13.8

The Quest to Find the True Age of the Universe and the Theory of Everything

John Gribbin

• Publisher: Icon Books
• Year: 2015
• Print ISBN: 9781848319189
• eBook ISBN: 9781848319196
• Topic: Physical Sciences
• Subtopic: Physics
• Index: Physics

The 20th century gave us two great theories of physics: the general theory of relativity, which describes the behaviour of things on a very large scale, including the entire Universe; and quantum theory, which describes the behaviour of things on a very small scale, the sub-atomic world. The refusal of the Universe to reveal an equation that combines these two great ideas has caused some people to doubt our whole understanding of physics.

In this landmark new book, popular science master John Gribbin tells the dramatic story of the quest that has led us to discover the true age of the Universe (13.8 billion years) and the stars (just a little bit younger). This discovery, Gribbin argues, is one of humankind's greatest achievements and shows us that physics is on the right track to finding the 'Theory of Everything'.

13.8 provides an eye-opening look at this cutting-edge area of modern cosmology and physics, and tells the compelling story of what modern science has achieved – and what it can still achieve.

PART ONE

How Do We Know the Ages of Stars?

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1 2.898

Prehistory: Spectra and the nature of stars

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The positivist philosopher Auguste Comte wrote in 1835 that: ‘there is no conceivable means by which we shall one day determine the chemical composition of the stars’. Unknown to him, the first step towards that understanding had, in fact, already been taken, and the process would be completed not long after his death in 1857.

Locating lines

That first step had been taken as early as 1802, when Comte was only four years old, by the English physician and scientist William Hyde Wollaston. In spite of being partially blind from 1800 onwards, Wollaston, one of the leading scientists of his day, made many contributions to optics. The 1802 discovery came when, following in the footsteps of Isaac Newton, he was studying the spectrum of sunlight passed through a slit to make a narrow beam and then through a glass prism to spread the beam into a rainbow pattern, the solar spectrum. He noticed that the colours were broken up by dark bands, and Wollaston counted two in the red part of the spectrum, three in the green and two in the blue-violet region. Wollaston mistakenly thought that these were simply gaps between the colours and did not pursue his investigation of the phenomenon. But his discovery triggered the interest of other researchers, most notably the German Joseph von Fraunhofer, who was able to produce much more detailed spectra in the second decade of the 19th century and eventually identified 574 separate lines. Today, all the dark lines in the solar spectrum, even more than he counted, are known as Fraunhofer lines. A short section of the spectrum has lines packed together, giving an appearance superficially rather like the lines of a barcode. But what causes them?

A partial answer came from the work of Robert Bunsen and Gustav Kirchhoff in Germany, in the 1850s and 1860s. Bunsen’s name is known to everyone who has studied chemistry thanks to the eponymous burner – although in fact this kind of burner was invented by Michael Faraday and the design improved by Bunsen’s assistant, Peter Desaga, who used the name of the more famous Bunsen, rather than his own, in marketing the improved device. But what matters here is not who invented the Bunsen burner, but what Bunsen and Kirchhoff did with it.

Early in the 1850s, the city of Heidelberg had been piped for the distribution of inflammable gas derived from coal to households and businesses – and to the scientific laboratories of the university. This was the inspiration for Bunsen’s work with the burner that now bears his name. The burner combines oxygen and the inflammable gas in a controlled way that produces a clear flame ideal for use in the ‘flame test’, by which substances are identified by the colour they give to a flame. Bunsen originally used coloured filters to calibrate these observations, but Kirchhoff pointed out that it would be possible to make a more detailed analysis using spectroscopy. Together, they built an apparatus which included a narrow slit for the light to pass through, a collimator to narrow the beam, a prism to spread the light out into a rainbow pattern and an eyepiece, like that of a microscope, to view the spectrum. Although Fraunhofer had used a prism and eyepiece combination in his work, this was the first time all these components had been assembled together in a single instrument – a spectroscope.

The Heidelberg team knew that when different substances were put in the clear flame of such a burner they burned with different colours. A trace of sodium, for example, makes the flame yellow, while copper colours the flame green/blue. So they analysed the light from these flames using spectroscopy. They found that each element, when hot, produced bright lines in the spectrum at precise wavelengths – in the yellow part of the spectrum for sodium, in the green/blue part of the spectrum for copper, and so on. (The yellow sodium lines were also known to Fraunhofer, who had used them to test the optical properties of glass, which had led to his investigation of the solar spectrum.) The German team soon realised that any hot object produces distinctive lines in the spectrum. One evening, from their laboratory in Heidelberg they were able to analyse the light from a major fire in Mannheim, some ten miles away, and identify lines produced by the presence of strontium and barium in the blaze.

A few days later, Bunsen and Kirchhoff were walking along the Neckar River, which flows through Heidelberg, and discussing what they had seen in the fire. According to legend, Bunsen remarked to Kirchhoff something along the lines of: ‘If we can determine the nature of substances burning in Mannheim, we should be able to do the same thing for the Sun. But people would say we have gone mad to dream of such a thing.’

Nevertheless, they turned their attention to the spectrum of the Sun and found that many of the dark lines identified by Fraunhofer were in the same part of the spectrum – at precisely the same wavelengths – as the bright lines produced by various elements when heated in the lab. The natural implication was that these elements are present in the outer layer of the Sun, but they are cooler than the layer below so as the light from the hot interior passes through this region they remove light from the spectrum at specific wavelengths instead of adding bright lines to it. Kirchhoff in particular developed this understanding of what was going on. Nobody at that time knew how the lines were produced – that would have to wait for the development of the quantum theory of atomic structure in the 20th century. But even without that understanding, in the 1860s it was now possible to find out what the Sun was made of – and, applying the same technique, what the stars were made of. Referring to their conversation by the river, Kirchhoff is said to have told his colleague: ‘Bunsen, I have gone mad.’ To which Bunsen replied: ‘So have I, Kirchhoff!’¹⁰ Kirchhoff’s discovery was presented to the Prussian Academy of Sciences in Berlin on 27 October 1859, now regarded as the date on which the discipline of astrophysics was born (although it was not given that name until 1890).

It had taken a mere three decades to prove Comte wrong. Well, not quite. In the remaining decades of the 19th century, astronomers were able to identify the presence of many elements also found on Earth in the spectrum of the Sun and, with less detail, in the stars. The natural assumption they made was that the overall composition of the Sun was rather like the overall composition of the Earth. But this turned out to be wrong. Stars are much simpler than that, and we now know that they (the Sun included) are mostly composed of hydrogen and helium, with just traces of the other elements. But at the beginning of the 1860s, nobody even knew that there was such a thing as helium. Its discovery marked the coming of age of solar – and stellar – spectroscopy.

Hunting helium

The leading light in the discovery of helium was the British astronomer Joseph Lockyer, who was, in the 1860s, an enthusiastic amateur observer of the Sun. (His job was as a clerk at the War Office in London.) He quickly picked up on the developments in spectroscopy being made by Bunsen and Kirchhoff, and applied those techniques to his solar studies. Using spectroscopy, he showed that the darkness of sunspots is caused by the presence of relatively cool gas near the surface of the Sun absorbing light from the hotter gas below it. His greatest achievement came on 20 October 1868, when he was able to analyse light from the outer layers of the Sun with a new spectroscopic instrument.

These observations followed hot on the heels of a spectroscopic study of the outer layers of the Sun during an eclipse visible from India on 18 August that year. The observations – the first eclipse studied since the suggestion by Kirchhoff that the Fraunhofer lines correspond to the presence of different chemical elements in the Sun – were made by the French astronomer Pierre Janssen. At that time, with the Moon blocking out the bright light from the surface of the Sun itself, he could detect lines in the spectrum of the material just above the surface. He noticed bright lines in the spectrum of this layer of the atmosphere of the Sun, known as the chromosphere, including a bright yellow line with a wavelength later measured as 587.49 nanometres, close to the lines associated with sodium. The spectral lines were so bright that Janssen realised that they could be observed even without an eclipse, and he made more observations before returning to Europe.

On 20 October of the same year, unaware of Janssen’s work, Lockyer used his new spectroscope to observe the solar atmosphere and found same yellow line. With impressive speed, both Janssen’s and Lockyer’s discoveries were presented to the French Academy of Sciences on 26 October 1868. But it was Lockyer who soon took things a step farther by claiming that the line must be associated with a previously unknown element, which he called helium, from the Greek word for the Sun, Helios.

This was a controversial claim. Many scientists preferred the idea that the line was associated with hydrogen subjected to extreme conditions of temperature and pressure. But in 1895 the physicist William Ramsay found that a previously unknown gas released by uranium produced a bright yellow line near to the sodium lines in the spectrum. He initially called this gas krypton, but when his colleague William Crookes pointed out that the line was in exactly the same place as the one found in the solar spectrum by Lockyer and Janssen he realised that it was, in fact, helium. (He later used the name krypton for another gas; nothing to do with Superman.) In effect, spectroscopy had predicted the discovery of helium on Earth, 27 years in advance.

By then, Lockyer had become a professional astronomer. In 1869 he was one of the founders of the scientific journal Nature, which he edited for the first 50 years of its existence, and in 1890 he was appointed Director of the Solar Physics Observatory in South Kensington, where he stayed until he retired in 1911. He was knighted in 1897, not least because of his discovery of helium.

As the discovery of helium shows, progress in astronomy proceeded on a broad front following the development of stellar spectroscopy, aided by new technological advances, not least photography, which, among other things, made it possible to keep a permanent record of stellar spectra that could be studied at leisure and compared with other spectra. But it makes sense here to jump forward to the 1920s and the next step towards an understanding of the composition of the stars, before doubling back to look at some of the other developments concerning their age.

Hunting hydrogen

That step was taken, appropriately, by someone born in 1900, with the new century. She also happened to be a woman, and it was still unusual for a woman to become a leading scientist in those days.

Cecilia Payne won a scholarship to Newnham College, Cambridge (the only way she could have afforded a university education) in 1919. She studied botany, physics and chemistry, but also attended a talk by Arthur Eddington about the eclipse expedition on which he had famously ‘proved Einstein right’ by measuring the way light from distant stars is bent by the Sun. This fired her interest in astronomy, and she visited the university’s observatory on an open night, asking the staff so many questions that Eddington took an interest and offered her the run of the observatory library, where she read about the latest developments in the astronomical journals.

After completing her studies (as a woman, she was allowed to complete a degree course but could not be awarded a degree; Cambridge did not award degrees to women until 1948) Payne looked for a way to pursue this interest. There was no chance of a career in research in England, where the only job opportunities for women scientists were in teaching, but through Eddington she met Harlow Shapley, from Harvard, on a visit to England. He offered her the chance to work for a PhD on a graduate fellowship (even though, technically, she was not a graduate), and in 1923 she left for the United States. Just two years later, she produced a brilliant thesis and became the first person to be awarded a PhD by Radcliffe College (also the first for work carried out at Harvard College Observatory). In it, she established that the Sun is mainly made of hydrogen. But, in a sign of the times, the idea was not fully accepted until two male astronomers independently came to the same conclusion.

Payne’s study of the solar spectrum made use of the then-recent discovery by the Indian physicist Meghnad Saha that part of the complication of the pattern of lines in a stellar spectrum (or the Sun’s Fraunhofer lines) was a result of different physical conditions in different parts of the atmosphere of a star. By the 1920s, physicists knew (as, of course, Bunsen and Kirchhoff had not) that atoms are composed of a tiny central nucleus, with one or more electrons at a distance from the nucleus. Dark lines in a spectrum are produced when an electron absorbs a specific wavelength of light, moving to a higher energy level within the atom, and bright lines are produced when an electron drops down from one energy level to another and emits radiation (in the form, we would now say, of a photon). An atom that has lost one or more of its electrons is called an ion, and the spectra of ions are correspondingly different (in a way that can be calculated) from those of the ‘parent’ atoms. Payne measured the absorption lines in stellar spectra and showed how the temperature (in particular) and pressure in the atmosphere of a star affects the ionisation of the atoms there. This makes for a more complicated pattern of lines than if all the atoms were in their un-ionised state.^a The spectra of stars differ from one another not because they are made of different things, but because of different amounts of ionisation in their atmospheres.

Payne’s great achievement was to unravel this complicated pattern of hundreds of Fraunhofer lines and work out what proportion of different elements in different stages of ionisation had to be present to account for the observations. Some idea of the difficulty of her task can be gleaned from the fact that her thesis was later described by the astronomer Otto Struve as ‘the most brilliant PhD thesis ever written in astronomy’. She worked out the proportions of eighteen elements in the Sun and stars, discovering that they all had nearly the same composition. But the big surprise was that according to her analysis the Sun and stars are made almost entirely of hydrogen and helium. If she was correct, everything else put together made up only 2 per cent of the composition of our nearest star, and of all stars. Most of the matter in the Universe was in the form of the two lightest elements, hydrogen and helium. This was almost literally unbelievable in 1925. Payne believed her results were correct, but when Shapley sent a draft of her thesis to Henry Norris Russell at Princeton for a second opinion, he replied that the result was ‘clearly impossible’. On Shapley’s advice, she added a sentence to the thesis saying that: ‘the enormous abundance derived for these elements [hydrogen and helium] in the stellar atmospheres is almost certainly not real’. But with the thesis accepted and her doctorate awarded, she wrote a book, Stellar Atmospheres, which began to persuade astronomers that the results were almost certainly real.

The change of mind was aided by the independent confirmation of Payne’s results by other astrophysicists. In 1928, the German astronomer Albrecht Unsöld carried out a detailed spectroscopic analysis of the light from the Sun; he found that the strength of the hydrogen lines implied that there are roughly a million hydrogen atoms in the Sun for every atom of anything else. A year later, the Irish astronomer William McCrea confirmed these results using a different spectroscopic technique.^b What this shows, more than anything, is that although Cecilia Payne was a brilliant researcher who got there first, this was a discovery whose time had come; given the technology of the 1920s it was inevitable that the discovery would be made sooner rather than later. In 1929, having carried out a similar analysis using a different technique, Russell himself published a paper confirming these results and giving due credit to Payne’s priority. Unfortunately, because of Russell’s established position in the astronomical community, for some time he was often cited as the discoverer by people who should have known better (or at least, should have read his paper properly).

Payne went on to a distinguished career in astronomy; in 1934 she married the Russian-born astrophysicist Sergei Gaposchkin and became known as Cecilia Payne-Gaposchkin. She remained at Harvard throughout her career, in spite of the low status and low pay she received as a woman. For many years, her official title was ‘technical assistant’, even though she carried out all the research and teaching duties expected of a professor. It was not until 1956 that she was promoted to become a full professor – the first female professor at Harvard. But, like most scientists, she was not primarily motivated by status or salary. In 1976, three years before her death, she was awarded the prestigious Henry Norris Russell Prize by the American Astronomical Society. No doubt she appreciated the irony. In her acceptance lecture, she said, clearly referring to her early work on stellar spectra: ‘The reward of the young scientist is the emotional thrill of being the first person in the history of the world to see something or to understand something.’ Even if someone else tells you it is ‘clearly impossible’.

Even at the end of the 1920s, however, astrophysicists had yet to grasp the full significance of the discovery that the Sun’s atmosphere is overwhelmingly rich in hydrogen. It would be nearly two decades before they would appreciate that even the interior of a star like the Sun is largely made of hydrogen (and some helium, but very little in the way of heavier elements). The longevity of this misconception was partly the result of an unfortunate coincidence, discussed later, which stemmed from the developing understanding of how hot stars are.

The heat of the Sun

Two temperatures are particularly important in our understanding of the nature of stars. The first is the temperature at the surface of the Sun; the second is the temperature at the heart of the Sun. They can be put in perspective with some basi...

Table of contents

• Copyright Page
• Contents
• About the Author
• Acknowledgements
• List of Illustrations
• Introduction: The Most Important Fact
• Part Zero: Prologue
• Part One: How Do We Know the Ages of Stars?
• Black White Plates
• Part Two: How Do We Know the Age of the Universe?
• Color Plates
• Glossary
• Sources and Further Reading
• End Notes
• Index
• Backinner
• Back Cover