No one knew what a black hole looked like before yesterday. Sure, we thought we knew, thanks to simulations and the now-famous black hole featured in the movie Interstellar.
But yesterday, scientists behind the Event Horizon Telescope unveiled an image of the 6.5-billion-solar-mass black hole at the centre of the (relatively) nearby galaxy M87. And a quick glance will show you that it doesn’t look anything like Gargantua, the black hole in the movie Interstellar. So, we asked the physicists behind both images to help explain the differences.
First off, if you’re completely lost, black holes are objects predicted by the theory of general relativity to have such an incredible gravitational field that light can’t escape once it enters a region called the event horizon. Today, scientists behind the Event Horizon Telescope unveiled an image of this phenomenon – not a photograph, but a reconstructed picture of the shadow that the black hole casts on the light behind it, created from data taken by eight telescopes around the world.
The black hole unveiled today looked more or less exactly the way that the Event Horizon Telescope scientists, guided by Einstein’s theory of relativity, expected it to look. Like, to an impressive degree. See:
The image of M87's black hole (left) compared to simulations (centre) and simulations blurred to match the telescope’s resolution (right). Screenshot: Akiyama et al (ApJL (2019))
But we, the pop culture-consuming public, might have expected something that looked a little bit more like this:
The black hole Gargantua created for the film Interstellar.
They’re not as different as you might expect. “The image in Interstellar is almost correct,” Kazunori Akiyama, postdoctoral researcher at the MIT Haystack Observatory who led the team that created the EHT’s image, explained to Gizmodo.
Perhaps most notably, the Interstellar black hole has a thin streak of matter around its centre, which M87's black hole seems to lack. That’s a simple difference to explain – initial evidence shows that we’re viewing M87's black hole from closer to one of the poles, rather than from head on. The disk of matter around M87 would be obscured by the observation angle, Akiyama explained. Take Saturn’s rings – they don’t cross the planet when you look at it from the top or bottom.
But we’re not looking at the black hole completely head on, and that’s the origin of the other main difference. M87‘s black hole seems to have a far brighter crescent-like shape on the bottom left. What you’re actually looking at is the fact that M87's black hole is probably spinning. The material orbiting the black hole would also spin, and spacetime itself would warp around the black hole. That means that material moving toward us would appear brighter, while material moving away from us would appear dimmer – which you can see in the M87 image.
“Christopher Nolan omitted that brightening because the human eye would likely not be able to discern the brightness differences on the two sides of the hole when the overall brightness is so extreme,” Kip Thorne, Cal Tech physicist and advisor on the film Interstellar, told Gizmodo. Nolan did take some artistic license with the appearance of the film’s black hole, as we’ve previously explained, including things like lens flare.
But there are other differences as well, explained Throne. The black hole envisioned by Throne had a much thinner, opaque disk of material. The black hole observed by the Event Horizon Telescope team appears to have a much thicker disk, but one that is somewhat more transparent to light. These are relatively minor points.
The Event Horizon Telescope will continue to take images, both of M87's black hole and of the black hole at the center of our own galaxy, the Milky Way. These images will create even clearer pictures – and will surely help science fiction produce more accurate visions of black holes than ever before.
Featured image: EHT/Interstellar