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Discovering the first millisecond pulsars


Who's sending us signals from space?
Antony Hewish Astronomer
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We decided to keep our tongues quiet until… until we knew what was going on. But there I was in Churchill College marking away, and at night wondering what the heck was going on. And I mentioned this quietly one day to a very distinguished man called Sir Edward Bullard, he was a fellow of Churchill College, and the advantage of Cambridge is that you have lunch with important people sitting next to you every now and again, and I mentioned it quietly to him, and said, 'Look, I'm getting this very odd signal at Lord's Bridge, what do you think it is?' He was a man of very wide experience and he said, 'Well, if it's narrow band have you thought about intelligent signals from outer space? Perhaps you're the first person to pick up aliens'. And, well, the thought had crossed my mind, but I dismissed it as being totally ludicrous. But there I was, sitting marking examination papers and couldn't do anything.

Well, a week later, I was able to get back to the lab and the first thing I did was to set up accurate timing. I got time, seconds, time pips on the… on the record and compared those with the… the pulses we were getting and found out that this thing was keeping time to better than a millionth of a second per day. I mean, it was absolutely repeatable; that was the first thing that happened and that was a shock. I mean, it was better than our clocks and, whatever it was, it was very odd. I mean, that… that was the first result.

The second was following up, actually, what Teddy Bullard had said, if it's narrow band… we were measuring the bandwidth, we were doing that anyway, but it turned out that this was a narrow band signal and that strengthened the… the possibility that we were picking up alien signals and, furthermore, the signal was… was dispersed, that's to say we measured the bandwidth of it and… from the shape of the bandwidth you know that the signal has traversed a certain distance of space because pulses travelling through space travel at slightly different speeds and this produces a characteristic… if you pick up over a finite bandwidth, you pick up what we call a dispersed signal, the pulse arrives slightly earlier at… at slightly shorter wavelengths and you can detect this in your… in your equipment. And we measured that and, from what we then knew about the density of electrons in the… in the interstellar space - I did a quick calculation which showed that this thing was… was about 40–50 parsecs away, which is about… what… that's about 100–150 light years or something like that. So here was a signal sending… coming from 150 light years, which is amongst the nearby stars, producing regular pulses. Well, it makes you think, doesn't it? It just looks so… so artificial. And I discussed this with Martin Ryle and we said, well, is it or is it not intelligence? Naturally, we kept our mouths tightly shut because any hint that we'd picked up intelligence from outer space and the lab would have been absolutely full of reporters and media.

So we kept our mouths tightly quiet and decided what we would do, and we decided that we ought to handle this through the Royal Society, which was the top scientific brass at the time and, if they first of all believed what we'd done, and then they… we'd require their advice to sort of handle this, because it isn't astronomy anymore, it's… it's almost politics, isn't it, if you pick up alien signals? But I decided also that the signals were accurate enough that I could detect motion of the source that was sending them using the Doppler effect. If you're picking up life on another planet, that planet is in orbit about some star, and if it's sending regular signals to you, they'll… they'll be Doppler shifted according to the orbit. And I… I would have been able to measure the orbital speed. And so when I got back to the lab and started continuing these timing measurements I decided to look for orbital effects, and that took me 3 weeks. And it's not easy to do because the Earth itself is moving through space and you have a huge effect to subtract first, which is the orbital motion of the Earth. But after 3 weeks I'd subtracted that off and decided there wasn't any orbital effect left. I mean, to begin with, it was a little bit worrying because there was a huge Doppler shift immediately detectable from the day-to-day measurements, but that was all due to the motion of the Earth. And when that was subtracted away there was nothing… nothing left; and that took me about 3 weeks to do.

While I was doing all that, Jocelyn was hunting through the records to see if there were any other sources that looked remotely similar to this because, if there's… there's one, there may be more. And she came up, actually, with another one quite quickly. And that, of course, doesn't remove the fact that it couldn't be intelligence because the moment you picked up intelligence, maybe there are others, too.

But anyway, the… by about January I had concluded in my own mind that it wasn't intelligence because there was no orbital effect detectable at all and Jocelyn had picked up another one which made it seem less likely, perhaps, and we decided to publish the result. But we had to have some theory to go with this and the only possibility… we knew it was a tiny object because only a tiny object can emit sharp pulses. I mean, if you turn the Sun on and off rapidly with a switch at intervals of a second, it wouldn't radiate sharp flashes because the radiation would come from different parts of the Sun at different times and you'd… you'd pick up a much time-broadened flash here. We knew that what was sending the signals couldn't be larger than a small planet like the Earth. And I went to the books and talked to people and saw that, well, there were these things called neutron stars which were only a few kilometres across, 10 or 20 kilometres, 10 miles or so in diameter, and if you flash those they could produce sharp signals. But there were also… there was just a faint possibility that a planet… that a star the size of the Earth could do it also. We all know that there are things called variable stars because the stars pulsate and there's a lot of theory to do with that. These pulsations are much too slow, but the smallest white dwarf star could possibly be a variable star that was almost fast enough. And when we finally published, I mean most of the… when I wrote that paper it was mostly describing exactly what we'd done and so on, rather than theory, but I… I ended up with some possibilities. And we suggested it might be radial pulsations of neutron stars or white dwarf stars. And it turned out in the end that the neutron star was the right answer, but not pulsation as I… as I had thought, but rotation. But we got pretty… pretty close to the… to the right answer and that was a very, very satisfying thing to have done.

Because we'd kept our mouths tightly shut over these months, I mean, I'm really talking about the… the first pulse came in on November 28th, I remember that day very well. I was lecturing at the time and Jocelyn phoned me to say what… what had happened. And it was in February that… that we actually published the paper and we kept our mouths shut. I thought that the local… our friends, as it were, around the group and in Cambridge should know about this before the rest of the world knew about it, but that seems a bit unfair since it's local work, and I gave a seminar in the… in the Cavendish describing all this. And that was early February and it was published… or was it late January? I've got the date somewhere in my diary, but 2 weeks later it was published in Nature. And then the balloon went up and everybody began to confirm it and it was very exciting. But, you know, that bit of work really was the… what I suppose you'd call the landmark in my career, which brought me more fame than I'd had from measuring the solar wind and all these other things. I mean, it was a discovery that opened up a new chapter of astrophysics and I feel immensely proud that we were able to actually do that here in Cambridge with quite modest funding. So that… that was a lovely period of my life.

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: Edward Bullard

Duration: 9 minutes, 34 seconds

Date story recorded: August 2008

Date story went live: 25 June 2009