“We were all getting together and asking: what does this thing mean?” says radio astronomer Michael Johnson at Harvard’s Black Hole Initiative about the glowing bright orange ring around the now iconic black hole the size of our solar system at the center of the monster elliptical galaxy M87 –the largest, most massive galaxy in the nearby universe–described by astronomers on April 10, 2019 as “paradoxical, intriguing, frightening” and “the end of spacetime.”
The Event Horizon Telescope (EHT) image marked the endpoint of years of work undertaken by a team of 200 scientists in 59 institutes across 18 countries. The project drew on data collected by eight telescopes whose locations range from Hawaii to the South Pole.The EHT team created the equivalent of a lens the size of planet Earth by integrating data from all the telescopes that were part of the project that’s 4,000 times more powerful than the Hubble Space Telescope.
“Russian Doll of Nested Rings”
The now iconic image captured light from the entire universe wrapping around the object in a nested series of rings. Peter Galison of Harvard, an EHT collaborator said, “As we peer into these rings, we are looking at light from all over the visible universe, we are seeing farther and farther into the past, a movie, so to speak, of the history of the visible universe.”
“Each successive ring,” says Johnson, “has about the same diameter but becomes increasingly sharper because its light orbited the black hole more times before reaching the observer. With the current EHT image, we’ve caught just a glimpse of the full complexity that should emerge in the image of any black hole.”
“What really surprised us was that while the nested subrings are almost imperceptible to the naked eye on images—even perfect images—they are strong and clear signals for arrays of telescopes called interferometers,” says Johnson. “While capturing black hole images normally requires many distributed telescopes, the subrings are perfect to study using only two telescopes that are very far apart. Adding one space telescope to the EHT would be enough.”
Mysteries of the Orange Ring
So, what is the faint, fuzzy orange ring? What mysteries does it harbor? Over the past year, the quest of the Event Horizon Telescope scientists to find answers has led them what they describe as “a cosmic hall of mirrors,” The international team of observational astronomers, theoretical physicists, and astrophysicists reports the Institute for Advanced Study, predict a striking and intricate substructure within black hole images from extreme gravitational light bending, where the black hole’s gravity takes light from all directions, warps it and beams it to Earth some 55 million light years away as 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.
To capture the eerie orange glow of the image, the 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 center 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? To help decode the image, reports New Scientist, Johnson reached out to some more theory-minded researchers, including Alex Lupsasca, with Harvard’s Center for the Fundamental Laws of Nature. “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.”
Since the image was released in 2019, continues New Scientist, physicists have run many models of the maelstrom around M87’s black hole called GRMHD simulations that combine general relativity with magnetohydrodynamics, which describes the behavior of the hot, ionised gases that surround the hole. Each simulation starts with some assumptions about what might be producing the radio waves – for example, matter spiralling inwards – and follows the waves that would be produced by such a source as the hole’s gravity bends their path, to predict what we would see on Earth.
It turns out that a wide range of possible sources lead to a fuzzy glow like the one seen by the Event Horizon Telescope, observed New Scientist, “the black hole stamps its form with such force that the emission’s true origin is hidden. But although the models weren’t useful in distinguishing between the sources, they revealed something unexpected and intriguing. They all predicted that there should be a very bright, thin ring embedded in the broad fuzzy orange one.”
“We’re Not Going to See Dinosaurs”
General relativity, said Lupsasca at a talk at the Institute for Advanced Study, predicts that embedded within this image lies a thin “photon ring,” which is composed of an infinite sequence of self-similar subrings that are indexed by the number of photon orbits around the black hole. The subrings approach the edge of the black hole “shadow,” becoming exponentially narrower but weaker with increasing orbit number, with seemingly negligible contributions from high order subrings. He argues that these subrings produce strong and universal signatures on long interferometric baselines. These signatures offer the possibility of precise measurements of black hole mass and spin, as well as tests of general relativity, using only a sparse interferometric array.
On Deck –A Radio Telescope in Space
“This movie is highly biased to stuff near the black hole. Each subring is also only around six days older than the last, so there is a limit to how much of the reflected universe just a few frames show us. We’re not going to see dinosaurs,” Johnson says. “When a black hole spins, it drags space-time into a kind of whirlpool around it”
“If you can resolve the super thin photon ring and put a ruler across it, now you are talking precision measurement,” says Lupsasca – perhaps to better than 1 per cent.
“To see the black hole rings, we probably need to put a radio telescope in space,”says Lupsasca..”As for that black-hole’s-eye movie of the universe, even the million-mile-wide radio array made possible by a dish at L2 would only be enough to show us a trailer, just three frames long. For a feature-length version, it is hard to imagine what kind of distant-future technology would be good enough. “Since the subrings get exponentially thinner, you need to increase your telescope size by roughly a factor of 10 for each additional subring that you want to see,” says Lupsasca. A radio array spanning from here to our next nearest star Alpha Centauri, over 4 light years away, would get us up to about 10 subrings.”
Again, alas, no dinosaurs…
Image credits: Shutterstock License