A team of astronomers has found indirect evidence of a supermassive black hole’s event horizon, providing further proof that these wacky objects actually exist in nature.
There are many wild ideas in science. But the theory of black holes — those objects so massive and compact that nothing, not even light, can escape — definitely makes the top of the list in my book.
Although the evidence for these grandiose objects is convincing, it isn’t 100% conclusive. So in a field that’s skeptical by nature, some astronomers are still working to prove that black holes are not just the concoction of theorists whose mathematics got away from them. When it comes to “something this extraordinary, we have to work harder before we’re willing to accept it,” says Ramesh Narayan (Harvard-Smithsonian Center for Astrophysics). It’s the carefully balanced dichotomy of wild ideas and obstinate suspicion that enables science to slowly zigzag toward the truth.
Now, a team of scientists including Avery Broderick (Perimeter Institute for Theoretical Physics, Canada), Narayan, and others has found further evidence of these elusive objects. The team peered into the very heart of M87, an elliptical galaxy whose supermassive black hole is 6 billion times heavier than the Sun. There, the astronomers saw indirect evidence for the monster’s event horizon: the point of no return that outlines the black hole itself.
The team used three radio telescopes in California, Arizona, and Hawai‘i to make the observations, which are part of the still-developing Event Horizon Telescope project. Their goal was to distinguish whether this object truly has an event horizon, which is characteristic of only a black hole, or if it actually has a surface.
As material streams onto any massive object, it heats up and emits torrents of high-energy photons. So the gas around the massive object should be extremely bright. But there will be a noticeable difference in that brightness depending on whether the object has a surface or an event horizon.
“If there’s a surface, then the stuff that falls in will splash onto the surface,” says co-author John Kormendy (University of Texas at Austin). And this splash will cause the material to glow even hotter and brighter. But if there isn’t a surface — like in the case of an event horizon — then from our perspective, the material eventually vanishes and therefore can’t keep boosting the glow.
So an object with an event horizon should appear darker than an equally massive object with a surface.
Under this assumption, the next few steps are relatively straightforward (but far from simple). First, the team measured the amount of material falling onto M87’s supermassive black hole. Second, the team calculated the expected brightness for an object with a surface. That number came in at roughly 100 billion times brighter than the Sun. Third, the team checked to see if their observations matched the expected brightness. If so, then the object has a surface. If not, then the object likely has an event horizon.
The team found an object that appeared darker than expected (by at least an order of magnitude) for an object with a surface. “So we conclude that the stuff is falling through an event horizon and giving out a lot less light than it would if it was bashing into a surface,” says Kormendy.
Black Holes Proven? Not Quite Yet.
Although very few astronomers doubt the existence of black holes, this result adds one more thread to the long spool of evidence that suggests black holes exist in nature. Previous studies looked at stellar mass black holes and the Milky Way’s own supermassive black hole, Sgr A*, and also found no evidence of a surface. The new M87 result expands the list of missing surfaces to heavier black holes beyond our galaxy.
It might not be the final nail in the coffin; the team is its own critic. “I will be the first to agree that this is not totally foolproof,” says Narayan. “It’s kind of proving something to be true, by proving that the opposite is not true. We’re proving that there is no surface so we’re saying there must be a horizon. That logical step is not 100% safe.”
The next nail in the coffin will come once the Event Horizon Telescope project is completed. In the next few years, the telescope will include antennas that span from the South Pole to North America, essentially creating a single Earth-size telescope that will cut through the gas and dust enshrouding these supermassive black holes and image the silhouette of the event horizon itself.
Avery Broderick et al. “The Event Horizon of M87.” Posted to arXiv.org April 5, 2015.
For more information on the Event Horizon Telescope project check out our February 2012 digital issue of Sky & Telescope magazine.