The closest astronomers have come to directly “seeing” a black hole happened last year, when the LIGO observatory detected the spacetime-warping gravitational waves radiating from a pair of black holes that collided some 1.3 billion years ago.
That’s cool. But for astronomers, it’s not enough. What’s eluded them is a view of the event horizon, the boundary of the black hole from which, when crossed, there is no return. After the event horizon, gravity is so intense that not even light can escape.
We’ve never seen a direct image of a black hole. But if an audacious experiment called the Event Horizon Telescope is successful, we’ll see one for the first time.
Why we’ve never seen an image of a black hole
The biggest problem with trying to detect a black hole is that even the supermassive ones in the center of galaxies are relatively tiny.
"The largest one in the sky [is] the black hole in the center of the Milky Way," Dimitrios Psaltis, an astrophysicist at the University of Arizona, said in 2015. "And taking a picture of it would be equivalent to taking a picture of a DVD on the surface of the moon."
What's more, because of their strong gravity, black holes tend to be surrounded by other bright matter in the process of being torn to bits. And it makes the actual dark spot hard to see.
That's why when hunting for black holes, astronomers don't usually try for direct observation. Instead, they look for evidence of the effects of a black hole's gravity and radiation.
“We typically measure the orbits of stars and gas that seem to circle around very dark 'spots' in the sky and measure how much mass is there in that dark spot,” Psaltis says. "If we know of no other astrophysical object that can be so massive and so dark as what we just measured, we consider this as very strong evidence that a black hole lies there."
Here’s a visualization of that concept:
You’re looking at 20 years of data on the stars that live near the supermassive black hole at the center of the Milky Way galaxy, called Sagittarius A. The stars, some many times more massive than our sun, are orbiting it.
“These orbits, and a simple application of Kepler's Laws, provide the best evidence yet for a supermassive black hole, which has a mass of 4 million times the mass of the Sun,” explains UCLA’s Galactic Center Group, which produced the animation.
Here’s another look at the same phenomenon. This video includes 16 years of images from the European Southern Observatory. This isn’t an animation; it’s real images of stars sped up by a factor of 32 million. Watch them dance around a mysterious blank center. There’s no known object other than a black hole that could force such massive stars into orbit.
And it’s this black hole — the mysterious Sagittarius A — that astronomers just might take a picture of in the coming days.
How the Event Horizon Telescope works
Because Sagittarius A is so small, and surrounded by so much occluding material, it’s going to take a huge telescope to see it. According to Nature, it would take a telescope 1,000 times more powerful than Hubble to get enough resolution to see it.
So how does the Event Horizon Telescope solve this problem? Conventional optical telescopes use bigger and bigger mirrors to see objects smaller and farther away in the universe. The Event Horizon Telescope is doing something similar: It’s creating a virtual telescope the size of the entire Earth.
The Event Horizon team is connecting radio telescopes at eight locations across the world — as far-flung as Hawaii and the South Pole — and instructing them all to look toward Sagittarius A for a few days. The network is the result of an international collaboration of 14 research institutions across the world.
These telescopes look like giant satellite dishes, and rather than picking up visible light like the telescopes you may have used in a backyard, they’re sensitive to radio frequencies, which our eyes can’t see. Astronomers will often take the radio telescope data and convert it to images our eyes can see.
Together, these eight telescopes have the power to “count the stitches on a baseball from 8,000 miles away,” as MIT explains. (The array will generate such a huge amount of data that it’s more efficient to fly the data from each of the telescopes to a centralized location than it is to transfer it over the internet. Data processing will take place at both the Max Planck Institute in Bonn, Germany, and the Haystack Observatory in Massachusetts.)
Event Horizon came online April 4, and it is expected to run until April 14. Along with observing Sagittarius A, it will also take a look at the supermassive black hole in the galaxy Messier 87, which is about 1,500 times more massive than Sag A.
But why bother?
Astronomers want to find the event horizon because 1) it’s freaking awesome, but also 2) in this extreme accretion disc region, Einstein’s 100-year-old theory of gravitation is put to the ultimate test. Supermassive black holes like Sag A also generate huge plumes of material that can extend the entire length of galaxies. Understanding how these plumes are generated will help astronomers understand how galaxies evolve. Scientists can also use the image to better test the idea of “Hawking radiation,” which suggests black holes are actually slowly losing mass over time.
But most of all, the direct imaging of a black hole will prove that studying even the most mysterious and powerful forces in the universe isn’t beyond our grasp.
Using a complicated technique called Very Long Baseline Interferometry, astronomers will be able to combine the data into one astronomical observation, and hopefully produce an unclouded image of a black hole.
The resulting image might show a dark spot surrounded by an accretion disk — a bright ring of matter that swirls around it. The disc will appear like a halo, as Nature explains:
Contrary to most artistic depictions of black holes, the accretion disk does not disappear behind the object the way Saturn’s rings can partly hide behind the planet. Around a black hole, there’s no hiding: gravity warps space-time, and here the effect is so extreme that light rays go around the black hole, showing multiple distorted images of what lies behind it. This should make the accretion disk appear to wrap around the black hole’s shadow like a halo
It may be months before the Event Horizon team assembles and publishes the image. And success is not assured: Even radio telescopes need clear skies to get an uninterrupted view of the heavens. But if it does succeed, it will be a huge success for astronomy, physics, and the power of worldwide collaboration.
Correction: A previous version of this post said the radio telescopes can pick up on a broad range of electromagnetic radiation, including ultraviolet and gamma radiation. This is wrong. These telescopes are only searching for radio frequencies.