Yesterday, humanity got its very first look at a black hole. The Event Horizon Telescope (EHT), a global collaboration which uses radio telescopes around the world to make one Earth-sized observatory, unveiled its pictures of the black hole at the centre of the distant M87 galaxy, the very first direct images of a black hole we’ve ever taken. Now, the even harder work begins: figuring out what it all means.
The images are the first proof that the event horizon – the line at which the black hole’s gravity is so strong that nothing, not even light, can escape – is real. It shows light from matter right next to the black hole bending around it to create a sort of bright halo, as predicted by Albert Einstein’s theory of general relativity.
“There are bizarre effects up close to a black hole,” says Priya Natarajan at Yale University. “We are starting to test Einstein’s theory of general relativity in a regime that we’ve never done before.” Even after the first image, though, many questions remain.
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The biggest is how general relativity, which governs gravity and the very large, and quantum mechanics, which concerns the very small, fit together. The event horizon is such an extreme distortion of space-time that both theories should be in play.
In fact, a black hole may be the only place in the universe where we have a chance of figuring out how they work together – but not with this picture.
“Can we really understand the hard stuff, like what’s going on at the quantum level at the event horizon? That’s not likely with this data,” says Janna Levin at Barnard College in New York. “Most astrophysical black holes are too big to really be exploring their quantum side.”
What we can learn are the specifics of supermassive black holes. For example, the M87 black hole imaged by the EHT is spewing out an enormous jet of plasma and energy. We know that this must be powered by the black hole spinning, but we don’t yet know how fast it rotates. Studying the asymmetry of the light around the black hole should give us the answer.
Speaking of which, we also don’t know where exactly the light in the images, against which we can see the black hole’s silhouette, comes from. Strictly speaking, the iconic orange glow is actually a tinted representation of colourless radio frequency photons, rather than visible light.
“You’re seeing photons that are just zipping around the black hole and they must come from very nearby, but their source is unclear,” says Kristin Madsen at the California Institute of Technology. They could be emitted by the disk of material circling the black hole, doomed to be swallowed down, or they could come from the base of the jet.
The EHT is also attempting to image the black hole at the centre of our galaxy, Sagittarius A*. Unlike M87 it does not seem to have a jet, but it is interesting in other ways – particularly because we may be able to see it changing over time.
The observations of M87 over the course of four days apparently show a small amount of variation, but it’s not significant enough to know whether this is real or an artifact of the imaging process. That’s part of the reason this first image showed the black hole in M87 instead of Sagittarius A*: it’s bigger and calmer, so it changes much more slowly.
“Changes that take a week on M87 take a few minutes on Sagittarius A*,” says EHT team member Michael Johnson at the Harvard-Smithsonian Center for Astrophysics in Massachusetts. “It’s like trying to take a picture of people but they’re all running around.”
The activity doesn’t make imaging Sagittarius A* impossible – it just makes turning the data from all the telescopes into one picture harder. Nevertheless, Johnson says, the collaboration will probably release a picture of our own galaxy’s black hole within the next year or so.
“We know that there are probably tens of thousands of smaller black holes around our galaxy’s supermassive black hole, and the little guys can fall into the big guy,” says Levin. “There’s all kinds of interesting things we can learn from looking at how the supermassive black hole changes.” We don’t actually know how matter falls into a black hole or how a black hole is connected to its jet, and watching one over time could help.
In the past year, the EHT has also added telescopes that will make the next sets of images twice as sharp, Johnson says. Improving the resolution much more than that would require major upgrades to all the telescopes, or expanding the network to include telescopes in space.
Eventually, years of observations will let us see how both M87 and Sagittarius A* change over time. One day we may even be able to make a movie of real black holes fascinating enough to rival Hollywood.
