In 1931, when a 27-year-old Russian physicist named George Gamow stepped off a ship in New York and surveyed his new home, Hubble’s discovery of the expanding Universe was already five years old. But it had not yet sunk deeply into the scientific consciousness. Although many scientists accepted Hubble’s proof that the Universe had indeed begun in a titanic explosion billions of years ago, nobody seriously thought that science could say anything about what happened in the Big Bang. The idea was simply too preposterous to consider.
Such a failure of nerve is common. Scientists may scrawl arcane formulae across blackboards with reckless abandon, but deep down they find it extremely hard to believe that nature really dances to the tune of their flimsy equations. When those equations describe the birth and evolution of the entire Universe – as Einstein’s did – it takes a brave man indeed to follow through their implications.
George Gamow would prove to be such a man.
Before emigrating to the US, Gamow had studied cosmology under Aleksandr Friedmann at the University of Petrograd. In Cambridge he had worked with Ernest Rutherford, who had created the science of nuclear physics, and in Copenhagen with Niels Bohr, who had created our modern picture of the atom. Gamow’s interests ranged far and wide, from the theory of stars to biology to popular science writing.1 But it would be in cosmology that he would make his mark.
Gamow would be wrong about almost everything. But his achievement was immense, for he would be the first person to take the Big Bang really seriously and use nuclear physics to predict what the earliest moments of creation were like. Decades later, others, following his lead, would go on to speculate about the first split second of the Big Bang.
On the face of it, what got Gamow thinking about the Big Bang seemed to have nothing whatsoever to do with the explosion at the beginning of time. In the 1930s, Gamow set out to explain where the chemical elements had come from. Where, he wondered, did oxygen and carbon come from, and iron and gold? Atoms of these elements made up everything in the Universe – our bodies, the Earth, the stars – so where did they come from?
When Gamow began thinking about this problem, astronomers already possessed an important clue. Over the years, the spectra of thousands of stars had been carefully examined. From the patterns of colours missing in each spectrum, astronomers were able to deduce which elements were absorbing the light. This enabled them to measure how common each element was in different parts of the Universe.
What they discovered was that the elements existed in roughly the same proportions absolutely everywhere. It was a clear indication that some common process had made all the elements in the Universe. Gamow guessed that originally the Universe had contained only a very simple ingredient, and that somehow all elements had been made from this ingredient. Gamow was not the first person to have this idea, but he would take it further than anyone else.
When Gamow started his quest, physicists knew that all the elements – from hydrogen, the very lightest, all the way up to uranium, the heaviest – were built from just three basic building blocks: tiny particles called protons, neutrons and electrons. Every atom consisted of a ‘nucleus’ – a tight clump of protons and neutrons – sitting like a sun at the centre of a cloud of furiously orbiting electrons.
The key thing that made a hydrogen atom different from, say, an atom of carbon or uranium was the number of protons in its nucleus, which was precisely matched by the number of electrons in its orbit. Hydrogen had just one proton and one electron, whereas carbon had six protons and six electrons. The nucleus of uranium, on the other hand, was a monster. It contained 92 protons and sat in the midst of a haze of 92 whirling electrons.
Protons and electrons were bound together by the ‘electric’ force between them. Electrons have a negative electric charge and protons a positive one. Nobody really knows what electrical charge is, only that the electric force between particles with different charges – electrons and protons – makes them attract each other, whereas the force between two similarly charged particles causes them to repel each other.
Neutrons have no electrical charge, which means they are unaffected by the electric force. They keep protons separated so they can live together inside a nucleus. Hydrogen needs no neutrons because it has a lone proton, but to keep the peace in a uranium nucleus 150 neutrons are needed. Without these, the electric force between all the positively charged protons would simply blast the nucleus apart.
Clearly there must be another force to counterbalance the electric force, or atoms could never exist. There is. The ‘strong nuclear’ force provides the glue which binds together neutrons and protons inside a nucleus.
Unlike the electric force, the nuclear force has an extremely short range. This means that protons and neutrons have to get very close together before they feel it. Once they do get close enough, it grabs them enormously tightly.
But although the nuclear force is strong, it is not overwhelming. The basic building blocks within a nucleus are able to rearrange themselves. Early in the twentieth century, it was discovered that this happens naturally in some ‘radioactive’ atoms. Their nuclei are unstable and sometimes spit out some of their neutrons and protons quite spontaneously, changing into other atoms in the process. And what nature can do, physicists soon learnt to do also. In 1932, the British physicists John Cockroft and E. T. S. Walton ‘split the atom’.2
The idea that atoms could be changed by adding or subtracting the basic building blocks – protons and neutrons – was the clue Gamow leapt on in his search for the origin of the chemical elements.
Gamow guessed that the Universe had started off with a mix of protons, neutrons and electrons, and that all the elements had been assembled from these. One of his later collaborators would call the mix the ‘ylem’. If the ylem were dense enough and hot enough, the protons and neutrons would start colliding and sticking together to make light elements, and the light elements in turn would collide with each other to make heavier elements.
There are many different reaction schemes you could imagine for building up the elements. But these would be determined by the known laws of physics once the initial mix was fixed. All you had to do was run through the calculations and see if at the end of the day you came out with the mix of elements we see around us today.
It was like trying to make a fruit cake without a recipe. One way would be to put together some likely ingredients and bake them in the oven. You could then compare the final cake with one bought in a shop. If the two were not quite the same, the ingredients could be modified and the cake baked again. The end result, after much trial and error, might be a perfect fruit cake.
Starting with protons, neutrons and electrons, Gamow was trying to cook up the precise mix of elements we now find in the Universe. The ylem would have to be extremely hot, that much was clear. Nuclei would stick together only if they collided at great speed, which meant at high temperature.3 At low speed, the electric repulsion between the protons in the respective nuclei would blast them apart long before they could get close enough to be gripped by each other’s nuclear force. At high temperature – and Gamow realised it needed to be billions of degrees – two nuclei would slam into each other so violently that they would overcome their mutual electric repulsion and get close enough together to be grabbed in a nuclear embrace.
But where in the Universe could temperatures of billions of degrees be found? It seemed a tall order to find a natural furnace that could reach a temperature of billions of degrees, and so forge all the chemical elements.
Gamow’s great insight was to realise that the entire Universe must have been such a furnace when it was very young.
If it were somehow possible to run the expansion of the Universe backwards, like a movie in reverse, we would see it get hotter as it got denser, just as the air in a bicycle pump heats up when it is compressed. Gamow was the first to realise that the Big Bang must have been a ‘hot’ Big Bang.
Gamow envisaged the early Universe as a seething mass of protons, neutrons and electrons compressed into a tiny, tiny volume. Something then triggered this mass to suddenly start expanding and cooling, and as it did so ‘nuclear reactions’ among the basic ingredients formed all the elements. All this would happen in the first few minutes after the Big Bang, before the expanding fireball became too cool and rarefied for nuclear reactions to continue to happen.
Gamow tried to bake the elements from several different cake mixes. One of his ideas, for instance, was that the ylem was a ‘superdense’ object made of protons and neutrons. This then broke up, like the primeval atom proposed by Georges Lemaître in 1931, and the huge amount of energy released heated the mix to billions of degrees.
To Gamow, the Big Bang was what happened when the ylem disintegrated spontaneously. Now, where the ylem had come from and what had triggered it to break up, Gamow had no idea. Like all scientists, he was trying to answer one question at a time.
Gamow realised early on that the ylem would not contain particles alone. Matter at any temperature gives out radiation, and the hotter it gets the more energetic the radiation. At a temperature of a billion degrees, matter produces intense gamma rays – enormously energetic radiation with a wavelength far shorter than visible light.
The early Universe, therefore, would have been a brilliantly bright fireball.
In such a fireball, light radiation could not travel any appreciable distance, the way it can in today’s Universe. The fireball would contain large numbers of free electrons, and these would greatly hinder its progress. Free electrons are particularly good at absorbing radiation and redirecting, or ‘scattering’, it.
Now light has the peculiar property that although it behaves like a wave when travelling through space, it acts like a stream of bullet-like particles when it interacts with matter.4 So in the fireball each particle of light – known as a photon – would repeatedly bounce off electrons.5
When a photon scatters off an electron, and the photon has more energy than the electron, the photon generally loses energy to the electron. Imagine a car hitting a motorbike. Since a car generally has more energy than a motorbike, the net transfer of energy is from the car to the motorbike. On the other hand, when a photon scatters off an electron and has less energy than the electron, the photon gains energy from the electron. A similar thing happens when two electrons collide with each other. The one with the most energy generally loses some to the other.
This tendency for energetic particles to share their energy with less energetic particles has an important consequence. If particles have time to interact with each other many times, then eventually a state will be reached in which the particles share the available energy as fairly as possible.
This is precisely what happened in the Big Bang fireball. Although the Universe was expanding rapidly, the interactions between photons and electrons, and between electrons and electrons, were proceeding at a much faster rate, so at every instant during the expansion the energy was shared out fairly between all the particles.
Any system in which the particles have reached such a steady democratic state is said to be in ‘thermal equilibrium’. Here, the word ‘equilibrium’ does not mean that the energy of each individual particle is unchanging. Just as before, all the particles continue to be involved in the same game of give and take. What stays constant is the number of particles in any given range of energy. As fast as particles are knocked out of the energy range, other particles are knocked in. Equilibrium here is therefore a statistical thing.
Now, matter in thermal equilibrium has a special place in the hearts of physicists. The reason is that it is simple for them to understand. In order to predict the properties of the whole system, they do not have to sit down and calculate the energy of trillions and trillions of individual particles all rushing about randomly. In thermal equilibrium, the statistical properties of the particles are predictable. In particular, the energy of the particles is distributed in a simple way which depends only on temperature and which physicists can easily compute.6 It does not matter whether the particles are gold atoms or protons and neutrons. If they are in thermal equilibrium at a particular temperature, their energy will be distributed in precisely the same way. All the nasty complexity of nature gets smeared out.
In reality, it is difficult to find matter in a true state of thermal equilibrium because to reach such a state a system of particles must be left to settle down for a long time. During this time, if any energy escapes or pours in from outside, then reaching equilibrium becomes harder. This means that the system of particles must be isolated from its surroundings.
But although the true state is elusive in nature, it is often approached closely. For instance, the interior of the Sun is close to being in thermal equilibrium. Deep in its interior, photons are bouncing around – undergoing repeated scatterings by free electrons – as if confined in a giant box. Only relatively few leak out through the surface and illuminate the Earth. But the best example of a state of thermal equilibrium was the Big Bang fireball. After all, it was confined in the box of the Universe, so there was no possibility of energy leaking either in or out.
Now radiation in thermal equilibrium with matter has a very special character. Just as the particles of matter have a simple distribution with energy, the particles of radiation – the photons – have a simple distribution with wavelength. The spectrum of this ‘thermal radiation’ is as familiar to physicists as the face of Albert Einstein. Its shape is described by a universal formula which depends only on the temperature of the material and does not depend at all on the nature of the matter that the radiation interacts with.
Radiation with an identical spectrum is emitted by a black surface that absorbs all the light that falls on it, so thermal radiation has become known as ‘black body radiation’. This is unfortunate because the term ‘black body’, with its connotations of black holes, only confuses people. The Sun and stars are, after all, good black bodies. But there you are. Most physicists use the term, so we are stuck with it.
A thermal, or black body, spectrum has a characteristic humped shape. The energy in any range of wavelengths rises very steeply as the wavelength gets longer, reaches a peak, and then falls off steeply again. The hotter the black body, the shorter the wavelength of the peak. For the Sun, the peak is at the wavelength of green light.
The reason why a black body spectrum drops off at very long wavelengths is easy to see. Think of thermal radiation confined in a box with opaque walls. Wavelengths that are longer than the dimensions of the box are excluded because they simply will not fit in the box. At short wavelengths, it is necessary to appeal to the photon nature of radiation for an explanation of the drop-off in the spectrum. The shorter the wavelength of a photon, the more energy it contains. So, at very short wavelengths, photons are simply too energy-hungry to be made.
In 1946, Gamow took on a research student called Ralph Alpher. In fact, Alpher was the one who first coined the word ‘ylem’ to describe the primordial mix of neutrons, protons and electrons jumbled together in a sea of high-energy radiation from which the elements were formed.
Gamow suggested to Alpher that he calculate the quantity of various atoms that would be produced in the cooling fireball and see if they matched the quantities observed in nature. Early in the work, Gamow and Alpher were joined by Robert Herman, a graduate student from Princeton.
Alpher and Herman carried out the calculations. But they also began thinking about the fireball radiation. Like Gamow, they realised it would have the spectrum of a black body. Energy was constantly being transferred between the light and matter as electrons constantly absorbed and scattered photons. The fireball radiation would keep its black body character even as the fireball expanded, stretching the photons to longer and longer wavelengths and cooling them. All that would happen is that the peak of the hump would shift to longer and longer wavelengths.
But Alpher and Herman realised something important that Gamow had missed. Today’s Universe should be filled with the remains of the heat of the fireball, greatly cooled by the expansion of the Universe.
Something enormously significant happened in the Universe about 380,000 years after the Big Bang, when the temperature of the expanding fireball dropped to about 3,000 degrees. Until then, the Universe was a seething mass of electrons and atomic nuclei forged in the first few minutes after the Big Bang. But suddenly it was cool enough for electrons to combine with these nuclei and form atoms. Very rapidly all the electrons in the Universe would be mopped up.
The effect on the fireball radiation was dramatic. With the electrons gone there was nothing to scatter the photons of the fireball. The rapidly cooling fireball suddenly became transparent to light.
In the language of physics, photons stopped ‘walking’ and began ‘flying’. By walking, physicists mean that the path of each photon was reminiscent of a drunkard’s progress. Each photon travelled only a short distance in a straight line before it encountered an electron and was ‘scattered’ in another direction.
But suddenly, 380,000 years after the Big Bang, everything changed. Atoms mopped up all the free electrons so photons could fly unhindered across space.7
After this ‘epoch of last scattering’, photons which had been unable to travel far in a straight line without running into an electron were suddenly able to fly unhindered. And they have been flying freely ever since, gradually losing energy as the Universe has grown in size.
At the epoch of last scattering, matter and light, which had been so intimately linked, went their separate ways. The photons of the fireball radiation have been flying across space for the past 13.7 billion years without ever meeting a particle.
The Universe continued to expand, stretching and cooling the radiation. By now it would be only a feeble glimmer. Today, Alpher and Herman predicted, the ‘background temperature’ would be –268°C, or just five degrees above absolute zero.8 The temperature of the background radiation is the temperature the Universe had long ago, but greatly reduced by the enormous expansion the Universe has undergone since.
Alpher and Herman published their prediction in a paper in the international science journal Nature in 1948. At first Gamow thought the idea unimportant. He argued with Alpher and Herman that although the Universe might be filled with this relic radiation from the Big Bang fireball, in practice it would be impossible to see it from the Earth. The problem was starlight. Gamow claimed it had the same energy density as the relic radiation, making it impossible to distinguish between the two.
Gradually, though, Gamow came round to Alpher and Herman’s view. He realised that he was wrong and that the fireball radiation would have a distinctive signature that would make it instantly recognisable to a telescope that was sensitive enough.
Despite this, everyone proceeded to forget about Alpher and Herman’s prediction. In fact, it would drop out of scientific sight for almost 20 years. One reason was that Alpher and Herman were themselves unaware that in the 1940s there existed telescopes in the world capable of searching for the cooled remnant radiation from the Big Bang. In the mid-1950s, they and their colleague James Follin did actually talk to radio astronomers at the National Research Laboratory and at the National Bureau of Standards about actually looking for the relic radiation. But they were told that the technology of the day was simply not up to detecting such weak relic radiation. This was wrong.
But the most important reason for the prediction being forgotten was that Gamow’s theory of how heavy elements were made was wrong. The theory worked well for helium – the simplest element after hydrogen. It predicted that about 25 per cent of the material emerging from the Big Bang should be helium. This is in extremely good agreement with what astronomers find when they study stars and the gas drifting through interstellar space.
But the theory failed miserably when it came to producing any heavier elements. The early Universe simply did not stay hot and dense long enough for successive thermonuclear reactions to build up elements such as carbon and iron.
As Fred Hoyle and his colleagues were to prove in the late 1950s, virtually every element heavier than helium has been manufactured since the time of the Big Bang – by reactions in the hot interiors of stars.
1. Gamow was one of those who guessed that short sequences of nucleic acid ‘bases’ along DNA might form a ‘code’ that carried the ‘blueprint’ for the proteins of our bodies. Francis Crick, James Watson and Maurice Wilkins proved him right and won the Nobel Prize in 1962.
2. In fact, Gamow was the one who told Cockroft and Walton that splitting the atom might be possible. The pair were awarded the Nobel Prize in 1951 for their achievement.
3. That is all temperature is: a measure of how fast the microscopic particles that make up a body are moving.
4. Why light has this wave/particle nature is one of the great mysteries of science. In reality, light is neither a particle nor a wave but something for which we have no word in our language.
5. The shorter the wavelength of light, the higher the energy of the photons. For instance, the photons of blue light have more energy than the photons of red light.
6. Strictly speaking, a temperature can be defined only for a body when it is in a state of thermal equilibrium.
7. A very similar process occurs in the Sun. Photons created by nuclear reactions deep in its heart are scattered repeatedly as they work their way up to the ‘surface’. The path they take is so contorted that they take about 30,000 years to get there. Once at the surface they are free and take only about eight minutes to fly to the Earth. Today’s sunlight is therefore about 30,000 years old.
8. Absolute zero is the lowest temperature attainable, and so has a special role in physics. When an object is cooled, its atoms move about more and more sluggishly. Absolute zero (which on the Celsius scale is equal to –273°C) is the temperature at which they stop moving altogether.