Featured image: The first photograph of a black hole to ever be recorded has been taken. Photo Courtesy of National Science Foundation
On April 10, 2019, members of the Event Horizon Telescope Collaboration (EHT) achieved an astronomy first: they published an image of a black hole’s shadow and accretion disk. But after a week of coverage from news and social media, what that means may still be a little nebulous.
The concept of an object that light could not escape was first proposed in 1783 by a brilliant natural philosopher John Michell as the “dark star,” but its insertion into popular thought would not happen until the mid-20th century when the term “black hole” was first coined.
Black holes were given a theoretical basis by Einstein’s theory of general relativity. “[The black hole] is one of Einstein’s predictions, and in Einstein’s case, an unwelcome prediction,” said Rob Stiling, professor of history at Seattle Pacific University, in an interview.
Karl Schwarzschild, one of Einstein’s contemporaries, offered a solution to the general relativity equations that completed it but allowed for strange phenomena where high densities of mass completely distorted spacetime. At the “Schwarzschild radius” of such a dense object, now referred to as the event horizon, time would stop and no light could escape.
Einstein actually rejected the possibility of a black hole existing in reality, but fellow scientists continued to develop relativity’s application to black holes.
“At this stage, the concept of a black hole is such a part of cosmology now, that it’s not really controversial,” said Stiling, “but it’s a really cool bonus that now we think we’ve seen one.”
The apparent gravitational effects of black holes have been viewed indirectly by their effect on local star orbits; the radiation signatures of early universe, matter-hungry quasars, black holes that blast extreme amounts of radiation; and rarely, gravitational lensing by quasars, where light from a distant sources bends around a massive object and appears multiple times.
While the existence of black holes was all but assured, one takeaway the imaging of M87* gave astronomers was its support for black hole mass prediction methods. Before the imaging of a black hole was feasible, astronomers used mass estimates based on either orbiting gas or stars, finding different values for each method. The recent measurement of the M87* black hole diameter gave a value very close to predictions made with star orbits.
In a star system like our own, a star’s accretion disk, the matter rotating around a central gravitational source, slowly cools and stabilizes into planets or asteroid fields, but the accretion disks of black holes at the centers of galaxies have a more destructive nature. Doug Downing, physics professor at SPU, used the analogy of trying to fill a dog dish with a hose.
“Most of the water splashes away from the dish,” explained Downing. “With the black hole, it is extremely unlikely that a particle [falling] towards it will hit it exactly, meaning that as [a particle] gets closer and closer to the black hole it will have to start going faster and faster because of angular momentum conservation. As all these particles orbit at high speeds they create friction and heat up.”
This process releases huge amounts of radiation all across the spectrum, from x-rays to radio waves, beyond what most stars put out with fusion. Ground-based radio telescopes look to these radio waves which move through space dust and our atmosphere alike, whereas visible light or x-ray are usually blocked.
People who have heard of the black hole in the center of the Milky Way may wonder why that black hole, Sagittarius A*, was not imaged instead of the super distant M87*. The Messier 87 galaxy, home of the recently imaged M87* black hole, is one of the most massive galaxies in the local universe.
The 53 million light-year away galaxy is nearly spherical and double the mass of the Milky Way. Sheperd Doeleman, the director of the EHT, explained this decision at a TED talk.
“The black hole in the center of the [Milky Way] galaxy is a thousand times less massive, but also a thousand times closer, so it looks the same angular size in the sky,” Doeleman said.
Parallax also likely played a role in the EHT’s decision to image M87* first. In the case of M87*, its distance was helpful, since the amount of wobble in a distant object is less pronounced than in a close object. The gravitational nudges of stars orbiting Sagittarius A* are very small but more apparent both because of its smaller mass and close proximity relative to M87*.
The Event Horizon Telescope Collaboration needed a telescope the size of a planet to produce an image of M87*. The EHT used a technique called interferometry, in which two or more smaller radio telescopes can combine their signal data to achieve the image resolution of a much larger telescope.
For perspective, the EHT stated that their current array offers resolution great enough to “read a newspaper in New York from a sidewalk café in Paris.”
The planned addition of more telescopes to the array will make this even better, though Doeleman also hopes to integrate space telescopes as a next step of the EHT array.
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The EHT monitored M87*’s radio emissions with eight high-altitude radio telescopes around the world, synchronized with hyper-precise atomic clocks.
Millions of gigabytes of data collected over 4 days in April 2017, went through supercomputers called correlators, which lined up the eight sets of data. Then, four different imaging teams extrapolated the missing pieces of image data with advanced algorithms. The four teams each developed their algorithms independently to avoid bias in their final image.
Katie Bouman, a MIT Ph.D student and member of one of the imaging teams, quickly gained fame when a photo of her excitement for the freshly-processed black hole image went viral. In her 2016 TEDx talk, she explained how the imaging teams’ algorithms fill in the missing information of images.
“We need a way to tell our algorithms what images look like without imposing one type of image’s features too much. One way we can try to get around this is by imposing the features of different kinds of images and seeing how the type of image we assume affects our reconstructions.”
If an algorithm can process an image different teaching materials and give the same end image, it is a sign that the algorithm works well and that human bias — like wanting to see a beautiful inferno of an accretion disk — has not altered how the algorithm performed.
Bouman attributed the tentative success of the EHT to the diversity of expertise represented.
“We’re a melting pot of astronomers, physicists, mathematicians and engineers, and this is what will make it soon possible to achieve something once thought impossible.”
M87* was the first black hole imaged, but the EHT plans to image Milky Way’s Sagittarius A* at some point in the future.
Though Sagittarius A* is much less active in accreting material than M87*, it is possible with time that the EHT will manage to catch Sgr A* in the act of consuming unlucky gas clouds, siphoning daredevil stars, or — with some great observations, planning and luck — the engulfing of its smaller, stellar-mass black hole neighbors.
