How your very existence is telling you that our Universe may not be the only one
‘As we look out into the Universe and identify the many accidents of physics and astronomy that have worked together to our benefit, it almost seems as if the Universe must have known that we were coming.’
Freeman Dyson
‘And I say to any man or woman,
Let your soul stand cool and composed before a million universes.’
Walt Whitman (‘Song of Myself’)
Look around you. The Earth is teeming with life. Nobody knows how it got started. But one thing is for sure. Life as we know it could not have begun without the element carbon. Atoms of carbon have a unique ability to join up with others of their own kind, creating a bewildering array of complex molecules. In our bodies, carbon-based ‘biomolecules’ perform myriad essential tasks, from metabolising the food we eat to responding to the light falling on the retina of our eye to encoding the information of inheritance in deoxyribonucleic acid, or DNA. We are carbon-based bipeds whose existence depends on carbon being common. After hydrogen, helium and oxygen, it is the fourth most abundant element in the Universe. The abundance of carbon actually tells us something rather interesting. It is telling us about a series of spectacularly unlikely coincidences in the properties of a handful of atomic nuclei. Not only are they responsible for your existence, but – even more than this – they strongly hint that our Universe is but one among an infinity of other universes floating like bubbles in an unimaginably huge ‘multiverse’.
It is an extraordinary conclusion to draw from the mere fact of our existence, but the logic turns out to be pretty inescapable. The first thing to realise is that the elements, including carbon, were not placed in the Universe on Day One by the Creator. Instead, the Universe started out with only the simplest of nuclear building blocks – protons and neutrons – and, subsequently, these have glued themselves together to form the nuclei of the 92 naturally occurring elements.
The evidence that the elements have been made – built up a brick at a time – is actually rather subtle. One of the most important clues comes from the abundances of different elements throughout the Universe. These can be estimated in many ways. One is by analysing the composition of rocks from the Earth’s crust and of meteorites from space. Such measurements were first carried out by the Swiss-Norwegian chemist Victor Goldschmidt in 1936. The abundance of elements can also be measured by examining the characteristic fingerprint they create in the light coming from stars, a technique used to effect by Cecilia Payne when she surprised everyone by discovering that the Sun is made overwhelmingly of the lightest elements, hydrogen and helium. It is interesting here to recall the words of the French philosopher Auguste Comte, who, in 1835, compiled a list of things he believed could never be achieved by science: ‘Never by any means shall we be able to study the chemical composition or mineralogical structure of the stars.’ Within 25 years, the German physicist Gustav Kirchoff showed that elements such as sodium, when heated in a flame, give out light at characteristic wavelengths and that such ‘spectral fingerprints’ can be used to identify different elements in the light from the Sun and stars. Comte was saved the ignominy of having to eat his words by having died before the discovery.
The analysis of the composition of the stars, Earth rocks and meteorites threw up a striking result. All across the cosmos, the elements appeared to be present in roughly the same relative proportions. As the American physicist Richard Feynman said: ‘The most remarkable discovery in all of astronomy is that the stars are made of atoms of the same kind as those on the Earth.’ Less striking, but equally important, is the pattern that is visible in the ‘universal’ abundances. In general, the heavier an element, the rarer it is in nature. In fact, so steep is the decline in the abundance of elements that uranium, element 92, is a billion times less common than element 11, sodium. It is easiest to see this on a piece of graph paper. If elements are plotted along the page, getting ever more massive from left to right, and their abundances plotted up the page, the result is a mountainside. From light elements on the left of the page, the mountainside plunges precipitously downwards to heavy elements like uranium on the far right.
Some elements, however, buck the trend of rapid decline in abundance with increasing mass. They stand out as significantly more common than their neighbours on the mountainside. For instance, there are hummocks at carbon, nitrogen and oxygen, and also around iron and its immediate neighbours. In addition, some elements are distinctly less common than their neighbours. For example, there are hollows on the mountainside at lithium, beryllium and boron. Why are some elements more common than expected and others less so? A strong clue can be found in a surprising place: Aston’s valley of nuclear stability.
Recall that, in Aston’s valley, nuclei with the lowest mass per nucleon – iron and nickel – are found at the bottom, and on the slopes rising on either side are nuclei with more and more mass per nucleon. Well, it turns out this simple picture does not tell the whole truth. As Aston refined his mass spectrograph and was able to measure the masses of nuclei ever more precisely, he found that the sides of the valley were not entirely smooth. There are little hillocks at locations where a nucleus has more mass per nucleon than its immediate neighbours, and there are little potholes at places where a nucleus has less mass per nucleon. The peculiar thing is that bumps on the abundance mountainside coincide exactly with potholes on Aston’s valley of nuclear stability, and hollows on the abundance mountainside coincide with hillocks on Aston’s valley of nuclear stability. The conclusion is unavoidable: there must be a connection. How common an element is must depend on the detailed properties of its atomic nucleus. It is a strong hint that nuclear processes are behind the creation of the elements – that the elements have been made.
Imagine that, high up on the slopes of a valley, someone lets loose a load of footballs. As they roll down the slope towards the valley bottom, they avoid the bumps on the slope and get trapped in the potholes. The correspondence between the cosmic abundances of the element and Aston’s curve shows that something like this must really have happened in nature. Nuclei must have been released high on the left-hand side of Aston’s valley of nuclear stability. They then ‘rolled’ down the slope towards the valley bottom, avoiding the bumps and lodging in the potholes. A nucleus high on the left-hand side of Aston’s valley of nuclear stability is a small, light nucleus. One that rolls down towards the valley bottom is therefore a light nucleus getting heavier and heavier as it accrues nuclear building blocks, one at a time. It is a light nucleus being built up into a heavier nucleus.
But if the elements were made – as all the evidence indicates – where were they made? The clue is in the temperature required for element building. Bigger, heavier nuclei have a bigger electrical charge than smaller, lighter nuclei. They therefore repel each other more fiercely, which means higher temperatures are required to smash them together hard enough to stick. The hottest places in the Universe appear to be stars like the Sun. Unfortunately, calculations by the English astronomer Arthur Eddington in 1925 showed that stars could not be the cosmic furnaces in which the elements were forged. According to him, the rotation of the Sun caused the material inside to circulate endlessly, and this endless circulation would keep the stuff of the Sun thoroughly mixed. So when hydrogen fused into helium to make sunlight, the helium ash would be mixed evenly throughout the star. The trouble was, this would continually dilute the hydrogen fuel of the Sun. As time passed, the Sun would gradually cool and go out. This was the opposite of what was required for an element-building furnace.
In the US, George Gamow was aware of Eddington’s calculations. They spurred him to begin looking about for another furnace that might be hot enough to forge the elements. He soon found one: the fireball of the Big Bang. In 1929, the American astronomer Edwin Hubble, working at Mount Wilson Observatory in southern California, had discovered that the galaxies – the building blocks of the Universe of which our own Milky Way is but one among billions – are flying apart from each other like pieces of cosmic shrapnel in the aftermath of a titanic explosion. We live in an expanding universe. And because it is expanding, an unavoidable conclusion is that it must have been smaller in the past. In fact, if the expansion is imagined running backwards like a movie in reverse, we come to a time when all of creation was compressed into an infinitesimally tiny volume. This was the moment of the Universe’s birth in the Big Bang, currently believed to have occurred 13.7 billion years ago.
Gamow picked up the idea of a Big Bang and ran with it. If the Universe had been smaller in the past, he reasoned, then it must also have been hotter (that old air-in-the-bicycle-pump effect again). The Big Bang must have been a ‘hot’ Big Bang. And if the Big Bang was hot, could it not have been the furnace in which the elements were forged out of some simple basic ingredient? The trouble was that time was in short supply. By the age of ten minutes, the Universe had expanded and cooled to such an extent that element-building processes were effectively choked off. It was actually a double whammy. By this time, the fireball of the Big Bang was no longer dense enough for nuclei to encounter each other frequently; and, when they did run into each other, they were moving too slowly to overcome their mutual repulsion. Gamow was not easily put off. Ten minutes simply had to be enough. ‘The elements were cooked in less time than it takes to cook a dish of duck and roast potatoes,’ he maintained.
Gamow’s optimism was misplaced, however. There was a more serious obstacle to element building than the limited time available. In nature there is no stable nucleus containing either five or eight basic building blocks. It means it is nigh on impossible to build any nuclei heavier than helium. After all, if a single nuclear building block – either a proton or a neutron – were to collide with and stick to a helium-4 nucleus, it would make a nucleus of mass 5. But no stable nucleus of this mass exists. And if two helium nuclei were to collide and stick, they would make a nucleus of mass 8. Once again, no such stable nucleus exists. There is no way to get beyond helium. Disappointingly for Gamow, the Big Bang could not have been the furnace in which the elements of nature were forged.1
Enter the British astronomer Fred Hoyle. Stars, to Hoyle, seemed a far more attractive furnace in which to forge the elements. After all, they remain dense and hot for millions, if not billions, of years, rather than the mere ten minutes of the Big Bang’s fireball. Since so much time was available, there was the possibility of relatively infrequent nuclear processes working their magic. All it would take would be one such rare process to leapfrog the troublesome mass-5 and mass-8 barrier and the road to the forging of heavy elements would be wide open. The trouble was that any such process would undoubtedly require very high temperatures. However, Eddington had shown that as stars fused hydrogen into helium, they gradually cooled and snuffed out. Hoyle was not discouraged. Out in space there were stars which were huge and bloated and pumped out as much as 10,000 times as much heat as the Sun. The existence of such ‘red giants’ – a prime example of which is Betelgeuse, burning brilliantly in the constellation of Orion – is evidence that there must be a way for stars to avoid the ignominious end envisaged by Eddington.
One way a star might do this, Hoyle realised, was if heavier elements became more common in its core than in its outer envelope. Since this would make the core denser than its surroundings, the powerful self-gravity of the core would crush it, heating it up. The temperature might easily soar to 100 million degrees, exactly what the doctor ordered for fusing together light nuclei to make heavier ones. At the same time, the intensely hot core of the star would pump out prodigious amounts of heat into the surrounding stellar envelope, causing it to puff up to giant proportions. And, as the material of the star inflated, it would cool until it merely glowed a dull red. This was a recipe for a red giant. It convinced Hoyle he was on the right track.
The trouble, of course, was that Eddington’s calculations showed that the material of a star was always thoroughly mixed together. But Hoyle was not to be put off. With astronomer Ray Lyttleton, he concocted a way of circumventing Eddington’s stellar show-stopper and forming a star with a super-dense, super-hot core loitering at the heart of a puffed-up and bloated envelope. It required the pair to postulate the existence of dense, cold clouds of hydrogen gas drifting about the Galaxy. Nobody knew whether such clouds existed. But if they did, Hoyle and Lyttleton pointed out, a star circling the centre of the Galaxy would inevitably plough through them, gathering about itself a thick mantle of hydrogen gas. Its interior – since it would be a mix of helium and hydrogen – would then be denser than its exterior. It was the recipe for creating a red giant with a super-dense, super-hot core.
Hoyle’s idea was ingenious, but it was unnecessary. Eddington was the pre-eminent astrophysicist of his day, but he suddenly realised he had made a stupid numerical error in his calculations. He was correct that the rotation of the Sun caused the material in its interior to circulate endlessly. However, he was wrong about the speed of that circulation. It was enormously more sluggish than he had estimated. In fact, it was so slow that it could not possibly keep the material of the solar interior blended. Without such mixing going on, the core would become ever more rich in helium as hydrogen burned. It would get denser, shrink and heat up. A star like a red giant, it turned out, was the natural and unavoidable fate of a star like the Sun.2
Hoyle’s hunch was right. Stars could, after all, get hot enough for element-building. But there remained the problem of the mass-5 and mass-8 barriers, which Gamow had discovered blocked the way to cooking up the elements in the furnace of the Big Bang. Hoyle looked around for a rare nuclear process that might leapfrog the barriers. And he found one. It involved not two helium nuclei but three. Could it be possible that, deep in the helium-rich cores of red giants, helium nuclei – alpha particles – came together in threes? If they stuck, the result would be a nucleus of carbon-12, which would neatly bypass the mass-8 hurdle.
This ‘triple-alpha’ process had actually already been considered by the American physicist Ed Salpeter in 1952. Salpeter realised immediately that the chance of three helium nuclei coming together simultaneously was so unlikely as to be effectively impossible – think of three blindfolded footballers blundering about a football pitch and all running into each other simultaneously at the corner flag. Instead, Salpeter focused his attention on two helium nuclei coming together. It might seem like a non-starter since, of course, gluing two helium nuclei together would make a nucleus of mass-8, which is unstable. But what Salpeter realised was that although such a nucleus – beryllium-8 – was unstable, it was not totally unstable. It hung about for a split second before falling apart. And, crucially, for that split second it was a sitting duck for a third helium nucleus.
The triple-alpha process, instead of requiring a ridiculously unlikely three-body encounter between helium nuclei, might be accomplished with more mundane two-body processes. Salpeter envisaged two steps. First, a pair of helium nuclei would collide and stick to make beryllium-8. Then, before the beryllium-8 nucleus had a chance to disintegrate, it would be struck by another helium nucleus to make a nucleus of carbon-12.
Salpeter’s two-stage triple-alpha process was far more likely than its single-stage version. Unfortunately, it was still not likely enough. When Salpeter carried out the calculations for the core of a red giant, he found the triple-alpha process could convert no more than a tiny fraction of a star’s helium into carbon. It was too inefficient. It was a dead end.
Hoyle was aware of Salpeter’s failure. However, he was unwilling to abandon the triple-alpha process because, quite frankly, it was the only game in town. Could there be a way to speed things up, he wondered? As he turned the problem over and over in his head, it struck him that there was indeed a way to boost the efficiency of the triple-alpha process. The trouble was, it was an awfully long shot.
Imagine a child on a swing. Say they are moving forwards and backwards once every five seconds. If you push the swing every three or every seven seconds, you will fail to boost the size of the arcs of the swing, and pretty soon you will have a protesting child on your hands as the swing comes to an erratic halt. Push the swing every five seconds, however, and it goes ever higher. Physicists say that the swing has a ‘natural frequency’ of one swing every five seconds. And it is a characteristic of any oscillating system like a swing that when the driving force – in this case, you pushing – matches its natural frequency, energy is transferred most efficiently. The oscillating system is said to be ‘in resonance’, or to ‘resonate’.
Now consider an atomic nucleus – specifically a nucleus of carbon-12. Imagine it as a bag containing a dozen nuclear building blocks. In reality, there is no such bag. However, the strong nuclear force which binds the building blocks effectively confines them to a small volume exactly as if they are in a bag. Now within the bag the nuclear building blocks jostle back and forth ceaselessly. Actually, the jostling is not entirely random. There is evidence that within a nucleus, the nucleons orbit in tight ‘shells’ reminiscent of the shells of orbiting electrons. But the key thing is that the whole bag has certain frequencies at which its contents naturally oscillate or vibrate.
Frequency is synonymous with energy, with sluggish, low-frequency vibrations containing little energy and violent, high-frequency vibrations a lot of energy. So each internal vibration of a carbon-12 nucleus corresponds to a particular vibrational energy. And energy was the key to Hoyle’s big idea for boosting the speed of the triple-alpha process. If three helium nuclei – or, equivalently, a beryllium-8 nucleus and a helium nucleus – collided and their total energy was exactly that of one of carbon-12’s natural vibrations, there would be a resonance. It would be like pushing the swing at its natural frequency. Only in this case what would be pushed would be the speed of the nuclear reaction that glued the components together to make carbon-12.
Of course, the nuclear reaction would be resonant only if carbon-12 happened to have an ‘energy state’ that exactly matched the combined energy of motion of three helium nuclei at the typical 100-million-degree temperature in the heart of a red giant.3 Hoyle put in the numbers and calculated the energy. It was 7.65 megaelectronvolts (MeV). Precisely what an MeV is is not important. Suffice to say, it is a unit physicists find convenient to express the energy of an atomic nucleus. The important thing is that if carbon-12 has an energy level at precisely 7.65 MeV, the nuclear reaction to make carbon-12 from three helium nuclei is resonant. Hoyle calculated how much carbon-12 would be forged in the heart of a red giant, assuming the 7.65 MeV energy state existed. It was appreciable. The speeded-up triple-alpha process worked. The mass-5 and mass-8 barriers were bypassed. The road to the building of all heavy elements was wide open. Everything depended on carbon-12 having a vibration energy at precisely 7.65 MeV. The question was, did it?
As luck would have it, in the spring of 1953, Hoyle was on a sabbatical from Cambridge University at the California Institute of Technology in Pasadena, where there was an experimental nuclear physics group. In fact, it had even dabbled in ‘nuclear astrophysics’. Its measurement of the speed of the crucial nuclear reactions in the CNO cycle were critical in showing that the CNO cycle could be the power source only of stars significantly more massive than the Sun. On arrival at Caltech, Hoyle wasted no time in going to the Kellogg Radiation Laboratory to see the group’s leader, Willy Fowler, and asking his question. Could carbon-12 have an energy level at 7.65 MeV?
He might as well have asked whether fairies were orchestrating the nuclear reactions in the heart of the Sun. Fowler thought he was in the presence of a lunatic. No theorist had ever been able to predict the precise energy of a compound nucleus. The mathematics was just too complicated. Although physicists rarely admit it, the only physical system whose behaviour they can predict with certainty is the two-body one: the Moon moving under the influence of the Earth’s gravity; an electron in a hydrogen atom orbiting in the electromagnetic grip of a proton. When it comes to three or more bodies, theorists are flummoxed. And carbon-12, with a dozen particles buzzing about in its nucleus like a tight knot of bees, is a ‘many-body’ system. It is totally beyond the power of theorists to predict its properties precisely. But that was exactly what Hoyle – a bespectacled young astronomer from England – was claiming to have done.
What made Hoyle’s prediction even more ridiculous was the crackpot logic behind it. ‘I exist and I am made of carbon, so the 7.65 MeV energy level of carbon-12 must exist,’ was the gist of it. In all of his research career, Fowler had never heard anything so extraordinary. A conclusion drawn from the observational fact that humans exist. An ‘anthropic’ argument. Biology determining physics. Scientific reasoning turned on its head.
It was highly probable Hoyle was wrong. On the other hand, Fowler adhered to the experimenter’s maxim: never close your mind to the unexpected. He rounded up the members of his small research group, who listened politely while Hoyle repeated his extraordinary argument for the existence of the 7.65 MeV state of carbon-12. Was there any possibility, Hoyle asked, that the experiments to date could have somehow missed it? Much of the technical discussion that followed went way over Hoyle’s head. Eventually, however, Fowler’s colleagues reached a consensus. If the 7.65 MeV state of carbon-12 had certain very special properties, yes, it was just possible that experiments might have missed it. The team decided to rejig their equipment and take a look.
For ten days, as the experiment proceeded, Hoyle was on tenterhooks. Each afternoon, he crept down into the bowels of the Kellogg Lab, the benevolent gift of a cornflake magnate, where Fowler’s colleague Ward Whaling and his team beavered away amid a jungle of power cables, transformers and diving-bell-like chambers in which atomic nuclei were fired at each other. And each afternoon, he crept back up again into the painfully bright Californian sunshine, relieved that his idea had survived one more day without being blown out of the water but anxious for the next day, and the next. On the tenth day, Hoyle was met by Whaling, who pumped Hoyle’s hand and gushed his congratulations. The experiment had succeeded. Hoyle’s prediction had been borne out. Unbelievably, there was an energy state of carbon-12 within a whisker of 7.65 MeV.
It was the most amazing result that Fowler had ever witnessed. He had not really believed that Hoyle’s outrageous prediction would be proved right. But it had – spectacularly. Like some omniscient god, Hoyle had peered into the heart of nature and spied something that mere mortals – or, at least, mere nuclear physicists – had been unable to see. He had maintained that the 7.65 MeV energy state of carbon-12 must exist because, if it did not, neither could human beings. To this day, Hoyle is the only person to have made a successful prediction from an anthropic argument in advance of an experiment.
Despite the spectacular triumph, however, Hoyle was not out of the woods. Once a carbon-12 nucleus formed inside a red giant, it was a sitting duck, just waiting to be struck by another helium nucleus. The result would be a nucleus of oxygen-16. All the good wrought by the triple-alpha process would be undone. Although carbon would be made, it would promptly be transformed into oxygen. The Universe would be carbon-free.
For carbon to be forged, it was necessary for a carbon-12 nucleus to vibrate at a very special energy equal to the combined energy of three helium nuclei at the typical temperature at the core of a red giant. Hoyle now realised that in order for some carbon to survive and not be transformed into oxygen, it was also necessary for oxygen-16 not to vibrate at a particular energy. Specifically, it must not have an energy equal to the combined energy of a carbon-12 nucleus and a helium nucleus at the temperature of a red giant. If it did, there would be a resonance and all carbon-12 would promptly be transformed into oxygen-16.
As part of its work on the CNO cycle, Fowler’s team had already measured the properties of the oxygen-16 nucleus. Hoyle pored over the data. There was a heart-stopping moment when he saw that oxygen-16 had an energy state very close to the energy to be avoided at all costs. But when he looked in detail, he found to his relief that the energy state was just out of range. Oxygen would indeed be made inside stars but, fortunately for human beings, not at the expense of carbon.
When Hoyle had time to think about what he had discovered, he began to marvel at the nuclear coincidence on which our existence so crucially depended. Beryllium-8 was unstable, but not so unstable that the triple-alpha process was impossible. Carbon-12 had an energy level at just the right place so that the triple-alpha process would be resonant and thus make appreciable quantities of carbon. And oxygen-16 had no energy level at a particular place, so that not all the carbon-12 would be transformed into oxygen-16. Without these three conditions being satisfied, the Universe would contain no elements heavier than carbon, or, alternatively, heavy elements but no carbon. Instead, everything was finely balanced to produce a Universe with roughly the same quantities of carbon and oxygen, both of which were essential for life.
Hoyle wondered what to make of this, and he came up with two logical possibilities. One was that there is a God who has fine-tuned the properties of the nuclei of beryllium-8, carbon-12 and oxygen-16 so we can be here. The problem with this option is that it is not a scientific one. A striking characteristic of science is that you get out more than you put in. A scientific explanation – often distilled into a formula or equation – is always simpler and more compact than the observations it summarises. If God fine-tuned things, the explanation – a complex supreme being – is as complex, if not more complex, than the thing for which an explanation is being sought. You get out less than you put in – the antithesis of science. The other problem with the God hypothesis is that one of the most striking things about the Universe is that it appears to be running perfectly well according to the known laws of physics, without any supernatural input.
But if a Creator did not fine-tune the energy levels of beryllium-8, carbon-12 and oxygen-16, what is the explanation for these unlikely nuclear coincidences? Hoyle came up with a stunning possibility. Perhaps our Universe is not the only one. Perhaps there are many universes, each with different laws of physics. In most the laws do not conspire to create nuclear coincidences for the creation of carbon, so there is no life. It is no surprise, then, that we find ourselves in a universe with the nuclear coincidences necessary for life. How could we not be? It is amazing, topsy-turvy logic. But, to Hoyle, it was the only thing that made sense. Incredibly, the fact that we exist as carbon-based beings may not simply be telling us about nuclear coincidences deep inside stars. It may be telling us that out there, in other spaces or other dimensions, there are a large number – perhaps an infinity – of other universes.
1. It would turn out, however, that the lightest nuclei such as deuterium – heavy hydrogen – and helium were made in the fireball of the Big Bang. In fact, after the ten-minute fury of nuclear reactions, roughly 10 per cent of the nuclei have become helium, a proportion we see all over the Universe and the prediction of which is touted as one of the great triumphs of the Big Bang model.
2. Hoyle was one of those scientists who was often right when he was wrong. Although his mechanism for making red giants was incorrect, the cold, dense, dark clouds of hydrogen gas he postulated did exist. They are the places where new stars are born. Not only that, but ‘accretion’ – the process by which Hoyle envisioned stars gathering hydrogen gas about themselves – is one of the most important and ubiquitous processes in the Universe. Among other things, it feeds the monster ‘supermassive’ black holes that lurk at the heart of just about every galaxy, including our own Milky Way.
3. In fact, a star must have a mass of at least three times that of the Sun to ever reach a temperature of 100 million degrees.