IN A way, it was the second signal that was the big one. The first signal I saw could have been a mistake. The second one meant this was something real. It took a while to realise what we had found: the very first pulsars, a new type of star. We’re still working out the true significance of the discovery today.
It was 1967, and we were looking for quasars using a radio telescope designed by Tony Hewish, my supervisor at the University of Cambridge. Back then, we knew only that quasars were very distant objects, with radio signals that grew strong and weak in an irregular way. But this new signal was strong, not weak, and came in absolutely regular short bursts.
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It didn’t look like interference, either, although that was often a problem for us. Our telescope was a tangle of 2048 radio antennas covering about 4 acres just outside the city (see picture, below). You pick up a lot of interference with such a vast collecting area. Once somebody mistakenly allocated our observation frequency to the local police.
The first unexpected signal was jammed into a quarter-inch of the chart recorder – just a pen moving mechanically over paper – which I’d set to run slowly for the longer quasar signals. So I ran the paper faster at the time of day the signal was appearing, to spread it out, a bit like a photographic enlargement. But nothing came. The signal had disappeared.
One of the first questions my colleagues asked was whether I had wired the telescope up wrong. I was used to that. For one thing, I was a junior doctoral student. For another, I was a woman. It had been worse in Glasgow, where I’d done my undergraduate degree. There, whenever a woman entered the lecture theatre, all the guys whistled, stamped, banged the desks and catcalled. Cambridge was more genteel, but also more snooty. I felt like an imposter there, as a girl from the provinces, from Northern Ireland. I was convinced that someone would find me out and then throw me out. I worked as hard as I could, so I’d have a clear conscience when that happened.
After a month or so, the signal reappeared. I immediately phoned Tony. If it was a signal, he said, it must be of human origin because it was so regular, pulsing once every 1.3 seconds, like a metronome beat. But I knew it couldn’t be. Stars rise and set 4 minutes earlier each day as Earth orbits the sun. It had been early August when I first observed the signal. Now it was November, and the signal had kept pace with the stars. If it was something artificial, like radio interference from someone driving around in a car with a badly suppressed alternator, they would have had to religiously get 4 minutes earlier every day.
It was an anxious moment when Tony came out to the observatory the next day to look over my shoulder, but, sure enough, the signal came. That was when we had to start thinking about what on earth – or off it – it might be. I called it LGM-1, for “little green men”, as a joke. But if it was a communication from an alien intelligence, they were using a bloody stupid technique. For one thing, the signal was amplitude-modulated. There are many ways that natural phenomena can modulate a signal’s amplitude. If you want to signal across light years of space, you wouldn’t use AM, you’d use FM – it makes for a more obviously artificial signal.
We managed to estimate the source’s distance. It was about 200 light years away: within our galaxy, but way beyond the distance our TV and radio signals had travelled into space since they’d started a couple of decades earlier. It really would have been a curious set of little green men signalling to our inconspicuous solar system.
Aliens begone
That was when we found a different signal and, a few weeks later, a third and a fourth, each with its own periodicity. That demolished the little-green-men hypothesis, unless lots of aliens were signalling to us from opposite sides of the universe. Instead, it must be some new kind of star. We didn’t know that when we published our – in February 1968. Of course, the media only latched on to one line in the paper saying we had briefly considered the signals might have originated on an alien planet.
I published under the name S. J. Bell, and at first the press didn’t realise I was a woman, let alone a young one. When they found out, I suddenly had reporters on the phone asking if I was brunette or blonde – no other colours were allowed, apparently – and what my vital statistics were, which I didn’t know. I was asked questions like how tall I was, and was that taller than Princess Margaret or not quite so tall? And then photographers were asking me if I could please undo the top buttons of my blouse. I have a sharp tongue and I would have loved to use it, but I wasn’t in a position to do so. The lab needed the publicity and I needed good references for my next job.
It was a similar story in 1974 when for the pulsar discovery and I wasn’t. I said at the time it was only right, because he was my supervisor, but I didn’t entirely believe it. I don’t think the snub was because I was a woman. It was because I had been a student. In those days, students just weren’t recognised. That’s changed for the better since.
“My colleagues asked if I had wired up the telescope wrong. I was used to that”
I got married soon after the pulsar discovery and moved away from radio astronomy, following my husband’s relocations for his job. I’ve had a varied career since: I’ve done astronomy in most bits of the spectrum, and been a lecturer, a researcher, a tutor and a manager. But I still feel a bit proprietary about pulsars, so I’ve kept a friendly eye on them.
We now know of 2500 or so pulsars dotted around our galaxy. Like any family, they are a diverse bunch. But they are all very compact, very massive, spinning neutron stars. As they spin, they swing a beam of radio waves around that regularly sweeps across Earth, a bit like a lighthouse beam. The signals are so regular that pulsars make very accurate clocks, allowing us to test Einstein’s theory of general relativity and how massive objects make gravity by warping the space-time around them.
So far, Einstein’s theory has scrubbed up really well, but we keep at it because gravity doesn’t fit in with the other forces of nature. It stands out alone and that’s a little bit suspicious. We’ve also only tested relativity in environments where gravity is relatively weak, like our solar system. Now we’ve found binary systems in which pulsars orbit other stars, so we can use these to test what happens when the gravitational effect is particularly strong.
We are also working to use disturbances in pulsar signals to detect gravitational waves affecting space-time. The LIGO experiment has detected gravitational waves sent rippling out when two stellar-mass black holes merge, but timing pulsar signals can allow us to see different things going on – what happens when two supermassive black holes at the centre of galaxies merge, for example.
These are all things I never could have imagined when I saw that first signal. I’m just glad I noticed it – and was persistent enough to believe it was real.
Jocelyn Bell Burnell was talking to Richard Webb
This article appeared in print under the headline “I put my finger on the pulsar”
