Podcast Extra: The first image of a black hole

Interviewer: Benjamin Thompson

Hi, listeners – Benjamin here. I mentioned in yesterday’s show that there was a story announced too late to make it into the regular News Chat. Turns out that this story – as I’m sure you’ve all heard by now – was about the first image of a black hole. Davide Castelvecchi, a senior reporter here at Nature, was in a press conference in Brussels yesterday when the image was unveiled, and he joins me in the studio to tell me a bit more about it. Davide, thanks for stopping by.

Interviewee: Davide Castelvecchi

Hi, thank you for having me.

Interviewer: Benjamin Thompson

Davide, maybe you can start by telling me specifically what this image is.

Interviewee: Davide Castelvecchi

It’s an actual picture of the supermassive black hole at the centre of the galaxy called M87, which is about 55 million light years away. It’s not the first time it’s been observed, but in all previous attempts it just looked like one blurred out spot.

Interviewer: Benjamin Thompson

Well, perhaps it’s a stupid question from me, but how do you go about taking a picture of a black hole if it’s black on a black background.

Interviewee: Davide Castelvecchi

Ha, so, it’s true you wouldn’t be able to see anything, but a black hole is often accreting matter. It’s surrounded by plasma that is orbiting, kind of like circling the drain, and getting superheated and emitting radiation as a result. And so that’s what you actually see – it’s the edge of the black hole against this maelstrom.

Interviewer: Benjamin Thompson

And the radiation that we can see is essentially backlighting the black hole then and giving us a silhouette which is what appears in the image I guess – this dark circle surrounded by an orange glow.

Interviewee: Davide Castelvecchi

Yes, exactly, and it’s the shape and size that we expected based on the predictions of general relativity – Albert Einstein’s theory of gravity.

Interviewer: Benjamin Thompson

Well, yesterday then six press conferences were held around the world, and as I say, you were at one of them in Brussels, and this is where this image was unveiled. What was the mood in the room at the time?

Interviewee: Davide Castelvecchi

So, when the slide came up, it was Heino Falcke, a German researcher based in the Netherlands, who put it up and the room went silent for a few seconds and then it erupted into an applause.

Interviewer: Benjamin Thompson

Well, let’s talk about the image itself. How did the researchers go about taking it?

Interviewee: Davide Castelvecchi

So, people knew that this black hole because it’s so large – it’s 6.5 billion times as heavy as our Sun – it’s very large, but still it’s so far away that they calculated it would take a radio dish the size of Earth to distinguish details in it and measure its actual size. But then they also realised that you don’t need to cover the entire Earth with a radio dish. What you can do is string together a network of many radio observatories spread around the globe.

Interviewer: Benjamin Thompson

Oh right, and these individual observatories have given the data and it’s sort of a patchwork affair to put them together to get to this final image.

Interviewee: Davide Castelvecchi

Yes, they had to record petabytes of data – that’s thousands of terabytes of data – and it was so much data that you couldn’t actually transfer it through the internet. You had to put the hard drives on aeroplanes and take them to a central location.

Interviewer: Benjamin Thompson

Well, why did they choose this M87 black hole in the first instance then? You say it’s really, really far away – I imagine there are ones closer, would that not be easier?

Interviewee: Davide Castelvecchi

Yeah, in fact they observed two of them and they are still not releasing data or results about the other one. The other one is the black hole at the centre of our own galaxy. It’s called Sagittarius A*, and the two of them are the largest black holes in the sky so that’s why they focused on these two.

Interviewer: Benjamin Thompson

So, if they’ve been looking at both, Davide, why have they announced the image from the furthest away black hole in the first instance?

Interviewee: Davide Castelvecchi

Because they haven’t had the time to look at the data from our own galaxy’s black hole yet. It’s going to be more complicated because it’s a smaller black hole – it’s more than 1,000 times smaller – which means that matter orbits it at a faster velocity. So, basically, during each night of observation, the light flickers and changes as you watch it, which makes it a lot more complicated to interpret but also potentially includes more information.

Interviewer: Benjamin Thompson

Well, sticking to this current image, what’s it telling us about the M87 black hole?

Interviewee: Davide Castelvecchi

Yeah, so first of all, it’s the right size and shape that you would expect it to be, and they also were able to measure the fact that the black hole is rotating and it’s rotating in a clockwise direction in the sky. There’s many other things that could still be in the data that they haven’t even had the time to look at, in particular they could try to understand how the black hole produces jets – these are these highly energetic beams that we see coming out of the galaxy M87 – they’re actually much larger than the galaxy itself.

Interviewer: Benjamin Thompson

So, if this is a starting point, what are the next steps?

Interviewee: Davide Castelvecchi

So, the next steps are, well, they already have a lot of data in the bag that they haven’t even look at about M87 and also about the black hole at the centre of the Milky Way. They’re going to continue to observe these objects every year. There’s a window of time around April of every year where the weather is supposed to be good at all the observing locations around the world at the same time, so the next observing run will be in April 2020, and it will include two or perhaps three more observatories than the previous runs. There’s also a dish that is planned for Namibia because currently the network doesn’t have any dishes in the whole continent of Africa. Also, they hope to be able to observe at a slightly shorter wavelength, which would improve even further the resolution so that we will be able to see smaller black holes, smaller details and so on.

Interviewer: Benjamin Thompson

Right, I mean, Davide, as I say, you were in the room when this announcement was made and you’ve been covering black holes for Nature for a long time – how important a finding do you think this work is?

Interviewee: Davide Castelvecchi

I think psychologically the impact will be tremendous. I don’t know yet how much information they will be able to extract, but judging from the first hints, it looks like it’s going to radically change the way we understand black holes and physics in general.

Interviewer: Benjamin Thompson

Well, Davide, thank you so much for joining me today. Listeners, we’ve got a couple of videos about this topic on our YouTube channel. One gives a three-minute overview of the new research and the other shows the joyous response from the researchers in the room when the image of the black hole was unveiled. Head over to youtube.com/NatureVideoChannel to watch them. I’ve been Benjamin Thompson. Thanks for listening, see you next time.