Gresham College Lectures

Pulsars, Microwave Ovens and the Radio Sky - Chris Lintott

March 04, 2024 Gresham College
Gresham College Lectures
Pulsars, Microwave Ovens and the Radio Sky - Chris Lintott
Show Notes Transcript

There have been two major revolutions in how we look at the sky - the shift beyond the optical to other wavelengths, particularly the radio, and the increasing attention paid to how objects change over time.

We start with the discovery of pulsars by Jocelyn Bell Burnell, explore how a microwave oven bamboozled astronomers, and discuss the latest research on Fast Radio Bursts, mysterious events detected in galaxies billions of light-years away.

This lecture was recorded by Chris Lintott on 21st February 2024 at Barnard's Inn Hall, London

The transcript and downloadable versions of the lecture are available from the Gresham College website:

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Good evening. It's a delight to be here and to talk to you all. Um, our story tonight is about what happens when astronomers get ambitious. When we look beyond, uh, the visible spectrum that we are used to seeing and that we're used to looking up at the night sky, uh, and utilizing and start to think about, um, wavelengths that aren't accessible to our eyes, to use technology to explore the universe. Um, and really it's a story of perseverance. It's a story of some, uh, amazing characters who've taken place part in the development of radio astronomy over the last century or so. Um, but it's also a story of remarkable discoveries. Um, and I thought I'd start off by talking about one that was reasonably close to home. This is an artist's impression, um, just to have something on the screen of a possible planet around our nearest star, or rather the nearest star to the sun. So Proximus Centura is just a matter of a few light years away, and we now know that this nearest star to us has at least one, actually at least two, and possibly more planets going around it. And the one that features in this artist's impression. Proxima B, which is a properly sci-fi kind of name. I think prox. And normally I'm, I'm not into astronomers naming things, but proximal B sounds good. Um, goes round the star every 11 days. So it's pretty close to its star, certainly much closer than Mercury is to the sun. But because Proxima Ura itself is a faint red dwarf, Proxima B may have the right kind of climate to support our kind of life. It's in what we call the habitable zone, clustered up against the pale red dot of its star. And so this is a place that we can think about looking for life in the universe. And that has been done. It's been done many times, but most recently by this magnificent dish. This is the Mur Young Parks Telescope. It's most famous as the dish in the film, um, which relayed the TV pictures from Apollo 11, uh, and the first walk on the moon to the waiting world. But it's a high class, uh, research facility, not just a communications dish. Uh, and in 2019, it spent, uh, a couple of months spending most of its time looking at Proximus Centura, looking for signals that might betray the p presence, not just of life, but of intelligence, uh, on, uh, this near on a planet going around this nearby star. A year later in 2020, the team who were doing this from a project called Breakthrough Listen Now, colleagues of mine in Oxford, but then based in Berkeley, um, realized that they thought they had something, that there was a signal in these observations that seem to be coming from a planet orbiting, uh, our nearest star. Uh, and this leaked. Uh, so anyone who, who likes the conspiracy theories, I've always told people that astronomers can't keep a secret. Turns out this is true. Ian sample at The Guardian got hold of this. And on the 18th of December, 2020 published a reasonably cautious article that says, scientists Looking for Aliens investigate radio Beam from nearby Star. And it went on to explain that while the signal will, they said, will probably turn out to be something other than life, um, this was the kind of thing, a narrow band signal at a particular frequency that moved as if it was on a planet, uh, that those who've spent years looking for alien intelligent life out there, uh, might have hoped to have found, to find it on the next star is, is really exciting. If there's an intelligent civilization next door, then surely we live in a properly Star Trek universe with intelligent species on most planets, or at least most m class planets. Uh, and then we can go on and explore the galaxy with our new neighbors. So this was hugely exciting and it builds on this long tradition of using radio telescopes for exactly this, for the search for extra terrestrial intelligence that goes all the way back to the 1960s. Uh, when scientists, uh, green Bank in West Virginia, uh, undertook what they called Project Osmo using spare time and spare resources because they didn't want to be seen to be spending government money listening for aliens on pointing their telescopes at the sky, looking for repeating signals in the radio that they thought might be, uh, setting might be signs of extraterrestrial intelligence. And of course, this is a staple of science fiction. You know how this works. You know how radio astronomers go about this work?'cause you've seen films, films like Contact. Uh, here is, uh, Jody Foster as radio astronomer listening in. Uh, you can tell she's serious 'cause the headphones are upside down, uh, to data from the very larger array just before in the film, a a signal comes in. But it's worth thinking about why radio is the astronomers tool for choice for this. There are advantages in looking, uh, or using the radio rather than the optical. One of them, of course, is that you can observe even when it's cloudy. And for an astronomer based in Britain, this is an essential feature of radio astronomy. So have we got the live feed running? If we could just turn up the sound. This is noise from a a, a very simple antenna acting as a radio telescope. And what you are listening to is a frequency is associated with a TV station that's over the horizon from this antenna, which is broadcasting live on the internet on live And what we're listening for is the moment when a meteor, a shooting star hits the upper atmosphere, just for that moment, it will ionize, it'll excite atoms in the earth's atmosphere, and then the signal from the distant TV station will bounce off that and be received by the antenna. So we should at some point hear a ping. Now, there isn't a meteor shower going on at the minute. If I did this in the middle of August, we'd get maybe, uh, as many as 80 an hour we're listening for sporadic meteors that maybe happen once every few minutes. Um, and I was hoping that by filling we'd hear something. But what I might do is just ask the text to let that play. Oh, there we go. That was just a gap. You know, atmospheric conditions interfere. This was gonna be great when it pinged early, but you could tell I haven't faked it. We might just let that play if that's not disturbing people, let's just let that play. And if you hear anything, my hearing's not the best, please just put a hand up. Uh, and then we'll have a moment of meteor observation. So yes, you can observe during the day and you can observe when it's cloudy, and you can observe in these interesting ways. Um, but there's a more fundamental reason to switch to radio astronomy as well. Um, that is, I can see the sound people at the back going, this is gonna ruin this lecture <laugh>. Um, I'm gonna keep going till we get a meteor there. Um, there's a more fundamental reason to, oh, there you go. Did you hear that? Yeah. There you go. Good. Excellent. One meteor detected. Kill that for now. Um, I recommend live even when you're trying to get to sleep, it's quite restful in a strange kind of way. Good. We've made an astronomical observation live from the stage that makes me very happy. Um, there's a fundamental reason not just observing in cloud during the day that makes you switch to radio astronomy. And it's a consequence of the fact that we are looking at, at wavelengths that are much longer than the visible light we're used to seeing with the visible light. You're at, um, nanometers. So, so very small wavelengths. The radio waves we use for astronomy range from about 10 centimeters up to tens or even hundreds of kilometers. So these are very long wavelength radiation. And what that means is that they pass through the galaxy relatively unimpeded. And so if you look for example, towards the center of the galaxy, using a normal telescope, one with a mirror and an eyepiece or a camera that's used to working in the optical, you get beautiful shots like this one from my friend Will Gator. But what you're seeing here is dust blocking our view towards the center of the galaxy. So this dark material actually obscures our view of, of much of our, of the system that we live in. We get to only see our local neighborhood. And this, for those of you who are here for my first lecture, uh, earlier in the year, this was why William Herschel's great project of mapping the Milky Way by counting stars fails. It fails because all he does is measure how much dust there is in the galaxy, absorbing and scattering visible light. But with radio waves, you can see right through this dust, you can see all the way, uh, to the center of the galaxy. And so we have a very different view of our surroundings once we switch to radio. And this was something that was apparent even in the early days of radio astronomy, um, radio astronomy, sort of, I was gonna say it happened by accident, that's probably unfair. But what is true is that it was developed by people who were not astronomers, who people with expertise in the new technology of radio communication that was growing up, uh, after the first World War in the twenties, and particularly into the thirties and forties. The man with the distinction of being the first radio astronomer ever, I think is is this chap. This is Carl Jansky. Um, he was working at Bell Labs in New Jersey in, uh, the United States. Um, and his task was to identify where background static was coming from, um, the kind of noise that interferes with radio communication. Um, and so to research this question, he built the, the rather magnificently jury rigged instrument. You can see here, this was known as his merry-go-round. So this is a set of simple antennae on, uh, bicycle wheels, um, that he could rotate. And the point here is that you then have a directional instrument. So you can tell if the noise is coming from over here or over here or over here and using this instrument. And after lots of painstaking study, uh, Jansky realized that most of the noise of the frequencies at which he was working, um, came from thunderstorms. So he was observing thunderstorms in the earth atmosphere. Um, and then the story gets interesting 'cause there's a long sequence of many of the observations I'll be telling you about tonight where somebody isn't content with doing half a job. I think Jankis bosses would've been very happy if he'd written up his results and said, most of the noise comes from thunderstorms. Can't do anything about thunderstorms. I guess you can monitor the activity. Job done, move on to next project. But he realized there was this other source of noise there that seemed to always be there. And initially he thought that it was coming from the direction of the sun in the sky. This was a remarkable discovery. No one had predicted that radio waves would come from the sun. And so jansky began to study this extra noise. Perhaps, unfortunately, or perhaps fortunately, over time, what happened was the whatever was causing this extra noise drifted away from the sun in the sky. So as the months rolled on, it turned out this wasn't the sun that was causing the, the noise. Um, but he did realize that it repeated, it appeared in the same part of the sky every 23 hours and 56 minutes. Now, that's the side day, that's the time it takes for the earth to rotate. Once 24 hours is the solar day. It's how long it takes to get back with the sun in the same position relative to the earth. If you pointed a star and wait a day, the earth will rotate you and you'll be back pointing at, at it 23 hours, 56 minutes later. That's why stars set four minutes earlier each day. It's why we get the changing of the constellations. So this observation, the fact that this noise was 23 hours, 56 minutes, uh, in repetition, meant that it was coming from the universe. And Jansky called it star noise, which is apparently the thread this evening. Is astronomers doing a good job of naming things? I like star noise. I think we should have stuck with calling this star noise. Um, and so he wrote up these results. He got some publicity, and he was essentially told by his bosses that he should go back to doing proper work. Um, and went back and, and never quite developed. So he died early in 1950 before the great burgeoning of radio astronomy. But we now recognize him as the, the first radio astronomer. He did get his star noise played on NBC radio. One of the people listening was the second great pioneer of radio astronomy and a hero of mine, groat. Reba Reba was a, an engineer who worked in suburban Illinois, lived in Wheaton, commuted to work in Chicago. Um, and as soon as he heard star noise on the radio, he wrote to Bell Labs asking for a job. They presumably went, we've already got one of these people distracted by the universe. And so gave him very short shift. He then, um, wrote to pretty much every astronomer in the United States to Yerkes to Harvard, to the new observatories in California, and said, I want to work on radio astronomy. And he basically got a set of reactions, which were what no one wanted to know about this guy with his vacuum tubes and his, his radios. This isn't astronomy. Astronomy is about making maps with telescopes, uh, and so on. And so he concluded in his writings at his time that the astronomers were afraid of this new science. And in a very great Reba sentence, he said, it was clear that nobody was going to do anything. So maybe I thought I should do something. And this is what he did. He built the world's first proper radio telescope. This is only four years after Jankis publication of his star noise. So this is a fully steerable dish. Uh, it's about 30 foot across, so that's nearly 10 meters across. Um, and he could swing this around the sky and make maps observing with it wasn't without its difficulties. He still had to go to work. So he'd go to work, he'd come back, he'd sleep for an hour, and then he'd observe him at night. This confused me when I found this out 'cause we've just said you can observe during the day if you're a radio astronomer, but the starter motors of cars at the time emit radio waves. And so during the day, he just detected traffic. Um, and so he would observe at night, get a couple more hours, sleep, go back to work and repeat the process. Um, the only hindrance to this very valuable scientific practice was that his mother who lived with him, um, used to hang her washing on the dish. And so that often had to be removed before observations could, could proceed. Reba got remarkable results out of this telescope, spending his weekends reducing the observations and turning them into maps. Um, these, uh, this is a map of the radio sky. So this is, you can imagine this is the celestial globe, two sides. This is the intensity of radio waves in a map that he produced after six years of effort in 1939. And what it really shows you is that there are places in the sky where there are bright sources of radio waves. The brightest of them at the top of this map is, uh, towards the constellation of Sagittarius. There are others in Nus, in Cassie p in the North Canis major and puppets in the South. But he managed to find that there are different sources. So it's not a uniform glow of radio waves that we're, we are bathed in. It's, uh, a universe where there are things that emit radio waves. He wrote to Otto Str, uh, who was a professional astronomer, who ran the main journal at the time, said in a deeply technical paper that explained the observations, showed these maps, and noted for the first time that our galaxy, the Milky Way, seemed to have spiral arms, that the structure of the galaxy is revealed in these maps. We hadn't been able to see that before. Um, str and the other astronomers didn't know what, still didn't know what to make of, of Reba. Um, they didn't publish his paper, but nor did they reject it. They sort of viewed it as a curiosity. And I think uniquely in the history of scientific, of astronomical publishing, they sent a delegation of proper astronomers to Wheaton outside Chicago to inspect this man and his telescope and see what he was talking about after the, he seemed to have passed the inspection. They published the paper, uh, many years after it'd been submitted. And, and Reba complained that it, that his data had been sat on until it got moldy, starting a lifetime of really not getting on with the scientific establishment. Uh, but he'd shown that radio astronomy, not just radio monitoring of a background noise was possible. Um, I could fill the rest of the lecture with Reba. He ends up in the fifties, employed as an astronomer, but he proposes a telescope that's bigger than anything we've got today, which the US government doesn't want to pay for. Uh, he then decides that the solution is to look for radio waves bouncing off ocean, uh, waves. So you use the ocean as a radio telescope essentially, uh, that takes him to Hawaii while in Hawaii, decides he doesn't want to come home. So he quits his job and stays in Hawaii for about 20 years. He ends up in Tasmania where he publishes a well sighted paper, uh, that explains that Tasmanian parrots are right clawed in the same way that some of us are right-handed. Uh, he also studies the genetics of beans, builds his own electric car, and has a house that's mostly made of aluminum to keep the energy use down. It's very advanced for the seventies. Um, he also built more radio telescopes, um, and was frustrated that he couldn't prove his theories of cosmology. He didn't believe in the Big bang, correct. Um, he acknowledged that other astronomers didn't agree with him, but he put that down to what he described as narrow-minded incompetence. Um, and thought that the problem was the earth's upper atmosphere. We had a demonstration earlier that the Earth's upper upper atmosphere can change properties of radio waves passing through it. That's how we heard the ping of the meteor. Um, and so Reba requested a intercontinental ballistic missile to be launched above his house thinking that its exhaust would change. This is all true, by the way, uh, that it would change the ionosphere and enable him to make his observations. Now, that didn't work. He wasn't allowed to use an ICBM, but he did have enough clout at the age of 70 that NASA fired the engines of a space shutter while it passed over his observatory in an effort to change the upper atmosphere. And, you know, I think aspiring to that sort of influence after a career spent studying parrots, making inventions and inventing a branch of science, I think this is, this is, there's a reason that he's a hero of mine. However, he left questions behind us. He left the astronomical mainstream. Mainly, what on earth are these things that are emitting radio waves? And in particular, what is this bright thing in the constellation of Sagittarius? Now we've seen Sagittarius already when I showed you this picture. Uh, this is an image of the middle of our galaxy. We're looking towards the center of the Milky Way. And the constellation of Sagittarius is right there looking a little like a celestial teapot for those who know their constellations. But working out what this thing was, had to wait until sharper images could be obtained. The trouble is that how sharp an image produced by a telescope is depends on two things. One is the size of the telescope. A bigger telescope will give you a sharper image, but it also depends on the wavelength of the light that you're looking at. And because radio waves have this long wavelength, you get blurry images. That's why in the 1930s when we were taking beautiful pictures of nebuli in the radio with sketching squiggles on a map, the solution is to have a very big telescope indeed. But building very, very big telescopes is very difficult. And so by the seventies, what people have learned to do is what's called interferometry. Um, combining light from many telescopes. This is the very large array in New Mexico. And though these are all individual dishes, they act as one telescope. So you feed the, the, uh, signal they receive either directly at the time through cables or later in the computer into a central system. And you get to some extent an image that's as sharp as if the dish was as big as the whole array. And these days, we can combine dishes from across continents to produce virtual telescopes that the size of the earth using, uh, an early version of this interferometer in the 1970s, um, it was a astronomers were able to show that that thing at the century of Sagittarius is small. It's less than a parex. So that's less than about three light years across. So even though whatever it is is bright enough that we see it from the, uh, center of the galaxy, more than 20,000 light years away, it's a small object. And it was called at that point, Sagittarius a star for shadowing GCSE grates by a couple of decades. Uh, the star is a chemical notation for being excited. And the astronomers in question thought it was exciting that they had this compact source. Now, the Galactic Center is complicated. Here is a modern radio image from the Meca telescope in South Africa via my colleague Ian Hayward and Co in Oxford. Um, if you can see, there's all sorts of stuff going on here. The, the compact source is the bright thing at the bottom that's sag a star, Sagittarius a star. You see there's a sort of bubble. You're looking at gas here. And so there's a bubble of gas just above, uh, that bright object. And then there's these mysterious arcs that sweep across this image from bottom left to top right. They're filaments of material and we don't understand yet what the cause of these structures are. We need to look more, more closely. We need to see if they change over time. We need to investigate them at different wavelengths. But we do now know what lurks at the center of that blob at the bottom of the image. And we know this not just because of radio, but because we followed up with other wavelengths. Having identified that the center of the galaxy is interesting, we can look in the infrared and using the infrared because we get a sharper image of the center. We've been able to watch over time. Stars move near this object. So this is a movie from a group led by Andrea Gates in, uh, California, um, from a telescope called cac, which is on monarchy on the big island of, of Hawaii. And the year is in the top right here. So this is 1995. Oh, I'm going to run almost 20 years worth of observations at the center of our galaxy. Um, this is an incredibly small patch of sky, uh, much less than a a thousandth of a degree across. So we're using all sorts of technology to get this incredibly sharp view. But what you can see over time is the, the stars move. They also move in orbit around some object that's at the center. Um, it's marked by the open star shape here, which I've added. That's not what stars look like <laugh>. Um, but it coincides with the position of S Jy star. So the radio source is right at the middle. And because we know about stars and because we know how massive these stars are, particularly this one that's coming down now with the yellow trail, that's, uh, S two, which we've observed complete a whole orbit, um, takes about 16 years to do. So we're able to measure the mass of the thing at the center, so sort of basic physics problem. And at the center, whatever SJ star is, it weighs 4 million times the mass of the sun. And all of that mass is in a volume that's smaller than that of our solar system. And theory tells us that if you put that much stuff in that smaller space, then it collapses to form what's called a black hole. So an object whose gravity is so immense that not even light can escape. And in Avie or a fe of radio astronomy, uh, organization called the Event Horizon Telescope used telescopes from all over the world over many years to release a couple of years ago an image in the radio of that very central object. And here it is, it's sort of an orange donut, uh, at the center of our galaxy. So this is s Jy star more or less. And what you are seeing here around the edge is very lucky light that escaped from just outside the event horizon of this black hole. The event horizon is the point of no return. And you can see the structure, the blobs are due partly due to the gravity distorting the light, and partly due to structure in the disc. Um, and in the center, this dark patch in the middle is what you can think of as the shadow of the large black hole at the center of our galaxy. So more than night, where are we set? 39. So more than, um, 80 years after Reba and Jansky discovered that there was a source in Sagittarius that was emitting radios, we could see the hot gas falling into the black hole, which is responsible from those radio waves. And if that's not exciting enough, when this image was released, if you were in the States, dunking Donuts would give you a free specially made glazed donut. Um, unfortunately only on that particular day and only in the us which made me extremely upset. Uh, so I call upon the donut makers of Britain to celebrate the center of our galaxy, uh, and this iconic image. If Gresham achieves nothing else this year, I think if we can get free donuts for everyone who's in the audience and those of you online, I think we'll have done well. So I've told one story about radio astronomy. I've told you about going from crude instruments that were designed to track noise, to steerable dishes, to interferometers that can give us very sharp images of objects like the black hole at the center of our galaxy. But there's another parallel story that happens that's really important and it mirrors what's happening elsewhere in astronomy as well. And that story is about the discovery of things that change in the sky. Fundamentally, there are only two kinds of astronomy. We map things and we watch them move, right? That's essentially, we're still a visual. I'm slipping up still observational science, right? Those are the things we could do. So I've talked about mapping, trying to take pictures of objects. We can also watch things change in brightness. And sometimes that can be really, really important. When in the 1950s and early 1960s, astronomers, particularly in Australia, particularly in Cambridge in the uk, had mapped the radio sky. We'd got to the point where there were a few hundred heading towards a few thousand sources, uh, in the sky. So we got a sky full of what were called radio stars of, of objects that we could observe. But there was a school of thought that said, those must be nearby. And there was a school of thought that said, these things must be in the distant universe. And one of the ways to resolve that is to take a closer look at them and determine whether they really are point sources like stars. In which case you might assume that they're distant, or if they're closer, then you might be able to see structure. Maybe they'll be shaped like a nebula or like a disc or something like that, in which case they're close enough for you to see them as more than points they must be nearby. And we can use an old way of distinguishing these things, which is to see whether they twinkle. Essentially there's a legend stroke observer's guide that, you know, twinkle twinkle little star. If you talk to people, if you read old observer's manuals, they'll tell you that planets, when they're seen in the sky, if you go outside tonight and look at Jupiter in the sky, they'll tell you that planets don't twinkle. And the logic is that though you can't see it with the naked eye, when you look at Jupiter, you're not looking at a point source of light. You're looking at something that's close enough that we can see a disc. And when that moves round the sky, it matters much less than when a point moves. And so stars twinkle more than planets. Personally, I can't see it, I can't distinguish the two, but maybe if you go outside, you can have a look. And this definitely works in theory. So the experiment that the Cambridge astronomers wanted to do was to observe as many sources in the sky as they could with greater time resolution. So instead of just adding up all the radio waves that were received over hours, looking at them minute by minute and even second by second to see whether, um, they were twinkling to resolve this issue of distance. And the task, uh, was undertaken by group, led by Anthony Huish. Um, but the work was carried out. The observational work in particular, once the telescope was built, was carried out by Jocelyn b Burnell, who's pictured here with the telescope. Now it's a weird telescope, it's a set of anni. Uh, it covered a large field, the area of which covered something like 57 lawn tennis courts. Um, and Jocelyn described her beautiful instrument as looking like something you could string peas along, which I think is fair enough while observing, while carrying out these observations. Um, what she's doing is she's looking at the output on graph paper of one of those old fashioned chart recorders with a pen that moves up and down to record the strength of the signal. And she noticed a little bit of scruff in 1967 just here above my finger, uh, labeled CP on here where the pen just scribbles for a second. That's an observation of an object that's changing rapidly and it turns out regularly in the sky. Now, natural things aren't supposed to change rapidly or regularly. This is the behavior that we associate with intelligence here on earth. Natural things change slowly and don't beat out a steady pulse. That's a biological trait. And Jocelyn and some of the other astronomers nicknamed this source LG M1, which stands for Little green men. Now, it's not clear that they ever thought this really was intelligence, though Jocelyn has told me about a time when she was cycling back from Cambridge, from the observatory into Cambridge, late one night, being annoyed that aliens had got in the way of her PhD and she's a mark of the kind of person doing this painstaking work. I think, you know, being annoyed, you've discovered aliens. Um, and in any case, they were reasonably relieved when they found a second one, LGM two, because they felt that the odds of two separate alien civilizations doing exactly the same thing were low. And so they felt then that they, they were sure that they'd found something natural, but what these things were was an important question. Um, and even before that, whether these things were real, this is the kind of thing that you might expect from an electronic artifact noise in the system that had been built. And so the first thing they did was go to a second telescope, adjust its chart recorder, which took a few weeks so that it could record rapid results. And then they pointed it the same part of the sky, or rather waited for the same part of the sky to pass overhead and expected to see the same signal. And with Jocelyn and the rest of the astronomers there clustered round the chart as the appointed hour and minute appeared, absolutely nothing happened. Disappointment all round this discovery that they'd been excited about, excited enough, according to the records, to keep from their colleagues. They were very careful not to tell anyone that they thought they'd found something this interesting and unusual, all faded away. It was a mistake. It was a problem in the wires, except that just as they were packing up, 20 minutes later, the chart jumped into action and there was a scribble. They'd calculated the position in the sky wrong by 20 minutes. If they'd been two hours late, then it would've been another 10 years, I think, before anyone had found these objects. So what are these things? Well, luckily the theorists had ideas and had been around, uh, uh, relatively quickly, and quickly came up with a solution. When large stars die, when they explode as supernova at the ends of their lives, they can produce dense objects, sometimes black holes, but for stars not quite big enough for their cause to come up to big black holes. We produce neutron stars. These are city size objects. They may be 10 kilometers across, but a typical neutron star will have 1.4 times the mass of the sun crammed into something the size of a city. So these are the densest objects that exist in the universe. Um, there's all sorts of terrible analogies, so you should take from this that they're very dense, but traditionally at this point in a talk, and astronomer will say something like, if you took a teaspoon of neutron star material, it would weigh, I picked this one off the web earlier, 900 times the mass of the great pyramid of Giza. Does that help you intuit how much that is? Or are we just sticking with very dense? I actually did one myself. I quite like this. So one teaspoon of neutron star material would weigh the same as 10 times as many humans as there are on earth. So all you have to do is imagine all the people on earth, which is easy, and then multiply them by 10 and then wave them. And that's the same as a teaspoon of this thing that's 10 kilometers long. We haven't got a very good analogy. It's really dense, is what I'm saying. Um, but how are these things producing these pulses? Well, it turns out because they're the core of a star, and stars rotate when you've got the core, it rotates pretty fast. And there are magnetic fields. They inherit the magnetic field of the star, and that can produce beams of radiation that shoot out from the magnetic pole of the neutron star. And so if you happen to be in a place in the universe such that one of these beams sweeps over you, then you see a regular pulse. And this can happen every few seconds, or it can happen every few, or it can happen a few thousand times a second. For the younger pulsars, they slow down. And so that's what these things became called. They're pulsars, they're pul pulsing stars, but they're the dead remnants of old stars still, uh, shining beams across the universe. And this is an interesting property because this is in some sense a clock. It's a thing that's counting, you know, just like lighthouses keep time and can be used to navigate, we can use pulsars to navigate around the galaxy. Uh, this is the principle behind the map that we've sent to outer space on the pro on the plaque that's attached to the pioneer spacecraft. The pioneers were launched to Jupiter and Saturn, and then they shot out into interstellar space and we sent this plaque, um, on them, um, a friendly greeting, um, from the 1970s, uh, a picture of the probe, I guess. So you know what you found, uh, a map showing that we are the fourth of, sorry, the third planet from the sun. And then this spiky thing here, uh, is a map of the galaxy. So we are at the center and then these lines, 14 of them point to different pulsars. And the little dashes tell you in units that multiply up compared to the size of a hydro atom, which is what's at the top, um, which pulsar it is. So it gives you the frequency of a pulsar and the 15th line points to the galactic center. So this should be enough information for alien astronomers who recover the pioneer probes to find us, which is a cheerful thought. Um, don't worry. The, the odds of the pioneers being found by, I dunno, there may be super intelligent satellite capture capturing beings who hate littering. I don't know. But the, the odds of the pioneers being found in deep space are low, I think. But it's interesting to me that pulsars were used for mapping. And indeed as clocks. Many of the most interesting things that have been done with these sources that have, have made up a large part of radio astronomy are to do with this ability to keep precise time. There was a result in the ear nine in the early 1990s by a team at Jora Bank who had been monitoring a particular pulsar. What they're expecting is a regular blip, blip, blip of a signal like my cartoon pulsar here. This should go on over time. You get occasional glitches and over a very long period of time, pulsar slow down, but they're expecting a regular bleep. What they found was that just by a little bit, sometimes the bleeps would arrive early and sometimes they would arrive slightly late. And there was a regular pattern to this change in timing. What they said was that there must be a planet around this pulsar and the gravity of the planet would pull the pulsar back and forth as it orbited. And so when it was nearer to us, we'd get a slightly early beep. And whether it's further away, we get a slightly later beep. It's a brilliant discovery. This is 1992. This is before other planets had been found around other stars. You could make exciting artist impressions of the things like this one, which bear probably no resemblance to reality, but look kind of impressive. Um, they announced this discovery, the first planets found around another object other than the sun to great fanfare. And this is really exciting because not only are these the first planets, how did they survive? Are they survivors from before the star that created the pulsar went supernova or did they form? Is there a second generation of planets that forms after, uh, after the supernova outta the dead breed? You get a rebirth of planetary systems. It's a fascinating idea. And unfortunately their discovery was wrong. The period they attributed to this planet around the pulsar, the time taken for it to go around the pulsar was precisely one earth year, which should ring alarm bells. What they'd done was that they've forgotten that the earth does not move in a circle around the sun. Sometimes in January, we're closer to the sun. In June, we're further away. I know it doesn't feel like that, but it's true. Um, and they just forgotten. They just made a mistake in the code. So they'd, what they detected was the earth moving. And Andrew Lyon, who led the team, had to stand up. He'd already been given a keynote talk at the American Astronomical Society in front of thousands of astronomers. And he had to stand up and fill an hour on the planet that it was very clear, didn't exist to his immense credit. He stood up on stage and he just said, as you know, we made a mistake. And then he sat down. Uh, and you know, I've been in similar circumstances, I've watched people in similar circumstances, bluster, uh, you know, this is an interesting technique that could be used to find planets. Preliminary results require some, you know, you get the idea, you've heard those talks, uh, but you didn't do that. And what was remarkable was there was a Polish astronomer in the audience who walked up to the microphone and said, I have a question. Well, I have a statement, really. And he said, inspired by your work, we've been monitoring pulsars two. And I found a pulsar that has three planets, two that are earth sized, um, one of which goes round in 67 days, one of which goes round in 98 days, and then one very close that's very small. It's still, I think, the smallest planet we know of just 2% of the earth's mass that whips around the pulsar every 25 days. And these are real, there's now a handful of pulsars where we can detect the presence of tiny planets next to something that weighs 1.4 times the mass of the sun because they're being pulled back and forth. And the precision of the timing is such that we're able to pick out the signatures of these planets. We can play other games with pulsar timing as well. In the seventies, two astronomers, uh, called Holson Taylor discovered two neutron stars in orbit around each other, one of which is a pulsar. So we get these blips, and again, over time monitored with our SIBO in particular. So this is a 30 year data set. Um, this is just the time taken for the, the pulsar to complete an orbit around the center of gravity of the system. And it's slowing down, it's slowing down by quite dramatic. It's now, uh, as of 2005, it was nearly 40 seconds, slower than it was when they first observed it, when the, uh, period was just over seven hours. And what's happening here is that as these two massive objects that are very close, they're close enough, they go round in just over seven hours as they spiral each other, they're losing energy. And so they're getting closer and closer and closer together, and they will eventually merge. But what's remarkable is that they're losing energy via a mechanism that Einstein's theory of relative details about, which is via something called gravitational waves. These are ripples in space itself. So these are massive objects moving fast enough that they cause ripples in space that spread out through the cosmos. And we can see the effect and we can predict actually how fast these things should be slowing down. And Halston Taylor won the Nobel Prize for testing this fundamental idea of, uh, general relativity. And then just last year, astronomers went one better. They monitored a whole host of different pulsars using telescopes from around the world. And here's a distracting graphic from their press release. Uh, 'cause you'll have gathered by now radio astronomy still doesn't give us great pictures of lots of these things. I mean, we got a donut, so that was nice. Uh, and we had that nice image at the center of the galaxy. But you know, we do have to result to, to pulse, to, to press release images in monitoring many, many, many pulsars arts. What these teams were able to discover was that the space that the earth travels through, and I guess we do too, isn't still there is an underlying background of ripples and waves that royal the surface of space as we travel through it, we live in this sort of unstable medium. It's a very subtle effect. It's only detectable 'cause of the precision with which we can make measurements of these pulsars. But we can see the effect of this rippling on our timings. And though two is a detection of gravitational waves, this time caused by colliding black holes in galaxies, hundreds of millions of light years away. And over the next 20 years, we'll make more of these observations and we'll perhaps even be able to start to say where those colliding galaxies are at the minute. All we know is that the sea is choppy. But I think detecting that is remarkable. And it comes from the precision with which we can monitor these objects that joscelyn discovered back in the 1960s. So pulsars are exciting, but there are more dramatic transience as well should end the lecture with a bang, I think, or at least a series of banks were back to parks, which in 2007 had detected a new type of source, a burst of radio waves coming at a particular set of frequencies from all over the sky, seemingly from the distant universe. So not associated with the Milky Way. Most of the pulsars that we see are in the Milky Way. These are bursts coming from the distant universe. And as astronomers as we've established a good at naming things, uh, we've called these the fast radio bursts because they're fast bursts of radio waves. They're FBS for short. Now, not all FBS looked alike. There was a subset of these things, um, that occurred at one particular frequency and the team of parks who were suspicious of them called them perons. Now, in Greek mythology, a perton is a stag with a human shadow. I'm not an expert on Greek mythology. We have other lecturers for that. So if somebody wants to gimme the context, that'd be good. But the point is they suspected these would be manmade, but they couldn't quite work out why only parks could see them. Other telescopes lacked the instrumentation, the expertise, the knowhow, the hutzpah, the capability to see these exciting objects. And they were published in about 50 different papers over the years until 2015. In 2015, a retired engineer, somebody who Jansky and Reba would've gone on with, I'm sure saw some of these papers and said that he knew exactly what the source of these perons was. There was to be fair, a clue in the data. Um, this is the time of day at which those things were observed. And as you can see, these celestial sources from the distant universe favor lunchtime at the observatory. The engineering question have worked on microwave ovens, and it turns out that the microwave ovens in the visitor center at parks, which were more than 25 years old, were malfunctioning. In particular, your microwave oven, if you open the door before it goes ping, which I have been known to do, I'm an impatient person. It's supposed to stop producing microwaves for all sorts of good reasons. These didn't. So every time somebody was impatient for their lunch, they created a signal which the telescope could detect. Uh, this had been happening for years. They now have some new microwaves. Perons don't exist, but fast radio bursts do. And we don't know yet what they are. They're mysterious sources that sometimes repeat, but don't always that mostly come from distant galaxies. But once didn't that appear at different frequencies. Um, and we're just at the beginning of working out what these objects are. The radio sky is much stranger than we thought it was. Teams in particular in Australia, led by people like Natasha Hurley Walker, who's an amazing scientist, have shown that there are things like pulsars, but which take hours to pulse rather than seconds. We don't know what they are, either one of them. The first one they found turned off in the two years between it, the data being taken and them going back to look on it, another one has been going for 30 years. There is a diversity of behavior that we don't understand, and we will have much more of this because of an effort called the Square kilometer Array, the SKA, which is going to plant dishes across Southern Africa and across Western Australia, sensitive to all different frequencies to look through the radio sky and to find out what might be out there. The SKA by some estimates will be capable of detecting airport radar on any of the nearest a hundred star systems, something like that. So that gives you some sense of, uh, the, um, capabilities that we're about to have the first dishes for the SKA on site. And they produced that galactic center image that I showed you earlier, and it's going to be a marvelous time. But I did leave you with proxima and BLC one. And I want to finish by saying that this candidate breakthrough, listen, candidate one, this possible signal from our neighboring planet turned out like most of these things to be not aliens. Uh, a careful look at the data showed that it repeated the signal, not just when the telescope was pointing at proximal, but also where it was pointing away from the star. And so it appears to be a kind of interference, perhaps from a satellite, perhaps from some malfunctioning radio equipment. It's a reminder, even as we build novel telescopes and novel technologies that though they can observe in the daytime, and though they can look through cloud radio, astronomers have life hard. They live in a noisy universe where a mobile phone on the moon would create a signal, millions of times brighter than anything else that they're trying to observe. And they live amongst us. And the observatories we build, though they're in relatively isolated places, have to deal with interference. And yet, through ingenuity and perseverance, the kind shown by Jansky and Reba, by Jocelyn Berne, uh, and astronomers ever since, we're able to listen carefully to the universe and just occasionally we hear a meteor when we're listening from one. Thank you very much. Thank you very much, Chris. That was wonderful. And we're all, I'm full up with questions, <laugh>, um, which is great. Um, trying to choose one. Let's start with an early one. So this is about the SKA. Why not share high and low frequency arrays in both South Africa and Australia for an intercontinental baseline? Yep. No, this is a technical question. So you'll see there's two types of telescope here. Um, so on the left are the things we're gonna put in Southern Africa and on the right is what's happening in Australia. Um, and they're designed for different frequencies. Um, and there's a split. So though it will work as one telescope, we'll observe different frequencies from different places. And it's just to simplify the engineering, really the original plan was to put everything on one site that would make life easier. But both sites are great, both countries, both South Africa and Australia were hugely, uh, supportive. And so just for simplicity, it's on two different continents and two different types of thing. Um, whether you'd want to mix it in the long run, sure. But for now, this is probably a 40 year effort to build this telescope. So later we'll mix it up. But for now, we just want our, our dishes out there. The nice thing about a radio telescope is that if you make it out of many dishes, you can start observing as soon as you put one down. And so we have telescopes on the ground now in both countries that are 1% of what we will end up with, but they're already producing amazing results. So we're, we're on the way. That's brilliant. And how might AI help with radio astronomy? Yeah, so the, the the, there's a bit of this story that's hidden that I didn't talk about in that the great advances aren't always driven by telescopes, they're driven by computing. And in particular, the ability to combine data from different observatories, um, makes an enormous difference. And that's a, a tale of can you afford a good supercomputer? So it's really the rising computing that has allowed us to do things like make this astounding image of the Milky Way at the center of the Milky Way. It's a, it's a computing problem. Um, machine learning helps by filtering signals. Um, perhaps the best way to see that is in Seti where when we're looking for aliens, what we've traditionally done is look for signals like BLC one, so narrow signals at a particular frequency on the grounds that those are likely to be either artificial or something interesting astronomically. But with more advanced artificial intelligence, we look for a greater range of signals. We can look for aliens chirping at us or signals that jump around and we can ask more generally what the, to, to find interesting things. So I think that will come, um, it's a long stretch to get to the data volumes the SK will produce. I think it's as much data every hour as is on the web every, uh, second. And so it's an awful lot of data, uh, to process before you start doing advanced, uh, machine learning and so on. Great. What's the fate of planets around pulsars? Are they gonna be, what's the fate of them? What's the fate of them and they, are they gonna be destroyed? Well, we think they stay there, the pulsar, because the pulsar has jets that come out the poles and the planets tend to be in the plane. Um, the jets don't sweep across the planets, so, so the planets will be fine. Um, the pulsar gradually spins down, becomes inert, and so you've got some planets around a faint cold dead star. It's probably not a nice place to go on holiday. Uh, but the planets will be fine in the long term and they'll remain there for us to observe. Brilliant. So what is the practical application, do you think? Half and for astronomers with Astronomy? Yeah, join Themselves. I mean, there is a traditional answer to this, which is that the technology development, particularly for radio astronomy, has all sorts of consumer, uh, electronic background. We usually claim the invention of wifi, uh, was via radio astronomist, which has now become their mortal enemy. So it's kind of, uh, a, a classical tragedy. Um, I think any sort of technological development helps and the kind of big engineering projects that power these, these radio telescopes, I think produce a skilled workforce in all of these places. That's why in South Africa in particular, there's a lot of excitement about the project, but it's also worth doing anyway, isn't it? It's kind of fun to know about the universe and so I'll always give that answer as well. Absolutely. We've got some, um, some questions sort of going back. So it was very interesting to see the Parks telescope at the beginning, and I remember the dish, the movie Yes. Um, and so on. And it's, it was interesting that Australia was one of the places that the first observations were coming from much more than other places. Is that because the atmosphere is very clear there or, Um, no, being in the south helps. So, so, um, there are other reasons to put telescopes on in, in Australia though, for optical astronomy, Australia has the deficit of not having any mountains. We'd like to be higher up please, for radio, this doesn't matter. And so it's always been a specialism. But being in the south House, there's this sad truth for those of us in the uk, in Europe, in the northern hemisphere, that a lot of the action is in the southern bit, the sky, particularly a good view of the galactic center, you need to be much further south than here. And so Australia has this prime view of Sagittarius, which climbs high in the sky, whereas from here we see it in the summer months, but only just on the horizon. So, uh, so the other bright sources, there's a galaxy called Centura a, that was one of the first galaxies to be studied. It's one of the brightest radio sources. You can't even see that from here. So go south as ever. The southern sky wind in astronomy. And back to history again, Reba, who you all love now, um, how was he actually capturing and recording the data back in 1944? That's a, a, a really good question. He's taking, he does have a chart recorder, but really he's doing long observation. So he's integrating on particular bits of the sky and he is literally reading off the signal strength. So he's making handwritten notes. So it's very old fashioned astronomy with some very high tech. And, and how do, do the radio signals then get conversed into optical representations? Yeah, so, so yeah, again, I can put up, you know, whenever I show you a picture, this is a good example. This was on the front page of almost every newspaper in the world as a photograph of the black hole at the center of the galaxy. And I hope I didn't say that, that 'cause the one thing it isn't is a photograph. So this is actually a computer model. So this is, uh, the result of a program that tried many, many, many millions of different shapes and then predicted what radio signals would be seen from the different telescopes. And then you go back and forth until they match. And so this is an image, but this isn't a picture of the radio waves that were detected by the telescope. Things like, so, so this is, it's sort of a construct. This is closer to being an image in the sense that if you had eyes that were sensitive at these wavelengths and you stared at this area, you'd see something like this. But again, it's a construction via a computer model of what the telescopes are seeing because we're doing this trick where we have many telescopes working together. So it, it's a much more complicated business than optical astronomy. Uh, people who are good at it, like Ian, who made this image, uh, essentially wizards. And, uh, we try not to ask them too many questions. Okay. Couldn't stop without that. Um, just, um, if we, um, uh, uh, alien, I sawers, if they happen to be pointing to us, what would they, what the thing that would be show up that we're throwing out? Yeah. Or would they have to be looking directly at us to see stuff? Yeah. No, this is a good question. So could aliens see us? I, I think it's interesting when you listen to the debates in the sixties and seventies about how to look for aliens. We are living at a time then where we are becoming radio loud. People are broadcasting TV out to the cosmos. And so, you know, there's 40 years worth of broadcast TV heading out, which would be pretty loud I think. Um, these days we do less of that. We narrow band to beam to satellites. We use fiber optics. And so I think it's less obvious than it used to be that we're going to be radio loud forever. So if they're watching right now, they would've picked up our tv. Um, I think airport radar is interesting 'cause even in advanced civilization, if you want to know whether invaders from the planet Z are coming, radar's a good idea. And radar involves broadcasting invaders from the planet Z aren't coming as far as I know, <laugh>, um, involves broadcasting out into space quite powerful. So I think radar's a good shout. So that's why we're thinking about airport radar. Um, there is also the story that I've told often, which is that the most powerful transmission ever sent into space was an advert for Doritos. Uh, and it's genuinely a true story and, uh, I'm not sure what aliens are gonna make of that if they find it <laugh>. Hello? I have two, but the second one's really short. Okay. Um, okay. Um, the first one was about Reba, that his data, um, sort of came about in 1939 and then was sat on and published in 1944. Yeah. Which is right in the, the chronologically where I imagine like just a lot of noise would be happening in the world. Did the second World War influence what he picked up on and did he map any of that? Yeah, not directly. He's in the middle of, uh, Illinois and so there is less of that of course, what the Second World War did do, and it's a part of the story I didn't tell, is that a lot of the material that's developed for radar, particularly in, in the uk but also with American help, becomes the equipment that radio astronomers use in the late forties and early fifties. So Giro Bank was founded, uh, to use radar equipment to look for meteors. Uh, and so there's a huge amount of technological development going on at the same time. And so that's the point really where Reba uh, loses touch with academic astronomy.'cause suddenly all these people who've rubbished his ideas go, oh, well this is great. Yes, now we have funding and equipment. We'll, we'll do all of that. Thank you. Uh, and so they move on now, short second question. Um, yes, it was about the donut. Um, I wondered whether it was sort of more washer shaped as in quite flat or genuinely donut Taurus Shaped. Uh, really good question. Yeah. So putting a thickness on this is, is hard partly 'cause what happens is if you've got the short question, long answer, uh, if you've got the black hole here, let's say there's a genuinely flat Taurus here, the trouble is that the stuff from the back, it's light gets bent and so it gets distorted up. And so getting the geometry requires understanding the bending of light around this black hole. So these arguments are still going on. Um, I don't actually know that we have a definitive conclusion yet, but the thickness of the disc would be really interesting. It effects things like how fast the black hole can feed on this material. Um, so maybe we'll return to that at, uh, at some point in a future lecture. But yeah. Great. Really great question. And I think I've talked long enough that you've forgotten the answer is, I don't know,<laugh>. Lovely. Hello. I wanted to ask you if, you know, uh, if there has been, uh, any work regarding the, the codification of the signal? So we don't really, as you know, so for BLC one that candidate, we didn't really get that far. We got as far as it appeared to be a signal coming from, um, the planet, but it didn't have a pattern to it. Um, so had it been real, the next task would've been to observe it with every radio astronom, every observatory that we got, and try and see some structure. So in science fiction, of course we are expecting to have sequences of prime numbers or structures for building a spaceship or so, but we've never really seen a signal with that structure. Um, the closest we've got is Jocelyn's pulsars. Were a regular pulse, so you can imagine that encoding a number or something like that. But we don't yet have anything complex enough that people have tried to decode, uh, anything. It's an obvious next step if we find a signal, but it will be a task for the world. I think one of the things I said in passing was that astronomers can't keep secrets. That's true. But we also literally can't keep se secrets. If I observed tonight, uh, what appeared to be a signal coming from an intelligent, uh, source out amongst the stars, the first thing I have to do is tell the US South America and Australia so that they can keep monitoring it because you wouldn't want it to drop out. And so we end up in a global network of people listening pretty quickly and then the signal becomes public. So when that happens, we can all decode it together. Thank you for the question,<laugh>. Lovely. I'm afraid that brings us to the end of our time. Let me just point out that Chris has got a book coming out very soon. It's going to be on sale at the next lecture, which has been moved to the 29th of April. It will be here in Conway Hall again. I do hope you'll all be able to join us. And meanwhile, I'd like you to join me in thanking Kristen Top very much. Thank you.