by Marcus Chown
16 The First Three Minutes by Steven Weinberg (Basic Books, New York, 1993).
17 Ibid.
* Dicke believed in an oscillating universe, in which the cosmos repeatedly expanded and contracted throughout eternity like a giant beating heart. If each cycle were to begin like the previous one, it would be necessary to destroy the elements built up in the previous cycle, probably inside stars. Dicke realised that extreme heat would do the trick, by slamming together nuclei so violently that they disintegrated into hydrogen. Thus he hit on the idea of the universe going through a hot dense phase, with heat radiation left over, for exactly the opposite reason to Gamow. This is the way real science is done; both men were right for the wrong reasons, and each wrong reason was different from the other. After all, we do not appear to live in an oscillating universe (as Dicke thought) and most of the elements were not forged in a hot Big Bang (as Gamow thought).
7
The holes in the sky
The black holes of nature are the most perfect macroscopic objects there are in the universe: the only elements in their construction are our concepts of space and time.
SUBRAHMANYAN CHANDRASEKHAR
Black holes are where God divided by zero.
STEVEN WRIGHT
Herstmonceux, Sussex, Autumn 1971
There was something terribly wrong about the blue star. It seemed to be orbiting a body that was not there. Two astronomers sat at a desk in an octagonal turret room of a fifteenth-century English castle and surveyed their puzzling observations.
It was the autumn of 1971, and Paul Murdin and Louise Webster had been sharing an office at the Royal Greenwich Observatory (RGO) at Herstmonceux Castle ever since the summer, when Murdin had returned from a seven-year stint in America. It was a large room, entered through a wooden door so low that Webster had to duck. When the castle had been built, the room had possessed only vertical slits, but a larger window had been added later. Through it, the moat that surrounded the picturesque castle could be seen and, beyond that, pastures dotted with grazing geese. On the wall beside the window, previous occupants of the turret room had recorded the date each year when the swifts returned to the castle.
The observations the two astronomers were poring over were of a star called HDE 226868. HDE stood for ‘Henry Draper Extension’, a catalogue of stars compiled between 1925 and 1936 by a small army of female astronomers at Harvard University and named after and paid for by the widow of an American doctor and amateur astronomer. Murdin had first become aware of HDE 226868 while at the University of Rochester in New York state, where he had carried out his PhD research and held several short-term postdoctoral jobs.
The big problem in astronomy, Murdin had realised while a student, was identifying something worth studying. As British humorist Douglas Adams remarked, ‘Space is big. You just won’t believe how vastly, hugely, mind-bogglingly big it is. I mean, you may think it’s a long way down the road to the chemist’s, but that’s just peanuts to space.’1 There are about two trillion galaxies, and many of those contain hundreds of billions of stars. Finding an interesting object is tantamount to finding an interesting grain of sand among all the ordinary grains on all the beaches of the world.
What was needed, Murdin realised, was some observational signature that might flag the fact that something unusual was going on. Together with the other graduate students at Rochester, he had often debated what might constitute such a sign; the emission of radio waves was one possibility, but Murdin had a nose for an interesting idea and zeroed in on the emission of X-rays as the newest and most promising sign.
X-rays are a high-energy type of light given out by matter when it is heated to hundreds of thousands, or even millions, of degrees. The Earth’s atmosphere screens out X-rays from space, which is unfortunate for astronomers, though not for life on Earth. However, in the late 1950s and early 1960s, the Italian–American physicist Riccardo Giacconi built the first crude X-ray telescope, which he and his colleagues launched on ‘sounding rockets’ that climbed above most of the atmosphere, before falling back to Earth. Their brief glimpses of the X-ray universe revealed a number of cosmic sources, and among them was Cygnus X-1, discovered in 1962, one of the brightest objects in the sky.
Unfortunately, early X-ray telescopes were so crude that they could rarely pin down where a cosmic source was more precisely than ‘in a particular constellation’. In the case of Cygnus X-1, the constellation was Cygnus, the swan. No one knew what a star emitting X-rays would look like, so the idea was to look for something unusual. At first, the area of the sky to be searched was so large that the task was all but hopeless, but by 1970, technical improvements in X-ray telescopes had improved the situation. Murdin noticed that the ‘box’ showing the possible location of Cygnus X-1 contained one star that was far brighter than the rest: HDE 226868. However, there did not seem to be anything strange about it.
By the time Murdin returned to England – to yet another short-term post, this time at the RGO at Herstmonceux in East Sussex – NASA had launched the first satellite carrying an astronomical telescope that was sensitive to X-rays. ‘Uhuru’ located a host of celestial X-ray sources, and Murdin obtained a preprint of its catalogue. The box pinning down Cygnus X-1’s location had shrunk considerably and was now only about a third of the apparent diameter of the full Moon. And crucially, it still contained HDE 226868. ‘The star was still waving a flag saying, “Look at me! I’m actually quite interesting!”’ says Murdin.
There things might have stayed had it not been for a piece of good fortune: Murdin sharing an office with Webster. An Australian just a tad older than Murdin, who was not yet thirty, Webster was working with Richard Woolley, director of the RGO, on a project to study how the stars move through space in our galaxy, the Milky Way.2
Murdin did not think the blue star HDE 226868 itself was the source of the mysterious X-rays – he and Webster had studied the star’s light and it was too ordinary.3 However, it was suspected that X-rays might be generated by matter from one star swirling down onto a compact, superdense companion, rather like water swirling down a plughole. Internal friction in such an ‘accretion disc’ would make the matter so hot that it glowed with X-rays. The question in Murdin’s mind was therefore: Did HDE 226868 have a companion that might be the source of the X-rays?
If the blue star varied its speed over time – approaching the Earth before later receding from it – it would be a sure sign that it was circling another star. Murdin did not even have to do any work to find out whether the star’s speed was changing, since his office mate and her team were already measuring the speeds of stars; he simply wrote the celestial co-ordinates of HDE 226868 on a filing card and handed it to Webster.
Webster was using the giant hundred-inch Isaac Newton Telescope, which was ridiculously situated at Herstmonceux, a location plagued by cloud, mist and rain (in 1979, at a cost greater than that required to construct it in the first place, it would be moved to the Roque de los Muchachos Observatory on La Palma in the Canary Islands). But this, it turned out, was another piece of good fortune for Murdin. Whereas the world’s best observatories, which were often at high-altitude, dry sites, where the ‘seeing’ was excellent, concentrated on the faintest and most distant objects in the universe, at Herstmonceux there was no choice but to focus on brighter objects, which could be picked out even in a murky sky. HDE 226868 was a good candidate.
Webster was using a super-sensitive ‘spectrometer’ to obtain the spectra of the stars. A star’s spectrum is a record of how its brightness varies with the frequency, or colour, of its light. The frequency is like the pitch of a sound; it gets higher when a star is moving towards us and lower when it is moving away from us. This phenomenon, a direct analogue of a police siren sounding shriller as it approaches and deeper as recedes, is known as the ‘Doppler effect’.
Fortunately, nature has given each type of atom a set of characteristic frequencies at which it gives out light, which acts like a fingerprint. To det
ect the motion of a star along the line of sight, it is necessary only to identify one such spectral feature in the star’s spectrum and then see whether it is shifted from the frequency it would have in a laboratory back on Earth.
Webster and her team took six spectra of HDE 226868; disappointingly, only one of the six showed any sign of motion. It was not promising, and Murdin was beginning to lose interest in the blue star. But since he was not the person doing the legwork, and there was a lone spectrum that offered some sort of promise, he decided to leave the filing card where it was and the blue star stayed in Webster’s observing programme.
Webster duly took another set of spectra, but when they came back, Murdin was astonished to see that they revealed motion. It was immediately obvious that HDE 226868 was in orbit around another star. When Murdin did the calculations, cranking the handle of what would today be regarded as a prehistoric mechanical calculating machine, he found it was circling once every 5.6 days.
The orbit deduced by Murdin showed why Webster’s first spectra had shown so little sign of motion. The Doppler shift reveals only the component of a star’s motion towards and away from the Earth. But by a piece of bad luck, five of the first set of spectra had been taken at a time in the orbit of HDE 226868 when it was travelling across the line of sight and so barely moving towards or away from us.
With hindsight, Murdin could see that the sole spectrum in the first set that provided a positive result was an erroneous one, caused by some instrumental problem that would never be tracked down. But it was another piece of good luck for Murdin; if it had not been for the sixth spectrum and its anomalous indication of motion, he would not have left the filing card with Webster and would not have discovered the companion of HDE 226868.
As Murdin and Webster sat with the latest data on their desk, their focus was on the mass of the unseen companion star. Having now obtained multiple spectra of HDE 226868, the two astronomers knew it was an extremely young, extremely hot and extremely luminous star, pumping out about 400,000 times as much light as the Sun. In 1971, the average mass of such a ‘type-O blue supergiant’ was believed to be about twenty solar masses. Using this mass and the orbital period of 5.6 days, it was possible to determine the mass of the companion. Actually, that was not entirely true – because the orbit of HDE 226868 was projected onto the two-dimensional sky and its true orientation in three-dimensional space was unknown, it was possible to say only that the companion was bigger than a certain ‘minimum mass’.
Murdin and Webster divided up the calculating work between them. Murdin went off to consult reference books in the library fifty metres down the corridor, double-and triple-checking that he was using the correct formula for deducing the unknown mass. It was a complicated expression and he wanted to make sure he had remembered it correctly.
The meeting between the two astronomers had lasted only an hour, but they were certain of their conclusion. The companion was at least four times as massive as the Sun and probably six times as big.4
At the time, the only compact stellar objects that were known were ‘white dwarfs’ and ‘neutron stars’, the latter having been discovered four years earlier in the guise of ‘pulsars’ by Cambridge graduate student Jocelyn Bell. But theory constrained both types of body to weigh less than a couple of solar masses, which left only one type of theoretical object as a candidate for the invisible star. Webster and Murdin looked at each other. She was calm and unflappable, as always, while he could hardly contain his excitement. The object they were both thinking of was a monstrous nightmare entity, whose existence had been predicted more than half a century earlier by a man dying in a field hospital bed …
The Eastern Front, Winter 1916
The sound of the guns woke him. Karl Schwarzschild could feel the dull pounding deep in his bones as he screwed his eyes closed against the winter sunshine streaming through the thin curtains. For a moment his mood was black. With the pain and discomfort he had been suffering, it had been hard to get to sleep, but he could not permit himself to succumb to self-pity; that way lay oblivion. He must cling at all costs to the good things. He glanced across to where his calculations lay on his bedside cabinet, terrified for a split second that they had been nothing but a dream. But no – thank goodness – the scientific paper he had written was exactly where he had left it in the early hours of the morning. Everything was true. With a pen and paper, he had revealed something extraordinary about the universe: that somewhere out in space there might exist celestial bodies so monstrous they were the stuff of nightmares.
The morning routine was exhausting and time-consuming. Nurses in starched white uniforms entered his isolation room. Kindly and gently, they mopped at the ugly weeping blisters that now covered most of his body and sat him in a chair while they changed his bloody sheets, before leaving him with a tray of soft bread and warm milk (though he would have preferred a beer).5 As he chewed at the soggy bread – the only foodstuff that did not further inflame his blistered mouth – he listened to the thud of the guns and pondered the chain of events that had brought him to this hospital on the Eastern Front.
When war was declared in August 1914, there had been no need for Schwarzschild to volunteer. Not only was he a man of forty but, as director of the Berlin Observatory, he held one of the most prestigious jobs in German science. But anti-Semitism was on the rise, and he was a Jew. It was not something he advertised. In fact, in his will, which he had penned on the eve of joining up, he strongly advised his wife not to tell his children he was Jewish until they were at least fourteen or fifteen.6 But though he lived a secular life and did not attend synagogue, he nevertheless felt very strongly that it was necessary to stand up and be counted; if Jews were to push back the tide of anti-Semitism, they must demonstrate beyond any doubt that they were patriotic Germans. That was why, as ominous events unfolded across Europe during the summer of 1914, he had resolved that if it came to it, he would put his life on the line to defend the Fatherland. In his eighteen months in the Kaiser’s army, Schwarzschild had run a weather station in Belgium, calculated shell trajectories with an artillery battery in France and, finally, been posted to the Eastern Front. It was there he developed the mouth ulcers. At first he thought they were the result of exhaustion. The winter of 1915 had been cold, and it was stressful being apart from his wife and family. But the ulcers had worsened, and within a month, blood-filled blisters had erupted all over his body, forming large areas of painful, raw-looking sores that eventually formed scabs. The blisters seemed to come and go in waves, flaring up and dying down unpredictably.
When he had arrived at the field hospital, the doctors were completely puzzled. It was several days before they diagnosed pemphigus vulgaris. Nobody knew what triggered the rare condition in which the body attacks its own skin, but it was known to be more common among Jews, particularly Ashkenazis from Eastern Europe.7 He was told that there was no known cure.
As a scientist, Schwarzschild wanted to know everything. The doctors, perhaps to spare him, were evasive, but it was patently obvious to him that his condition was potentially life-threatening. The skin, after all, is the body’s largest organ. It is through the skin that we sweat, and if it is compromised, the body has no means of avoiding over-heating. Not only that but the skin provides a barrier against infection; if it is breached, the body is left open to attack by every alien microorganism.
The paper he had just finished was the second he had begun while with his artillery battery at the Eastern Front. He began to check through the calculations. Had he made a mistake? Did his results hold up? He had no one to share his thoughts with. These past weeks he had known what it felt like for Newton, ‘voyaging through strange seas of thought, alone’.8 So fresh was the new theory of gravity he was using that he knew he was one of the first people, if not the very first, to understand and master it. Apart, of course, from its genius creator.
*
Albert Einstein had crossed paths with Schwarzschild on only a handful of occasions,
and they had exchanged little more than pleasantries. The reason was that the Berlin Observatory was in Potsdam, just outside the city, while Einstein’s workplace, the Kaiser Wilhelm Institute for Physics, was in the suburb of Dahlem, nearer the centre. Despite this minimal contact, however, Schwarzschild had followed Einstein’s decade-long struggle to find a theory of gravity that was compatible with his special theory of relativity with ‘burning interest’.9
The special theory of relativity contradicts Newton’s theory of gravity in several ways. Whereas Einstein recognised that nothing can travel faster than light – the cosmic speed limit – Newton assumed that the gravity of a body like the Sun is felt everywhere instantaneously, which is equivalent to saying that gravitational influence propagates at infinite speed. And whereas Einstein recognised that all forms of energy are sources of gravity because all forms of energy have an effective mass, Newton assumed that mass alone is a source of gravity.*
The fact that light energy has an effective mass has an observable consequence, which Einstein realised before he achieved his goal of finding a theory of gravity that was compatible with relativity. The path of starlight that passes close to the Sun on its journey to the Earth should be bent by solar gravity, and when the First World War broke out in 1914, Schwarzschild’s colleague Erwin Freundlich had been in the Crimea with two companions, planning to observe light bending during the total solar eclipse of 21 August.10 Unfortunately, they were thrown into prison by the Russians as enemy aliens and had limped back to Berlin in late September, in one of the first prisoner exchanges of the war.
