Dame Jocelyn and the pulsars
August 17, 2007
“University is for the dissemination of knowledge,” said the Dean of Science at McGill on Tuesday evening. “We don’t keep it in a box.” Therefore members of the general public were invited to a lecture on Tuesday evening by Jocelyn Bell Burnell, former president of the Royal Astronomical Society and a Dame of the British Empire since last June. I was there because our son George is one of the pulsar researchers at this week’s conference and because I was interested in the topic in any case.
What is a pulsar? It’s a distant, dying, neutron star, the size of Montreal Island, but a thousand million million million million tonnes massive, spinning (for example) at eleven revolutions per second, and if you pick up that frequency on a radio telescope it sounds like an old tractor. However, a signal from the fastest pulsar yet discovered has just been observed by someone in the audience (a student at McGill), and that one emits a more definite note: a top E.
Forty years ago this month, Miss Bell was mapping the sky for quasars, while studying at Cambridge University under the direction of Antony Hewish (who incidentally was also in the audience; she referred to him as “Tony”). The students had spent two years constructing their own radio telescope from posts, wires and cables strung out over a field the size of 57 tennis courts. They only had one computer that was used for their interferometer and they had 30 metres of paper print-out to analyse every day. On August 6th, 1967, she noticed a peculiarity along the line drawn which was first assumed to be caused by local interference from a pirate radio station or a faulty piece of equipment. Only after eliminating these possibilities did the scientists dare to conclude that this signal was coming from outer space. Because “it makes one feel better to give something a name”, they jokingly called this source the LGM, Little Green Men, but the lack of a Doppler effect in the signal (which Dame Jocelyn dramatically demonstrated in the lecture hall by swinging a beeping kitchen timer round her head on a long piece of string) led the team to conclude that the pulsations detected must come from a rotating object with a very small diameter and a very big mass, some 200 light years away.
“The physics of these objects is extreme,” she told us. Pulsars are formed from the ultra compression of a massive star with an iron core, like the stars in the Pleiades cluster, which has exploded in a supernova (the Crab Nebula is the after effect of such an explosion) leaving a spinning remnant so dense that it is the equivalent of the six billion people on earth all crammed into a space the size of a thimble. This object has an iron crust and “God knows what in the centre!” as Dame Jocelyn put it. The work required to climb 1cm on such a body is approximately equivalent to the work required to climb Everest, on Earth. The atmosphere on a neutron star is only 2cm thick and its gravity bends light to an angle of 30° over its horizon. A clock (if you could make one!) would be forced to tick twice as slowly here as on Earth, and if you were so foolhardy as to approach the star-remnant you would suffer “spaghettification” of your body which wouldn’t do you any good at all.
How do they know such things? I wonder. They are still speculating about the shape of a pulsar’s magnetic field. It is assumed by some that this would be the same as for Earth, but “there was a big argument about this in our meeting today,” said George.
The weakness of the signal received on earth from a pulsar (about 1800 of which have been discovered so far) was demonstrated by the distribution of thin strips of paper to each member of the audience, on which was written the message:
In picking up this piece of paper, you have used a million times more energy than a radio telescope receives from all known pulsars per year.
Apparently pulsars have planets and are extraordinarily symmetrical, “round to 1mm in 69000 km … and an even rounder one has just been discovered.”
If my notes make no sense you should download the webcast of the lecture, click on the slide show, and listen to what the lecturer really said. She was an excellent speaker who finished her presentation by quoting the poem Planetarium by the feminist American poet, Adrienne Rich.
Dame Jocelyn Bell-Burnell is a Quaker from Northern Ireland who was educated at the Mount School in York and is now the president of its old scholars’ association. In our basement we happen to have a copy of the Swarthmore Lecture she gave at Aberdeen University in 1989: Broken for Life, which is an exploration of the “role of the person who is not whole,” and an attempt to answer the question, “can the wounded person offer something to the rest of us? Can brokenness be life-giving?” Her conclusion has nothing to do with astrophysics:
…the more we have faced pain the more whole we are, and the more capable we are of suffering and of loving … Through our vulnerability we become channels of comfort and consolation. By our faithfulness in risking ourselves we open barriers and enable God’s grace to flow in the world.