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Heart of darkness

Black holes and the 2020 Nobel Prize in Physics

Black holes are the most mysterious objects in the universe; matter and space folded into an enigmatic knot from which not even light can escape. The 2020 Nobel Prize awarded research into black holes, thinking about the unthinkable, and seeing the unseeable. But just what did these extraordinary scientists do? Tune in as we try to unravel the puzzle.

The Royal Swedish Academy of Sciences this year jointly awarded the Nobel Prize in Physics, with one half to Roger Penrose “for the discovery that black hole formation is a robust prediction of the general theory of relativity", and the other half to Reinhard Genzel and Andrea Ghez "for the discovery of a supermassive compact object at the centre of our galaxy".

What makes these discoveries exciting and what do they tell us about life, the universe, everything? Two of our leading physicists, **Geraint F. Lewis **and **Peter Tuthill**, help shed some light on the science of black holes.

**PETER TUTHILL **

My name's Professor Peter Tuthill. I'm here to kick things off for Sydney Ideas talk - *Heart of darkness: black holes, and the 2020 Nobel Prize in Physics.*

We'll just get going in a minute. But first, I'd like to acknowledge that we are hosting this virtual event from the lands of the Gadigal people of the Eora nation.

We want to acknowledge the traditional custodians of the various lands on which you all work today, and the Aboriginal and Torres Strait Islander people for joining us for this event.

We pay our respects to elder's past, present and emerging and celebrate the diversity of Aboriginal peoples and their ongoing cultures, and the connections to the lands and the waters of New South Wales.

And in particular, while I'm introducing an astronomy talk, I'd like to mention the heritage of Aboriginal astronomy with sophisticated knowledge used for navigation, timekeeping, and many cultural practices. You can find out a lot about that; there's a lot of research nowadays on those areas.

So to move forward, this talk is going to be presented by myself and Professor Geraint Lewis. So Geraint is going to take away the first part, which talks about the theory and ideas behind this Nobel Prize. And I'm going to take the second half. I'm Professor Peter Tuthill. And I'll just hand over to Geraint.

**GERAINT F. LEWIS **

Okay, so thank you all for being here. Yes. So it's a very exciting year for the Nobel Prize in Physics, because it was awarded for one of the really cool and interesting ideas in all of astrophysics in all of the things that we know in the universe.

And these are these objects known as Black holes. And the Nobel Prize this year, was awarded to three people, the Nobel Prize can go to a maximum of three people.

And in fact, the Nobel Prize this year was actually split into two different pieces, one side to do with the theoretical aspect of black holes. And that's the part that I will be talking about. And the other half, which was awarded to Reinhard Genzel, Andrea Ghez was to do with a technical side of how we observe black holes.

So what I'm going to do is, I'm just going to give you a very brief summary of what we think Black holes are, and what Roger Penrose who's the old guy here, what his contribution was.

And the actual citation was for the discovery that black hole formation is a robust prediction of the general theory of relativity.

So to understand what this actually means, we need to go back a few years, we need to go back back to the 1600s to this guy. So this is Isaac Newton, and here he is pondering the apple falling from the tree.

And what Isaac Newton did for us is he wrote down a mathematical rule, which many of you will have seen in high school for how gravity works.

He said that gravity was related to mass. And if you have two massive objects, then there's a force between them. And that force is proportional to the masses.

So the more massive the objects, the bigger the force, and is inversely proportional to the square of the distance apart. So this mathematical rule, coupled with the other mathematical rules that Newton gave us for gravity and motion; has proven to be very, very powerful.

I mean, when we look at an apple falling from the tree, or the moon orbiting the Earth, or the planet orbiting the sun, or the sun orbiting the galaxy, we can use Newton's law of gravity, the equations that were written down now almost 400 years ago to describe this motion in detail.

Now, when Newton was working back in the 1600s, people had this mathematical rule, but they still didn't really understand what the universe was made of. And one of the big mysteries was what was the nature of light.

And at the time, there was sort of like an argument going on about whether or not a light was a wave, some sort of wavy phenomena, like a wave on the ocean, or it was made of particles, little discrete pieces.

And even Newton himself thought that if light was made of little chunks of stuff, whatever this stuff is, that it should be influenced by gravity. That gravity should tug on the path of light ray, and deflect its motion.

Now, roughly 100 years after Newton had his original ideas, this guy came along; This is John Michell. And you know, he was one of the great polymaths of science, he did a lot of work in magnetism in astronomy.

But he also thought about this question of gravity. Especially about how gravity could influence the path of a lightray. And what he realised is that if light is made of chunks of stuff, then if light is trying to get away from a massive object, then it should be influenced by that object's gravity.

And in the same kind of way that if we're standing here on the surface of the earth, and we throw a ball up in the air, it comes back down and we catch it; he sort of realised that if the gravity of an object was strong enough, the same should happen to light. That light would try to escape, but it couldn't overcome the gravitational pull of the object, and it would fall back.

And what he realised is that if this was the case, this object, this Dark Star, would be invisible, because no light would be able to get away from it.

And this was the first notion that we have of these objects, which we now call black holes, things where gravity is so strong that not even light can get away.

The question of gravity remained essentially in the same sort of picture as Newton gave us for several hundred years, until the start of the 1900s, and the arrival of this guy, and this is possibly the most famous scientist who ever lived.

This is Albert Einstein. And part of his fame came from what he told us with regards to gravity.

He essentially threw out the picture that Newton has given us, and said that gravity is not a force in the way we think of forces, but is more to do with how space and time themselves can be bent and warped by the presence of mass. And the thing that we think of as gravity is actually objects responding to this curved space time.

Einstein's general theory of relativity is now how we talk about gravity if you want to work out how the GPS system works, that depends upon motion of objects and things in different kinds of gravitational fields, you have to use Einstein's general theory of relativity.

So it superseded the Newtonian picture. The thing with Einstein though, unlike Newton, which, who had that one simple equation; Einstein's general theory of relativity is mathematically a lot more challenging. And I actually love this picture, this is a wrecked train somewhere in the desert in the middle of South America.

And somebody came along with a can of paint one time, and they wrote onto this train, which for me, is one of the most beautiful equations that we have in the universe. This is what's known as the Einstein field equations. And this is how you relate the geometry of space time to the distribution of mass.

Now, Einstein always admitted that he found math challenging, and lots of good friends who were very good at maths and they helped him.

And when he looked at his equations, even though he realised how powerful his equations were, he thought to himself, well, maybe there are no solutions that I can write down for, you know, if I want to work out the gravitational field of an object.

These equations are just so complex, so messy, that I don't think there's a way to write down solutions. So that was in 1915, when he derived these equations.

But of course, when you make a prediction like that, there are no solutions, somebody comes along and makes you look like a fool very quickly.

And for Einstein, it was this guy. This is Karl Schwarzschild. And again, Karl Schwarzschild was a mathematician. But he was a mathematician in a very difficult time. Of course, this was 1915-1916, World War I was raging, and Schwarzschild was actually serving on the Eastern Front. And people say that war is you know, hours of boredom interspaced with moments of terror.

And in his hours of boredom, he worked on the equations of Einstein's general relativity. And what he realised is that he could make some assumptions.

What he wanted to do was calculate the structure of space time outside of a spherical object, something like the earth or the sun, what does the gravity look like outside of that object. And he found that he could work with equations, he wrote down this particular mathematical equation, which we now know as the Schwarzschild Solution.

And it's what we use to calculate the gravitational effects of the Sun or of the earth. But what he realised is that in this particular picture, what he could do is he could make his object smaller and denser.

And that, of course, increases the strength of gravity around that object, you make it smaller and dense, again, increases the amount of gravity, and you realise it gets to a point where you've squeezed it so much, that the object just completely collapses down into a point.

And that point, its gravitational field is so intense, that light can't escape. So what we've got here is our mathematical description of a black hole. And if you want to see where this particular equation is used, if you've seen the movie Interstellar, that picture we've got at the top there, that's a representation of how we expect a black hole to look.

That black hole's gravity is so strong that all the light rays in its vicinity have been distorted. And it's this equation coupled with Einstein's other equations in relativity that allow us to calculate this particular picture. So that's great. That's a very interesting finding that we have in the mathematics of relativity.

So after Schwarzschild had written down his equations, the next question was, well, is that a mathematical oddity? Or is this something physical that we have in the universe?

And so people started thinking about, well, how could black holes be formed? How do you get so much mass crushed down into a little volume to get the gravitational, so strong that light can't escape. So we can think about the picture that Newton gave us, right?

So this is Isaac Newton again, pondering and what we've got here, this is just a little animation, it's supposed to represent a sphere of mass. And we're going to just let it go and let gravity do its thing.

And what we've got in this particular situation is gravity is pulling out all that mass, and the mass has been shrinking down and shrinking down getting denser and denser, gravity is getting stronger and stronger, and down, it goes down it goes.

And eventually all of this mass arrives at the centre at exactly the same time, and you get all the mass in a tiny point, you have the equivalent of a black hole. So that's great. In Newton's picture, it works, we can get all the mass to get squeezed down, and we can produce this point.

We've created this object called a black hole. Now, where in the universe would we find a mechanism that can squeeze mass into a tiny volume, and there is an obvious culprit. In this picture, on the left, here, we have a galaxy. So this is galaxy has a few hundred billion stars in it.

But the one that I want to draw your attention to is that bright one on the lower left hand side. That is a supernova. That is an exploding star.

And that star has collapsed and exploded and it now outshines all of the other billions of stars in that galaxy put together. So the collapse of these objects, these massive stars into supernova seemed like the ideal place where you could create black holes.

What I've got on this side, this is a computer simulation, we do lots of stuff on computers these days, this is meant to be the inner part of a massive star, as that star is basically becoming unstable, and tripping over into this this supernova regime where it's gonna blow its self apart. The timescale at the top that's in milliseconds, this stuff all happens very quickly, the start stars cores got crushed, nuclear reactions are starting to go wild, things are starting to get really out of hand.

But the key thing is that if you look at this picture, it's not spherical. It's kind of messy, it's kind of lumpy. It's kind of not that perfection that we saw with regard to Newton's picture of collapse. So let's go back to Newton again and say, okay, Isaac, what if I take a distribution of mass, make it roughly spherical, but let's not make it perfect, let's make it lumpy and bumpy, and tell that to collapse? Well, this is what happens.

The collapse seems to start off pretty much the same as we saw before. But some stuff rushes to the centre first and rushes through and comes out the other side. Other stuff arrives later.

And because we didn't start off with a perfectly smooth mass distribution, then we don't get this perfect collapse down into a point.

And there was real worry, there was a worry that this picture that Newton has says that if you have an imperfect mass distribution, then you can't form black holes. So maybe that solution that Schwarzschild found that was a mathematical oddity, and people were kind of happy with that, until the 1950s and 60s, where we started to see signs in the universe of objects that looked like there were black holes.

So how could they afford? And this is where this man came on the scene. So this is a younger Roger Penrose, he's now 80. This was him working back in the 1960s. And again, Roger Penrose, he's a mathematician, and he's a polymath at that; in that he worked on lots of different areas in mathematics and mathematical physics.

And in the mid 1960s, he turned his attention to this question of how do you get mass to collapse into a perfect object that we would think of as a black hole? And what he did, being a mathematician, he, as you can see, in the diagrams behind him, he thinks differently to a lot of people.

He thinks in terms of geometry, right, instead of just writing down equations, he dreams in shapes and spaces. And this picture that we have on the right, this is from his famous paper, where he considered the mathematics of a collapsing object in terms of Einstein's theory, but also in terms of the geometric picyure, he called them tracked surfaces, etc. But he was just trying to think about the overall geometry of what was going on.

Which is important, because remember, Einstein's theory is a theory of the curvature of space and time, which sounds like a geometric thing. So once he had done his work, he showed that objects when they collapse, they naturally form black holes. So what do we mean by that, so here we have our collapsing mass distribution again, and it's lumpy, it's bumpy, it's not perfect, and we're going to let it collapse.

And as it starts to collapse, it does exactly the same thing as it did in terms of Newton's picture. But as the density goes up, and the gravitational field goes up, and the curvature of space time becomes more and more and more, you find that curvature funnels the mass tighter and tighter and tighter.

And what Penrose showed is that it doesn't really matter how lumpy your mass distribution is, as long as you can squeeze it past a certain limit, then gravity is going to win. Gravity is going to take over, and it's going to collapse the mass down, and it's going to form a black hole. So this is what Penrose was awarded the Nobel Prize for, remember this is work in the 60s.

And it's now 2020. So it's been a long time coming. But I don't want you to go away from this thinking that the question of black holes in the universe is now solved because of this work that Penrose did.

In fact, they've even become more mysterious. And I'm just going to show you this little video before I finish. This one I just borrowed off YouTube, just to give you an idea of what we're talking about.

Now, I said you have to crush mass down. And so this is our sun many, many times more massive than the Earth. If you want the sun to become a black hole, you have to crush it down, and you have to crush it down and you have to crush it down and keep crushing it and keep crushing it until it's about three kilometres across, then gravity wins. A black hole is guaranteed.

And it seems like supernova are the ideal places for doing precisely this kind of crushing to turn the mass of the Sun into a black hole.

The earth you'd have to crush it down into the size of a peanut. Now as I mentioned, we're starting to see black holes out there in the universe, and some are more massive than the Sun.

So this particular black hole, this has a mass roughly four times the mass of the sun. So you can imagine starting off with a giant star, it's collapsing and crushing mass down to give you this bigger black hole. But we're nowhere near the biggest. Here we have another black hole that we've measured out in the universe. This one is about the size of the planet Mars.

So how much mass do we need to crush down to give you this black hole? Well, not one or three or whatever number of masses of the sun. But actually, you've got to take essentially 1000 times the mass of the sun to squeeze in, to give you a black hole of this size.

Maybe we created smaller black holes and they merged together to give you this big one. But we have a real mystery. The real mystery of the universe is that the biggest black holes that we know are enormous.

And this is now zooming out just to give you the scale. And we're almost there. So the biggest black holes that we can see, they dwarf the solar system in terms of their intense gravitational field.

And how much mass do they have - this one is one of the most massive, okay, so here we have 1000 suns in one direction. And in the other direction, maybe it's a few hundred thousand times the mass of the sun? Well, no, this is just the start of it, we've actually got to add all of this.

And then a few more of these. And then a few more, and then build it up a little bit. And what we find is that the biggest black holes that we have in the universe are millions to billions of times the mass of the sun.

And we've really struggled to understand how these objects form. Before I finish and hand it over to Peter, as I mentioned, Roger Penrose is a very interesting character. He's worked in an awful lot of areas.

He's written some interesting books. If you're interested in physics, if you're interested in mathematics, I recommend that you take a look at these books.

But even if you're not that into physics or mathematics, look up the concept of a Penrose tile. This is another area that he was thinking about how do you cover a floor with a tiling pattern that doesn't repeat.

So that's a summary of the theoretical side. So let me just stop sharing my screen and I'll pass it on to Peter and he will talk about the technical side with regards to observe in black holes.

**PETER TUTHILL **

Thanks Geraint. So, as Geraint mentioned, I'm talking about the second part of this kind of hunt, I suppose the first part being the intellectual or conceptual pursuit of these objects as an idea.

But then you want to go out and see if it's a, you know, if they really exist out in the universe. So this process proceeded in almost exactly the opposite fashion to the story Geraint just illustrated.

And that is that an enormous tonne of extremely high technology was thrown at it to accomplish this. And it sort of maybe throws the theoretical work into sharp relief, because that was really accomplished with a pencil and paper. It's a very remarkable tribute to the human mind. It's a point I'll come to later in the talk, but the citation for Reinhard Ganzel and Reinhard Genzel, Andrea Ghez who shared the other half the prize.

The citation actually doesn't mention that they found a black hole they found a "supermassive compact object". And we'll come to that terminology a little later in this talk. It's interesting that they chose that language.

So I'm going to adopt some imagery from a favourite author of mine, Lewis Carroll, to illustrate the absurdity of this quest, because we're trying to find an object, which is really, by definition, something that can't be seen.

Black holes are very, very black. they emit no light, no energy, in fact, they violate causality. You can't have a causal process coming out of a black hole. One way to think of a black hole is it's kind of a waterfall of space.

At the event horizon of a black hole, space is falling in onto the black hole at such a rate that any object even an object moving at the speed of light will be brought to a standstill. And that reminds me, of course, famous analogy with the Red Queen, who, no matter how fast you run, you simply stand still, when you're in those Carol's Looking Glass land.

So this is already sounding like an absurd quest, we're trying to find something which by definition, can't transmit any information to you.

If you add on top of that, the usual kind of problems that astronomers like Geraint and I face, which is that we live in a very, very large, dark universe. And we've tried to find a very small object by comparison.

So even though the black holes are enormous, for scale, we're trying to find something that's about the size of a football on the moon. That's the sort of apparent size that it would present in the night sky.

So it's a very tiny black object that doesn't want to be found in a very dark, black sky. So astronomers had a few ideas about how you would go about such a crazy quest.

And I'll kind of go through these three ideas in turn. One is to sort of look for the fireworks, you know, we have an object here, which should cause some mayhem with its gravity; should throw the furniture around.

So we might not be able to save the object itself. But we should be able to witness the the effect it's having on its environment.

Maybe more fundamentally, one thing that a black hole can't hide is the fact that it's a gravitating body. So if we follow the gravity down the hole, then at the end of that quest, we should eventually come up with the black hole if they exist.

And the third thing I'll talk about is almost the most absurd of the three, which is that, well, maybe an object which can't be imaged, can be made imaged, after all, so I'll come to that at the very end.

So the first part of the quest following the fireworks was almost immediately an absurdly successful. We went looking for fireworks, and almost everywhere we looked, we found fireworks.

The left hand image you see here is a spectacular high energy jets coming out of an active galaxy, we call an AGN, so stuff is coming out here.

And it's being forced out into the, into the universe in such a fashion that these jets span the voids between the galaxies, it's an enormous structure. We found big ones like this.

And we found small ones like the one over here, microquasar SS433, which is kind of your backyard variety of the same phenomena.

This is a solar mass style object that's throwing off jets. And here we see an entire galactic sized object that's doing the same thing. So we already have the idea that there's a span of these objects, and they span this range of scale. I'll just illustrate at the bottom here. Messier87 is one of the most famous of these objects.

It's that that object in the lower left corner there, and if you zoom in on that with a radio telescope, you see this spectacular jet structure that kind of wraps around, and if you zoom in on the jet itself, you can actually even make these time lapse movies; where you can witness stuff being thrown out, almost at the speed of light.

So we see furniture getting chucked around, and we assume there's a big black gorilla in the middle.

Most astronomers, as Geraint pointed out, have been reasonably well convinced that black holes exist since the sort of 1950s since these kind of images started being made. However, there's this maxim that extraordinary claims such as the existence of something as crazy as a black hole do require extraordinary evidence.

And are we perhaps guilty of, you know, painting the red roses on a white rose tree? by Lewis Carroll's famous playing cards here? Are we guilty of trying to paint reality with the colours we want to see? Are we going to get found guilty by the Red Queen?

So strangely enough for science, and this doesn't really often happen; the entire field turned into a kind of a semi legalistic kind of debate. People would find more and more absurd situations where they thought a black hole had to exist.

And then people would publish papers saying, well, no, actually, maybe it could be something else. Merely seeing the furniture getting thrown around doesn't mean it has to be a gorilla. Maybe there's some other object than a black hole, causing those crazy high energy jets.

So here's a question for everyone to think about. If our own sun tomorrow suddenly collapsed into a black hole, that would be bad news for life on Earth. But the question really is, how would we all die?

And I often ask this to sort of school students and lectures I give. And I get all kinds of interesting answers about event horizons and spaghettification by tidal forces.

But in fact, to an astronomer, the answer is, well, we'd all freeze to death. So the only thing that would happen to the earth is that we just wouldn't get any sunrise tomorrow, and eventually, it would just get colder and colder.

So this is a problem. If you're looking for a black hole, what we want to do is to probe the strength of the local gravity. And our own solar system gives us a good model for this. As we get closer and closer in, we can see the gravity gets stronger and stronger, or another way to put that as the curvature of space.

And we can see Mercury's buzzing around in higher gravity here, it has to go faster. Mars is out here and weaker, gravity can go slower.

So we have a probe, we can probe how strong the local gravity is just by witnessing the motion of objects.

And you've seen pictures like this, cartoons like this before, where we see the gravity getting stronger and stronger as it gets closer in.

But what would actually happen if we got to the surface of something. So here's a cartoon about the earth, what would happen if we could dig a hole right into the middle of the earth and sort of climb down into the earth.

A lot of people's intuition might be that the gravity would get stronger and stronger as you got towards the middle.

But in fact, and that's in fact, what this cartoon says it says the space gets more and more curved, it's kind of like this red line here, we get more and more curvature, more and more gravitational field. In fact, the opposite happens, as soon as you start digging under the earth, what happens is, there's earth above you pulling you upwards as well.

And if you're at a point right at the middle of the earth, there's an equal distribution of mass all around you, and you get pulled in all directions, and you're in fact, weightless.

There is no orbit in the middle of the earth. So the true picture is this blue one, we have the same gravity at large distances. But as soon as we burrow beneath the surface of the earth, gravity starts to decline.

If the earth on the other hand was a black hole, that wouldn't be the case, we would see the gravity get stronger and stronger as we got closer and closer to the central source. So this gives us a clue. This gives us a key, a way in.

What we need to do; it's useless measuring things out here, out beyond the surface of the earth, we couldn't tell if it was a black hole, or a distribution of matter like the earth. But once we go below the surface, we can start to see the difference.

The black hole model has a big difference to the actual true, expected gravitational field. So the whole game is about trying to push this curve and trying to find these cases where we can get close in. So what is a good target to try and get close into?

Well, I'd just briefly like to throw an Australian connection. The centre of our own galaxy is an obvious candidate, we saw those fireworks in other galaxies. Maybe our own galaxy has a black hole.

And the centre of our galaxy, just a shout out to the home team was first found with this amazing telescope, firstly at Dover Heights, and then later, it was firstly at Potts Hill and later at Dover heights, overlooking Sydney, on the sea cliffs.

So somewhat after the centre of the galaxy was first identified. It already became apparent that well, yes it probably does look like those other ones we see out in the universe where there's something happening in the middle that looks like it's got all of the fingerprints of a of a black hole.

So even in the 1970s, there are papers saying, well, probably there is a black hole there. But what you really want to do is burrow down and follow that gravity.

So what we did is we studied the stars as they got closer and closer to the middle. And this plot shows the enclosed mass or the mass you expect interior.

And if what you expected to see was you were burrowing into something like burrowing into the earth, or going into a distribution of stars, we expect to see this curve tail off according to this dotted line here.

But what we're seeing instead was the curve flattened off. So what that meant was something heavy and massive, seemed to be interior or smaller than anything you could measure.

And we're pushing this down from sort of the size of the galaxy down to the size of the local galaxy, and then down to quite small areas like you know, sort of areas of a local stolen neighbourhood.

So the question becomes what could possibly lie in closed in this tiny volume that's got on this axis, there's a million solar masses, few million solar masses, so something very small and compact must lie here.

And this is where these legal arguments come in? Well, it looks like it's a black hole, it smells like a black hole, but does it have to be a black hole?

And for reference, this is about six kilogrammes per cubic kilometre, astronomers are going to tell you nup it's got to be a black hole, it can't be anything you could hide in there that heavy.

But in actual fact, I just did a calculation before this talk, that's only one teaspoon placed every 200 metres through that volume.

That doesn't sound so absurd. I mean, an astronomer will tell you, they won't stay there. And that they would have to collapse. But still, you have to rely on theory to tell you those kind of things.

The problem got further exacerbated almost immediately, in the same year, some of these extra galactic other galaxies, the same game was played, let's try and drill into the middle. And we find these rotation curves which show the rotation getting faster and faster as you get to the middle.

And these tell you that there's like 10 times more mass in the middle of this galaxy, enclosed in the same volume. So we have 30 million solar masses inside one parsec or so.

So it became this game of sort of who can take the pain, where does the absurdity level break and you say, okay, it has to be a black hole. So at this point, we enter this actual era.

In fact, the work I just showed you, some of that came from these groups, that's Reinhard Genzel and Andrea Ghez here.

You want to drill down to the middle to prove that it can't be anything but a black hole, you want to push that inner limit smaller and smaller. And to do so this is where the shock and awe technology comes in. I don't have time to describe how these work.

But there are technologies now available to astronomers, and they give you these spectacular sort of lightsaber fights on the sky.

When you're at the observatory, it's truly awesome to see. But what they're able to do is to sharpen up the picture, to undo the blurring that the Star Light encounters when it comes to the atmosphere.

And you get this very clear image. And of course, a clear image is what you need to zoom in on the middle. Once you've zoomed in on the middle, you can pull that noose still tighter around the black hole.

And the closer they got, the more apparent it became that there really must be something right at the middle that's very, very heavy indeed. So this is the data from the Keck UCLA group, Andrea Ghez's group, using this advanced adaptive optics technology.

What you're seeing here is almost 30 years of work by these groups. And what they're seeing is a bunch of stars whizzing around very, very fast, around an unseen dark object right at the middle.

So it's now we've kind of constricted that noose around the black hole by factors of 100. So the astronomers by now even the sceptics have almost all thrown in the towel, it has to be really a relativistic black hole object.

But at this point, the story takes an interesting turn. So for the last few years, you know, I've known both of these scientists fairly well. So I've worked with Andrea in the past, and I've kind of been on watch, I've been expecting maybe she might get nominated for the prize one day. But then just lately, a couple of years, a year or so ago, one and a half years ago, there was this amazing late entrant to the field and I'm going to talk about the third idea for going looking for these objects. Just going out and looking and seeing if you can see a black hole.

But it turns out you can image something that can't be seen we do so every day. It's called a shadow or a silhouette. Here's an image of the moon blocking the light from the sun.

So in an almost identically analogous experiment, very audacious astronomers built this sort of distributed telescope with radio dishes spanning the globe, from the South Pole, almost to the North Pole, and from Hawaii up to Europe and Spain.

And they linked all these up with this amazing amount of technology. And they made this image in the middle you see is actually a real image of the shadow or the silhouette cast by a black hole.

So this is pretty much to me proof positive that a black hole has to exist here, there's actually a decrement of light, you see an event horizon. That's the singularly defining feature of a black hole. So this is really truly a stunning tour de force force of technology.

And having got this I was kind of expecting, Andrea and Reinhard might have to wait because this just looks like such a slam dunk.

So it's an interesting question about why the Nobel for them and why now and maybe you can, you can take it from me that I kind of expect another prize for the same, almost the similar science with the event horizon telescope.

But I think some of the answer to why a Nobel and why now comes from this game changing instrument called the gravity interferometer VLTI, I don't have time to go into how they do it, but they basically link up all these telescopes, these are big eight metre telescopes.

And that's kind of like a truck sized object, they link up all these enormous telescopes to make one vast instrument that can really probe extremely close, very, very fine resolution. So they saw these objects, this star, which has the romantic name of S2.

That's one of these objects that zooms super close to the black hole. It's one of the ones you saw in the animation just before. And with this instrument, it's so exquisitely precise that you can see all of these new effects come to play.

You can witness that gravity doesn't behave the way Newton expected, we get this strange precession, we get the precession of the orbit, which is predicted by General Relativity, but not by Newtonian physics.

And in fact, amazingly enough, this instrument's so powerful, it can see the motion of a star every day, from across the galaxy from 225,000 light years away, we're watching a star march across the sky every single day.

Of course, that stars going at a few percent the speed of light. So it's a very fast star. But still, the technology required to do this is really awe inspiring. I'll just give you one final result from the gravity team.

They actually zoom right in on the middle. And it turns out that they saw a little every now and then the very object right in the middle that's flare up and you see stuff. It's actually not the black hole you're witnessing.

It's the black hole's lunch, it's stuff that's about to get funnelled in on the black hole. And they watched this blur of light, zoom around in about 20 minutes, going at about 30% the speed of light. So this is really just a new box of candies that we've only just opened.

There'll be amazing results coming out of these new machines that probe right down into those areas of physics that are otherwise inaccessible.

And just to finish finally, I'll finish off again here with another one of these absurd pictures from Lewis Carroll, where the king and the queen are arguing with the executioner about how to chop the head off a cat which doesn't have a body.

The cat eventually fades into a grin and it seems to me that with these results, we now have almost captured the Cheshire Cat's grin.

It seems just such an absurd thing to have set out to do and I'm just in awe that we've managed to do it. So on that note, thanks, everybody. I think we're probably signing off now. Okay.

**GERAINT F. LEWIS **

Thank you all.

**ANNA BURNS **

Thanks for listening to the Sydney Ideas podcast. For more information head to sydney.edu.au/Sydney-Ideas.

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*A Fortunate Universe: Life in a Finely Tuned Cosmos*, which examines why the physical properties of the universe appear to be just right for complexity and life and *The Cosmic Revolutionary’s Handbook: or how to beat the Big Bang *that describes how we know what we know about the universe.

*Event image: first image of black hole released by the Event Horizon Telescope (10 April 2019) via Wikimedia Commons / diagram from Penrose’s 1965 paper “Gravitational Collapse and Space-Time Singularities”, Phys. Rev. Lett. 14, 57. / composite: Geraint F. Lewis*