Black Hole seen for the first time

15 August 2017

Interview with

Luciano Rezzolla, Institute of Physics in Frankfurt

When it comes to telescopes, bigger is better! This year marks the beginning of a new era. Radio telescopes dotted across the globe have joined up to create a telescope the size of the Earth. The initiative is called the Event Horizon Telescope and the aim is to capture the first images of a black hole. Luciano Rezzolla, from the Institute of Physics in Frankfurt, is part of the collaboration and joined us. Chris Smith asked him to explain what exactly is a black hole...

Luciano - A black hole is one of the most cherished objects in our imagination nowadays and yet, as a scientist, we don’t have any definitive observation that confirms their existence. A black hole is an object which gravity’s so strong that they posses a surface which is called the event horizon where nothing, not even the lightest particle, can escape, and the lightest particle is, of course, light. So one can think of black holes as objects where gravity has the ultimate word, and where life cannot move out of this very specific surface, the event horizon.

Chris - How do we know then that black holes exist if no-one’s ever seen one?

Luciano - The idea is to use the indirect evidence. For instance, in the case of the supermassive black hole at the centre of our galaxy, you can convince yourself there’s a black hole because you see stars that are moving around an object which doesn’t emit any light, which is extremely massive - 4 million solar masses and yet you don’t see it. So the motion of stars near a supermassive black hole is a way of telling that there is a black hole. What we would like to see is “a smoking gun” for presence of black hole and that is the presence of an event horizon. That’s what we’re trying to do with this project - the event horizon telescope.

Chris - I suppose it’s complicated this, because astronomy is largely based on looking at things using light and, if you’re trying to see something that doesn’t let light escape, that must be tricky?

Luciano - It’s actually quite frustrating. But we do have an option here and that is given by the fact that if a black hole is immersed in a bath of radiation, of light, then there’s going to be a region out of this bath which will be darker because some of the light will not be able to reach us. This is actually what we are trying to measure and take a picture of, this region which is less luminous. It is what we call the shadow of a black hole, so through radio telescopes we’re trying to reproduce the shape of this shadow.

Chris - Right, so in other words by looking out how the light that you can see behaves you can infer what must be influencing the light in that way and, therefore, work out what must be there to do that and then you can see whether your observation is a theory?

Luciano - That’s right, that’s correct. And the shape can be telling a lot of information because different black holes have different theories of different properties will give you a different shadow. So, by measuring the properties of the shadow we will know a lot about the properties of the black hole.

Chris - Now why has the Event Horizon Telescope been convened to do this? We’ve got lots of telescopes on Earth - why do this with them?

Luciano - The problem is that light in wavelengths that are not radio tend to be absorbed, so what you want to have is radio telescopes measuring this shadow. Radio telescopes are actually ideal for this, not only because they can get the only radiation that reaches us, but also allows us to have the highest resolution. In order to have an idea of the high resolution that you need, it is like taking a picture of an orange which is placed on the surface of the moon. It’s a humongous resolution and the best telescope to give us this resolution is radio telescopes because there is a very simple rule in the resolution and that is that given a certain wavelength, you want to have the largest possible telescope to see this wavelength. In the case of the radio telescope you can get very large dishes, hundreds of metres. As a matter of fact even 100 metres would not be enough. What you really would like to have is a radio telescope which is as big as the whole planet. You may think this is just impossible. Actually it is possible and people have pioneered this technique which is called very large baseline interferometry decades ago. So the principle is you can’t have a single telescope, why don’t you have two telescopes as far apart as possible, and you just make sure they record exactly the same radio wave front. That’s what you do by synchronising these two telescopes with atomic clocks. In this case the larger is the better and if you can have more than two telescopes picking up the same wavefront the better because even the detail of this wavefront would be more precise. That’s what the Event Horizon Telescope collaboration has done this spring. It has set up a very large campaign where all the biggest telescopes scattered across the planet have recorded data, and this data we are now analysing.

Chris - You’ve even got the telescope in Antarctica which has made some recordings, but you haven’t got the data from that yet because you haven’t got the hard disks back from Antarctica have you?

Luciano - This is the most advanced technology to do this type of job. At the end what you really need is an aeroplane getting this data that is stored in these discs. But that’s it - you can’t win with the solar winter so we have to wait for planes to be able to land at the south pole telescope and pick up the data. Which actually is very important because the south pole telescope is the one that can always look at the galactic centre. While other telescopes would be as the Earth rotates they would not be seeing the galactic centre during some time of the day. But the most important piece of the puzzle is actually still missing and that’s why we hope that once we have that data we will have the most accurate measurements ever made of the galactic centre.

Chris - So you merge the data from all of these individual telescopes, which have all looked at the same event at the same time so you know that they’re all seeing it from a slightly different viewpoint and that should, therefore, give you this very deep detailed picture. But what will the picture that ultimately emerges when you put this sort of montage or this mosaic together, what will this look like?

Luciano - This will depend a lot on the quality of the data. This is data that is taken over several days, but we hope we will see some evidence of this shadow. So an evidence of a region where there’s going to be a lack of luminosity, while nearby there’s going to be a lot of emission. This is what our numerical simulations are telling us we should see.

In practice, I’m not able to tell you the quality of the data up until we have processed it. We might be very lucky and see something which is very suggestive of the presence of a horizon, or we might have to try again and remove some of the uncertainties to do, for instance, with the fact that the light would be scattered and so the image would be blurry.

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