UPDATE: The EHT’s first image has been released! See: This Is the First Image of a Black Hole
Tomorrow, on April 10, the Event Horizon Telescope (EHT) will make an international announcement about a “groundbreaking result” from the global collaboration. Further details as to what this result actually is are under wraps, but as the EHT’s mission is to image a supermassive black hole for the first time, the expectation is that it will be a historic day for humanity. We may actually see what a black hole — more precisely, a black hole’s event horizon — really looks like.
But we already know what a black hole looks like, right? There have been countless science fiction imaginings of black holes over the years and, most recently, the Matthew McConaughey movie “Interstellar” depicted what is touted as the most scientifically-accurate sci-fi black hole ever.
Interstellar’s black hole, called “Gargantua,” is a sight to behold and many physicists and CGI experts went out of their way to base that thing on the physics that is predicted to drive these monsters. Physics heavyweight Kip Thorne even advised on how this rotating black hole — a supermassive one at that — should look and behave, based on earlier work by Jean-Pierre Luminet (ScienceAlert has a great article about this).
Back to reality, the EHT may well be presenting its own “Gargantua moment” tomorrow when the first results are presented. The EHT is a global network of radio telescopes all dedicated to probing the final frontier of general relativity. Black holes are the most extreme gravitational objects in the universe and the supermassive monsters that lurk in the cores of most galaxies are true behemoths.
The EHT currently has two targets it hopes to image, the supermassive black hole in the core of our galaxy, the Milky Way, and one inside the massive elliptical galaxy, M87. With a mass of four million Suns, our galaxy’s supermassive black hole is called Sagittarius A* (Sgr A* for short) and is located approximately 25,000 light-years away. But M87’s monster dwarfs our comparatively diminutive specimen — it’s a super-heavyweight among supermassive black holes, with a mass of a whopping 6.5 billion Suns.
In a wonderful stroke of cosmic luck, although M87 is 50 million light-years away, some 2,000 times further away than Sgr A*, it’s also approximately 2,000 times more massive. This means that both Sgr A* and M87 will appear approximately the same size in the sky to the EHT. They are also two wonderful targets to study, as both are very different in nature.
Now, back to Gargantua. As this CGI beauty is based on real physics theory, and assuming the first EHT image doesn’t throw the fidelity of general relativity into doubt, both Gargantua and the two EHT targets should, basically, look the same. Sure, there’s going to be differences based on mass, jets of material, size of accretion disks and other details, but will the EHT first image bear any resemblance to the Interstellar rendering?
Short answer: no, it should look something like this:
Long answer: It’s all about wavelength. Over to gravitational wave astrophysicist Dr. Chiara Mingarelli, of the Flatiron Center for Computational Astrophysics (CCA), who’s tweet inspired this article:
Gargantua was created with human vision in mind. Our eyes are sensitive to visual wavelengths, from 380 nanometers (violet) to 740 nanometers (red), and movies are very much based on what humans can see (I hear infrared movies are rubbish). But the EHT cares little for nanometer wavelengths — the EHT is all about seeing the universe in millimeter wavelengths, which means it can see things our eyes can’t see. It is a network of radio telescopes all working together as one planet-wide virtual telescope via a clever method known as very long baseline interferometry. By viewing a black hole target at these wavelengths, astronomers have the ability to see straight through the accretion disk, dusty torus (if it has one), jets of material and other nonsense floating around the black hole.
Here’s a few frames from the simulation Dr. Mingarelli is referring to above, wavelength increasing from nanometers to millimeters, left to right:
The EHT can see right up to the innermost limit, just before nothing, not even light, can escape the gravitational grasp of the event horizon. Any hot plasma or dust that would otherwise obscure our view of the horizon are transparent at wavelengths more than one millimeter, so we can see the radiation emitted by the hot, turbulent material that is being tortured by the extreme environment right at the horizon.
Gargantua is a glorious rendering of what a supermassive black hole might look like if we could take a trip with Matthew McConaughey and co. (give or take some CGI sparkle for dramatic effect). What the EHT sees is the shadow, or the silhouette, of a black hole’s event horizon — that will likely be either perfectly circular or slightly oblate, if general relativity holds. That’s not to say that Gargantua doesn’t look like Sgr. A* or M87 in visible wavelengths as Hollywood intended, it’s just that the EHT will lack most of Gargantua’s CGI.
So, I’ll be waking up far earlier tomorrow to watch the EHT announcement and keeping my fingers crossed that we’ll finally get to see what an event horizon really looks like.