By their very name, black holes exude mystery. They’re unobservable, uncontrollable and—for more than 50 years after their first prediction in 1916—undiscovered. Astronomers have since found evidence of black holes in our universe, including a supermassive one at the center of our own Milky Way. Yet much remains unknown about these cosmic enigmas, including what exactly happens to the stuff that they suck up with their titanic gravity.
Fifty years ago, physicist John Wheeler helped popularize the term "black hole" as a description for the collapsed remnants of supermassive stars. According to Wheeler, who coined and popularized several other famous astronomy terms such as "wormholes," the suggestion came from an audience member at an astronomy conference where he was speaking, after he had repeatedly used the phrase "gravitationally collapsed objects to describe the cosmic giants.
“Well, after I used that phrase four or five times, somebody in the audience said, ‘Why don’t you call it a black hole.’ So I adopted that,” Wheeler told science writer Marcia Bartusiak.
Wheeler was giving a name to an idea first explored by Albert Einstein 50 years earlier, in his influential theory of general relativity. Einstein's theory showed that gravity is a result of the distortion of space and time by the mass of objects. While Einstein himself resisted ever acknowledging the possibility of black holes, other physicists used his groundwork to flesh out the galactic monsters. Physicist J. Robert Oppenheimer, of atomic bomb fame, dubbed these bodies "frozen stars" in reference to a key feature outlined by physicist Karl Schwarzschild soon after Einstein published his theory.
That feature was the "event horizon": the line surrounding a black hole at which it becomes impossible to escape. Such a horizon exists because, at a certain distance, the speed required for any atom to break away from the black hole's gravity becomes higher than the speed of light—the universe's speed limit. After you cross the event horizon, it is thought, all of the matter that comprises you is shredded apart violently by intense gravitational forces and eventually crushed into the point of infinite density at the center of the black hole, which is called a singularity. Not exactly a pleasant way to go.
This detailed explanation of death via black hole, however, is theoretical. The intense gravity of black holes distorts the passage of time so much so that to observers outside the black hole, objects falling into one appear to slow down and "freeze" near the event horizon, before simply fading away. (Which sounds a lot nicer.)
In other words, despite the importance of this event horizon, scientists have never actually directly proven its existence. And because of the difficulty of even finding black holes (because light cannot escape them, they are invisible to most telescopes), much less observing them, there haven't been many chances to try. In the absence of convincing proof, some astrophysicists have theorized that some of the objects we call black holes might be dramatically different than what we’ve come to believe, with no singularity and no event horizon. Instead, they could be cold, dark, dense objects with hard surfaces.
This black hole skepticism began attracting its own skepticism, however, as telescopes finally captured black holes in the act of something extraordinary. In the last seven years, "people started seeing stars falling into black holes," says Pawan Kumar, an astrophysicist at the University of Texas at Austin, where incidentally Wheeler taught theoretical physics for a decade. "These are very very bright things that can be seen from billions of light years away."
More of these bright, relatively quick star swallowings have since been observed. Last year Kumar decided that these light emissions would make a good test for proving the existence of the event horizon. "Most people in the community assumed there is no hard surface," Kumar says. However, he stresses, "in science, one needs to be careful. You need proof."
So in 2016, Kumar and his collaborator Ramesh Narayan, of the Harvard-Smithsonian Center for Astrophysics, worked to calculate what kind of effects you would expect to see if a star being swallowed by a black hole was really colliding with a hard surface. It would be akin to smashing an object against a rock, Kumar says, creating intense kinetic energy that would be emitted as heat and light for months—or even years.
Yet a scan of telescope data over three and a half years found no instances of the light signatures that he and Narayan calculated would be released if stars struck a hard-surface black hole. Based on probability, the researchers had predicted that they should have found at least 10 examples over that time period.
Kumar calls this research, published this year in the journal Monthly Notices of the Royal Astronomical Society, a "good-sized step" toward proving the event horizon's existence. But it's still not quite proof. A hard-surface black hole could theoretically still exist within his study's calculations. But the radius of that surface would have to be within about a millimeter of the black hole's Schwarzschild radius, or the point at which the speed necessary to escape the gravity of it would equal the speed of light. (Note this the Schwarzschild radius is not always the same as an event horizon, since other stellar objects have gravity, too).
"The limits this paper places on the radius of a possible solid surface—4 thousandths of a percent outside the Schwarzschild radius for a supermassive compact object—is impressive," says Bernard Kelly, a NASA astrophysicist who wasn't involved in this research.
Kumar already has research in the pipeline to narrow that limit even further, to the point where it would be almost certain that no hard-surface black holes could possibly exist. That, for him, would be reliable proof that traditional black holes are the only kind of black holes that occupy our universe. "If it is completed, it will pretty much in my view close the field," Kumar says. "We will have firm evidence that Einstein's theory is right."