What does a black hole actually look like? What telescope can make such an observation? And what could possibly go wrong?
Astronomers across the world are gearing up this week to make a bold attempt at taking the first ever image of a black hole. This daring idea will require the international cooperation of a telescope the size of the Earth and has the potential to turn everything we know about gravity on its head.
So what does a black hole actually look like? What telescope can make such an observation? And what could possibly go wrong?
What does a black hole look like?
Astronomers believe that the majority of galaxies like our Milky Way host supermassive black holes at their centers. Supermassive is really an official term and suggests masses from hundreds of thousands to a billion times the mass of our Sun. The supermassive black hole at the center of our galaxy, known as Sagittarius A* is 4 million times the mass of our Sun crammed into a space with a diameter less than roughly a third of the distance between the Earth and the Sun.
Despite their expected ubiquity and the fact that we have a supermassive black hole in our own galactic backyard, no one has ever imaged a black hole directly. That’s because, as the name implies, they are black meaning that they are so dense that even light cannot escape. Einstein’s equations of general relativity predict that the strong gravitational pull around such dense objects will warp the space around them, causing light to follow a curved path.
The German astronomer Karl Schwarzschild further determined that, as you get closer and closer to a black hole, this warping will eventually become so strong that light will bend back inward towards the black hole, unable to escape. This boundary that marks the point of no return is known as the event horizon or, in some cases, the Schwarzschild radius. Despite what Laurence Fishburne may tell you, there’s no coming back once you have passed beyond a black hole’s event horizon. Although you can check out my previous episode on what happens when you fall into a black hole.
Even without direct observations, we can observationally deduce the presence of a black hole in a few different ways. Tracking the motions of stars orbiting the center of our galaxy, for example, reveals that a massive yet optically invisible object must lurk there in order to explain how fast those stars are moving. Much of this work has been led by Astronomer Andrea Ghez and you can watch the animations of these stellar motions around our galaxy’s central supermassive black hole at her research group’s website.
We can also view supermassive black holes indirectly if they are actively accreting material. As matter falls into a black hole, it forms a very hot, swirling disk. Newly added material will emit high energy photons as it hits the disk, in the form of X-rays which can then be observed. And, of course, an entirely new method of observing black holes was introduced just last year when the first gravitational wave detection revealed two merging intermediate-mass black holes.
But all of these methods only offer an indirect look at the black hole. An experiment running from April 5th–April 14th will attempt to directly image the event horizon of our own black hole, Sagittarius A*. As material like gas and dust approaches the black hole, it speeds up and emits energy which can be observed as a ring or crescent at the location of the event horizon. The black hole is expected to cast a shadow on a portion of this ring, and that shadow holds clues to the mass and size of the black hole that lurks within. The shape of that shadow is further predicted by Einstein’s theory of general relativity to be circular, a prediction that these observations can test directly.