It’s been a hot week for science. Thanks to the Event Horizon Telescope, an algorithm created by 29-year old PhD graduate Katie Bouman, and a lot of hard work, humanity got its first photo of a black hole – M87 in the galaxy Messier 87, some 55 million light-years away. It wasn’t made with visible light: it was made with radio-frequency waves (1.3 mm, since you ask). What I want to show you is what that photo actually reveals. Well, other than a ring of fire surrounding a bottomless pit, of course. And it’s very cool.
How cool? Get this. M87 is a huge black hole – its mass is 6.5 billion times that of our Sun. The central shadow in the photo is about the size of our solar system. The torus you can see orbits it once every two days. But it’s also 55 million light years away. So that makes this photo an incredible technical achievement, made possible by creating a virtual Earth-sized telescope out of eight installations across the globe, then technically interpolating the resulting images. The technique is well known – it’s called interferometry. But it’s never been done to this scale before.
That’s not the only awesome thing. See that shadow? Inside it is the event horizon. Because of the incredible degree of space-time distortion that exists around a black hole, and the way that light is bent around it, the event horizon would show us an infinite number of repetitions of itself, if we could see it. And not just the part facing us, either. All of it, including the bits facing away. So yeah, the middle of the black circle is an infinite-hall-of-mirrors-with-360-degree vision. If you could see the event horizon, of course. Actually, you can’t.
It’s probably a good thing you can’t see what’s inside the event horizon. That’s where time and space are distorted to an infinite degree – a ‘singularity’, which is a mathematical point. The laws of physics totally break down here. Even cause-and-effect don’t necessarily happen in the order we expect. This means that anything can happen. Entropy doesn’t exist. It might spontaneously generate Cthulthu, for instance. Or Sauron. Or a pair of fluffy dice. Or something we cannot comprehend. Or, in very boring fashion, a stream of undifferentiated particles. Or stuff might spontaneously vanish. Who can tell? Luckily the event horizon, which is where the escape velocity of the black hole exceeds the speed of light, protects us from the effects.
So – what do we actually see in the photo? It’s pretty close to what theory predicted. There’s an accretion disk around the black hole, formed by material spiralling into it, and that’s fairly hot. And there’s a black bit in the middle which, intuitively, you’d suppose is the event horizon – the ‘reality’ of a black hole, to our senses, where the escape velocity exceeds light-speed, so it appears black to us. Actually, thanks to spin, it isn’t exactly. So what are we seeing?
Let me explain. Black holes spin. Spin, in fact, is one of only two factors that technically characterise them. The other is mass. The mechanisms of that were worked out in the early 1960s, by a New Zealand mathematician, Roy Kerr. In a way it stands to reason: black holes are originally formed from a star, which of course will be spinning – and which will retain its angular momentum. As it collapses into a black hole, conservation of momentum means the rate of spin and hence radial velocity increases drastically. Like, right out to light-speed. This also causes the event horizon to be non-spherical: it bulges at the ‘equator’. Kerr figured out exactly how that then integrated with Einstein’s General Theory of relativity.
Technically, every black hole that spins is called a Kerr Black Hole, in honour of his key contribution to our understanding of them. And this also gives us a very different meaning to the black hole picture.
The fact of the black hole spinning means that relativistic tidal effects also exist – time and space aren’t just bent, they’re being twisted like taffy, around and around. Now, it’s possible for light at a specific distance from the event horizon to actually orbit a black hole. This region is known as the ‘photon sphere’. In a spinning black hole, the photon orbit can exist only around the equator, and there are two points where it occurs, one orbiting in the opposite direction to the other. Thanks to relativistic effects, the orbits can be very strange and don’t necessarily match the equator.
The thing is that again thanks to General Relativity, there is a point beyond which no stable orbit can exist around a Black Hole – everything inside that simply spirals in and is eaten. That gives us an innermost distance for the photon sphere. It’s specifically known as the ‘Innermost stable circular orbit’ (ISCO), and its radius is (predictably) r(ISCO). Why is this important? See that ring in the photo? The innermost boundary of the ring – that’s the r(ISCO) distance. So the black centre you see is not the event horizon; it’s actually the distance at which objects can stably orbit. The event horizon itself is inside that area.
The cool part is that from the radius, it’s possible to work out the rate of spin (one method involves the black hole having a black-body curve from the accretion disk, which I’ll explain in another post). Anyhow, as the spin rate increases, the value of r(ISCO) decreases, until finally it’s almost touching the event horizon. So we can see from this photo that M87 is likely not spinning at the maximum potential rate. It’s been theorised that the 5000-light year long gas-jet pumped out from the accretion disk (not visible in the close-up photo) might limit the rate.
Because the black hole pulls light that doesn’t fall into it around, you get to see the back of the accretion disk as well as the front. It’s extreme gravitational lensing. But wait, there’s even more. Now, check out the bright area. This brightness is caused by the fact that the accretion disk is moving around the black hole at a fair fraction of the speed of light. When the particles move towards us, doppler effects increase the apparent frequency, which in the photo appears brighter.
So what we’re seeing here is basically the very edges of the laws of physics – twisted, turned and stretched in every direction. All predicted, of course, by Albert Einstein over a century ago, and by Roy Kerr fifty years back. And who’d have ever imagined that we’d ever be able to see the effects? See what I mean by cool? I love physics.
And if you want to check out the papers that the team wrote, they’re here: https://iopscience.iop.org/journal/2041-8205/page/Focus_on_EHT
Copyright © Matthew Wright 2019