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upstart writes in with an IRC submission:

After decades of effort, scientists are finally seeing black holes:

While working on his doctorate in theoretical physics in the early 1970s, Saul Teukolsky solved a problem that seemed purely hypothetical. Imagine a black hole, the ghostly knot of gravity that forms when, say, a massive star burns out and collapses to an infinitesimal point. Suppose you perturb it, as you might strike a bell. How does the black hole respond?

Teukolsky, then a graduate student at the California Institute of Technology (Caltech), attacked the problem with pencil, paper, and Albert Einstein's theory of gravity, general relativity. Like a bell, the black hole would oscillate at one main frequency and multiple overtones, he found. The oscillations would quickly fade as the black hole radiated gravitational waves—ripples in the fabric of space itself. It was a sweet problem, says Teukolsky, now at Cornell University. And it was completely abstract—until 5 years ago.

In February 2016, experimenters with the Laser Interferometer Gravitational-Wave Observatory (LIGO), a pair of huge instruments in Louisiana and Washington, reported the first observation of fleeting gravitational ripples, which had emanated from two black holes, each about 30 times as massive as the Sun, spiraling into each other 1.3 billion light-years away. LIGO even sensed the "ring down": the shudder of the bigger black hole produced by the merger. Teukolsky's old thesis was suddenly cutting-edge physics.

"The thought that anything I did would ever have implications for anything measurable in my lifetime was so far-fetched that the last 5 years have seemed like living in a dream world," Teukolsky says. "I have to pinch myself, it doesn't feel real."

[...] But no one could be sure those black holes actually are what theorists had pictured, notes Feryal Özel, an astrophysicist at the University of Arizona (UA). For example, "Very little that we have done so far establishes the presence of an event horizon," she says. "That is an open question."

Now, with multiple ways to peer at black holes, scientists can start to test their understanding and look for surprises that could revolutionize physics. "Even though it's very unlikely, it would be so amazingly important if we found that there was any deviation" from the predictions of general relativity, Carroll says. "It's a very high-risk, high-reward question."

[...] In September 2019, Teukolsky and colleagues teased out the main vibration and a single overtone from a particularly loud merger. If experimenters can improve the sensitivity of their detectors, Ohme says, they might be able to spot two or three overtones—enough to start to test the no-hair theorem.

[...] In the meantime, the sudden observability of black holes has changed the lives of gravitational physicists. Once the domain of thought experiments and elegant but abstract calculations like Teukolsky's, general relativity and black holes are suddenly the hottest things in fundamental physics, with experts in general relativity feeding vital input to billion-dollar experiments. "I felt this transition very literally myself," Ohme says. "It was really a small niche community, and with the detection of gravitational waves that all changed."

[The story provides a well-written and eminently readable history of research into black holes. Best treatment on the subject I've ever seen. --martyb]

Original Submission