Cosmic Log
Astronomers have used machine learning to sharpen up the Event Horizon Telescope’s first picture of a black hole — an exercise that demonstrates the value of artificial intelligence for fine-tuning cosmic observations.
The image should guide scientists as they test their hypotheses about the behavior of black holes, and about the gravitational rules of the road under extreme conditions.
After years of observation and weeks of rumor-mill rumblings, astronomers today unveiled their first image of the supermassive black hole at the center of our own Milky Way galaxy, Sagittarius A*.
Technically, the picture from the Event Horizon Telescope project doesn’t show light from the black hole itself. After all, a black hole is a gravitational singularity so dense that nothing, not even light, can escape its grip. Rather, the picture shows the “shadow” of a black hole, surrounded by the superheated, glowing gas that surrounds it.
And technically, the picture may not match what folks might see with their own eyes up close. Rather, the readings come from eight observatories around the world that combined their observations in radio wavelengths.
Nevertheless, the new view of Sagittarius A*, or Sgr A* for short (pronounced “sadge-ay-star”), serves to confirm in graphic terms what astronomers have long suspected: that our galaxy, like many others, has a supermassive black hole at its heart.
Today’s revelations follow up on the Event Horizon Telescope’s first-ever black hole image, which was released in 2019 and showed the supermassive black hole at the center of M87, an elliptical galaxy about 55 million light-years away.
Sgr A* is much closer — a mere 27,000 light-years from Earth, in the constellation Sagittarius. But there’s nothing to fear from this black hole: It’s relatively quiescent, in contrast to the galaxy-gobbling behemoths that are standard science-fiction fare.
Our galaxy’s black hole is thought to hold the mass of 4 million suns within an area that’s roughly as big around as Mercury’s orbit. Checking those dimensions against the image data serves as a test of relativity theory. Spoiler alert: Albert Einstein was right … again.
“We were stunned by how well the size of the ring agreed with predictions from Einstein’s theory of general relativity,” EHT project scientist Geoffrey Bower said in a news release. “These unprecedented observations have greatly improved our understanding of what happens at the very center of our galaxy and offer new insights on how these giant black holes interact with their surroundings.”
The EHT’s findings about Sgr A* are the subject of a special issue of The Astrophysical Journal Letters — and to whet your appetite for all that reading material, here are three videos that summarize the past, present and future of black hole imaging:
Why are black holes so alluring?
You could cite plenty of reasons: They’re matter-gobbling monsters, making them the perfect plot device for a Disney movie. They warp spacetime, demonstrating the weirdest implications of general relativity. They’re so massive that inside a boundary known as the event horizon, nothing — not even light — can escape its gravitational grip.
But perhaps the most intriguing feature of black holes is their sheer mystery. Because of the rules of relativity, no one can report what happens inside the boundaries of a black hole.
“We could experience all the crazy stuff that’s going on inside a black hole, but we’d never be able to tell anybody,” radio astronomer Heino Falcke told me. “We want to know what’s going on there, but we can’t.”
Falcke and his colleagues in the international Event Horizon Telescope project lifted the veil just a bit two years ago when they released the first picture ever taken of a supermassive black hole’s shadow. But the enduring mystery is a major theme in Falcke’s new book about the EHT quest, “Light in the Darkness: Black Holes, the Universe, and Us” — and in the latest installment of the Fiction Science podcast, which focuses on the intersection of fact and science fiction.
The first-ever picture of a black hole is the gift that keeps on giving — in the form of new insights into the dynamics behind the mysterious phenomenon and new evidence that Albert Einstein was right.
The validity of Einstein’s theory of general relativity has been proven time and time again over the course of the past century. But physicists keep coming up with new ideas for tweaking the theory’s equations in unorthodox ways.
To figure out how much leeway there could be for variations on Einstein’s theme, researchers took a closer look at the supermassive black hole at the center of the galaxy M87.
M87’s black hole, which lies about 55 million light-years from Earth, was featured in a history-making close-up last year, produced by a radio astronomy collaboration known as the Event Horizon Telescope. The achievement is likely to win the EHT collaboration a Nobel Prize as soon as next week.
The team behind the relativity-checking research, published this week in Physical Review Letters, measured the size of the black hole’s shadow — that is, the dark central region from which light rays can’t escape, due to the gravitational pull of a singularity that’s 6.5 billion times as massive as our sun.
The predicted size of the shadow could vary, depending on which theory of gravity you go with. But in M87’s case, the size matched up precisely with Einstein’s theory.
The latest test of general relativity in the strong gravitational field regime near black holes (Psaltis et al. 2020) constrains deviations from GR by a factor of 500 better than historical constraints!
Image Credit:
Dimitrios Psaltis and Raquel Fraga-Encinas pic.twitter.com/HzaWC1NKZ8— Event Horizon 'Scope (@ehtelescope) October 1, 2020
“Using the gauge we developed, we showed that the measured size of the black hole shadow in M87 tightens the wiggle room for modifications to Einstein’s theory of general relativity by almost a factor of 500, compared to previous tests in the solar system,” the University of Arizona’s Feryal Özel, a senior member of the EHT collaboration, said in a news release.
“Many ways to modify general relativity fail at this new and tighter black hole shadow test,” Özel said.
The Event Horizon Telescope’s findings add to a bonanza of black hole data from the Laser Interferometer Gravitational-wave Observatory, or LIGO, and Europe’s VIRGO detector.
“Together with gravitational-wave observations, this marks the beginning of a new era in black hole astrophysics,” said lead study author Dimitrios Psaltis, a University of Arizona astronomer who recently finished his stint as the EHT collaboration’s project scientist.
And the EHT isn’t stopping with last year’s image. Just last month, the collaboration unveiled a “movie” that shows a wobbling pattern of emissions from the surroundings of M87’s black hole. The analysis of black hole dynamics over time, published in The Astrophysical Journal, was created by feeding more than a decade’s worth of observations into a computer model.
Eight observatories around the world contributed to the initial round of observations for the Event Horizon Telescope project. For the EHT’s next campaign in 2021, there’ll be three more observatories on the case, in Arizona, Greenland and France.
The added capacity should result in higher-fidelity images — not only of M87’s black hole, but also of Sagittarius A*, the supermassive black hole at the center of our own Milky Way galaxy.
Up-close views of black holes could well shine a light on another prediction made by general relativity, known as the no-hair theorem. This theorem states that the characteristics of black holes are completely determined by their mass, spin and electrical charge.
If the theorem is correct, all black holes with the same values for those three attributes would be identical to each other. Any other distinguishing characteristics for black holes and their history — their “hair,” metaphorically speaking — would disappear forever inside the black hole’s event horizon.
In a theoretical paper that was published months after the death of British physicist Stephen Hawking, he and three co-authors argued that black holes might be surrounded by a distinctive kind of “soft hair” that’s left behind as they evolve. This would have deep implications for general relativity, and for a long-running, almost metaphysical debate over what happens when something falls into a black hole.
Was Einstein right, or was Hawking? Thanks to gravitational-wave detectors and the Event Horizon Telescope, we could well find out.
WASHINGTON, D.C. — Scientists today shared the first picture to show the immediate surroundings of a galaxy’s supermassive black hole, captured by a network of radio telescopes that adds up to what could be considered the world’s widest observatory.
A project called the Event Horizon Telescope delivered a fuzzy view of the dark monster at the center of an elliptical galaxy known as M87. The edge of the black hole’s dark circle, known as the event horizon, was surrounded by the bright glare of superheated material falling into the black hole.
“This is a remarkable achievement. … It’s almost humbling in a certain way,” EHT project director Shep Doeleman, an astronomer at the Harvard-Smithsonian Center for Astrophysics, said during a news briefing here at the National Press Club.