NASA
Andy Lawrence
University of Edinburgh, UK
We only have actual pictures of two black holes, taken using the Earth-sized Event Horizon Telescope. These images look like doughnuts seen face-on, with a central hole, but the discs aren’t really doughnut-shaped. They look that way because the super-strong gravity near the black hole bends the light right round in a circle, leaving a dark central patch. That distortion means it would look that way from whichever direction you looked at it.
However, we believe that most giant black holes are surrounded by a much larger, roughly doughnut-shaped, disc of dusty muck. We have never actually been able to take a picture of one of these, but we think they are there because we sometimes see cones of light either side of where we think the black hole is. One way to explain this is that the fat disc blocks the light coming out sideways, while letting it escape out of the poles. We also notice that we seem to be missing about half the black holes we think we should see – because the disc of muck is hiding some of them, depending on which direction we are looking at them from.
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The super-strong gravity near a black hole bends the light round in a circle, leaving a dark central patch
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Some astronomers (including me) aren’t convinced these blocking structures are actually rotating doughnuts. They could be a wedge-shaped outflowing wind or a normal thin disc that is tumbling over itself as it falls towards the black hole. If only we could get a picture, we would know!
Ron Dippold San Diego
California, US
They do tend to look like “blobby doughnuts”, especially given the low resolution of the Event Horizon Telescope – it’s an engineering marvel that it works at all! But they do look different depending on whether you are looking edge-on to the disc or down at a pole, and there are exceptions to this rule too.
Of course, you can’t actually see a black hole: by definition, no light can escape it. What you are seeing is the glowing accretion disc of everything that is currently orbiting it. If you look at the black hole from above a pole, you will see the giant, glowing ring around a dark shadow. Due to relativistic warping, part of the light that would normally go away from you is actually pulled back around the back of the black hole and out towards you, so what you are seeing on the “left” side is the left part of the ring, but also part of the right part of the ring!
On the other hand, if you look edge-on (as seen in the movie Interstellar), the light from the ring in the back of the black hole gets bent upwards and downwards around the black hole, so you see bright arcs above and below the actual disc closest to you. There is still a shadow region, the actual black hole, but now it looks like an oval between the arcs.
Odds are, you aren’t looking down the pole of any specific nearby black hole – which is good, because the near-light-speed jets they emit from their poles can be millions of light years long and could easily fry you. So you won’t see it as exactly symmetric. Then again, odds are you also aren’t looking at it exactly edge-on either, so you will see something between the two views, and this does average out to “doughnut shape with some blobs”.
What are the exceptions? First, there are theorised to be a lot of lone black holes floating around in the voids of space that have nothing to swallow. Without an accretion disc, the only way you would even be able to tell one was there would be by how it was gravitationally lensing the light from behind it. Second, it is also theorised that some stars actually have small black holes lazily orbiting inside them. If the black hole is small enough, this should be surprisingly stable for billions of years. In that case, the easy answer is that you just see the star.
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