Nothing gets out of a black hole---not even light. Once a star, a planet, a piece of dust, or even a single photon crosses the limit known as the event horizon, it’s not coming out again. Pulled into the crushing gravity of the singularity at the black hole’s heart, it vanishes from the universe.
That’s a big problem if what you really want from a black hole is a photograph. By definition, it’s impossible. All light getting sucked in means no light reflects back---so a black hole is invisible, across the spectrum. And, duh, invisible objects don’t show up in photographs.
But thanks to a new telescope, Tim Johannsen, an astrophysicist at the Perimeter Institute and the University of Waterloo in Ontario, Canada, may be able to get a black hole pic after all. A loophole in physics means he might be able to see not the black hole itself, but its shadow. And, no big deal, but that photo just might overturn Albert Einstein’s theory of general relativity.
So...wait. Black holes have shadows? Sort of. As gas and dust and other cosmic material approaches a black hole, “that stuff heats up, like millions and millions of degrees,” Johannsen says. That superheated matter swirls around the black hole in what’s called an accretion disk; because it’s so hot, the accretion disk emits a lot of light. Some of those photons zoom out towards Earthbound telescopes, while others cross the event horizon and fall into the void. So when astronomers look at a black hole, what they expect to see is a ring of bright light---the accretion disk---surrounding a circle of nothingness. That circle of nothingness is the shadow. (The black hole itself is just a single point within.) You can see a model of that here:
At least, that’s the idea. No one has ever actually seen a black hole’s shadow. “Despite their enormous mass, black holes are also exceedingly small,” says Avery Broderick, Johannsen’s colleague at the Perimeter Institute and the University of Waterloo. Seen from Earth, the shadow of Sagittarius A*, the supermassive black hole at the center of the Milky Way (also known as Sgr A*, which astrophysicists pronounce “Saj-A-star”) is just 1/35,000,000th the width of the Moon, or 50 microarcseconds wide.
Here’s where that new telescope comes in. Maybe. Johannsen, Broderick, and their colleagues hope the Event Horizon Telescope will be able to resolve Sgr A*’s shadow. The EHT is actually nine telescopes (and counting), all working together and each located in a different spot on Earth. Coordinating those telescopes’ observations allows them to work as one big telescope that is, in essence, as big as the planet. The bigger your telescope, the higher your resolution. “The Event Horizon Telescope has the capability to produce the highest-resolution images in the history of astronomy”, Broderick says. “That means, for the first time, we can see what happens right down in the immediate vicinity of black hole event horizons.”
Scientists working on the EHT hope to see images in the spring of 2017. But they already have some ideas of what they’ll get. General relativity describes gravity not as a force drawing two objects together, but rather as the warped spacetime that governs each of those objects movements. Concentrate a big enough mass in a small enough region of spacetime, and its gravity will be inescapably huge—voila, you’ve got a black hole. If that sounds weird to you, well, it took 50 years for astronomers to discover that black holes were real objects, not just a quirk of general relativity’s math.
The problem is, general relativity is really good at describing giant things like stars, but breaks down utterly when it comes to really teeny tiny things like photons and quarks. To talk about those, you need a different theory: quantum mechanics. The central problem in physics today is that the theories are fundamentally incompatible. To figure that out, physicists are keen to find places where the theories overlap or break down---like, for example, the event horizon of a black hole.
General relativity doesn’t just predict the existence of black holes. It also precisely describes the size and shape of their shadows. Sgr A*’s shadow is supposed to be perfectly circular and 50 microarcseconds wide. “What would it look like if general relativity were wrong?” wonders Broderick (and just about every other astrophysicist on the planet). There are two possibilities. “The shadow could be more egg shaped,” says Johannsen. “That would be a smoking gun for a GR violation.” It might also be slightly smaller or bigger than general relativity predicts. All he needs to figure it out is the picture from the EHT. (Johannsen and Broderick just published a paper outlining their strategy in Physical Review Letters.)
And what if Sgr A*’s shadow doesn’t look the way general relativity says it should? Well, that would be great. If the results held up, physicists could start looking for alternative theories of gravity that did predict the shadow's size and shape. Success wouldn’t mean the new theory would automatically be the successor to general relativity, of course. But it’s a good way to figure out which theories might be on the right track, so you can give their other predictions a closer look.
Johannsen’s favorite possibility involves extra dimensions. A shortcoming of general relativity is that it doesn’t explain why gravity is so much weaker than the other fundamental forces. “Let’s assume there is another space dimension. Gravity would immediately penetrate that and become kind of diluted,” Johannsen says. In other words, gravity isn’t weak, it’s just working across more dimensions than the other forces. Amazingly, theories that predict those extra dimensions also predict a different size for Sgr A*’s shadow. In a couple years, finally proving---or falsifying---this weird new physics could “literally be as ‘easy’ as putting a ruler across the image,” Johannsen says.
“We’re getting this amazing opportunity to finally put Einstein to the test around the most enigmatic and striking predictions of this theory,” Broderick says. If Einstein is wrong, general relativity won’t go away—it’s too good at what it does. It just won’t be the whole story anymore. Isaac Newton was plenty right about how gravity worked here on Earth; Einstein revolutionized our understanding of the universe. But the universe is big enough to have room for someone to come along and do it again.
