Wonders of the Universe
Page 10
The tale of how we learnt to read the history of the stars in their light began with the work of Isaac Newton in 1670. In his ‘Theory of Colour’, Newton demonstrated that light is made up of a spectrum of colours, and that with nothing more complicated than a glass prism you can split the white light of the Sun into its colourful components. Almost 150 years later, the German scientist Joseph von Fraunhofer made a startling discovery about the solar spectrum whilst calibrating some of his state-of-the-art telescopic lenses and prisms. Lying within the solar spectrum, Fraunhofer documented the existence of 574 dark lines; there were literally hundreds of gaps – missing colours in the Sun’s light. Unaware of the significance of this discovery at the time, Fraunhofer carefully mapped their positions in great detail. He went on to discover black lines in the light from the Moon and planets, and from other stars. These are now known as Fraunhofer lines.
Further work by two more of the great German scientists of the nineteenth century, Gustav Kirchhoff and Robert Bunsen (perhaps best known to schoolchildren everywhere as the inventor of the Bunsen burner), finally gave meaning to these lines. They surmised correctly that these black spectral lines were the fingerprints of the chemical elements in the atmosphere of the Sun itself. Across 150 million kilometres (93 million miles) of space, the light of our star had carried the signature of its constituents to us.
Kirchhoff and Bunsen’s discovery was purely empirical – they had observed that when gases are heated on Earth they do not simply glow like a piece of hot metal, they give off light of very specific colours – and interestingly those colours depend only on the chemical composition of the gas and not on the temperature. In particular, each chemical element gives off its own unique set of colours. The element strontium, for example, burns with a beautiful red colour, sodium with a deep yellow, and copper is a haunting emerald green.
The two German scientists also noticed that the missing black lines in the solar spectrum corresponded exactly to the glowing colours of the elements. There are, for example, two black lines in the yellow part of the Sun’s light that correspond exactly to the two distinct yellow emission lines of hot sodium vapour. You will be familiar with this mixture of two very slightly different yellows – it is the colour of sodium streetlights.
Interestingly, Kirchhoff and Bunsen had no idea why the elements behaved in this way, but this didn’t matter if all you wanted to do was to match the signatures of elements observed on Earth with the signatures in the light from the Sun and stars. It wasn’t until the turn of the twentieth century that an explanation for this strange behaviour of the elements was discovered. The answer lies in quantum mechanics, and the spectrographic work of physicists and chemists such as Kirchhoff and Bunsen was a major motivating factor in the development of the quantum theory. Elements emit and absorb light when the electrons surrounding their atomic nuclei jump around. The key insight that led to quantum theory was that electrons can’t exist anywhere around a nucleus like planets around a star, but they are instead placed in specific, very restrictive ‘orbits’. The deep reason for this is that electrons do not always behave as point-like particles of matter. They also exhibit wave-like properties, and this severely restricts the ways in which they can be confined around the atomic nucleus. What happens at a microscopic level when an atom absorbs some light is that an electron jumps to a different, more energetic, orbit and it emits light when the electron falls back from a higher to a lower energy orbit. The difference in energy between the lower orbit and the higher orbit must correspond exactly to the energy of the light absorbed or emitted.
In the early nineteenth century, German scientist Joseph von Fraunhofer documented the existence of 574 dark lines within the solar spectrum. This diagram is a visual representation of these Fraunhofer lines.
Spectographic investigations have revealed that Sirius, the dog star, is metal-heavy, with an iron content three times that of the Sun.
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Isn’t it simply wonderful that just by looking at the light from those twinkling stars we can tell what those fiery worlds, so far away, are made of?
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Although Polaris, the pole star (top and middle), is 430 light years away, we know by looking that is has about the same heavy element abundance as our sun, but markedly less carbon and a lot more nitrogen. Vega (bottom), meanwhile, as the second-brightest star in the northern sky, consists of only about a third of the amount of metals as our sun.
However, quantum theory also stipulates that light should not always be thought of as a wave. Just like electrons, light can behave as both a wave and a stream of particles. These particles are called photons. Now, here is the key point: photons of a particular energy correspond to a particular colour of light, so red photons have a lower energy than yellow photons, which have a lower energy than blue photons. Since each element has electrons in unique orbits around the nucleus, this means that each element will only be able to absorb particular photons in order to move its electrons around into higher energy orbits. Conversely, when the electrons drop from higher to lower energy orbits, they will only emit photons of a particular energy and therefore a very particular colour. This is what we see when we observe the elements emitting or absorbing particular colours of light. We are in a very real sense seeing the structure of the atoms themselves.
When looking at a spectrum of light from our sun you can see hundreds of Fraunhofer lines, and each and every one of those corresponds to a different element in the solar atmosphere which absorbs light as it passes through. From sodium in the yellow, through iron, magnesium, and all the way across to the so-called hydrogen alpha line in the red, the signatures of each of the elements are encrypted in the solar code.
So by looking at these lines in precise detail you can work out exactly which elements are present in the Sun. This turns out to be roughly 70 per cent hydrogen, 28 per cent helium, and the remaining 2 per cent is made up of the other elements.
It is worth repeating here that you can apply this theory not only to the Sun, but for any of the stars you can see in the sky – which allows us to measure the constituents of their atmospheres with extraordinary accuracy. Isn’t it simply wonderful that just by looking at the light from those twinkling stars we can tell what those fiery worlds, so far away, are made of?
These spectrographic investigations of the light from the cosmos have confirmed what our scientific intuition suggested to us: wherever we look, we only ever see the signatures of the set of ninety-four naturally occurring elements that we have collected and identified here on Earth.
So it is clear that we are connected in a very real sense to the whole of the Universe – with its hundreds of billions of stars across billions of galaxies – because we are all intrinsically made of the same stuff. And, as we will explain, there is one very simple reason for that: everything in the Universe shares the same origin
THE EARLY UNIVERSE
In order to understand where we come from we have to understand events that happened in the first few seconds of the life of the Universe. When the Universe began it was unimaginably hot and dense – we literally don’t have the scientific language to describe it. It was beautiful in a very real sense. There was no structure, there was certainly no matter, and it was exactly the same whichever way you looked at it. It’s a difficult concept to grasp, but we can get some idea of what happened to the early Universe by looking at the behaviour of one of the most common substances on Earth: water.
Water is one of the most common substances on Earth, but it can produce some of the most spectacular geological wonders on our planet. The El Tatio Geysers in Chile are just one example of water’s awesome activity.
© Charles O’Rear/CORBIS
One of Earth’s most incredible natural wonders, the El Tatio Geysers make up the largest geyser field in the world. As they are located at a height of 4,200 metres (13,800 feet) in the Chilean Andes, they are also the highest.
EL TATIO GEYSERS, CHILE
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gh in the Andes Mountains, in the far north of Chile, you will find the spectacular El Tatio Geysers. Erupting at 4,200 metres (13,800 feet) above sea level, this is one of the geological wonders of Earth’s Southern Hemisphere. Not only is it one of the largest geyser fields in the world, it is also one of the highest. For those who journey here to witness the eruption of the jets of water skywards there is only one time to visit – sunrise.
In the early morning, as the Sun begins to peer over the horizon, the combination of super-heated water and freezing cold air produces a rare phenomenon. Like all geysers, the boiling water delivered to the surface by the geological plumbing bursts out and flashes into steam, forming the majestic columns. But here, because of the high altitude and bitter temperatures, the steam rapidly condenses and returns to its frozen state, covering the ground with sheets of ice. It is surely one of the most spectacular naturally occurring locations on the planet in which you can see water in all three of its phases: liquid, vapour and solid ice. It is this rapid transformation of water through its three familiar phases that provides us with a nice analogy to discuss events that happened in the very early life of the Universe.
A water molecule is made up of two chemical elements: oxygen and hydrogen. Oxygen and hydrogen atoms are symmetric when they are alone and uncombined. This particular use of the word symmetric is perhaps unfamiliar; what is meant in this context is that the atoms themselves would look the same no matter what angle you viewed them from. In the language of physics, this is called rotational symmetry. A perfect sphere has perfect rotational symmetry, because whichever way you look at it or spin it around it looks exactly the same. When an oxygen atom combines with two hydrogen atoms to form a water molecule – H2O – this rotational symmetry disappears because the water molecules have a particular shape – there is an angle of 105 degrees between the hydrogen and oxygen atoms. A physicist would say that the symmetry is now broken, because the water molecule has a distinct orientation. We can break the symmetry of water still further by cooling down all the molecules until they stick together and solidify into ice. Now the crystals of ice are beautiful and almost impossibly intricate; full of structure and a complexity that completely hides the perfect symmetry of the original atoms, and also the simple but different symmetry of the water molecules themselves.
Approximately 70 per cent of Earth’s surface is covered by water. At the El Tatio Geysers you can see water in all its three forms. Walking through pools of water on the ground, I held a sheet of glass in the geysers’ steam and watched ice crystals form on it.
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Exactly like the journey of steam to ice, of chaos to order, this was the Universe in transition. A transition where the structure and substance of all the particles of matter emerged for the first time.
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The important point here is that all this complexity emerged when the symmetry was broken, but we did nothing to the water itself to break its symmetry other than cool it down. So although it looks for all the world as if a master sculptor sat down and chiselled out beautiful patterns in the ice, this intricacy and beauty emerged completely spontaneously out of building blocks that are themselves utterly symmetric.
Physicists call this process spontaneous symmetry breaking, and it is this idea that lies at the heart of our understanding of the early Universe
THE BIG BANG
Thirteen billion years ago the Universe began in the event called the Big Bang. We don’t know why. We also don’t know why it took the initial form that it did. This is one of the unsolved mysteries that makes fundamental physics so exciting. The first milestone we can speak of in anything resembling scientific language is known as the Planck Era, a period that occurred a mind-blowing 10–43 seconds after the Big Bang. When written in full, that number has 42 decimal places: 0.00000000000000000000000 0000000000000000001 seconds. That’s not very long at all. This number can be arrived at very simply because it is related to the strength of the gravitational force. It is so incredibly tiny ultimately because gravity is so weak – and we don’t know the reason for that, either! At that time the four fundamental forces of nature that we know of today – gravity, the strong and weak nuclear forces, and electromagnetism – were one and the same force, a single ‘superforce’. There was no matter at this stage, only energy and the superforce. This is what a physicist would call a very symmetric situation.
As the Universe rapidly expanded and cooled it underwent a series of symmetry-breaking events. The first, at the end of the Planck Era, saw gravity separate from the other forces of nature, and so the perfect symmetry was broken. Around 10–36 seconds after the Big Bang, another symmetry-breaking event occurred which marked the end of the Grand Unification Era. This saw the strong nuclear force (the force that sticks the quarks together inside protons and neutrons) split from the other forces. At this point the Universe underwent an astonishingly violent expansion known as inflation, in which the Universe expanded in size by a factor of 1026 (that’s 100 million million million million times) in an unimaginably small space of time – it was all over in 10–32 seconds. This was when sub-atomic particles entered the Universe for the first time, but they weren’t quite what we see today because none of them had any mass at all.
Careful scientific study leads us to conclude that the building blocks of our Universe are fundamentally hydrogen and helium.
Up until this point this story is theoretically well-motivated but experimentally relatively untested. The next great symmetry-breaking event, however, which occurred 10–11 seconds after the Big Bang, is absolutely within our reach because this is the era we are recreating and observing at CERN’s Large Hadron Collider. It is called electroweak symmetry breaking; at this point the final two forces of nature – electromagnetism and the weak nuclear force – are separated. During this process the sub-atomic building blocks of everything we see today (the quarks and electrons) acquired mass. The most popular theory for this process is known as the Higgs mechanism, and the search for the associated Higgs Particle is one of the key goals of the Large Hadron Collider project.
We are now on very firm experimental and theoretical ground. From this point on we know pretty much exactly what happened in the Universe because we can do experiments at particle accelerators to check that we understand the physics. The emergence of the familiar particles and forces we see in the Universe today happened, we believe, as a result of a series of symmetry-breaking events which began way back at the end of the Planck Era. The concept of spontaneous symmetry breaking in the early Universe is exactly the same as for the transitions from water vapour to liquid water to ice. Complex patterns emerge without prompting – just as a result of falling temperature – and these patterns obscure the underlying symmetry of the initial state. So just as the seemingly infinite complexity of snowflakes masks the simple symmetry of oxygen and hydrogen atoms, so the array of forces of nature and sub-atomic particles we see as the building blocks of the Universe today obscures the symmetry of the early Universe.
There is now one final step needed to arrive at the protons and neutrons – the building blocks of the elements – and the first elements themselves. This began around a millionth of a second after the Big Bang, when the quarks had cooled enough to become glued together by the strong nuclear force to form protons and neutrons. The simplest element, hydrogen, consists of a single proton. So after only a millionth of a second in the life of the Universe, the first chemical element had made an appearance. After three minutes, the Universe was cold enough for the protons and neutrons themselves to stick together to form helium. With two protons and one or two neutrons in its nucleus, helium is the second-simplest chemical element. There were also very, very small amounts of lithium, with three protons, and beryllium, with four protons – the third-and fourth-simplest elements. And this is pretty much where the process stopped. After three minutes the Universe had the four distinct forces we know of today – gravity, the strong and weak nuclear forces, and electromagnetism, and
was composed of roughly 75 per cent hydrogen (by mass) and 25 per cent helium. This is the story of the creation of the simplest chemical elements and of successive symmetry-breaking events in the early Universe
A computer simulation of an event showing the decay of Higgs Bosen producing four muons (white tracks). This image shows how the Higgs Bosen might be seen in the CMS detector from the Large Hadron Collider at CERN.
CERN / SCIENCE PHOTO LIBRARY
SUB-ATOMIC PARTICLES
Our understanding of the structure of matter has increased in the last century. Originally, atoms were thought to be the basic building blocks of life, but Rutherford’s famous diffraction experiment proved that matter consisted mainly of space, with each atom containing a very small dense nucleus surrounded by a cloud of electrons. Further investigation showed that each nucleus was composed of protons and neutrons and that each proton was composed of up and down quarks. We have now reached what is believed to be the smallest particles possible – scientists have now discovered that all matter is composed of 9 particles and 4 forces, plus the hypothetical Higgs Boson. The search for the basic building blocks of matter has used matter colliders, which can produce the very high energies that are required to recreate the temperatures in the early Universe, when these sub-atomic particles originally existed.