First images of a black hole

This week, scientists have just announced the first images of a black hole. Chris Smith was joined in studio by astronomer Carolin Crawford from the University of Cambridge, to explain how this was done, and what it means...

Carolin - Well, just first to stress, it’s not actually an image of a black hole. It’s the closest we can get, but you are seeing effectively the silhouette of the event horizon around a black hole. So you’re more or less sitting in the shadow it casts towards us. So that hole that you see, I mean if you look at the image it’s this beautiful bright annulus with a dark circle in the middle - that’s not a black hole that’s the shadow of the event horizon.

Chris - It looks like a bit of a bagel in the sky, doesn’t it? It’s a long way away though isn’t it?

Carolin - It’s 55 million light years away and it sits right at the centre of a colossal giant elliptical galaxy called M87. And we’ve already weighed the mass of the black hole, it’s six and a half billion times the mass of our Sun.

Chris - How do you know that?

Carolin - We know that because a black hole you can’t see it, but you can measure its gravitational effect on matter around it, in this case it’s ionised gas, and stars in the vicinity you look at how they’re moving, you can work out what gravity they’re responding to, you can weigh the black hole. One thing that has come out of this that’s nice is those estimates match very nicely the size of the event horizon we see here, so there’s a consistent story about the mass within this black hole.

Chris - How were the images assembled or prepared?

Carolin - The images were taken with something called the Event Horizon Telescope which is where you mock up a radio dish the size of the Earth by linking together eight widely separated radio telescopes. The reason you do that is that if you increase the effective diameter of your telescope you get very good, what we call angular resolution, that’s the ability to see details. And if you increase the size of your radio dish to that of the Earth well, you can get a resolution that you could detect a grapefruit on the surface of the moon and that is ideally matched to the event horizon around the supermassive black hole M87 and also the supermassive black hole at the centre of our galaxy called Sagittarius A*.

Chris - Yes, I was interested that they chose to report on the bigger one was a lot further away, though, than the one in our own galaxy. Do you think they’re saving the best till last? Do you think they’ve got some big reveal up their sleeves?

Carolin - Well, the nearer one - nearer, I mean it’s still 26,000 light years away. The thing about Sagittarius A*, it's 4 million times the mass of our Sun, so it’s a thousand times smaller, it’s a thousand times nearer but it’s also much more variable. Changes in brightness are on a much faster timescale so actually correlating all the data from all these eight different telescopes is a lot harder work. So there will be a big reveal, they’re just being more careful and they’re taking longer to do it.

Chris - And what do you think they will do next then because now we can image black holes, will they just turn their devices on as many black holes as possible?

Carolin - Well first of all, there are improvements. I mean, obviously, we’re going to look at more supermassive black holes - different ones, but you can improve the algorithms to try and sharpen the images. We can observe at other wavelengths and try and improve the angular resolution, the detail we can see and see fine structure around the black hole. And if we really wanted to be adventurous we could perhaps increase, in the long future, the diameter of this potential radio telescope, put a radio dish on the Moon. Again, get to far more detail and perhaps see changes in real time of how this material is accreting onto the black hole, almost get a movie of this accretion as it happens.

Chris - We’ve had a whole slew of questions on our forum: nakedscientists.com. This persons says: how many kinds of black hole are there? Do they come in different flavours?

Carolin - Oh yes, there are definitely different flavours of black holes. There are two kinds that we regularly observe; you have the ones that, we call them stellar sized, but in fact they’re probably ten times the mass of our Sun and they’ll go from about ten solar masses up to about 60 solar masses, and they’re the ones that you find like in the spiral arms of galaxies, they come from the death of stars.

But then you get the behemoths that live at the centres of galaxies, and these are what we call the supermassive black holes. And these are the ones that can be millions, if not billions times the mass of our Sun.

Those are the astrophysical black holes that we actually observe. There are predictions that there could be something called primordial black holes that maybe formed either during or just after the Big Bang. But we don’t observe those yet and so that’s more speculation.

Chris - That question came from Mad Atheist on our forum who also says how many of these black holes involve a singularity? Do they all have one; is that an obligatory thing with a black hole?

Carolin - Yeah, that’s what defines it as being a black hole. Singularities where you have something that goes to infinite value and we’re talking about basically infinite density. So, by definition, all black holes are the singularity

Chris - And what happens, he is asking, when you get to the event horizon to the speed of light? Does light inside a black hole change speed or is it still running at the speed of light?

Carolin - Who knows! Once things go over the event horizon our physics break down. I couldn’t speculate.

Chris - Geordie F wonders, is there any way we can know anything related to the mass distribution inside the black hole i.e. over the event horizon? This goes back to the point the person was asking about knowing about the speed of light or not inside the black hole. I guess the answer’s going to be no.

Carolin - The answer’s no. The event horizon is literally the horizon beyond which we can see no event and we can do the thought experiments, but it’s completely locked away. Anything that’s going on beyond that boundary is completely locked away to us at the minute. There’s no way we can work it efficiently mathematically or observationally.

Chris - Were you excited when you saw the initial papers come out?

Carolin - Very excited. The data were taken two years ago so everybody’s just been on tenterhooks for this image that we knew that was coming. So it’s very satisfying.