The Shadow of a Black Hole

The invisible manifests itself through the visible: so say many of the great works of philosophy, poetry, and religion. It’s also true in physics: we can’t see atoms or electrons directly and dark matter seems to be entirely transparent, yet this invisible stuff makes and shapes the universe as we know it.

Then there are black holes: though they are the most extreme gravitational powerhouses in the cosmos, they are invisible to our telescopes. Black holes are the unseen hand steering the evolution of galaxies, sometimes encouraging new star formation, sometimes throttling it. The material they send jetting away changes the chemistry of entire galaxies. When they take the form of quasars and blazars, black holes are some of the brightest single objects in the universe, visible billions of light-years away. The biggest supermassive black holes are billions of times as massive as the Sun. They are engines of creation and destruction that put the known laws of physics to their most extreme test. Yet, we can’t actually see them.

A simulation of superheated material circling the black hole at the center of the Milky Way. Credit: Scott C. Noble, The University of Tulsa

Black holes are a concentration of mass so dense that anything that gets too close—stars, planets, atoms, light—becomes trapped by the force of gravity. The point of no return is called the event horizon, and it forms a sort of imaginary shell around the black hole itself. But event horizons are very small: the event horizon of a supermassive black hole could fit comfortably inside the solar system (comfortably for the black hole, that is, not for us). That might sound big, but on cosmic scales, it’s tiny: the black hole at the center of the Milky Way spans just 10 billionths of a degree on the sky. (For comparison, the full Moon is about half a degree across, and the Hubble Space Telescope can see objects as small as 13 millionths of a degree.)

Both the size and nature of the event horizon make it difficult to observe black holes directly, though indirect observations abound. In fact, though black holes themselves are strictly invisible, their surrounding regions can be extremely bright. Many luminous astronomical objects produce so much light from such a small region of space that they can’t be anything other than black holes, even though our telescopes aren’t powerful enough to pick out the details. In addition, the stars at the center of the Milky Way loop close enough to show they’re orbiting an object millions of times the mass of the Sun, yet smaller than the solar system. No single object, other than a black hole, can be so small and yet so massive. Even though we know black holes are common throughout the universe—nearly every galaxy has at least one supermassive black hole in it, and thousands more smaller specimens—we haven’t confirmed that these objects have event horizons. Since event horizons are a fundamental prediction of general relativity (and make black holes what they are), demonstrating their existence is more than just a formality.

However, confirming event horizons would take a telescope the size of the whole planet. The solution: the Event Horizon Telescope (EHT), which links observatories around the world to mimic the pinpoint resolution of an Earth-sized scope. The EHT currently includes six observatories, many of which consist of multiple telescopes themselves, and two more observatories will be joining soon, so that EHT will have components in far-flung places from California to Hawaii to Chile to the South Pole. With new instruments and new observations, EHT astronomers will soon be able to study the fundamental physics of black holes for the first time. Yet even with such a powerful team of telescopes, the EHT’s vision will only be sharp enough to make out two supermassive black holes: the one at the center of our own Milky Way, dubbed Sagittarius A*, and the one in the M87 galaxy, which weighs in at nearly seven billion times the mass of the sun.

The theory of general relativity predicts that the intense gravity at the event horizon should bend the paths of matter and light in distinct ways. If the light observed by the EHT matches those predictions, we’ll know there’s an event horizon there, and we’ll also be able to learn something new about the black hole itself.

The “gravitational topography” of spacetime near the event horizon depends on just two things: the mass of the black hole and how fast it is spinning. The event horizon diameter of a non-spinning black hole is roughly six kilometers for each solar mass. In other words, a black hole the mass of the sun (which is smaller than any we’ve yet found) would be six kilometers across, and one that’s a million times the mass of the Sun would be six million kilometers across.

If the black hole is spinning, its event horizon will be flattened at the poles and bulging at the equator and it will be surrounded by a region called the ergosphere, where gravity drags matter and light around in a whirlpool. Everything crossing the border into the ergosphere orbits the black hole, no matter how fast it tries to move, though it still conceivably can escape without crossing the event horizon. The ergosphere will measure six kilometers across the equator for each solar mass inside the black hole, and the event horizon will be smaller, depending on just how fast the black hole is rotating. If the black hole has maximum spin, dragging matter near the event horizon at close to light speed, the event horizon will be half the size of that of a non-spinning black hole. (Spinning black holes are smaller because they convert some of their mass into rotational energy.)

When the EHT astronomers point their telescopes toward the black hole at the center of the Milky Way, they will be looking for a faint ring of light around a region of darkness, called the black hole’s “shadow.” That light is produced by matter that is circling at the very edge of the event horizon, and its shape and size are determined by the black hole’s mass and spin. Light traveling to us from the black hole will also be distorted by the extreme gravitational landscape around the black hole. General relativity predicts how these effects should combine to create the image we see at Earth, so the observations will provide a strong test of the theory.

If observers can catch sight of a blob of gas caught in the black hole’s pull, that would be even more exciting. As the blob orbits the black hole at nearly the speed of light, we can watch its motion and disintegration in real time. As with the ring, the fast-moving matter emits light, but from a particular place near the black hole rather than from all around the event horizon. The emitted photons are also influenced by the black hole, so timing their arrival from various parts of the blob’s orbit would give us a measure of how both light and matter are affected by gravity. The emission would even vary in a regular way: “We’d be able to see it as kind of a heartbeat structure on a stripchart recorder,” says Shep Doeleman, one of the lead researchers on the EHT project.

Event Horizon Telescope astronomers have already achieved resolutions nearly good enough to see the event horizon of the black hole at the center of the Milky Way. With the upgrades and addition of more telescopes in the near future, the EHT should be able to see if the event horizon size corresponds to what general relativity predicts. In addition, observations of supermassive black holes show that at least some may be spinning at close to the maximum rate, and the EHT should be able to tell that too.

Black holes were long considered a theorist’s toy, ripe for speculation but possibly not existing in nature. Even after discovering real black holes, many doubted we would ever be able to observe any of their details. The EHT will bring us as close as possible to seeing the invisible.

Go Deeper
Picks for further reading

Event Horizon Telescope
Learn more about the science and technology of the EHT at the experiment’s official website

Living Reviews in Relativity: The confrontation between general relativity and experiment
Clifford M. Will, a physicist at the University of Florida, on experimental tests of general relativity.

The New York Times: Black Hole Hunters
Dennis Overbye follows Sheperd Doeleman and the drama of the Event Horizon Telescope’s first observations from the summit of an extinct volcano in Southern Mexico.

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Matthew Francis

    Matthew R. Francis is a science writer, physicist, public speaker, educator, and frequent wearer of jaunty hats. He contributes a weekly column about astronomy and space to The Daily Beast. His writing has also appeared in Ars Technica, Slate, Nautilus, Aeon, and a variety of other publications. A former college professor and planetarium director, he holds a PhD in physics and astronomy from Rutgers University. Image courtesy of Tony Hitchcock.