Читать книгу Human Universe - Andrew Cohen - Страница 15

The history of science is littered with crunching moments of conflict, debates and disagreements that divided opinion in the most passionate of battles. The wonderful thing about science, however, is that the debates can be settled when facts become available. Science and ‘conservative common sense’ famously clashed in 1860 when Thomas Huxley and Samuel Wilberforce fulminated over the new theory of evolution published by Darwin seven months earlier. I imagine Wilberforce’s indignant reddening cheeks shaking with righteous outrage as he denied the repugnant possibility that his grandfather was a monkey. None of his relatives was a chimpanzee, by the way; we simply share a common ancestor with them around 6 or 7 million years ago. But the ‘unctuous, oleaginous and saponaceous’ bishop, as Disraeli once called him, was having none of it. This might be a little unfair to the great Victorian orator and bishop of the Church of England, but in the case of evolution he was firmly on the wrong side of reality. Few great leaps in knowledge occur without dividing opinion, and this is entirely appropriate. Extraordinary claims require extraordinary evidence, and the great scientific discoveries we are celebrating here are utterly extraordinary. The trick as an educated citizen of the twenty-first century is to realise that nature is far stranger and more wonderful than human imagination, and the only appropriate response to new discoveries is to enjoy one’s inevitable discomfort, take delight in being shown to be wrong and learn something as a result.

The world of astronomy had its moment of intellectual sumo in what has become known as the Great Debate. The year was 1920, and two eminent astronomers found themselves stuck on a train together travelling the 4000 kilometres from California to Washington to discuss the greatest cosmological question of the day. The younger of the two men, Harlow Shapley, we have already met. He had just published his data suggesting that the Milky Way galaxy was much larger than previously suspected. This, however, was where he believed the universe stopped; Shapley was convinced our galaxy was the beginning and end of the cosmos. His travelling companion thought otherwise. Heber Curtis had been studying a misty patch of light known as the Andromeda Nebula. He was convinced that this was not part of our galaxy, but a separate island universe of billions of other stars.

It is not known what they discussed on the train, but the debate itself took place at the Smithsonian Museum of Natural History throughout the day and night of 26 April. At stake was the scale of the universe itself, and both men knew that the question would ultimately be settled by evidence rather than debating skills. The human race had already been shunted from the centre of the universe by Copernicus, and now faced the possibility that the Milky Way galaxy itself was part of a multitude, stretching across millions of light years of space. The question wasn’t settled that evening, but the experienced Curtis, perceived as the underdog because of the magnitude of what he was suggesting, landed significant blows. Curtis observed that the Andromeda Nebula contains a number of novae – exploding stars that shine temporarily, but brightly, in the night sky – but he also noted that the novae in Andromeda appeared on average to be ten times fainter than any others. Curtis asserted that Andromeda’s novae appear dimmer simply because they are perhaps half a million light years further away than those in the Milky Way. Andromeda is therefore another galaxy, claimed Curtis, which strongly implied that the other so-called nebulae were other galaxies too. This was the very definition of an extraordinary claim, and the extraordinary evidence came only four years later.

In 1923 a photo of Andromeda, taken by a 33-year-old astronomer called Edwin Hubble, further fuelled the Great Debate. It’s only a photograph but, just like Anders’ Earthrise, it belongs to a rarefied group of images that have transformed our perspective. Aside from their scientific merit, such images assume great cultural significance because of the ideas they generate and the philosophical and ideological challenges they pose. They also carry with them, in the shadows, personal stories. Someone would have taken a photograph of Andromeda, someday, and discovered what Hubble did. But Hubble took this one, and his story therefore becomes inextricably intertwined with it. Some don’t like their history presented in this way, but science is richer when its stories include people as well as ideas; curiosity is, after all, a human virtue. Hubble may never have taken the photograph had he followed through on a promise to his father to practise law. Reading jurisprudence at Queen’s College, Oxford, as one of the first Rhodes Scholars, Hubble aimed to fulfil his father’s wishes, but John Hubble died before Edwin finished his degree. The death of his father encouraged Edwin to ditch law and revisit his childhood passion for astronomy. He left Oxford for the University of Chicago, joined the Yerkes Observatory and received his PhD in 1917 with a thesis entitled ‘Photographic Investigations of Faint Nebulae’. After brief service in the US Army at the end of World War One, Hubble obtained a position at the Mount Wilson Observatory. He found himself at the controls of the largest, most powerful telescope on the planet, and with the knowledge and good sense to point it at the most intriguing and controversial object in the night sky: Andromeda. Just like Curtis before him, Hubble could make out distinct features within the misty patch, but the newly commissioned 100-inch Hooker telescope allowed him to see much more detail. On 5 October 1923 he took a 45-minute exposure, found three unidentified specks that he assumed were new novae and marked them all with an ‘N’.

To confirm his findings Hubble needed to compare this plate with previous images of Andromeda taken at Mount Wilson. The following day he made the journey down to the basement archive where the observatory’s collection of images was catalogued and stored. To Hubble’s delight, two of the specks were indeed newly discovered novae – what we now know to be the bright nuclear flares of white dwarf stars as they accrete gas and dust from a nearby companion. But it was the third speck that he found most interesting when he compared it to previous images. As Hubble scanned back through the Mount Wilson catalogue he discovered that the star had been captured before; in some plates it appeared brighter, whereas in others it appeared dim or not present at all. Hubble immediately grasped the importance of his discovery. The third speck was a Cepheid variable, the type of star Henrietta Leavitt had studied two decades earlier. In one of the most famous corrections in scientific history, Hubble crossed out the letter ‘N’ and replaced it in red ink with the letters ‘VAR’ followed by a very understated exclamation mark.

Hubble had discovered a cosmic yardstick in Andromeda, and it was a trivial matter to calculate the distance. The new star varied with a period of 31.415 days, which, following Leavitt, implied its intrinsic brightness was 7000 times that of our Sun, and yet it appeared so dim in the night sky that it was invisible to all but the most powerful of telescopes. Hubble’s initial calculations revealed that the star was over 900,000 light years away from Earth, a staggering distance when the size of our own galaxy was estimated to be no more than 100,000 light years across. Hubble, with the help of Leavitt’s ruler, laid the Great Debate to rest. Andromeda, the distant patch of light in the night sky, is a galaxy; an island, according to current estimates, of a trillion suns. Current measurements put the giant spiral at a distance of 2.5 million light years from the Milky Way, one of around 54 galaxies gravitationally bound together to form our galactic neighbourhood known as The Local Group.