Jocelyn Bell and the Lighthouses of the Sky

By Maddie Geddes-Barton

In 1967 Bell was working on her PhD in astronomical physics: the study of outer space. The specific aim of her research was to test a new radio telescope designed mainly by her supervisor Anthony Hewish.

The Cavendish team of astronomers working on the project were specifically interested in searching for quasars. Quasars are compact sources of radio waves, found a long way from the earth in outer space. The group hoped to detect them by observing the ‘twinkling’ of the signals these quasars produce. Radio waves from radio sources detected on the earth ‘twinkle’ in a similar manner to light waves from stars. We see stars twinkle because light waves interfere with our atmosphere. Likewise, the radio waves emitted by quasars will twinkle as they interfere with the solar winds surrounding our sun. This twinkling phenomenon is known as scintillation and is more pronounced in compact radio wave sources like quasars. The team hoped to use the new telescope to detect these highly scintillating sources and, hopefully, successfully identify them as quasars.

When the telescope was used in July, Jocelyn Bell was responsible for operating the telescope and analysing the data. Two months into the experiment she became aware of a ‘bit of scruff’ on her records which did not appear to come from a scintillating source or from man-made interference with her equipment. Checking back through her records she noticed that this unexpected signal had occurred before. She decided to investigate the anomaly further and took a series of faster chart recordings, focusing on the specific area of sky. For two weeks she was unable to detect anything. Finally at the end of November the ‘bit of scruff’ appeared again on her fast recording. She calculated that the unidentified source of radio-waves was pulsing with extreme regularity at a very high rate, each pulse 1.3 seconds apart. At the time it was assumed that the only emitters of radio pulses were star-like objects. The rate at which her signal was pulsing was much too high to have come from an object as large as a star. So, when Bell took her findings to Hewish he dismissed them as man-made interference. There were no known objects in the space that exhibited this sort of behaviour. However, upon checking Bell’s records Hewish found he was forced to agree that the signal did indeed come from outer space.

After further investigations had been made by the Cavendish team, it transpired that the duration of the pulse was so short (1.6 milliseconds) that it must have been emitted from a planet sized object. Was it possible that this signal was in fact little green men in a far off galaxy trying to get in touch? The Cavendish team were stumped as to any other explanation. Bell returned to her data analysis and discovered a second ‘bit of scruff’ on her records. It came from an entirely different part of the sky. It seemed a little ridiculous that two different alien civilisations were both signalling to us. In Bell’s words, ′it was very unlikely that two lots of little green men would both choose the same, improbable frequency, and the same time, to try signaling to the same planet Earth.′ The only other explanation available to Hewish and Bell was that a new natural phenomenon had been discovered.

Bell was given the green light by Hewish to continue investigating these strange pulsing signals and by January she had discovered four of them. They wrote a paper on their strange new discovery which was published that year in the scientific journal, Nature. The discovery of this inexplicable phenomena caused a great stir within the scientific community and theorists began work on the search for an explanation. Hewish himself proposed that the signal could be due to pulsed emissions from a neutron star, which is what remains of a star at the end of its life. Thomas Gold, an American physicist, went on to provide a more satisfactory explanation later that year, now widely accepted as scientific fact. He suggested that the pulses did indeed come from neutron stars. However, Gold claimed that instead of the regular pulsing being a natural result of the manner in which the radio waves are emitted from the star, the source pulsed because it was rotating very fast. Imagine a lighthouse far out at sea. Although the beam shines all the time it appears, from a distance, to flash not rotate.

Bell’s discovery provided evidence of these (at the time) largely theoretical ideas about what happens to stars at the end of their lives. It also revealed that these neutron stars display some almost unthinkable behaviour. Our sun rotates once every 27 days. These pulsars (because they are so small and dense) rotate about once a second. Pulsars have since been discovered which rotate once a millisecond. Imagine something the size of a small planet rotating once a millisecond, it’s almost inconceivable. Pulsars have turned out to be some of the most exotic objects in our galaxy. They have been used to confirm the existence of gravitational radiation as predicted in Einstein’s general relativity as well as to study the solar corona (the radiation we see in a solar eclipse). They are also some of the most accurate time keepers known to man and are currently being exploited by international timekeepers to improve the definition of terrestrial time (the time standard on the surface of the earth).

In the light of the ground-breaking nature of the discovery Antony Hewish and Martin Ryle (another member of the Cavendish team) were awarded the Nobel prize for physics in 1974. That Jocelyn Bell was overlooked, despite her key role, remains one of the most outrageous decisions in the history of the prize. Some people have put this oversight down to institutionalized sexism within the scientific establishment and especially within the world of physics. In 1999 Bell was finally awarded a CBE for her services to astronomy. The Vela pulsar and its surrounding pulsar wind nebular Composite Optical/X-ray image of the Crab Nebula, showing synchrotron emission in the surrounding pulsar wind nebula, powered by injection of magnetic fields and particles from the central pulsar.

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2 Responses to Jocelyn Bell and the Lighthouses of the Sky

  1. P.Hellings says:

    First sentence of this article contains a BIG error: it is not “astrological” but “astronomical” !
    Astronomy is a science, astrology is something you may believe or not, but it has not an scientific foundation. I have never seen a university where you can obtain a master degree or a PhD in asrtology.
    Sincerely
    prof.dr. Paul Hellings
    Group T univ. college, Leuven, belgium

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