Physicists have been theorizing about black holes for generations. Now science has made it possible to see one.
On April 10, astronomers working with the Event Horizon Telescope unveiled a “photograph” of the monstrous black hole that sits at the center of the M87 galaxy, 54 million light years away. Viewed simply as an image, it is neither impressive nor straightforward: It presents a sort of nondescript, blurry, half-glazed doughnut, showing not the black hole itself but the shadow-like distortion it carves in the surrounding illumination. Yet, to a thinking mind, the image reflects the glory of understanding, and to an alert imagination it opens new portals into space, time and deep history.
The making of the image was a tour de force of science and technology. The black hole is enormous, with a radius of roughly 9 billion miles (or one hundred times the distance from the Earth to the sun) and a mass equivalent to two quadrillion Earths. But because it is so far away, it occupies only a tiny portion of the sky, so we must use a very large, exotic telescope to see it. In fact, the Event Horizon Telescope isn’t a single instrument but a system of eight radio dishes at six far-flung locations in Hawaii, Arizona, Spain, Mexico, Chile and Antarctica. Astronomers used precise atomic clocks, accurate within a trillionth of a second, to synchronize data from all these places and then stitched it all together using supercomputers.
The concept of a black hole goes back to the 18th century, when the English astronomer-clergyman John Michell calculated that a sufficiently large star couldn’t shine because light wouldn’t move fast enough to “lift off” and escape the star’s gravity. But Michell’s conjecture outran the physics of its time, which didn’t understand light, gravity or stars well enough to support it.
The foundations for the modern understanding of black holes weren’t laid until the early 20th century, building on James Clerk Maxwell’s theory of electromagnetism and Albert Einstein’s theory of relativity. In 1939, J. Robert Oppenheimer and Hartland Snyder wrote “On Continued Gravitational Contraction,” the most important paper in the history of black holes. They considered what would happen to a dust cloud of any size and uniform density as it collapses through the force of gravity. Observers within the dust cloud would find themselves surrounded by matter, and they would experience a universe broadly resembling our own, which would eventually end in a “big crunch.” But observers outside the collapsing cloud would see it wink out into a black hole, as gravity at its boundary increases beyond the ability of light—or any form of matter—to escape.
Oppenheimer and Snyder’s work made it clear that black holes were a plausible outcome of known physical processes. It also makes the astonishing suggestion that a black hole can be a universe and vice versa, differently viewed. I don’t think physics has fully digested this idea, even today.
It would be surprising if M87’s black hole revealed fundamentally new physical processes. Such gigantic black holes have low density and exert only weak forces on the outside, albeit on a grand scale. But by comparing and contrasting the central black holes in different galaxies, we will learn about how galaxies form and evolve. Our own Milky Way galaxy also harbors a central, giant black hole, hidden from direct observation by enshrouding dust.
Seeing the black hole image, my mind flashed back to an earlier iconic image, the “Earthrise” captured by Apollo 8. The black hole is much bigger and more imposing, as a physical object, but also much less complex—not to mention less user-friendly—than our Earth. It’s a wonderful world that is home to both.
Originally appeared on April 17, 2019 on The Wall Street Journal website as ‘Face to Face with a Cosmic Wonder‘
Frank Wilczek is the Herman Feshbach Professor of Physics at MIT, winner of the 2004 Nobel Prize in Physics, and author of the books Fundamentals: Ten Keys to Reality (2021), A Beautiful Question: Finding Nature’s Deep Design (2015), and The Lightness of Being: Mass, Ether, and the Unification of Forces (2009).