What does a black hole actually look like? For generations, scientists argued over whether black holes actually existed or not. Sure, there were mathematical solutions in General Relativity that indicated they were possible, but not every mathematical solution corresponds to our physical reality. It took observational evidence to settle that issue.
Owing to matter orbiting and infalling around black holes, both stellar-mass versions and the supermassive versions, we’ve detected the X-ray emissions characteristic of their existences. We found and measured the motions of individual stars that orbit suspected black holes, confirming the existence of massive objects at the centers of galaxies. If only we could directly image these objects that emit no light themselves, right? Amazingly, that time is here.
In theory, a black hole is an object that cannot hold itself up against gravity. Whatever outward forces there are — including radiation, nuclear and electromagnetic forces, or even quantum degeneracy arising from the Pauli exclusion principle — must be equal and opposite to the inward force of gravity, or collapse is inevitable. If you get that gravitational collapse, you will form an event horizon.
An event horizon is the location where the fastest speed attainable, the speed of light, is exactly equal to the speed necessary to escape from the gravity of the object inside. Outside of the event horizon, light can escape. Inside the event horizon, light cannot. It’s for this reason that black holes are expected to be black: the event horizon should describe a dark sphere in space where there should be no light detectable of any type.
We see objects in the Universe that are so consistent with the expectations for a black hole that there are no good theories, at all, for what else they might be. Furthermore, we can calculate how large these event horizons should both physically be for a black hole (proportional to a black hole’s mass) and how large they should appear in General Relativity (about 2.5 times the diameter of the physical extent).
As viewed from Earth, the largest apparent black hole should be Sagittarius A*, which is the black hole at the center of the Milky Way, with an apparent size of approximately 37 micro-arc-seconds. At 4 million solar masses and a distance of around 27,000 light years, it should appear larger than any other. But the second largest one? That’s at the center of Messier 87, over 50 million light years away.
The reason that black hole is so huge? Because even at that incredible distance, it’s over 6 billion solar masses, meaning it should appear as roughly 3/4ths the size of the Milky Way’s black hole. Black holes are well-known for emitting radiation in the radio portion of the spectrum, as matter accelerates around the event horizon, but this gives us a brilliant way to attempt to view it: through very-long-baseline interferometry in the radio portion of the spectrum.
All we need, to make that happen, is an enormous array of radio telescopes. We need them all over the globe, so that we can take temporally simultaneous measurements of the same objects from locations up to 12,700 kilometers (8,000 miles) away: the diameter of Earth. By taking these multiple images, we can piece together an image — so long as the source we’re imaging is radio-bright enough — as small as 15 micro-arc-seconds in size.
The Event Horizon Telescope (EHT) is exactly such an array, and it’s not only been taking data from all over the world (including in Antarctica) for years, it’s already taken all the images necessary of Sagittarius A* and of Messier 87 you could hope for. All that’s left, now, is to process the data and construct the images for the general public to view.
We’ve already taken the data necessary to create the first black hole images ever, so what’s the hold up? What are we poised to learn? And what might surprise us about what the Universe has in store?
In theory, the event horizon should appear as an opaque black circle, letting no light from behind it through. It should display a brightening on one side, as matter accelerates around the black hole. It should appear 250% the size that General Relativity predicts, due to the distortion of spacetime. And it should happen because of a spectacular network of telescopes, in unison, all viewing the same object.
Normally, the resolution of your telescope is determined by two factors: the diameter of your telescope and the wavelength of light you’re using to view it. The number of wavelengths of light that fit across your dish determines the optimal angular diameter you can resolve. Yet if this were truly our limits, we’d never see a black hole at all. You’d need a telescope the diameter of the Earth to view even the closest ones in the radio, where black holes emit the strongest and most reliably.
But the trick of very-long baseline interferometry is to view extremely bright sources, simultaneously, from identical telescopes separated by large distances. While they only have the light-gathering power of the surface area of the individual dishes, they can, if a source is bright enough, resolve objects with the resolution of the entire baseline. For the Event Horizon Telescope, that baseline is the diameter of the Earth.
I’m so pleased that the Event Horizon Telescope, and imaging the event horizon of a black hole directly, will be the topic of October 3rd’s Perimeter Institute public lecture: Images from the Edge of Spacetime, by Avery Broderick.
We’ll be live-blogging it right here, starting just a few minutes before its official start time of 7 PM Eastern Time (4 PM Pacific Time), and you can follow along by watching the video below. See you here, then, and hopefully we’ll learn all the answers then!
(All updates, below, will have the timestamps in bold in Pacific time, with screenshots where appropriate from the lecture itself.)