The field at the centre of the universe: Cambridge's outdoor pulsar pusher
Radio astronomy, sheds and high-explosive ordnance
Posted in Geek's Guide, 12th February 2016 14:15 GMT
Geek's Guide to Britain A field full of bits of old wire and an abandoned garden shed: it doesn't look like the place where Nobel prize-wining research was conducted, pushing the frontier of radio astronomy.
But it was. This is the Mullard Radio Astronomy Observatory, at Lords Bridge – site of a disused railway station just outside Cambridge – which is part of the University Of Cambridge Cavendish laboratory Astrophysics group.
It was at Lords Bridge that physicists discovered pulsars, a breakthrough in our understanding of the Universe.
Pulsars, or pulsating radio stars, are a type of neutron star. Neutron stars are very dense, small (typically about 20km in diameter), smooshed-together remnants of massive stars whose centres have collapsed after a supernova. They are subject to enormous gravitational pressure, so much so that their protons and electrons have combined to form neutrons, hence the name. A neutron star is considered a pulsar when it rotates at enormous speeds, emitting regular bursts of radio waves, X-rays and gamma rays from above its magnetic poles, producing streams of light which appear to "pulse" because of the rotation. Around 1,000 pulsars are now known to exist.
Pulsars have since been employed by physicists trying to prove the existence of gravitational waves, as theorized by Albert Einstein. Physicists looked for proof of gravitational waves by judging the interference they might cause with the pulses emitted by millisecond pulsars.
In the end, gravitational waves were proved to exist using other means.
Pulsars have been used in the hunt for extrasolar planets – planets orbiting stars other than our Sun. Scientists in 1992 found three planets circling a pulsar called PSR B1257+12. Evidence suggests such planets are are created by the debris blown off the surface of the dying star before it collapses.
And some believe that because of their fixed location and their regular blips, spacecraft could navigate using pulsars as a kind of compass or GPS. The European Space Agency (ESA) in 2012 commissioned Geek's Guide destination the National Physical Laboratory (NPL) and Leicester University to investigate the feasibility of using pulsars to navigate in deep space. Whether this is feasible is another matter.
But, here we are. With a hut. In a field. It’s about as far from the cathedral of science as somewhere like the Large Hadron Collider as its possible to get. It looks like a very old set of Doctor Who: open fields, pen recorders and the lingering fear of unexploded bombs. I’ll come back to that.
The hut where pulsars were discovered. Photo: Simon Rockman
I’m here as a guest of Cambridge Wireless’ Heritage SIG and my guide is Peter Doherty, who for the last 17 years has been building equipment at the site. Steeped in the history and with a passion as much for the history of endeavour as for the discovery of the unknown, Doherty is one of those people whose infectious enthusiasm comes from being a person who clearly enjoys his job.
And this must be a pre-requisite for working at Lords Bridge, which was a station on the Varsity line, the train service between Oxford and Cambridge shut down in 1967.
But there are some more ominous ghosts in this past. The whole reason the site became available to the University of Cambridge is that during the World War II it was a bomb dump, storing high explosives and mustard gas for distribution to the local British and American airfields. One of four secret Forward Filling Stations, it had stored 2,000 tones of explosives, including two 250-tonne containers of mustard gas. Many thousands of tonnes of gas were made at the site.
It was deemed too unsafe for general use, and so was given to the University.
After the WWII, the chemical weapons, which Britain officially never had, proved hard to destroy. In an accident in 1955, a tank of 20 tonnes of gas exploded when a worker on top was careless with a blowtorch. The accident report says there were no casualties, but a couple of rescuers were awarded medals, so it’s unlikely that it was quite as minor as you’d be led to believe from the reports of the time. Eventually the munitions were loaded into ships that were deliberately sunk at sea.
Since the war, the odd shell has been turned up and it’s unlikely that the site is now completely clear. This has been a boon for the local wildlife and for the radio astronomers of the University Of Cambridge.
The war, however, had a second, more positive affect: the introduction of radar, which helped advance radio astronomy.
The remains of the 4C radio telescope at Cambridge. Photo: Simon Rockman
Radar is credited with helping turn the Battle of Britain by giving the RAF time to scramble and intercept the Hitler’s Luftwaffe. But for all that glory, there did exist technical issues and work was needed to filter out unwanted, extra-terrestrial signals from radar reception.
The problems of interference were highlighted when radar failed to spot three German battleships during one incident. At the time this was blamed on German jamming but subsequently turned out to be Sun spot activity. Research on which frequencies and where signals came from became very important.
Enter Nobel Prize winner Martin Ryle, of the Radio Astronomy group of the Cavendish Laboratory of the University of Cambridge – Cavendish was opened in 1874 under the direction of another Geek’s Guide to Britain notable, James Clerk Maxwell.
After a brief stint at Cavendish’s ionospheric research group, Ryle worked on the development of radar during WWII for the RAF.
After the war, at Cambridge, Ryle investigated the effect of radio emissions from the Sun that were interfering with radar equipment.
Ryle's WWII raid
Funds were limited, but Ryle managed to obtain a large amount of radio equipment from the Royal Aircraft Establishment at Farnborough.
Officially this was bought as army surplus, but Doherty gives the impression that Ryle just rocked up at Farnborough with five trucks and helped himself with no one minding much. Among the equipment was some captured German radar equipment, including several 3m and 7.5m steerable Wurzburg radio antennas.
Ryle became head of a new radio astronomy group at Cavendish, breaking new ground in electronics and building a team who’d go on to lead radio astronomy, including an individual named Antony Hewish.
The group was driven by the desire to build a new generation survey telescope but squeezed by space. A new site was needed. The eventual location would be a disused wartime Air Ministry bomb storage facility outside Cambridge: Lords Bridge. The observatory was Christened the Mullard Radio Astronomy Observatory and opened in 1957.
The basic principle behind all of the antennas at Mullard is Earth rotation. That is that the sky is scanned not by moving the dish, but by having the antennas widely spaced to measure interference patterns and the movement of the Earth being harnessed to scan. A scan can take 12 hours.
The first telescope to be built at Lords Bridge was the 4C, which was the first large aperture synthesis telescope and was made up of one fixed element and one moveable element. Its total length was 450 metres, and its width 20 metres. It used 64km of reflecting wire. This was built in 1957, and became operational in 1958. Running at 178MHz, it completed its survey of the sky in 1965 with results published in 1965 and 1966. The 4C found nearly five thousand radio sources and was fundamental in establishing many characteristics of galaxies.
When it was operational, the old Varsity line was still running, a fact that affected the placement of some parts of the antenna.
The Cambridge Wireless heritage SIG troop into the control room, photo: Simon Rockman
The site is now home to the One Mile and Half Mile telescopes. Built in 1964, the One Mile was the first “super-synthesis” instrument. It had three 120-tonne, 18 metre dishes: two were fixed while the third could be moved along an 800-metre track to take up station at 60 different points. While the track is straight, one end is raised by 5cm to allow for the curvature of the Earth. The telescope ran at 408MHz and 1.4GHz.
The Half Mile antenna was added in 1968, mainly to make observations of the distribution of hydrogen clouds in nearby galaxies. In 1972 two more nine-metre dishes were added, one fixed and one on the track. To get a full scan with the antennas in different oppositions along the track took 30 days.
Running mainly at 1.4GHz, the Half Mile telescope was fantastically productive during its 15-year lifetime, and led to 50 published papers and 20 PhD theses. It was the first telescope to produce good maps of M31 in the Andromeda constellation and M33 in Triangulum.
Yes, yes, we were getting to the pulsar part
It was 20 years after Mullard opened that that Nobel-prize winning work on pulsars took place. That rather beaten and weathered-looking hut that I mentioned earlier is connected to a field of dipole antennas and was used by Jocelyn Bell and Hewish as their base for hunting quasars. The antennas look more like a vineyard than a scientific research establishment.
What they accidentally discovered was pulsars.
Most radio sources in space are large objects, such as whole galaxies, but Bell and Hewish were hunting for a smaller source -"quasi-stellar" objects, or quasars. They hoped that the twinkling radio sources could point them in the right direction. Two months into her observations, Bell spotted of a bit of “scruff" on the records. It didn't look like a charged particle cloud effect or man-made interference. Bell looked back and found that it had occurred before - and had always come from the same area of the sky.
Bell spotted the "scruff" on
a pen recorder trace". Photo: Simon Rockman
What was it? It was a sharp burst of radio energy emitted on a wavelength of 3.7m at regular intervals. Bell stepped up her investigations and discovered that the pulses had an interval of 1.3 seconds, far too fast for something as large as a stars. Hewish was of the opinion that it was a man-made signal reflecting back but research showed no possible sources and another radio telescope could also pick up the signal, eliminating an equipment fault. The pulses were coming from well outside the Solar System. Of course the hope was that it was generated by intelligent life. A 16-millisecond duration meant that the source could be no larger than a small planet.
What Bell had found was the first-known recording of something that became called a pulsar and was eventually recorded as Cambridge Pulsar – CP - 1919
Hewish won the Nobel Prize for the discovery in 1974; Bell didn’t, getting her PhD instead. This lead it to be called the “No Bell prize” by some.
Many of Bell and Hewish’s dipoles are still in place but they will be removed as the local farmer takes over the fields. It's a shame that, given the significance of the field and the hut, no efforts are being made to preserve the site.
The computing systems at Lords Bridge pre-date magnetic storage, with paper tape being the preferred media. There was no actual computing done at the site; tapes were taken to the university for processing. The astronomer would configure the telescope by punching a tape with the required co-ordinates and then take it into the university to use EDSAC.
This would produce another paper tape. This was then taken back to Lords Bridge where the second tape would be used to control the telescope for its run. Signals were recorded on a third tape, which was taken back to the computer for analysis and combined with other results, which were then saved to magnetic tape.
The control room for the One Mile telescope feels like a Jon Pertwee era Dr Who set, with gas-discharge clocks plotters. Something notably absent from the radio astronomy side is anything resembling a PC. There are a few oscilloscopes and lots of paper tape punching and winding equipment but the whole place feels as though it’s in a time warp.
In addition to the radio telescopes, there is one optical telescope, the Cosmic Anisotropy (CAT) Telescope housed in a former munitions bunker. Like the radio telescopes, it relies on the planet's rotation to conduct scans, with a series of mirrors used to build up the image. This image is fed into the bunker, which was covered with earth during the 1980s to provide a cool, environment for the complicated optics.
When can we visit?
Alas, visiting the site is near-impossible for most – it’s not generally open to the public and I was a guest of the Cambridge Wireless’ Heritage SIG. However, the University does have an outreach programme and is keen to hear from other groups, schools and organisations that want to visit.
If you do happen to be in the area, or if simply want to bag some drive-by bragging rights, then you’ll find Cambridge is a wonderful part of the world. It’s not far from Junction 12 of the M11, which means a trip could easily be combined with a visit to the excellent Imperial War Museum at Duxford, which is just off Junction 10.
The One Mile control - like you've travelled in time... backwards. Photo: Simon Rockman
On my visit we ate at the Three Horseshoes in Harston, a classic English pub, serving classic English Thai food. It was excellent.
While there are lots of great places to stay around Cambridge, I’d recommend actually staying in Cambridge, and taking the opportunity to wander around the city. In particular, I’d recommend the Regent Hotel, and while there go for a coffee at Hot Numbers, where some of the brews look like science experiments. It’s a good place to visit to bring your thoughts back down to Earth after the observatory.
While there, you can ponder the Mullard Radio Astronomy Observatory. It is an incredible place responsible for a major development in our understanding of the Universe. This, and its awesome appearance. It's almost as though there is a full map of the universe and the whole 150 acres of the Mullard is a microscopic dot with a sign that reads: “You are here.” ®
By car: Leave the M11 at exit 12 for the A603 and head through Barton. The observatory's entrance is on the left-hand side of the road.
N/A - not open to the general public