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Who's sending us signals from space?


Picking up pulsars for the first time
Antony Hewish Astronomer
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I mean, it really was a dramatic thing. And it's the sort of research which not… many people are lucky enough to do because, you… you have to be doing these outlandish things like I was doing, doing a rather strange experiment to pick up these unusual signals. But the antenna, it turns out that interplanetary scintillation is just the same timescale. It fluctuates on a timescale of a 10th of a second, which is exactly what you need to pick up the solar wind; the… the solar wind fluctuations are about the same timescale as pulsar periodicities and during that survey, which I set up to map the whole sky, we suddenly discovered pulsars coming in.

And it was my graduate student [Jocelyn Bell Burnell] who has achieved a lot of fame for this who first saw there was something funny going on. We were carrying out a complete sky survey and that was going to take 12 months; we were scanning the sky in strips and using twinkling to measure the angular sizes of things and locate the quasars. And that was going very nicely. But she… it was in August 1967, we'd built… I'd planned the antenna, designed it and got it moving in 1965; it was operating first of all in 1967 and it was only a couple of months after we'd begun the sky survey, and Jocelyn, who'd come in at the beginning of the antenna construction, she pointed out… our records were just pen recordings, it wasn't digital and done in computers, she was analysing it all by hand. We had lots and lots of paper charts and she was rolling these charts on and looking at them on her desk and there was one faint… what we thought was a faint quasar, but it was showing much too strong scintillation in the middle of the night, which… which made it stand out on the… on the records. You see, the solar wind has a density which varies inversely as the distance from the Sun; it goes as… as one over the distance square, as you… as you get out into space, obviously, because of the conservation of energy – conservation of mass rather – and in the middle of the night you only expect sort of 10%, 5% scintillation. And here was a weak source showing 100% scintillation and, not only that, it wasn't always there in the sky for reasons which I won't elaborate on. But… but to measure angular sizes you need… you need many, many measurements as the sources approach the Sun and as… as they go away on the other side at… at different times of the year in order to use the solar wind, as it were, as an… as an angular size measuring instrument. It's an interferometer of varying baselines essentially so… so you have to have many, many observations of a source to measure its angular size properly. And so we had repeated observations; we were making a full sky survey about once per week and some weeks this source wasn't there at all in the sky. Other times there it was, showing… showing this faint twinkling. And so, what… what on earth was it? That… that was the first question. I mean, we looked at it and said, is it real or isn't it? Well, it could easily have been radio interference because interference, as you know, in long wave length radio astronomy, low frequency, you get lots of terrestrial interference of one sort or another and we had plenty of that. But this was repeating at almost but actually not quite the same sidereal time, star time, and that made it look as though it probably was a genuine source in the sky, but it wasn't always there. And why was it showing 100% scintillation in… in the middle of the night?

My first thought was we'd picked up a source like the radio Sun but much further off. I mean, if the Sun can produce radio waves which fluctuate, then other stars can too, obviously and put a star like the Sun much, much further away, then it's going to produce a signal which is sometimes there and sometimes isn't there. And these stars that do this… stars that do this are known as flare stars, and I contacted optical astronomer friends and said, do you know of a flare star with these coordinates? And this was David Dewhurst who was our contact then at the observatories here. I asked him whether there were… there were any sources in that direction and he said, no, he didn't know of any. And then I thought, well, could it be some astronomer doing an experiment causing radio interference, which would, if they were tracking a certain source in the sky, they would be doing things at the same sidereal time, perhaps, and causing radio interference. So I got on to the Royal Greenwich Observatory here, Donald Lynden-Bell, whom I sure you know, was working down there, he was a friend of mine, and I actually… wrote him a letter, he's still got that letter, and I said: 'What do you know about astronomers who might be doing experiments which could cause radio interference at such… and such a time?' He didn't know anything about that and he said, no, nothing there. And then I thought, well, maybe it's signals being reflected from the ionosphere back down into our telescope which would give you a source at some declination, fixed declination, and not at… not at others. I mean, it wasn't normal interference, obviously. And my mind was just going round and round and Jocelyn didn't have any ideas either.

And this was actually the time when I decided that we really had to have a rather closer look at the signal to see what it was. I mean, did it look like radiation from a star like the Sun? Did it have the same time structure? When you're doing a survey you have to economise on… on chart paper and so on, we were only using the slow chart speed. I decided that we must run a chart much faster to see actually what this strange nocturnal scintillation really looks like, what's the intensity of the function of time? Does it look like scintillation or does it look like something else? And Jocelyn fixed up another recorder we had for another experiment, I got her to transfer that to the… to the equipment and she started measuring this source daily and the wretched thing disappeared. I'd said it wasn't always there on… on the recordings. And we're now talking about the autumn of 1967, that's… that's when all this was going on. So Jocelyn, who was a very diligent student, kept on repeating these measurements, but finally got fed up because nothing was there when she looked on this fast chart speed, nothing at all. And so she gave up in disgust and I was of course in mid-term, having a busy teaching term anyway, so I didn't bother too much about it then and began to think, well, it's all gone away, you know, it was interference after all.

But then, in November, it suddenly reappeared quite strongly and Jocelyn got back onto it and ran a fast record and bonk, bonk, bonk… and there were… there were these pulses coming through. Well, an artificial signal like that, what… what on earth could it be? So I mean, my instant reaction was, well, it has to be some terrestrial interfering signal because no natural source is going to produce clock-like… clock-like pulses. And would you believe it, at that moment I had to spend all my time as an examiner marking. In those days we had a scholarship examination which had to be marked at great speed round… round about Christmas time and, instead of being able to get to the lab and do… do follow-up work on this, I was in Churchill College, which you know well, and marking scholarship papers. And I was doing that for a whole week.

During that time… we obviously didn't say anything about these signals because I knew that if it was leaked that we were picking up strange signals with the… with the new radio telescope the place would fill up with the press and the media and so on and serious work would be impossible. This had happened once before because when Sputnik first went up we were the first people outside of Russia to know the orbit of that satellite because we used simple interferometers when the Lunik went up to locate its… its orbit. And the press descended on… on us at the… at the radio observatory, which was then, we hadn't moved out to… well, we had moved to Lord's Bridge but we were still operating close to Cambridge and we set up the interferometer there. And this was Bruce Elsmore and colleagues, who actually did this. The press invaded and you just can't do serious research work with the press around falling over things.

Antony Hewish (1924-2021) was a pioneer of radio astronomy known for his study of intergalactic weather patterns and his development of giant telescopes. He was awarded the Nobel Prize for Physics in 1974, together with fellow radio-astronomer Sir Martin Ryle, for his decisive role in the groundbreaking discovery of pulsars. He also received the Eddington Medal of the Royal Astronomical Society in 1969.

Listeners: Dave Green

Dave Green is a radio astronomer at the Cavendish Laboratory in Cambridge. As an undergraduate at Cambridge his first university physics lecture course was given by Professor Hewish. Subsequently he completed his PhD at the Cavendish Laboratory when Professor Hewish was head of the radio astronomy group, and after postdoctoral research in Canada he returned to the Cavendish, where he is now a Senior Lecturer. He is a Teaching Fellow at Churchill College. His research interests include supernova remnants and the extended remains of supernova explosions.

Tags: Jocelyn Bell Burnell

Duration: 10 minutes, 2 seconds

Date story recorded: August 2008

Date story went live: 25 June 2009