More than half a dozen scientific press conferences are set for 10 April, raising hopes that astronomers have for the first time imaged a black hole, objects with gravitational fields so strong that even light cannot escape. Although their existence is now almost universally accepted, mostly from the effect of their gravity on nearby objects, no one has actually seen one.
Black holes themselves are entirely dark and featureless. The giant ones at the centers of galaxies are also surprisingly small, despite containing millions or billions of times the mass of our sun. To make observing them yet more difficult, those giants are shrouded in clouds of dust and gas. But streams of superhot gas swirl around the holes, emanating radio waves about a millimeter in wavelength that can penetrate those clouds.
Two years ago, an international collaboration known as the Event Horizon Telescope (EHT) corralled time on eight different radio telescopes around the world to try to image the supermassive black hole at the center of our galaxy, known as Sagittarius A*, and another at the center of nearby galaxy M87. They used a technique known as interferometry to combine the output of the globally scattered instruments to produce images as if from a single dish as wide as Earth. A dish that large is needed to see the details of something that would fit easily within the orbit of Mercury and is 26,000 light-years away.
Their 5 nights of observing produced 4 petabytes of data. If that amount of data was music stored as MP3s, it would take 8000 years to play. The team has spent the past 2 years correlating, calibrating, and interpreting the data and they are now preparing to show us the results.
If the EHT has an image, it may reveal the shadow of the black hole’s event horizon, the point of no return for anything falling in toward the black hole, against a backdrop of the bright swirl of gas in orbit around it. The size of that shadow and the shape of the swirling gas, lensed by the hole’s gravity, will help confirm many theories about these enigmatic objects.
As we wait for this week’s announcement, Science spoke with someone who has spent much of his career imagining what black holes might look like. In February, in anticipation of the EHT results, Jean-Pierre Luminet of the Paris Observatory in Meudon, France, released an illustrated history of black hole imaging that records decades of progress from pen-and-ink drawings to supercomputer simulations and Hollywood movies. This interview has been edited for clarity and brevity.
Q: What prompted you to start to work on visualizations of black holes back in 1978?
A: The challenge was to show something of an object that is by definition invisible, plus my natural interest in optical illusions and space-time distortions, and moreover the fact that nobody had previously had the idea to calculate something realistic!
Q: Were you surprised by the curious shapes that you discovered?
A: Not at all, because before writing a computer program with equations I always try to get a preliminary idea from geometrical considerations. In this case, simple geometrical reasoning suggested that, due to gravitational lensing, no part of the disk could be hidden, even its rear side! And simple considerations about the relativistic rotation of the disk implied that a strong Doppler shift would cause a strong asymmetry of the apparent flux.
Q: The first image you produced, using a pen and ink, was impressive. What was the reaction to it from astronomers and the public?
A: As I was very young it was my former Ph.D. adviser Brandon Carter who began to publicize my work by showing the picture at a meeting of the Royal Society in London. After I received reprint requests from all over the world, my picture was reproduced in popular science magazines such as Scientific American, Sky & Telescope, and so on, and in monographs by other astronomers.
Q: Soon after, astronomers realized that some, if not all, galaxies have a supermassive black hole at their centers. What challenges did this pose?
A: The difficulty in visualizing the environments of massive black holes at galactic centers is that you don’t know if the accretion structure is a thin disk (as in most simulations), a thick one (like a 3D torus), or a cloud of gas, or if you have jets and so on. For instance, in very active galactic nuclei and quasars the accretion flow is very important and the disk is probably thick. Luckily, Sagittarius A* and M87* are not active galaxies, so the hypothesis of a thin disk is reasonable. It depends also (but not so much) on whether the black hole is rotating or not.
Q: With Jean-Alain Marck in the 1990s you moved on to sweeping animations of movement around a black hole. Did this help with understanding, or was it more for public engagement?
A: It was essentially to provide more engaging (colored, animated) pictures for the public. Curiously enough, the scientific community considered these simulations as a game and did not realize their future importance.
Q: What do you think of the visualizations produced for the movie Interstellar?
A: I wrote a lot of things about this in my blog. In short, geometrically good but physically wrong because they neglected the physical properties of the accretion disk and the Doppler shift effects.
Q: Has the EHT added impetus to the visualization field as people try to figure out what they would actually see?
A: Sure. I stopped my history of black hole imaging in 2002 precisely because as soon as imaging a black hole with the EHT became a possibility, there was a burst of so many simulations that I couldn’t fit them in. Sadly, most of these simulations did not cite our pioneering ones.
Q: What sort of image do you think the EHT team will reveal this week?
A: It will depend on many factors: for instance, on the inclination angle of the observer with respect to the accretion disk. If it is almost face-on, the luminosity asymmetry due to the Doppler shift will not be strong. Also, on the environments of Sagittarius A* and M87*, on the thickness of the disk (if there is one), and so on. I suspect for various reasons that M87* should provide a cleaner image than Sagittarius A*. In any case, if there is a thin accretion disk (as I’m sure is the case for M87*) the image should not be far from one of the views calculated by Jean-Alain Marck in 1989.