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Afternoon, everybody, and welcome to this Friday's physics colloquium or physics kilowatt hour special, of course, by definition.

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But this one is particularly special because it is being given by Jocelyn Bell.

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Burnell Well, it's a great pleasure to introduce to you this afternoon.

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50 years ago, of course, Jocelyn was the first person ever to see the signal from a pulsar, and she will tell us more about that this afternoon.

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But before she does so, I'd just like to tell you a little bit about her subsequent career.

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So she she was an undergraduate in Glasgow. She graduated with a doctorate from Cambridge in 1969.

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About which one minute I'm sure she worked and she's worked as a number of universities.

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She worked at Southampton University College, London, the Royal Observatory, Edinburgh.

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She was a professor of physics in the Open University for a long time.

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She also is was as many years visiting professor at Princeton University, which, of course, is an honour not accorded to all that many.

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And she also served a period as dean of science at the University of Bath.

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She's been president of the Royal Astronomical Society.

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She's been president of the Institute of Physics when she retired as president of the Institute of Physics.

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Her successor, unfortunately, died a few months later.

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And it's absolutely typical of Jocelyn that she immediately stepped into the breach and took the job over again, having just got out the other side.

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And she did that until about Choo Choo successor was elected in due course.

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Jocelyn's commitment to the physics community has always been absolute, and her service to the physics community has been fantastic.

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So we were very lucky in the end to persuade her to come to to Oxford,

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where she's now been a visiting professor and a fellow of Mansfield College for many years.

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Subsequent to that, she became president of the Royal Society of Edinburgh,

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which she currently holds, and she was elected pro-chancellor of the University of Dublin in 2013.

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So this has been a career full of events, full of great service to the community,

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and it's a great pleasure to welcome you today to talk to us about what started 50 years ago.

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Jocelyn Belbin. Thank you very much, John, for that generous introduction.

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And thank you all for turning out on a Friday afternoon.

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What I thought I would talk about is a bit about the discovery of pulsars.

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This is pitched primarily as other grad students. A synopsis of the properties of pulsars and then some of the heavier physics that

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pulsars are giving us information or lack of information on to do with gravity.

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Our understanding of gravity to do with very condensed matter and to do with the prince.

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The equivalence principle. Einstein or not.

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But I'm going to start. In 1965 when I turned up in Cambridge, roar from the northern fringe of the country and got presented with a set of tubes,

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as did every incoming grad student to radio astronomy, except a few peculiar people called theoreticians.

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And they're not microelectronics tubes. This is heavy duty stuff, and they've been very heavily used and it was very nice to get given the set.

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I partly chose,

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partly was assigned to a project working with Tony Hewish to use a new technique called interplanetary scintillation to find more quasars.

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And the first thing we had to do was build an enormous radio telescope.

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Tony had got a grant of about £12,000 from.

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What on earth was it called than a saucy movie?

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Something like that. That sum of money then would have bought starter houses for three young married couples.

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Just to give you an idea of the scale. It wasn't a big grant as grants go, but we don't advertise that too loudly.

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With its half a dozen of us worked for a couple of years building a telescope.

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And this slide I tried to convince grad students is the typical working conditions for a student.

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It was certainly fairly typical of my working conditions.

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Here you see the ends of some low loss air space cable.

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The other end is down by the other hut. It's so precious, you can't coil it up and take it indoors.

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And if, like me, you're putting the connectors on either end.

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You work through the winter in these little hubs.

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And now we're testing the importance of these cables.

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We have a slotted waveguide here and some electronics.

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And what you can't see is about 400 yards of cable back to the nearest mains plug.

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Working in the field literally is quite difficult.

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For example, the wind cools the smouldering ion and it won't sodapoppin it.

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It's it's annoying. This is Don Rolfe,

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who was assigned to me as technical assistant rather reluctantly on his point

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part as he'd never worked with a woman before and wasn't sure he wanted to.

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And quite a lot of my effort went into trying to convince him that what we needed to do next was his idea, not my idea.

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And sometimes it worked, but it didn't always.

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This is the finished product. It looks homemade because it is homemade.

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Wooden posts are the most striking things.

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They're merely to keep the antennae and open cave, open wiring out of the wet grass, which would be an electrical shock.

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The next most noticeable thing are perhaps these slanting bars.

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They carry a reflecting screen strands of wire that run along here, and they're tilted because the sun is up there.

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And we're using a solar based technique, which I'll explain in a minute.

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These nice bluey stuff is copper. Very valuable, has subsequently been pinched, but you're still there at this time.

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And there's a whole row of antennae there, half wave antennae, 81.5 megahertz.

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So that's 3.7 metres wavelength, distinctly low frequency.

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And there's 248 of these antennae. There's over a thousand wooden posts.

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There's 120 miles of wire and cable. But more importantly, let me tell you how it works.

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Young slits, too. Slits. You get an interference pattern, dark and light fringes.

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If the slips are coast close together, the fringes of current spacing is small.

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If the slit is large, if the slits are far apart.

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The fringe spacing is fine. This is operating a bit like young slips because although it's physically in one unit electrically, it's in two halves.

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It's an interferometer. Back to the slips.

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If the slips are fine, then there's good spread of the fringes.

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If the slips are broad, then this fringe pattern is governed by the diffraction pattern of the wide slit.

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Here we have two things right adjacent to each other.

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So the fringe pattern is one faint fringe, one bright fringe, one faint fringe.

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And that's it. Which is quite good if you've got lots of things happening.

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As an undergraduate in Glasgow, at least for a project I had used transistors.

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But when I came to Cambridge, it had transistors. Noisy, unreliable.

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We use valves and they did for quite a bit longer.

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I think they could have used transistors, but they were sticking with what they knew.

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So this telescope took us two years to build, and when it was built, the rest of the team melted away.

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And as a grad student, I was left to run it or actually to get it operating and run it.

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But thanks to Don, the technicians tenacity, it worked the first time it was switched on, which is very unusual for a radio telescope.

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No. What was all this about? Well, we were using a technique called interplanetary scintillation.

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Let's work off the bigger diagram. The sun is somewhere over here.

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Earth is done here. Here's a radio telescope and there's a quasar somewhere away.

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Way up there. And coming off the sun is a solar wind travelling a few hundred kilometres per second.

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And it's not uniform. It contains density irregularities, which I have drawn as clouds.

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Small fluctuations in electron density.

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It's a plasma, so there are free electrons. And the size of these clouds, blobs expands as they get further away from the sun.

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Now, if you have a radio galaxy somewhere up here, it's broad on this scale and you're viewing it through this kind of cloudy medium.

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Doesn't have a big effect.

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But if you have a very, very compact object, then as this cloudy medium blows past, you see the thing dimly bright, dimly bright.

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It fluctuates. And here's a typical signal that you get from this sort of situation.

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There's a time scale of one second. Up till then, radio astronomers had rarely used timescales this short.

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But Tony had got a hint of this fluctuation, and we decided we would run with a much shorter time constant.

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And that's key to the whole story. At that stage, we were only about 20 quasars.

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And one of the reasons Tony got this grant was it's a brilliant way of picking up quasars.

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He also pitched that we could measure the angular diameter.

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That really didn't work, to be honest. And these days, there are better ways of measuring angular diameters.

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But then it was the only one to tackle angular diameters, you know, a few seconds of arc.

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So it's quite easy.

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You just observe the sky and anything that twinkles simulates this equation and anything that doesn't is a radio galaxy.

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Except that you've got four acres of collecting area and the faintest whistle of

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radio interference anywhere within a hundred miles will swamp your cosmic source.

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Equally, if the sun misbehaves and produces radio waves, you get a lot of interference.

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So you get used to identifying interference along with identifying quasars.

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But in six months observing with this telescope, I find about 100 further quasars.

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So it certainly worked as a project and I got my thesis on that.

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At that time, Cambridge University had one computer is occupied, a room not quite as big as this, but a big room.

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And it had less memory than your laptop. And very few people had time on it.

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Martin Ryle, who was the head of the radio astronomy group, did because they were doing aperture synthesis,

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they were doing Fourier inversions, and the computer was a big help.

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You're doing four weird inversions. Every other academic had grad students.

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Maybe still true. So our output came out on chart paper and most of the time chart paper ran relatively slowly.

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So in a day we only got three metres, 30 metres.

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It took four days to scan the sky totally.

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So one scan sky was about 120 metres.

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But having operated the telescope for six months, I had over three miles or five kilometres of this stuff.

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And I can assure you, I have looked at every fraction of an inch of that, sometimes twice over.

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So this is a sample of it. Just occasionally in this vicious fix of stuff, there was half a centimetre of signal that I couldn't properly classify.

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It didn't look exactly like a quasar, and it didn't look exactly like interference.

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And the first few times I loved it with a question mark and passed on.

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Now, I guess most people in this room are physicists. So I wonder if you share this ceiling with me.

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Most acute when I was an undergraduate. You're revising for an exam.

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There's something you still don't understand and you've got to learn it.

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Parrot fashion and it really bugs you. Things you understand, you know, you can reconstruct if you go to question on it, that's fine.

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But the things that you didn't understand, or at least in my case, lodged at the back of my brain.

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For attention later. And clearly this thing lodged at the back of my mind because it was something I didn't understand.

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And after seeing it a few times, my brain said, You've seen something like this before, haven't you?

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You've seen something like this before from this bit of the sky, haven't you?

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And then it's easy. Because this radio telescope relies on the Earth's rotation to do the coverage in this direction and.

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Right ascension. But we can control the beam in elevation in declination by phasing between the different rows.

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And I have the charts filed in shoeboxes by Declination Strip.

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So you get out the shoe box for that strip of the sky, you get out the records.

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This would be a great place because you need lots of long floor and you spread them out and you line them up.

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Well, yep, I saw that one. That's one I logged. It's not.

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They're not. They're not. They're. Might be there.

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I didn't notice that. Yes.

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And I logged that. And here's this one. And they're all lined up in.

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Right. Ascension. They're coming from the same bit of the sky, the same spot amongst the stars.

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Now before I pass on, this is an example of this very, very weak signal.

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The the truly interference fringes have been smoothed out.

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It's been high, passed, filtered, and we're left with the scintillation.

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Only this isn't scintillation because the spikes only go up and scintillation goes up and down.

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This is some very low level interference and it looks different.

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People at the front may be able to see that.

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You can see the chart paper between the spikes and the spikes go up and down, whereas here's a block where you can't see anything between the spikes.

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You're doing thorough analysis when you say that kind of thing.

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You're looking at the frequencies present in the signals and you're looking at the amplitudes and signs of the coefficients for each frequency.

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That was actually the first time we saw it, but I didn't recognise it.

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It was. A month or two later before it began to click.

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At this time of year. This bit of sky is in the night sky.

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It's about as far away from the sun as you can get. And yet I'm doing a solar based technique.

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Usually you twinkling caused by the solar wind. There ain't much solar wind in the night sky.

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So what's making this intimate? It's clearly a very peculiar thing.

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I discussed it with Tony and we decided that what we needed was to get an amplification.

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Basically to spread this out a bit. And the way you do that with charts is easy.

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You just run the chart paper faster. The high speed works, except the pen recorder gets through the rule of paper in 20 minutes.

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And if you just switch to high speed, the grad student lives at the telescope, putting a fresh roll of paper in every 20 minutes, day and night.

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So that's not such a good idea. Instead, what clearly we had to do was the grad student to go out to the observatory about here,

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switch to high speed, let it run to about here and switch it back to normal.

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It's been over the month of November and during the whole of the first 28 days of 27 days of November, I go out to the observatory.

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The dedication switched to the high speed recorder and make high speed recordings of receiver noise.

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The thing has disappeared. Tony's livid.

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It's a flare star. And you've been gone and missed it. Most of you will know some of the younger people may not know.

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Grad students are like the cat they're for kicking. So I got kicked out.

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One day I skipped. I thought after 27 or 28 days of this stuff, this thing is now transiting at lunch time.

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And there's a very interesting lecture on ageing in.

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In the university, and I decided to go to it. And next morning I come out and the thing is reappeared.

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And that's the one day I've skipped in the whole of several months.

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So I don't leave the observatory. I stay there till the transit and.

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This is what happens. These are artificial time pips broadcast every 1 seconds and this ignore the top one into the percent of a pulse.

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No pulse pulse pulse, no pulse post, post, post, pulse, pulse post.

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Several missing. And then back in face on beat.

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And you can see, even as the pen charts rolling, that those pulses are equally spaced.

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But as soon as that transit was over, I got it out. Spread it on the floor, moved along with bits of paper with ticks on it,

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and established that it was constant and it had a period of one in the 30 seconds.

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And at that point, I dared to phone my supervisor, Tony.

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He was in Cambridge, in the university, in an undergraduate physics laboratory,

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probably dealing with some twits of a physics undergraduate who thought that his diffraction grating had two lines per inch or something like that.

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And then his Twitter for grad students opens up and says, Hey, Tony, you know that funny, scruffy signal?

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It's a string of pulses, one in the 30 seconds apart. Oh, well, that settles it.

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It's man made. I'm not good at marshalling arguments quickly.

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I knew it wasn't man made. I knew it. Kept sidereal time, not human time.

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But I couldn't marshal the argument quickly enough. Anyway,

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Tony was interested enough to come out to the observatory the next day and stand looking over my shoulder as we switched to the high speed recorder.

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And that's really scary. This thing's been absent for a month.

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It's appeared the day I wasn't there. It's appeared the day I got it.

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Is it going to appear on the third day? And it did.

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And he saw it with his own eyes. And he established that the spirit hadn't changed since yesterday.

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And then we started wondering what on earth it was.

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Tony finally became convinced that it kept sidereal time, so the idea that it was manmade interference really doesn't work.

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And we had to come up with some other explanation.

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So it's not local radio interference.

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We got a colleague, Paul Scott and his graduate students who also had a radio telescope on site with its own receiver,

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which also worked at 81.5 megahertz for the mega cycle for a second at that time, but in their mind, same frequency.

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And they turned their telescope on and we sat and waited.

201
00:22:00,850 --> 00:22:07,660
And nothing happened. It had already appeared in my telescope beam and we knew it was there and posting.

202
00:22:09,490 --> 00:22:16,960
What had happened was Robin had miscalculated by 5 minutes when it would appear in his telescope beam.

203
00:22:18,360 --> 00:22:23,310
And it did, but not after until after an agonising 5 minutes.

204
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If he'd made a mistake at 25 minutes, we'd have all packed up and gone home.

205
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The story might be very different.

206
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It became clear because I kept observing this thing that the posters were short, which implied the object was small,

207
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but the posters were maintaining period, which implied that it was big because it had large reserves of energy.

208
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It wasn't getting tired. So it's small and it's big.

209
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Yeah. A good example of what you have to do in physics.

210
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Be really precise about the question you're asking.

211
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It's small in the sense of angular diameter.

212
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It's big in the sense of mass. What you might think meant big angular diameter.

213
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But let's just be strict. It's big in terms of mass, and we now know that neutron stars fit that bill perfectly.

214
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But at the time it was a little puzzling.

215
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John Pilkington managed to get a dispersion measurement, not for the benefit of any students present.

216
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You would have seen dispersion with light. Every time you see a rainbow or shine, light through a prison.

217
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There's also dispersion of radio frequencies, because when a radio wave passes through a space with some free electrons,

218
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the high frequencies travel faster than the low frequencies.

219
00:23:53,670 --> 00:23:58,560
Radio hams have known about this for a long time. Unidentified Vertical Whistlers.

220
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Coming from a lightning strike on the far side of the earth, radio wave travels round, high frequencies arrive before the low frequencies.

221
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So we thought, we wonder, is there any dispersion on these pulses?

222
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And by setting up two receivers at slightly different frequencies don't.

223
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Pilkington showed that the high frequencies arrive first. And by making a guess that the number of free electrons in space.

224
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We came up with a distance of 200 light years, which puts it beyond the solar system, but well within our own galaxy.

225
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I go back a side or two? This, I regret to say, was my original terminology.

226
00:24:44,080 --> 00:24:57,940
It was a joke which has never gone away, and Tony was still hung up on the idea that this was man made artificial.

227
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So he thought we ought to test and see if it his little green men.

228
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If it's little green men, they probably live on a planet and their planet goes round their sun and there will be Doppler shifts.

229
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Therefore, on the pulse period, closer spacing as it approaches, larger spacing as it retreats.

230
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So the grad student continues going to the observatory and switching to the high speed recorder and so on.

231
00:25:27,830 --> 00:25:33,200
And we studied this post period and we found a doctor shift.

232
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But it's the Doppler shift due to the motion of the earth around the sun because moving observer also gives the Doppler shift.

233
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We saw no sign of any further Doppler shift.

234
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We know Christmas time almost.

235
00:25:52,270 --> 00:25:55,540
Actually, I know the dates. It's the 21st of December.

236
00:25:56,830 --> 00:26:01,960
And I go down to Tony's office to talk about something with them.

237
00:26:02,830 --> 00:26:06,520
And the door's normally open, but this occasion, the door shut.

238
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So I knock and Tony has come in and put my head around the door.

239
00:26:12,080 --> 00:26:14,300
Jocelyn, come in and shut the door.

240
00:26:15,080 --> 00:26:22,100
So I went in and shut the door and there was a discussion going on that I think I should have been part of right from the beginning.

241
00:26:22,130 --> 00:26:25,070
There was Tony, there was Martin Ryle, the head of the group.

242
00:26:25,430 --> 00:26:32,240
It was John Shakeshaft, who is one of the editors of monthly notices, and they were having a discussion about how the [INAUDIBLE] we published this.

243
00:26:33,190 --> 00:26:39,100
We've only got one. We have no real intelligence about what it is.

244
00:26:39,520 --> 00:26:43,030
There are some ideas floating around, but there's only one.

245
00:26:44,780 --> 00:26:48,110
And we didn't resolve the issue. And I went home for supper.

246
00:26:48,560 --> 00:26:55,010
Really very cross. Why do some lot of literary men have to pick my telescope on my frequency to signal

247
00:26:55,010 --> 00:26:59,510
to us just when I'm trying to get a Ph.D. and I've only got six months of money left.

248
00:27:04,580 --> 00:27:10,520
The main program was still going on, and there were still stacks and stacks of paper charts coming in.

249
00:27:11,030 --> 00:27:14,390
And my logbook says No.

250
00:27:14,420 --> 00:27:19,310
A thousand foot behind with a chart analysis. No. 2000 foot behind with the chart analysis.

251
00:27:20,870 --> 00:27:29,329
And I came back in after supper to do some more chart analysis and I was looking at a patch of the sky which is around.

252
00:27:29,330 --> 00:27:37,190
Right, Ascension 11 hours 33. There's a very, very strong radio source called Cassiopeia A.

253
00:27:37,210 --> 00:27:48,310
That's 12 hours difference from that. Cassiopeia, A in Britain does not set north, is there?

254
00:27:49,000 --> 00:27:53,829
So Cassiopeia A is high in the south. It goes down in the west.

255
00:27:53,830 --> 00:27:57,130
It grazes the horizon and it comes up again.

256
00:27:57,790 --> 00:28:01,630
And it's so flipping strong, you can see it through the back of the radio telescope.

257
00:28:02,730 --> 00:28:09,570
But you're also seeing it through a great thickness of ionosphere and they on a sphere can also cause scintillation.

258
00:28:09,570 --> 00:28:12,030
So there's immense sort of turbulence.

259
00:28:12,900 --> 00:28:23,760
And because of CAS, a lower culmination, there was six or eight inches of chart recording that I just couldn't use and I usually just passed over it.

260
00:28:24,300 --> 00:28:28,800
But this night the turbulence was slightly less and I suddenly noticed.

261
00:28:31,720 --> 00:28:37,470
It's 5 to 10. The Cavendish is locked at ten and you can be locked in or locked out.

262
00:28:41,940 --> 00:28:46,350
I'm going to Ireland tomorrow with my boyfriend for Christmas to announce our engagement.

263
00:28:46,560 --> 00:28:52,920
I need to be there. And this thing's transiting at about 3:00 in the morning.

264
00:28:56,150 --> 00:28:59,960
So I have all the charts of that strip of sky out on the floor lined up and.

265
00:29:00,440 --> 00:29:03,890
Yes, no, no, no. We have maybe on this one.

266
00:29:04,280 --> 00:29:07,160
Right. I need to go out to the observatory.

267
00:29:07,520 --> 00:29:16,970
So bundle, charge out as the janitor locks the doors behind you and go to the observatory, 2:00, 3:00 in the morning and December and very cold.

268
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And in those circumstances, something I think something in the receivers didn't always work and it was only at kind of half or a third power.

269
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When I got out there, it was at half or a third power.

270
00:29:33,370 --> 00:29:43,750
So I flicked switches and I breathed on it and I swore at it and I got it to work for 5 minutes at full strength,

271
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and it was the right 5 minutes on the right setting.

272
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And then came pulse, pulse, pulse, pulse, pulse, one and a quarter seconds apart.

273
00:29:57,710 --> 00:30:02,420
No. Your guess by the amount of time I spent describing that, that for me is the sweet moment.

274
00:30:03,290 --> 00:30:14,290
It's not little green men. There are two lots of little green men on opposite sides of the universe singing to earth at 81.5 megahertz y.

275
00:30:15,770 --> 00:30:21,780
Y. Signalling to Earth. And why, apparently using an amplitude modulated technique.

276
00:30:22,880 --> 00:30:29,990
Now. This has to be something stellar. So I left a message for Tony, went off to Ireland.

277
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Tony kindly kept the survey running while I was away,

278
00:30:34,280 --> 00:30:40,010
which meant putting in the inquest and paper in the chart recorder and piled the charts on my desk.

279
00:30:40,570 --> 00:30:55,880
Amanda and I came back sporting an engagement ring, which I was incredibly proud of, but I wore it to the lab, which was a very, very bad mistake.

280
00:30:57,250 --> 00:31:00,400
Because back in the 1960s, married women didn't work.

281
00:31:01,490 --> 00:31:07,190
It was shameful if a married woman had to work. It meant that he couldn't earn enough money to keep them both.

282
00:31:08,460 --> 00:31:15,240
And mother certainly didn't work because it was absolutely proven that if mother worked, the kids were delinquent.

283
00:31:19,540 --> 00:31:25,900
Beware of proof. Anyway, I reappeared.

284
00:31:26,620 --> 00:31:30,970
Big pile of charts. Okay. What I had to do. Busy analysing some more.

285
00:31:33,810 --> 00:31:37,960
Pretty routine. Oh, yeah. Which ones? That. Any.

286
00:31:38,850 --> 00:31:42,030
It's not either. Wow.

287
00:31:42,660 --> 00:31:46,670
Okay, I'll just finish this chart and then we'll get out the other ones for this.

288
00:31:48,690 --> 00:31:53,819
What? Oh 833 and oh 954.

289
00:31:53,820 --> 00:32:00,780
The posts are people. This one actually was quite strong and I had missed it because I thought it was a quasar.

290
00:32:01,960 --> 00:32:05,590
It was so strong, the pen was hitting the end stops top and bottom.

291
00:32:08,400 --> 00:32:12,720
But at this point, Tony appears and I say, look, happy New Year, Tony, look at this.

292
00:32:15,420 --> 00:32:26,320
How many more of you missed? Go back through all your old records.

293
00:32:27,610 --> 00:32:31,930
So I did, but we didn't find any more. And those two were ultimately confirmed.

294
00:32:33,310 --> 00:32:38,740
So we had four. Wrote up paper on the first one.

295
00:32:41,590 --> 00:32:47,889
A super intelligent journalist asked, Have we ever considered extraterrestrial life from being truthful characters?

296
00:32:47,890 --> 00:32:56,190
We said yes. And for a couple of weeks, we got no work done, you know, just take a taxi to the observatory.

297
00:32:56,200 --> 00:33:03,009
CBC wants you. And so on. And I find that very, very difficult,

298
00:33:03,010 --> 00:33:09,460
because what would typically happen was they would ask Tony about the astrophysical significance of the discovery.

299
00:33:10,240 --> 00:33:16,780
And they turned to me for the human interest, which means they wanted to know my vital statistics,

300
00:33:17,780 --> 00:33:20,880
whether I was brunette or blonde and how many boyfriends I had.

301
00:33:22,180 --> 00:33:25,930
And I found this very difficult. I felt like a bit of meat.

302
00:33:27,060 --> 00:33:33,969
And I would have loved to have been rude. But you're a grad student who hasn't finished a thesis.

303
00:33:33,970 --> 00:33:38,620
You haven't got a job to go to. You're going to need references from the lab.

304
00:33:39,130 --> 00:33:44,730
The lab could do with the publicity. I'd better be not rude.

305
00:33:46,090 --> 00:33:50,760
But it wasn't. It wasn't a totally comfortable experience.

306
00:33:53,400 --> 00:33:57,420
So when I get onto some proper physics 35 minutes later.

307
00:33:57,440 --> 00:34:03,390
Yeah, sorry about that. So.

308
00:34:05,020 --> 00:34:10,540
What type of stars are these? Stars.

309
00:34:10,570 --> 00:34:17,740
Most stars that you see in the night sky are shining because they're fusing hydrogen to helium or maybe helium to carbon.

310
00:34:19,050 --> 00:34:22,590
And most stars and their life after that.

311
00:34:23,490 --> 00:34:29,730
But more massive stars like the Pleiades. They're currently probably fusing hydrogen to helium.

312
00:34:30,660 --> 00:34:37,200
They're doing it very fast. The process goes as temperature to the power 17.

313
00:34:38,080 --> 00:34:43,340
And these are massive stars. So the temperature in the middle because of the pressure is high.

314
00:34:43,350 --> 00:34:46,860
So they're very, very luminous and they have a relatively short life.

315
00:34:48,650 --> 00:34:56,120
They will go beyond fusing helium to carbon. They do fusion reactions right up to where they've got an iron nickel core.

316
00:34:57,290 --> 00:35:02,540
And as you probably know, the binding energy of the iron and nickel bits of the.

317
00:35:04,240 --> 00:35:13,390
Periodic table mean that you can't fission hydrogen and get energy efficient iron and get energy out or fuse iron and get energy out.

318
00:35:14,230 --> 00:35:20,170
So these stars end up with an iron nickel core and are unable to generate any more energy.

319
00:35:21,580 --> 00:35:25,150
And the net result is a collapse followed by a massive explosion.

320
00:35:28,390 --> 00:35:34,690
This is a very famous example of these large Magellanic Clouds in the Southern Hemisphere sky,

321
00:35:35,920 --> 00:35:40,690
lots of lovely hydrogen gas, H alpha emission, all this pink stuff,

322
00:35:40,960 --> 00:35:49,510
millions of little stars, one picked out with an arrow, and for the benefit of the non astronomers, the arrows added after the photographs taken.

323
00:35:52,210 --> 00:35:55,280
And this is the star we had to identify with an arrow.

324
00:35:55,340 --> 00:35:58,930
It turns out it was one of these massive stars and it exploded.

325
00:36:00,610 --> 00:36:09,610
And in the explosion, 90 or 95% of the stellar material is thrown out and the other 5% collapses,

326
00:36:09,610 --> 00:36:13,390
at least in some of these explosions, to form a neutron star.

327
00:36:18,580 --> 00:36:25,840
The most famous example these days is the Crab Nebula, which is the remains of the star that exploded about a thousand years ago.

328
00:36:26,770 --> 00:36:32,770
It was a considerable problem. It's radiating by synchrotron radiation, this nebula.

329
00:36:33,280 --> 00:36:36,910
But it doesn't seem to be getting tired. The electrons.

330
00:36:37,900 --> 00:36:39,340
Are somehow being replenished.

331
00:36:40,000 --> 00:36:47,920
And it was Jeffrey Burbage who said in the 1960s, there's two kinds of astrophysics, the study of the Crab Nebula and the study of everything else.

332
00:36:49,540 --> 00:36:57,370
It turns out that the exploding star left a pulsar about there, and the energy of the pulsar is what energises the nebula.

333
00:36:59,900 --> 00:37:03,170
A cartoon of what I'm talking about.

334
00:37:03,800 --> 00:37:14,630
The ones I always catch is the movie. So let's start with its very compact star spin axis vertical magnetic axis inclined.

335
00:37:15,550 --> 00:37:19,240
And coming out to the magnetic axis is a radial beam.

336
00:37:20,110 --> 00:37:23,470
When it shines on your radio telescope, you see a pulse.

337
00:37:26,220 --> 00:37:31,530
Slightly more detail in this one. Again, spin axis, vertical, magnetic axis inclined.

338
00:37:32,070 --> 00:37:39,570
It's the magnetic field lines over the pole which form a kind of funnel shape which was quite a lot of hand-waving,

339
00:37:39,570 --> 00:37:46,290
I think still means that you get a beam of radio waves which comes out here and then sweeps around the sky.

340
00:37:47,870 --> 00:37:56,330
And it also means that there'll be a lot of pulsars that we don't see because the beam doesn't shine on our face, maybe 20%.

341
00:37:57,640 --> 00:38:01,300
I think it's fair to say that the emission mechanism is still a bit of an issue.

342
00:38:02,200 --> 00:38:06,009
There are a few people who think they have cracked it. We don't know.

343
00:38:06,010 --> 00:38:16,200
The rest of us understand what they're talking about. Well, we're dealing, therefore, with the course of some of these massive stars,

344
00:38:16,800 --> 00:38:22,650
things that weigh typically a few times 10 to 27 tons, a little bit heavier than our sun.

345
00:38:23,610 --> 00:38:29,060
They have a radius of ten kilometres and I don't mean tens or something.

346
00:38:29,100 --> 00:38:34,820
I mean ten kilometres. And so you have a [INAUDIBLE] of a lot of mass in a very small ball.

347
00:38:35,430 --> 00:38:40,020
And the average density is the average density is like the density of the nucleus of the atom.

348
00:38:41,240 --> 00:38:47,120
So there are strong surface gravity effects, tidal effects and lots of other things too.

349
00:38:48,860 --> 00:38:58,220
Because of the strong gravitational force, light gets bent and you can get bent over the surface, around the surface of one of these stars,

350
00:38:59,030 --> 00:39:01,609
not quite as dramatically as it does around a black hole,

351
00:39:01,610 --> 00:39:09,830
but nonetheless on from one spot on the surface of a neutron star pulsar, you'd see 20 or 30 degrees over.

352
00:39:10,280 --> 00:39:16,540
So you can see most of the stars standing in one place. Gravity also redshifts light.

353
00:39:16,540 --> 00:39:20,710
So if there were little green men to us, they look like little red men.

354
00:39:21,760 --> 00:39:28,750
But there aren't. And it also affects the rate at which clocks goes go roughly a factor of two.

355
00:39:29,260 --> 00:39:32,560
So a clock on the surface of a neutron star would.

356
00:39:33,400 --> 00:39:40,690
Tick every 2 seconds rather than every second. There is enormous gravitational force.

357
00:39:41,500 --> 00:39:46,540
I've worked out that the work you do climbing a mountain that's one micron high on

358
00:39:46,540 --> 00:39:51,730
one of these stars is comparable to the work you do climbing Everest here on Earth.

359
00:39:53,440 --> 00:39:56,920
And because of the strong gravity, the atmosphere is compressed.

360
00:39:57,760 --> 00:40:01,210
In the absence of electrical forces, the atmosphere is about this thick.

361
00:40:02,450 --> 00:40:09,950
So if there were little green men, they'd need to get their nostrum stone at foot, and they'd be a different shape.

362
00:40:10,720 --> 00:40:17,150
But actually there are electromagnetic forces, and that inflates the atmosphere to be about the height of this bench.

363
00:40:18,050 --> 00:40:25,560
But again, you'd have to get your nostrils down there. There's a very strong gradient of gravity.

364
00:40:27,060 --> 00:40:31,440
And it will stretch anything that comes close.

365
00:40:32,490 --> 00:40:38,550
So suppose you're. Suppose you're going to visit one of these and you're going to land feet first.

366
00:40:39,600 --> 00:40:44,910
So they do lightweight land on the pole so that as you go in, your body gets stretched.

367
00:40:45,920 --> 00:40:51,500
But actually the difference in force between your head and your feet is such that it would pull the body apart.

368
00:40:52,420 --> 00:40:59,470
And your body will land. Plop, plop, plop. The stretching is apparently called spaghetti ification.

369
00:41:02,140 --> 00:41:09,430
So don't go visit a poster. There's a huge magnetic field, probably about ten to the eight Tesla,

370
00:41:10,150 --> 00:41:16,810
where there's an assumption in there and that is that their radio radiation is magnetic dipole or the energy losses.

371
00:41:17,050 --> 00:41:18,610
Magnetic dipole radiation.

372
00:41:20,120 --> 00:41:29,000
And if you spin that kind of magnetic field of the typical pulsar period, you get 10 billion volts, a centimetre, something like that.

373
00:41:30,110 --> 00:41:35,900
So the electromagnetic effects are extreme and quite hard to analyse, therefore.

374
00:41:37,670 --> 00:41:41,330
And all that magnetic field included rotates as a solid body.

375
00:41:43,160 --> 00:41:47,690
The we believe the pulse period that we see is the rotation period.

376
00:41:48,170 --> 00:41:51,760
And we see rotation periods between one point 4 milliseconds.

377
00:41:51,760 --> 00:41:56,990
That's 700 hertz down to periods of 10 seconds.

378
00:41:57,650 --> 00:42:07,100
This is an interesting interestingly fast pulsar because there are some theories that say if a pulsar tries to spin faster than that,

379
00:42:07,370 --> 00:42:10,370
it will be braked. I don't mean broken.

380
00:42:10,370 --> 00:42:19,510
I mean brakes will apply because the theory says in the very centre of the star you get rossby instabilities set up.

381
00:42:19,940 --> 00:42:27,650
They will produce gravitational radiation and that will carry away energy which effectively prevents the star from spinning any faster.

382
00:42:28,660 --> 00:42:33,250
So, of course, the hunt designed to find an even faster one to prove that theory wrong.

383
00:42:34,030 --> 00:42:36,760
But as far as I know, that's where we still stand.

384
00:42:38,090 --> 00:42:45,470
And so we've got relativistic speeds in with all the other lovely gubbins to make some really difficult physics.

385
00:42:51,810 --> 00:42:55,440
Today. We know of about about two and a half thousand of these.

386
00:42:57,150 --> 00:43:02,350
Most of them are in the radial. The visible has proved a bit disappointing.

387
00:43:02,370 --> 00:43:06,210
There are some, but not many. X-rays produce a few.

388
00:43:06,840 --> 00:43:10,200
The really surprising thing is the number that have turned up in gamma rays.

389
00:43:10,860 --> 00:43:14,280
There are now a number of gamma ray astronomy satellites,

390
00:43:14,790 --> 00:43:22,470
and I think the current tally is around 200 known in gamma rays, some of which are very, very hard to see in the radio.

391
00:43:23,290 --> 00:43:29,090
So. Mission mechanism must be from a different place in some of the gamma ray ones.

392
00:43:33,210 --> 00:43:37,680
Most of the 2500 are isolated single pulsars.

393
00:43:38,070 --> 00:43:44,280
But there's a number in binaries. There's one binary where both companions are pulsars.

394
00:43:45,360 --> 00:43:52,900
And there's one triple system. And there are, I think, about three or four with planets.

395
00:43:54,190 --> 00:44:07,280
You can do this quite easily. Pulsar planet is the planet that roamed the pulsar moves just a little bit.

396
00:44:07,790 --> 00:44:11,150
You can't see the planet, but you're into the Doppler shift on the pulsar.

397
00:44:13,500 --> 00:44:20,830
So. A few with planets and we can explain some of them, but not all of them.

398
00:44:20,860 --> 00:44:26,420
Some of them are really, really hard to explain, but they clearly are planets and planets, plural.

399
00:44:26,440 --> 00:44:35,459
In that particular case. And the received wisdom is that there's about 100,000 pulsars in the galaxy.

400
00:44:35,460 --> 00:44:39,450
But since I'm in Oxford, I think the figure is whereas I was 20,000.

401
00:44:41,150 --> 00:44:45,670
Could be. Could be? Yep. Numbers open for debate, which is good.

402
00:44:48,540 --> 00:44:51,540
When you get ten to the power, 27 tons spinning.

403
00:44:52,660 --> 00:44:57,430
It keeps spinning and it's the devil's own job to make it change its spin.

404
00:44:58,320 --> 00:45:02,130
So these pulses come round very, very accurately.

405
00:45:03,210 --> 00:45:07,910
And the period derivatives are between ten to the -12.

406
00:45:07,920 --> 00:45:11,280
That's a pretty grotty pulsar, 210 to the -21.

407
00:45:12,430 --> 00:45:18,550
They're comparable with the best terrestrial clocks. And at one point we thought they might become the new time standard.

408
00:45:19,090 --> 00:45:24,340
But the US list is on the case and they're going to get a better clock.

409
00:45:26,930 --> 00:45:31,460
But they're still pretty good and they're dotted throughout the galaxy.

410
00:45:32,660 --> 00:45:41,030
So now we have clocks that we can use for some experimental relativity tests on an astronomical scale.

411
00:45:44,220 --> 00:45:52,080
This is, strictly speaking, the spin down rate. There is, of course, inevitably a bit of noise or jitter or whatever you want to call it.

412
00:45:52,620 --> 00:45:58,500
But for the most stable pulsars, we can measure post arrival times to tens of nanoseconds.

413
00:46:00,580 --> 00:46:03,850
And through the Doppler effect that I've already described.

414
00:46:05,650 --> 00:46:13,030
We can measure the radius of a binary, a pulsar in a binary system to a few microns.

415
00:46:15,460 --> 00:46:21,100
And the orbit of the radius of that orbit might be half a million kilometres.

416
00:46:21,820 --> 00:46:25,510
So an accuracy of a few microns in half a million kilometres.

417
00:46:26,500 --> 00:46:28,510
These are fantastic clocks.

418
00:46:32,270 --> 00:46:40,040
Some of the most interesting physics that I'm going to talk about in the next 10 minutes are from pulsars in binary systems,

419
00:46:40,040 --> 00:46:45,650
where pulsars are twinned with something else. Most often they're twinned with an ordinary star.

420
00:46:46,370 --> 00:46:50,030
Sometimes they're twinned with another neutron star, which is not a pulsar.

421
00:46:50,660 --> 00:46:56,990
In one case, it's twinned with another pulsar, and in one case, we have a pulsar in a triple system.

422
00:46:57,680 --> 00:47:02,050
We would love a pulsar twin with a black hole. Yet.

423
00:47:04,500 --> 00:47:09,420
So this is the first pulsar ever found in a binary system, 1974.

424
00:47:10,140 --> 00:47:13,260
And it turns out to be a very close binary system.

425
00:47:13,620 --> 00:47:19,980
And it turns out to be relativistic. And because of relativistic effects, the orbit changes.

426
00:47:21,180 --> 00:47:25,410
This is the work by Hudson Taylor, for which they got a Nobel Prize.

427
00:47:26,280 --> 00:47:31,560
This graph has date along the bottom. 1974 goes up beyond this.

428
00:47:31,800 --> 00:47:35,560
This is one of the orbital parameters. These are the data points.

429
00:47:36,030 --> 00:47:39,930
You can't see the error bars because they're about 20% the size of the dot.

430
00:47:41,170 --> 00:47:51,240
And the line is Einstein's prediction. For the change in that orbital parameter due to the emission of gravitational radiation.

431
00:47:52,700 --> 00:47:54,050
And it's a brilliant fit.

432
00:47:55,250 --> 00:48:03,890
So to say they got the Nobel Prize because this was the first and very good evidence for the emission of gravitational radiation.

433
00:48:08,390 --> 00:48:19,190
The fit is good to the predictions of general relativity within 0.3%, but they continued to observe it because they'd like to beat that number down.

434
00:48:21,530 --> 00:48:27,980
The double pulsar where we've got two pulsars in a binary system are fairly recently discovered.

435
00:48:28,370 --> 00:48:38,210
It's a very tight binary system. The orbital period is about two and a half hours, which means the pair would fit inside the sun, actually twice over.

436
00:48:38,570 --> 00:48:48,220
It fit within the radius of the sun. And because they're so close and the orbital period so short, the speeds are fast and it's relativistic.

437
00:48:50,270 --> 00:48:59,160
Oh, I think we skip the rest of that. So we've had to learn a bit about relativistic orbits.

438
00:49:00,150 --> 00:49:05,040
Kepler told us about the five parameters we need to define a classical orbit.

439
00:49:05,510 --> 00:49:09,990
Okay, if it's relativistic, you need another five parameters.

440
00:49:10,920 --> 00:49:15,570
The advance of the periosteum. The way the ovals turned around.

441
00:49:16,410 --> 00:49:24,030
The gravitational redshift. I've mentioned that earlier. The change in the orbital period due to emission of gravitational radiation.

442
00:49:24,900 --> 00:49:27,570
And two quantities that we call the SHAPIRO DeLay,

443
00:49:28,560 --> 00:49:35,130
which is a bit which is the temporal equivalent of the way a light ray gets bent as it passes a massive object.

444
00:49:37,660 --> 00:49:44,590
And for the double pulsar. They have been studying it carefully for ever since it was found.

445
00:49:45,460 --> 00:49:49,360
They have got all these parameters, all five of them.

446
00:49:49,840 --> 00:49:52,960
In fact, they've got a few extra as well.

447
00:49:54,040 --> 00:50:02,620
In fact, they've got five extra parameters. So we've got five independent tests of R with this system.

448
00:50:04,170 --> 00:50:09,870
And the way they have done it is they make a mass mass plot, the mass of the two pulsars,

449
00:50:10,260 --> 00:50:13,500
and then they put on top of this all of the other parameters.

450
00:50:13,860 --> 00:50:17,070
So there's the advance superior strong omega dot.

451
00:50:17,100 --> 00:50:20,640
That's this one which has a small error bar.

452
00:50:21,150 --> 00:50:26,010
There's the gravitational redshift, which is gamma, which is this curvy one, and so on.

453
00:50:27,070 --> 00:50:32,290
And if general relativity is correct, all these lines intersect to the point.

454
00:50:33,870 --> 00:50:40,920
They're intersecting somewhere in. There will be some with quite big air of ours, but a blow up of X is like that.

455
00:50:42,020 --> 00:50:46,670
I think these error bars are 68% confidence, but.

456
00:50:48,650 --> 00:50:49,160
So.

457
00:50:51,240 --> 00:51:01,950
They're able to say already that the theory is okay to within 0.02% and they are seriously squeezing other theories of gravity other than Einstein's.

458
00:51:02,910 --> 00:51:11,010
Not I don't think they have eliminated any yet, but they're really struggling to survive and the work is going on.

459
00:51:11,010 --> 00:51:20,260
It's being led by Michael Kramer in Bonn. This is for the solid state physicists, and this is going to have to be pretty fast.

460
00:51:20,890 --> 00:51:24,370
These objects are dense. They're very solid.

461
00:51:25,060 --> 00:51:33,070
And the solid state physics is huge fun. There's several hundred theories about what the structure would be.

462
00:51:33,460 --> 00:51:39,760
I'm going to pick out one or two features which are either curious or maybe instructional.

463
00:51:40,780 --> 00:51:46,420
So we're going to start at the surface. We're going to take a ten or 11 kilometre journey into the centre,

464
00:51:46,930 --> 00:51:51,850
and in that ten kilometre journey, the density is going to go up ten orders of magnitude.

465
00:51:52,920 --> 00:51:56,220
So I'm going to deal with ten orders of magnitude of density in 5 minutes.

466
00:51:58,310 --> 00:52:05,180
One of the theories that I find really entertaining says that the surface of these objects is a thin iron polymer.

467
00:52:07,120 --> 00:52:12,880
You didn't know iron was a polymer, but where there's a strong magnetic field.

468
00:52:13,390 --> 00:52:22,180
Atoms aren't spherical. They're slim, cylindrical. And in the strong magnetic field, it can come with a large lama radius or a small radius.

469
00:52:22,510 --> 00:52:26,020
So large, small, large, small, large, small.

470
00:52:26,620 --> 00:52:34,570
Make a polymer. A polymer strand, then the polymers will bind together beautifully or parallel to the magnetic field.

471
00:52:35,590 --> 00:52:42,430
Conductivity this way is like copper. Conductivity this way is like asbestos at the poles.

472
00:52:42,430 --> 00:52:51,709
It looks like a badly shaped hedgehog. And these stars start spending fast.

473
00:52:51,710 --> 00:52:56,770
They're quite a blink. They have to adjust their shape as they cool and age.

474
00:52:57,400 --> 00:53:02,710
But this iron skin has a young's modulus. About half a million times that of iron of steam.

475
00:53:05,750 --> 00:53:09,649
Nice idea. Might not be true. Go inside.

476
00:53:09,650 --> 00:53:14,510
That'd get worse. Gets a bit more normal body centre, cubic lattice.

477
00:53:16,010 --> 00:53:20,780
But if I go into just a tiny bit of quantum mechanics for the students shooting

478
00:53:20,880 --> 00:53:28,870
Schrodinger potential well energy levels two electrons in each two you run out of energy,

479
00:53:28,920 --> 00:53:32,840
you run out of electrons somewhere up here, which is called the Fermi Energy.

480
00:53:35,000 --> 00:53:39,709
Okay. Go a little bit further into the neutron star.

481
00:53:39,710 --> 00:53:46,260
The density is greater. The well shrinks. What happens to energy levels when the world shrinks?

482
00:53:48,480 --> 00:53:53,830
It's like squeezing a tube of toothpaste. So the Fermi Energy isn't here.

483
00:53:54,280 --> 00:54:01,710
It's up at ceiling level. Over here is a radioactive nucleus that wants to do a bit of decay.

484
00:54:03,640 --> 00:54:07,330
The electron coming out can have a range of energies, but there is a maximum.

485
00:54:08,290 --> 00:54:16,370
And let's say it's at this level. And it's trying to get into this potential world where everything's full up to ceiling level.

486
00:54:17,690 --> 00:54:21,540
And it can't get in. So the decay is prevented.

487
00:54:23,180 --> 00:54:29,150
So you end up with nuclear neutron rich nuclei because beta decay is prevented.

488
00:54:31,040 --> 00:54:33,079
Go a bit further in the potential woes.

489
00:54:33,080 --> 00:54:42,500
This width and the Fermi energy is way up at one point at 1.89 MTV covering the mass difference between a neutron and a proton.

490
00:54:43,590 --> 00:54:49,770
And a proton press. One of those electrons can make more neutrons and the nuclei get even more neutron fat.

491
00:54:54,470 --> 00:55:02,690
They get so fat that when you've got a density of about 10 to 14, neutrons drip out of the nucleus and you have free floating neutrons.

492
00:55:04,220 --> 00:55:09,470
They're probably super fluid. Standard BCS theory two.

493
00:55:09,470 --> 00:55:13,990
Spins net result zero. Superfluidity.

494
00:55:14,140 --> 00:55:18,790
Not superconductivity, unless you've got electrons and protons, which you might have.

495
00:55:19,210 --> 00:55:30,060
But superfluidity is bad enough. Superfluid liquid can be quantised into what's called Feynman on Sagar Vortices.

496
00:55:30,750 --> 00:55:36,340
Tiny, tiny whirlpools. About a million of them per square centimetre.

497
00:55:37,240 --> 00:55:38,980
So you've got all these little whirlpools.

498
00:55:40,460 --> 00:55:46,850
You've got an inclined magnetic field and there are some theories that say the magnetic field can be in flux tubes.

499
00:55:47,690 --> 00:55:53,600
So you've got two kinds of spaghetti at an angle inside a rapidly rotating body.

500
00:55:54,690 --> 00:56:01,560
Yippee. Closer in.

501
00:56:01,740 --> 00:56:10,830
This is work that has been done in Oxford. You get these pasta shapes, what's known as pasta shapes, the nuclear configuration.

502
00:56:11,520 --> 00:56:17,730
And then you hit the nuclear density at about two by 10 to 70 kilograms per cubic metre.

503
00:56:18,690 --> 00:56:22,950
And the very centre of the neutron star, we don't have a lot of data.

504
00:56:24,000 --> 00:56:34,740
And I'm not sure we're going to get a lot of data because the accelerator data is not new tronic, it's hydraulic plutonic.

505
00:56:34,920 --> 00:56:37,920
It's perhaps the word I want and this might be different.

506
00:56:38,820 --> 00:56:42,299
And so the theories here, well, there's no constraints.

507
00:56:42,300 --> 00:56:45,360
So can you imagine? Oh, it's quite clues on plasma.

508
00:56:45,360 --> 00:56:49,030
It's mesons, it's quarks, it's Bose-Einstein condensate.

509
00:56:49,680 --> 00:56:55,020
Yeah, we don't know and I'm not sure how we will know.

510
00:56:55,590 --> 00:57:03,400
But anyway. Large Mars pulsars, it turns out, can constrain large mass pulsars.

511
00:57:03,880 --> 00:57:12,550
If you can establish their mass can constrain the theorists. These curly lines are some of the hundreds of theories.

512
00:57:12,910 --> 00:57:23,230
Green ones involve quarks. Pink ones involve strange green, strange quarks, the pink, nuclear arms and other exotic stuff.

513
00:57:23,440 --> 00:57:32,530
And these are blue under nucleons. And this pulsar mass eliminates at least some of the models.

514
00:57:33,780 --> 00:57:39,090
And there is a second now really massive poster also up at the same sort of level.

515
00:57:39,540 --> 00:57:42,600
He's put more bottles on this diagram, but it's the same idea.

516
00:57:44,430 --> 00:57:49,170
This one is in a very tight orbit with a white dwarf.

517
00:57:50,150 --> 00:57:56,420
And it's the first time we've got a massive pulsar in a very tight orbit.

518
00:57:58,510 --> 00:58:04,660
This particular post far post r is a good testbed for looking for scalar gravity fields.

519
00:58:05,670 --> 00:58:13,100
Always worth checking out that there are such things and also checking out where there is dipolar gravitational waves.

520
00:58:13,830 --> 00:58:17,190
So I would watch the work being done by Antoni artists.

521
00:58:17,230 --> 00:58:21,150
He's enormously impressive. Probably still a postdoc.

522
00:58:24,620 --> 00:58:28,190
And this one will be very quick. I won't go on much longer.

523
00:58:28,790 --> 00:58:34,009
You know about the equivalent principle. You don't need to read that one equivalence principle,

524
00:58:34,010 --> 00:58:41,840
apparently first tested repeatedly by Galileo dropping things off the Tower of Pisa and resistance was a problem.

525
00:58:43,640 --> 00:58:47,870
It was repeated by the astronauts when they got to the moon.

526
00:58:48,200 --> 00:58:56,660
This is actually not the original one. The video there is too grainy, but there's a hammer and there's a feather and they're falling.

527
00:58:57,830 --> 00:59:02,720
Simultaneously in the environment with a lack of air.

528
00:59:03,290 --> 00:59:08,450
But the gravity here is not very strong. So this is only testing the weak equivalence principle.

529
00:59:09,860 --> 00:59:14,389
To test the strong equivalence principle. Keep your fingers crossed, folks.

530
00:59:14,390 --> 00:59:21,410
We have not found a pulsar orbiting another star and the pair are orbiting a third star.

531
00:59:21,890 --> 00:59:26,120
And the gravity here is serious. So this is strong field stuff.

532
00:59:26,630 --> 00:59:33,170
So that's like the hammer and that's like the feather. And do they fall in the same way in the gravity of the star?

533
00:59:33,950 --> 00:59:37,190
Or put differently, does this binary orbit change?

534
00:59:38,220 --> 00:59:43,610
Because the strong equivalence principle is violated. They're working on it.

535
00:59:43,790 --> 00:59:48,740
We don't know the answer yet. The rumours, I think, come and go is the fair thing to say.

536
00:59:49,190 --> 00:59:55,280
But I hope that very soon we'll have a test of the equivalence principle in a strong gravitational field.

537
00:59:56,330 --> 00:59:57,440
I think I've said all of that.

538
00:59:59,430 --> 01:00:08,219
So police are astronomers are continuing to hammer various aspects of relativity theory because as you probably well know,

539
01:00:08,220 --> 01:00:11,430
there are some problems with general relativity.

540
01:00:13,840 --> 01:00:16,930
Singularities doesn't mesh to our quantum theory.

541
01:00:18,170 --> 01:00:21,850
And the force of gravity doesn't fit with the other forces.

542
01:00:21,850 --> 01:00:25,670
So maybe at some very fine level there's a problem.

543
01:00:26,420 --> 01:00:33,340
So they're continuing to chase it. I'm going to live with that and end with slides.

544
01:00:33,580 --> 01:00:40,420
We've got postal work going on in Oxford. This was probably only about two thirds of the group could be present.

545
01:00:41,320 --> 01:00:42,430
Thank you for your attention.