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Do black holes look the same from any angle?

Ron Dippold
San Diego, California, US

In theory, yes; in practice, no. First let鈥檚 distinguish the parts of a black hole. In the centre is the singularity, where all physics breaks down because the maths goes to infinity. The part we can 鈥渟ee鈥� is the event horizon, where almost no light can escape the gravitational pull. We can鈥檛 actually see it, because no light except faint Hawking radiation can escape to hit your retina, but we can tell where we stop seeing things 鈥� the central shadow.

Then there is the photon sphere, which is where light that comes in at just the right angle goes into tight orbit around the black hole. It eventually falls in or escapes, and we see the escaping light as a bright ring around the central shadow.

The biggest part of the black hole environment is the accretion disc, which is all the matter it has sucked up rotating in a giant disc around it. The extreme forces in this region cause the disc to glow. Some of the accretion disc is doomed to spiral in and be eaten, but some of it will be blown out in massive relativistic jets at the black hole鈥檚 poles 鈥� these jets can approach light speed and be light years long!

So, the short answer is that the accretion disc and relativistic jets make a black hole look very different from different directions.

But what if we ignore them and just look at the event horizon? The (unproven) 鈥渘o-hair theorem鈥� posits that the only things you can measure about a black hole after it forms are its mass, its electric charge and its angular momentum, or spin. The mass and charge are the same from all directions, but the spin can be measured 鈥� it is what determines where the accretion disc is and which direction it rotates. If you somehow had a black hole with no spin, it would appear the same from all directions. But as soon as any matter falls in, it imparts spin, so it is very hard to have a black hole with zero spin.

What about a naked singularity, with no event horizon covering it? This continues to be a hot topic of debate. No naked singularity has ever been seen and anything dense enough to become a singularity would normally have an event horizon hiding it. But it is theorised that something spinning fast enough could shrink its event horizon to the point that the singularity was exposed.

Kip Thorne made simulations of the optical effects for observers orbiting a black hole for the film Interstellar

So practically, you can always tell where you are in relation to a wild black hole. But at some point, it might be possible to make a small, spin-less black hole in a lab that would be as symmetrical as your control over it allowed.

Nick Canning
Coleraine, County Londonderry, UK

First clarify what you mean by 鈥渢he look鈥� of a black hole. It isn鈥檛 the same sort of thing as 鈥渢he look鈥� of Earth鈥檚 globe seen from space. That involves light scattering off the planet鈥檚 surface, but light can鈥檛 scatter off the surface of a black hole鈥檚 event horizon. The only optical clue to the presence of a black hole is its gravitational lensing effect on light from matter falling into the hole, heating up and emitting light while still outside the horizon, or light from background stars on the other side of the sky to the observer looking towards the hole.

According to theory, an event horizon鈥檚 surface is perfectly uniform and smooth, so if the distribution of matter and stars is uniform in all directions outside the horizon, then the optical observations would be the same for all directions around the black hole. However, if the stars aren鈥檛 uniformly placed, then visual observations will change as we orbit the hole.

Theoretical physicist Kip Thorne made simulations of the optical effects for observers orbiting a black hole for the film Interstellar. These agree with the later images of the supermassive black hole at the Milky Way鈥檚 centre.

To answer this question 鈥� or ask a new one 鈥� email lastword@newscientist.com.

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