now, we have only inferred the presence of black holes by detecting their effect on nearby matter. We believe there are black holes 10 to 24 times the mass of our Sun, and supermassive black holes millions or billions times that mass. We believe they are the core of most large galaxies, and that they have an instrumental role in shaping them. But they are so massive and dense that nothing can escape their pull, making a direct observation impossible.
“They’re cloaked by an event horizon, where their gravity prevents even light from escaping,” Doeleman said. “And yet, the matter that falls onto the event horizon is superheated, so that before it passes through, it shines very, very brightly.”
This is what we’re seeing in the first images of M87’s black hole—photons are being pulled into a temporary and unstable orbit, before reaching their final destination over the event horizon. This photon sphere, and the shadow of an event horizon, appears to confirm what has so far only existed in theory and assumption. “We now know, clearly, that black holes drive large scale structure in the universe from their home in these galaxies. And we now have an entirely new way of studying general relativity and black holes that we never had before.”
An Expected Surprise
Eight ground-based radio telescopes were used to form an Earth-sized field of “unprecedented sensitivity and resolution” to capture the images, and they were met with relief. “We could have seen a blob,” Doeleman said. “We could have seen something that was unexpected, but we didn’t see something that was unexpected. We saw something so true.”
The captured images are astonishing, and even more so when compared to a simulation of the M87 black hole, blurred to the expected resolution of the telescope. They look almost identical. Our predictions, guided by science more than a hundred years old, appear to match up with our observations in outer space.
Credit: Akiyama et al and ApJL
Albert Einstein’s general theory of relativity was first published in 1916 and is still regarded as one of the most securely founded theories of all time. The theory proposes that gravity arises from the curvature of space-time. Massive objects weigh down the fabric of the cosmos—and an object massive and compact enough has the ability to deform space-time to form a black hole.
Einstein’s theory is comprised of 10 field equations, and when we talk about black holes, Karl Schwarzschild’s solution is perhaps the most significant. The German astronomer understood that the escape velocity from the surface of an object depends on its mass and radius. A rocket must be travelling at least 11.2 kilometres per second to escape Earth’s gravity. If an object is massive enough, with a small enough radius, the escape velocity reaches the speed of light. As far as we know, nothing can travel faster than the speed of light, and so this object—this black hole—is inescapable.
The general theory of relativity not only predicts the existence of black holes, but it also posits that event horizons should be circular. Using Schwarzchild’s solution, the EHT team was able to come to the conclusion that the event horizon of M87’s black hole has a radius of roughly 20 billion kilometres, weighing in with the mass of 6.5 billion suns.
It appears New Zealand mathematician Roy Kerr has also been proven right, after solving Einstein’s field equations himself more than 50 years ago. The Kerr solution had proposed all black holes rotate, triumphing over Schwarzchild’s assumption that they remain static. Observations show the M87 black hole is spinning clockwise. “Overall,” the paper states, “the observed image is consistent with expectations for the shadow of a spinning Kerr black hole as predicted by general relativity.”
Over and over, experiments have backed Albert Einstein’s theory: the unusual orbit of Mercury, gravitational time dilation here on Earth, the discovery of gravitational waves in 2015, and now, the first images of a black hole. Avery Broderick, associate professor at the University of Waterloo’s physics and astronomy department, said, “Einstein’s equations are beautiful, and so often in my experience nature wants to be beautiful… Just for that reason alone, and the long history of Einstein being proven right here, I suppose we’re not terribly surprised.”
The black hole at the centre of Messier 87 wasn’t the only target of the Event Horizon Telescope project. It was hoped the Milky Way’s very own black hole, Sagittarius A*, would have been the first object of its kind photographed. But with Sgr A* amounting to the mass of just four million suns, capturing an image has proven difficult. The team assured the world they’re working on it, and much more.
New telescopes will be added to the project, hopefully sharpening the images and providing more detail for scientists unravelling the mysteries of space from the ground. This includes the study of powerful jet streams of high-energy particles, fired some 5,000 light years into space, as seen in this Hubble image from 1998.
Credit: J. A. Biretta et al., Hubble Heritage Team (STScI /AURA), NASA
Scientists have even higher hopes. These images could provide clues in the long search for an overarching theory to combine the general theory of relativity and quantum mechanics. The two theories have been successful in describing the very big and the very small, but refuse to cooperate, especially when it comes to gravity. Fresh findings about black holes could finally make sense of quantum gravity, the elusive field of theoretical physics that aims to marry the two.
When asked about the implications in the search for dark matter, the response was “not yet”. The effort it took to compile these images alone is a massive accomplishment. The findings that could come out of this data will likely span decades, involving scientists from various fields around the globe, in the same spirit of international collaboration that brought us here in the first place. As Avery Broderick said, “mysteries abound around black holes” and there’s always plenty more to find out.
Featured image credit: EHT Collaboration