Brian Cox OBE FRS is a Professor of Particle Physics at the University of Manchester and the Royal Society Professor for Public Engagement in Science. His many highly acclaimed BBC television documentaries include, most recently, Human Universe and Forces of Nature.
Jeff Forshaw is Professor of Theoretical Physics at the University of Manchester, specializing in the physics of elementary particles. He was awarded the Institute of Physics Maxwell Medal in 1999 for outstanding contributions to theoretical physics.
We dare to imagine a time before the Big Bang, when the entire Universe was compressed into a space smaller than an atom. And now, as Brian Cox and Jeff Forshaw show, we can do more than imagine: we can understand. Over the centuries, the human urge to discover has unlocked an incredible amount of knowledge. What it reveals to us is breathtaking.
UNIVERSAL takes us on an epic journey of scientific exploration and, in doing so, reveals how we can all understand some of the most fundamental questions about our Earth, Sun and solar system and the star-filled galaxies beyond. Some of these questions – How big is our solar system? How fast is space expanding? – can be answered from your back garden; the answers to others – How big is the Universe? What is it made of? – draw on the astonishing information now being gathered by teams of astronomers operating at the frontiers of the known Universe.
At the heart of all these questions – from the earliest attempts to quantify gravity, to our efforts to understand what dark matter is and what really happened at the birth of our Universe – is the scientific process. Science reveals a deeper beauty, connects us to each other, to our world, and to our Universe and, by understanding the groundbreaking work of others, reaches out into the unknown. What’s more, as UNIVERSAL shows us, if we dare to imagine, we can all do it.
For their specific help with various parts of the book, we’d like to thank Richard Battye, Sarah Bridle, Mike Bowman, Bill & Pauline Chamberlain, Ed Copeland, Mrinal Dasgupta, Neal Jackson, Scott Kay, Kevin Kilburn, Peter Millington, Tim O’Brien, Michael Oates, Subir Sarkar, Bob Seymour and Martin Yates. Particular thanks go to Mike Seymour, with whom we have had many enjoyable and helpful discussions.
Special thanks are also due to the team at Penguin and especially Tom Penn, our editor, and Tom Etherington, who produced the figures.
Thanks also to Diane and Sue for their continued guidance and support.
Finally, we would like to thank Peter Saville for his influence on the book, which extends beyond the beautiful cover design.
This book has been a long time in the making and we are deeply grateful for the support and encouragement of our families.
The following is a list of some basic maths and physics that may be helpful.
POWERS OF 10
In cosmology and particle physics we often encounter numbers that are either very big or very small. To help write such numbers we use exponential notation. For example, 1 million, which is equal to 1,000,000, can be written 106. This should be read as ‘10 to the power of 6’, which means it is equal to 10 multiplied by itself 6 times. Tiny numbers are written with negative powers, so one billionth, which is equal to 1/1,000,000,000 = 0.000000001, is written 10−9.
UNITS
Very often we deal with quantities that carry units. The simplest example might be a distance, such as 1 kilometre = 0.621 miles. As in the case of kilometres and miles, we always have the freedom to choose the units we want to use to measure something. Although metres and kilometres are convenient units for stating the typical distances we encounter in our daily lives, they are not very convenient in cosmology and particle physics. More commonly we will want to use light years, megaparsecs, ångstroms and nanometres. These can all be converted into metres as follows:
1 light year = 9.46 × 1015 metres
1 megaparsec = 1 Mpc = 3.26 × 106 light years
1 nanometre = 10 ångstroms = 10−9 metres
1 femtometre = 10−15 metres
Likewise, in everyday circumstances it makes sense to measure energies in joules (e.g. a 20 watt light-bulb radiates energy at a rate of 20 joules per second), but when discussing particles of atom-size and smaller it makes more sense to use the electronvolt (denoted eV), which can be converted into joules using:
1 eV = 1.60 x 10-19 joules
We will often abbreviate units, e.g. 1 nm = 1 nanometre or 1 km = 1 kilometre or 1 mega-electronvolt = 1 MeV.
Numbers that carry with them an associated unit are called dimensionful numbers, and it is common to want to combine dimensionful numbers by multiplying or dividing them. The simplest example of this is when we take a distance and then divide it by a time in order to get something that is a speed. For example, a car moves 100 km in 2 hours, therefore its speed is 100 kilometres divided by 2 hours = 100 km/2 h = 100 × 1 km/2/(1 h) = 50 × 1 km/1 h = 50 km/h. Obviously you could see that the car moves at 50 km per hour straight away – but we chose to show the various intermediate ways we could have written the ratio because simple manipulations like these are sometimes performed in the text.
We encounter two particularly elaborate dimensionful numbers in the book: the Gravitational constant, G = 6.67 m3/s2/kg and the Hubble constant, H = 68 km/s/Mpc. These units might look abstract, but in fact they are easy to comprehend. For example, divide G by a distance squared and then multiply it by a mass and you end up with a number with the units of acceleration (m/s2): if the distance is the radius of the Earth and the mass is the mass of the Earth, then the corresponding acceleration is the acceleration due to gravity, for an object dropped close to the Earth’s surface. Likewise, the units of the Hubble constant tell us that a galaxy 1 Mpc away recedes from the Earth at a speed of 68 km/s.
Occasionally we make use of elementary algebra. For example, the calculation of the speed of the car above could be carried out using the formula v = d/t where d = 100 km and t = 2 hours, to give v = 50 km/h. A formula can be transformed into another equally valid one by performing the same operation on each side of the equals sign, e.g. v × t = vt = d/t × t = d. In this case, we multiplied both sides of the equation by t to deliver an equation telling us that the distance d is equal to the product of the speed, v, and the time, t. Notice how we can write a product of two numbers either using an explicit multiplication symbol (v × t) or, more simply, as vt. We always denote division by a backslash symbol (e.g. d/t × t means ‘d divided by t multiplied by t’, which is simply equal to d).
ELEMENTARY PARTICLES
Atoms are the building blocks of the ordinary matter we encounter on Earth. They are approximately an ångstrom across, and most of their mass resides in a tiny central nucleus built from protons and neutrons. Protons are only about 1 femtometre in diameter and they carry positive electric charge. Orbiting around the nucleus are the electrons, which carry negative electric charge such that the entire atom is neutral. The simplest atom is called hydrogen and it consists of a single proton and a single electron – it is the most abundant type of atom in the Universe. The way that the electrons are arranged around the nucleus governs the way an atom interacts with other atoms, e.g. to produce molecules. The entire list of atoms can be collated in the Periodic Table.
We now know that protons and neutrons are themselves built from smaller particles called quarks and gluons. The gluons mediate the strong nuclear force, which binds the quarks together. In total there are six types of quark, although only the lightest two of these are used in building protons and neutrons. The electrons are also part of a bigger family of particles called leptons – the muon and tau leptons are like heavier versions of the electron. The remaining leptons are three electrically neutral neutrinos.
Apart from the electromagnetic force, which causes particles with opposite electric charge to attract each other, and the strong nuclear force, there is also the weak nuclear force. This force is much weaker than the other forces, except in very high-energy interactions, and it is able to make neutrons turn into protons with the emission of an electron and a neutrino. This feature plays a key role in the nuclear fusion processes that take place in the centre of the Sun, causing it to burn.
Just as gluons mediate the strong force, the electromagnetic force is mediated by photons, which can also be regarded as particles of light. The weak nuclear force is mediated by the W and Z particles.
The Standard Model of particle physics is a very precise mathematical framework based on quantum theory that describes how all of these (i.e. the six quarks, the six leptons, and their anti-matter partners) elementary particles interact with each other through the exchange of photons, gluons and W and Z particles. The Standard Model involves one more particle, the Higgs particle, whose interactions with the other particles are responsible for their having mass. Photons and gluons do not interact with the Higgs particle and they have zero mass.
The Standard Model does not include the gravitational forces between particles, and it does not include dark matter in its list of particles. These are two of the reasons why it is generally regarded as being incomplete.
We dare to imagine a time when the entire observable Universe was compressed into a region of space smaller than an atom. And we can do more than just imagine. We can compute. We can compute how hundreds of billions of galaxies emerged from a single subatomic-sized patch of space dwarfed by a mote of dust, and there is precise agreement between those computations and our observations of the cosmos. It seems that human beings can know about the origins of the Universe.
Cosmology is surely the most audacious branch of science. The idea that the Milky Way, our home galaxy of 400 billion stars, was once compressed into a region so vanishingly small is outlandish enough. That the entire visible congregation of billions of galaxies once occupied such a subatomic-sized patch sounds like insanity. But to many cosmologists this claim isn’t even mildly controversial.
This is not a book about knowledge handed down from on high. More than anything, it is about how we – all of us – can gain an understanding of the Universe by doing science. You might think that it’s impossible for the average person to explore the Universe in much detail: don’t we need access to Hubble Space Telescopes and Large Hadron Colliders? The answer is no, not always. Some fundamental questions about our Earth, our Sun, our solar system, and even the Universe beyond, are answerable from your back garden. How old are they? How big are they? How much do they weigh? We will answer these questions by doing science. We will observe, measure and think. One of the great joys of science is to understand something for the first time – to really understand, which is very different from, and far more satisfying than, knowing the facts. We will make our own measurements of the motion of Neptune, follow in the footsteps of the pioneering cosmologist Edwin Hubble in discovering that our Universe is expanding, and make an apparently trivial observation standing on a beach in south Wales.
As the book unfolds, our gaze will inevitably turn outwards towards the star-filled galaxies. To understand them, we will rely on observations and measurements that we cannot make ourselves. But we can imagine being a part of the teams of astronomers who can. How far away are the stars and galaxies? How big is the Universe? What is it made of? What was it like in the distant past? The answers to these questions will generate a cascade of new ideas, and, before the book is finished, we will be equipped to enquire about the origins of the Universe. Science is an enchanting journey of exploration. It is an exciting, rewarding process and one that leaves scientists with a feeling of being better connected to the world around them. It leaves a sense of awe and humility too; a feeling that the world is beautiful beyond imagination and that we are very privileged to be here to witness it.
Before we begin our journey, however, we will allow ourselves a glimpse of the destination. What follows next is the story of how our Universe evolved from a subatomic patch of space into the oceans of galaxies we see today. Perhaps, by the end of the book, you will judge that it might just be true.
Consider the Universe before the Big Bang. By ‘Big Bang’ we mean a time 13.8 billion years ago when all the material that makes up the observable Universe came into being in the form of a hot, dense plasma of elementary particles. Before this time, the Universe was very different. It was relatively cold and devoid of particles, and space itself was expanding very rapidly, which means that any particles it may have contained were moving away from each other at high speeds. The average distance between particles was doubling every 10−37 seconds. This is a staggering, almost incomprehensible, rate of expansion: two particles one centimetre apart at one instant were separated by 10 billion metres only 4 × 10−36 seconds later; more than twenty times the distance from the Earth to the Moon. We do not know for how long the Universe expanded like this, but it continued for at least 10−35 seconds. This pre-Big Bang phase of rapid expansion is known to cosmologists as the epoch of inflation.
Let us focus on a tiny speck of space a billion times smaller than a proton, the atomic nucleus of a hydrogen atom. At first glance, there is nothing particularly special about this tiny patch. It is one small part of a much larger, inflating Universe, and it looks much the same as all the other patches that surround it. The only reason this particular patch deserves our attention is that it is destined, over 13.8 billion years, to grow into our observable Universe: the region of space containing all the galaxies and quasars and black holes and stars and planets and nebulae visible from Earth today. The Universe is far bigger than the observable Universe, but we can’t see it all because light can only travel a finite distance in 13.8 billion years.
Before the Big Bang, the Universe was filled with something called the ‘inflaton’ field; a material thing, like a still ocean filling space. The gravitational effect of the energy stored in the inflaton field caused the Universe’s exponential expansion, and this is the origin of its name: it is the field responsible for inflating the Universe. On the whole, the inflaton field remained undisturbed as the Universe expanded, but it was not perfectly uniform. It had tiny ripples in it, as required by the laws of quantum physics.
By the time our observable Universe was the size of a melon, the period of inflation was drawing to a close as the energy driving it drained away. This energy was not lost, however; it was converted into a sea of elementary particles. In an instant, a cold, empty Universe became a hot, dense one. This is how inflation ended and the Big Bang began, delivering a Universe filled with the particles that were destined to evolve into galaxies, stars, planets and people.
We do not currently know which particles were present at the moment of the Big Bang, but we do know that the heaviest particles soon decayed to produce the lighter ones we know today: electrons, quarks, gluons, photons, neutrinos and dark matter. 1 We can also be confident about the particles that populated the Universe when it was around a trillionth of a second old, because we are able to re-create these conditions on Earth, at the Large Hadron Collider.2 This is the time when empty space became filled with the Higgs field, which caused some of the elementary particles to acquire mass.3 The weak nuclear force, responsible for the reactions that allow the stars to shine, became distinct from the electromagnetic force at this time.
A millionth of a second after the Big Bang, when the hot plasma had cooled to 10 trillion degrees celsius, the quarks and gluons formed into protons and neutrons, the building blocks of atomic nuclei. Although this primordial Universe consisted of an almost uniform soup of particles, there were slight variations in the density of the soup – an imprint of the quantum-induced ripples in the inflaton field. These variations were the seeds from which the galaxies would later grow.
One minute after the Big Bang, at around a billion degrees, the Universe was cool enough for some of the protons and neutrons to cluster together in pairs to form deuterium nuclei. Most of these then went on to partner with additional protons and neutrons to form helium and, in tiny amounts, lithium. This is the epoch of nucleosynthesis.
For the next 100,000 years or so, little happened as the Universe continued to expand and cool. Towards the end of this time, however, the dark matter gradually began to clump around the seeds sown by the ripples in the inflaton field. Regions of the Universe where there was a slight excess of dark matter grew denser, as their gravity pulled in yet more matter from the surroundings. This is the start of the gravitational clumping of matter that will eventually lead to the formation of galaxies. Meanwhile, photons, electrons and the atomic nuclei bounced and zig-zagged around, hitting each other so frequently that they formed something resembling a fluid. After 380,000 years, when the observable Universe was a thousand times smaller than it is today, temperatures dropped to those found on the surface of an average sun-like star, cool enough for electrons to be captured in orbit around the electrically charged hydrogen and helium nuclei. Suddenly, across the Universe, the first atoms formed and the Universe underwent a rapid transition from a hot plasma of electrically charged particles to a hot gas of electrically neutral particles. This had dramatic consequences, because photons interact far less with electrically neutral atoms. The Universe became transparent, which means the photons stopped zig-zagging around and started to head off in straight lines. The majority of these photons continued onwards, travelling in straight lines for the next 13.8 billion years. Some of them are just arriving at our Earth today in the form of microwaves. These ancient photons are messengers from the earliest times, and they carry a treasure trove of information that cosmologists have learnt to decode.
As the Universe continued to expand, its denser regions, composed mainly of dark matter, became ever denser under the action of gravity. Hydrogen and helium atoms clustered around the dark matter, and swirling atomic clouds grew until the densest regions collapsed inwards, increasing the pressure and temperature at their core to such an extent that they became nuclear furnaces; the fusion of hydrogen into helium was initiated, and stars formed across the Universe. A hundred million years after the Big Bang, the cosmic dark ages came to an end and the Universe was flooded with starlight. The most massive stars had brief lives and, as they ran out of hydrogen fuel, they began to fuse heavier elements in an ultimately futile battle with gravity: carbon, oxygen, nitrogen, iron – the elements of life – were made this way. When the fuel finally ran out, these stars scattered the newly minted heavy elements across space as they ended their lives as bright planetary nebulae or exploding supernovae. In a final flourish, the violent shock of each exploding supernova synthesized the heaviest elements, including gold and silver. New stars formed from the debris of the old, and congregated in their hundreds of billions in the first galaxies. The galaxies, numbered in hundreds of billions, were moulded into the giant filamentary webs that criss-cross the Universe by the gravitational pull of the dominant dark matter.
4.6 billion years ago in the Milky Way galaxy, a gas cloud enriched in stellar debris collapsed to form our Sun. Shortly afterwards, the Earth formed from the remains of the cloud. Then, 4 billion years ago, in a great ocean created from hydrogen formed in the first minute of the Universe’s life and oxygen forged in long-dead stars, the geochemistry of the young Earth became biochemistry: life began. In 1687 Isaac Newton published the Principia Mathematica. We’ve obviously skipped a bit of biology.
This is the broad outline of the story of the evolution of the Universe, from before the Big Bang to Isaac Newton. It seems that collections of atoms on a cooling cinder, in possession of a precious thing called science for barely an instant, have found a way to glimpse the fires of creation. The rest of this book is the story of how we did it.
The Earth is 4.55 billion years old, give or take 50 million years. This is a figure consistent with independent measurements of the age of the Universe, which place the Big Bang 13.8 billion years ago. It is also consistent with physical biological evidence and our understanding of evolution by natural selection, which suggest that the first living things appeared on Earth around 3.8 billion years ago. The life cycles of stars fit into this timeline too. The age of our Sun is estimated at 4.6 billion years, and similar stars are predicted to live for around 10 billion years before they die. More massive stars have much shorter lifetimes. There must have been time for at least some stars to live and die before the Earth formed, because the Earth is made out of heavy chemical elements like iron, carbon and oxygen: elements that are made inside stars. Leaping forward in time, the basalt columns of the Giant’s Causeway in Ireland were formed 60 million years ago, around the time the dinosaurs became extinct. The oldest living tree is a bristlecone pine that lives in the White Mountains in California. It is – as of 2016 – 5066 years old.
All these dates are determined using very different kinds of science, but, remarkably – impressively – they fit together without contradiction. There is nothing special about this particular list; we chose this eclectic bunch simply because they reflect a variety of different ‘old’ things, and we could have chosen a different list. This raises the question: how do we know how old something is? Age is not a trivial thing to determine, especially for very old things, because it must be inferred indirectly. We can’t sit around and watch while the Universe evolves from the hot plasma of its birth. We can’t even point to direct evidence for the age of the oldest tree; nobody was around to write about it and record the date when it was a tiny sapling. But we don’t need to have been present: knowledge can be acquired indirectly if we do a little detective work to collect evidence and then apply simple logic to draw conclusions. This book is all about taking a scientific approach to securing knowledge of the world around us. This approach is incremental – a framework of knowledge grows over time as we understand more about the Universe – and it sits in stark opposition to haphazard thinking: you don’t build a computer by trial and error and you are prone to mistakes if you don’t entertain the likelihood that you may be wrong. We trust our lives to scientific knowledge, in hospitals and aeroplanes, and exactly the same type of thinking can be used to great effect elsewhere in our lives. In this book, we will show how far it is possible to travel in understanding the Universe by taking simple, reasoned steps coupled with careful observations. In this chapter, we are going to begin by exploring the science that allows us to measure the age of things with such confidence and precision.
Let’s begin with the age of the Earth. A very obvious way to start is to look at what we can see: to ask whether there are any features on the Earth’s surface that might give us a clue to its age. To take a careful look at Nature, in other words, and see what we can work out from simple observation. For example, we know that river valleys are cut by flowing water, and that coastlines are subject to erosion. These are features that change with time; therefore, observing them carefully and understanding the physical processes that formed them should allow us to estimate their ages. On larger scales still, could the familiar shapes of the continents and oceans also tell us something about the way they have evolved, and how long it has taken them to do so?
Figure 2.1 is a map of the Atlantic Ocean and the landmasses that surround it. South America and Africa in particular look as if they fit together. Let’s suppose this fit is no accident and make a proposal: the continents were snuggled together at some time in the past, and have been gradually moving apart ever since. If this theory is correct, then we can make a rough estimate of the age of the Atlantic Ocean. Of course, this isn’t a new idea – Alfred Wegener’s idea of a global-supercontinent that broke up over time as a result of continental drift is over 100 years old. The point here, and throughout this book, is that we can uncover the science for ourselves – we want to follow in the footsteps of the great scientists, to appreciate how irresistible progress comes from simple thoughts. As a first step, we need to confirm that the broad outline of our hypothesis (that South America and Africa were once joined and have been moving apart ever since) is plausible by checking whether the Atlantic is still growing today. If it is, we can measure the current rate of separation of the continents, and – if we make the further assumption that this rate has stayed constant since the time that the continents began to separate – we will be able to make an estimate of the age of the Atlantic. There are a lot of assumptions here, but let’s get on with it and see what we find.
If we were very committed experimentalists, we could measure the movements of the continents ourselves. We could pack a couple of GPS receivers into a rucksack, fly to the eastern coast of Brazil, fix one of the receivers to the ground, fly back across the Atlantic to northwest Africa – a distance of around 4000 km – and set up the second GPS receiver. Over the next few years, we could monitor how the receivers move relative to each other. We don’t need to do this, because geologists have already been making such measurements for many years. Quite wonderfully, apart from using GPS receivers, the distance between North America and Europe has also been measured using a pair of radio telescopes (one in Europe and one in the USA) each focused on a distant quasar. Quasars are active galactic nuclei that most probably originate as matter accretes onto super-massive black holes in the centres of galaxies, and they are among the brightest objects and therefore the most distant we can see. Because they are so far away, they serve as excellent fixed points on the sky, which is important for triangulating the distance between Europe and the USA. We describe the measurement in a little more detail in Box 1. Do you remember those school science experiments where you had to begin with the heading ‘Apparatus: two large radio telescopes and a grid system comprising active galactic nuclei over a billion light years from Earth’?
Figure 2.3 shows a summary of the results measuring the present-day rates at which the various tectonic plates are moving. It shows that the Atlantic, between northern Brazil and northwest Africa, is currently expanding at a rate of 2.5 cm per year, which is the speed at which fingernails grow.
The distance between two radio telescopes on the Earth’s surface can be determined using a technique known as Very Long Baseline Interferometry. The two telescopes look at the same distant object in the sky, and from the difference in arrival time of light signals – determined using very precise clocks accurate to 1 second in 1 million years – the distance between the telescopes can be determined to millimetre accuracy. Quasars are so bright that they are visible at distances of many billions of light years, and being so far away guarantees that they appear still during the time of the measurement. Over twenty years, telescopes in Westford, Massachusetts, and Wettzell, Germany, have been used to determine the rate at which the Atlantic is opening between Europe and the United States. The data are shown in Figure 2.2, which shows a rate of spreading in this region of 1.7 cm/year. Satellite laser ranging, which involves bouncing laser light off satellites, and GPS measurements are also used along the length of the North and South Atlantic, and give consistent results.
Working on the assumption that the continents have always been moving apart at this rate, we can now estimate the age of the Atlantic Ocean: 4000 km × 40 years/metre = 160 million years. If this figure is a good estimate, then we now also have a minimum age for the Earth – because obviously it can’t be younger than the Atlantic Ocean.
We’ve just done what could be described as a ‘back of the envelope’ calculation. Obviously, we’d like to know if our number is anywhere near correct; after all, we did make a bold assertion and a very bold assumption. We asserted that the continents were once part of a single landmass and assumed that they have been moving apart at a steady rate ever since. Let’s examine these assumptions more closely and try to judge how reasonable they are.
Look back at the map in Figure 2.1. It also shows the topology of the Atlantic Ocean’s floor. The great range of underwater mountains running down the centre is called the Mid-Atlantic Ridge. This ridge clearly mirrors the shape of the continents on either side; it’s also bang in between the two continents, in the middle of the Atlantic, and is currently spewing out material from the Earth’s interior: lava that solidifies and forms a crust. This suggests a mechanism that could explain why the continents are continuing to move apart today: new ocean crust is being formed along the Mid-Atlantic Ridge.
All of which seems to indicate that our assertion is in good shape. We could, of course, have been fooled by a series of coincidences: (i) that the coastlines appear to fit together and match the shape of the Mid-Atlantic Ridge; (ii) that the Mid-Atlantic Ridge lies midway between the continents; (iii) that the lava erupting from the Mid-Atlantic Ridge has nothing to do with the currently observed widening of the ocean. But although we can be pretty confident that these are not simply coincidences, nothing we have established so far implies that the separation of these two continents has been proceeding at the same rate for over a hundred million years, and we must admit that, at this stage, this assumption is a blind guess.
The age of sea-floor rocks in the Atlantic is determined by exploiting the fact that sea-floor basalt is magnetized in a stripy pattern, as shown in Figure 2.5. The stripes, typically some tens of kilometres wide, are formed as new rock spews out of the ridge and becomes magnetized by the Earth’s magnetic field. When the rock freezes, the magnetic orientation gets frozen within it. The stripes appear because the Earth’s magnetic field flips its direction from time to time, and these changes of direction are encoded in the rocks. We can therefore map the temporal evolution of the sea floor by measuring the barcode-like pattern of stripes, known as ‘polarity chrons’, so long as we have some method of setting the timescale for the flips in the Earth’s magnetic field. And we do: radiometric dating methods have been used to date rocks in other locations, such as on-land lava flows. The barcode patterns match.
In December 1968 and January 1969, the Glomar Challenger, a scientific research drill-ship, acquired a very important dataset by drilling a series of seventeen holes in the equatorial and South Atlantic, many of them traversing the Mid-Atlantic Ridge. The samples the Glomar Challenger collected were dated mainly by paleontological methods, which involved looking for tiny fossils in the sample cores and matching them with known stages in the evolution in the flora and fauna of the oceans (whose ages themselves are fixed using radiometric methods). The shipboard scientists involved analysed the cores and found an age–distance relationship from the Mid-Atlantic Ridge that is remarkably consistent with the assumption that the sea floor has been spreading at a constant rate. They found sediments sitting directly above the sea-floor with ages ranging from 10 million years for samples 200 km from the ridge, all the way to 70 million years for samples taken 1300 km from the ridge, corresponding to a sea-floor spreading rate of close to 2 cm/year.
So let’s bring in some serious science. For decades, geoscientists have meticulously examined ocean floors across the globe and determined the age of the rocks on the seabed. This is a difficult task, and requires some beautiful science that we will discuss in a moment (see also Box 2). For now, let us just present the data, which is shown in Figure 2.4. There is a very clear pattern in the Atlantic: the youngest rocks lie along the Mid-Atlantic Ridge; the oldest are to be found bordering the continents. This fits very nicely with our proposal that the Atlantic was formed by sea-floor spreading from the Mid-Atlantic Ridge; if we are right, the rocks on the seabed should indeed get progressively older the further we travel from the ridge. Notice also that there are no sharp transitions where the rocks suddenly get much older, nor are there any extended regions where the rocks are all the same age. This is what we would expect if the rate at which new rock is being formed along the Mid-Atlantic Ridge has remained roughly constant during the time that the continents have been moving apart. The final observation we can make is to look at the age of the rocks lining the ocean floors along the edges of the continents. These are dated to be around 180 million years old – in broad agreement with our back-of-the-envelope calculation.
We haven’t yet described how we go about dating rocks directly. But we can say that our suggestion that the Atlantic Ocean was created by the continents drifting apart due to geological activity along the Mid-Atlantic Ridge is consistent with the measured age of the rocks on the ocean floor.
Logical consistency and the accumulation of evidence are very important features of the way modern science works. Consider, for example, what would happen to our previous logic if the Atlantic were significantly younger than 160 million years. For the sake of argument, let’s go with Bishop James Ussher, and say it is around 10,000 years old. This rather casual level of precision is doing the good bishop a disservice, because he was very specific. He asserted that the world was created on the evening of 22 October 4004 BC. The bishop performed his calculations in the late seventeenth century, using historical records and the Bible. We, on the other hand, are operating on the back of an envelope, which means we are content to work with round numbers.
If we want to accommodate an Atlantic that is 10,000 years old, but still accept that the two continents were both close together at some point, then the rate at which the continents moved apart would have had to have been much faster than the currently observed 2.5 cm per year. Instead, we would require an expansion rate of the order of 400 metres per year for most of the 10,000-year period.
The problem with an expansion rate of 400 metres per year is that the rocks along the Atlantic shores are measured to be 180,000 years old, a date that is in good agreement with the 2.5 cm per year spreading rate. If we insisted on a 10,000-year-old Earth, then it must follow that the rock ages are wrong by precisely the same factor as the spreading rate estimate is wrong. This would be quite a coincidence.
With Bishop Ussher’s dating still in mind, a second possibility might be that the continents were never in fact close together, but instead they were originally created 4000 km apart, 10,000 years ago. In that scenario, the fact that the observed drift rate of 2.5 cm per year just happens to be consistent with the age inferred from dating the rocks must be regarded as a meaningless coincidence, not least because we would also need to reject as wrong the methods used to date the rocks. In addition, we’d also have to suppose that the two continents and the Mid-Atlantic Ridge all fit together quite by accident. It is clear then that the case for a young Earth requires we reject the most obvious interpretation of the facts and appeal instead to coincidence and error. We have only been studying the case of the Atlantic Ocean so far and we will meet some more examples of very old things in due course. It is up to you to judge the extent to which the evidence is convincing.
The reason that it is so difficult to make an argument against the Atlantic Ocean being around 160 million years old is that independent measurements, relying on completely different science, combine to provide a consistent picture of what happened. It is easy to cook up a scenario, however fanciful, that casts doubt on some measurement or other. But it is usually extremely difficult to argue for a radical change in one area without making large parts of the whole interlinked edifice inconsistent. Given that the scientific edifice is also the thing that keeps your lights on, keeps aircraft in the sky and makes your computer work, this is not usually a sustainable position to take. Our modern scientific world-view is a universal one, and this is a key reason why it is so robust and successful.
One of the most precise ways of dating old rocks is through radiometric techniques. The key idea is that certain types of atoms are radioactive, which means that they can spontaneously transform into atoms of a different type. (In Box 3, we provide a primer on the basics on atoms and radioactivity.) This transformation process is known as radioactive decay. If we know the rate at which a particular type of atom decays, then by counting the number of those atoms present in a rock sample we can obtain a measure of how much time has passed since it was formed. We do not need to know what causes atoms to decay (for that we have to understand some quantum physics); we just need to know the rate at which atoms decay, which is called the half-life. The half-life expresses how long, on average, it takes for half the atoms in a sample to decay. For example, if we know that a rock sample initially contained N radioactive atoms, and we measure that it currently contains N/4 atoms – that’s to say, a quarter of the radioactive atoms originally present – we can deduce immediately that two half-lives have elapsed since the rock was formed.
Some atoms have short half-lives of much less than one second; others have long half-lives, reaching into the billions of years. If we want to determine the age of a rock, the best way would be to count atoms whose half-life is not too different from the age of that rock. If the half-life is much less than the age, most of the radioactive atoms will have decayed away, and we will have a difficult job counting the small number that remain. If the half-life is much greater than the age, very few atoms will have decayed and we might struggle to determine any significant deviation from the initial number. None of this would be of any practical use if radioactive atoms were rarely found in rocks. Fortunately, they are relatively common.
You may have already noticed that there could be a flaw in our plan. How could we possibly know how many radioactive atoms were present in a given rock when it first formed? This might seem to scupper the half-life dating procedure. However, there is a beautiful way to sidestep the problem, known as the isochron method.
In order to understand the isochron method, let’s look at a specific atom, rubidium-87, which we will write as 87Rb. Rubidium is chemically similar to potassium, about as abundant as zinc, and often found in common, potassium-rich minerals in rocks. It is radioactive, with a very long half-life of 48 billion years. Being barely radioactive is a bonus for dating the oldest rocks on Earth because, as we will see, they have ages of several billion years. When a rubidium atom decays, it converts into an atom of strontium-87 (87Sr). We can count the numbers of 87Rb and 87Sr atoms in a sample of rock. The clever part of the isochron method is to exploit the existence of a different sort of strontium atom, strontium-86. 87Sr and 86Sr are different isotopes of strontium; the only difference is that 86Sr has 1 less neutron in its nucleus. This means that they are chemically identical – and this is crucially important. Also crucially, 86Sr cannot be produced through radioactive decay, which means that any 86Sr now present in the rock sample was there when it was originally formed.
In a sample from the rock we want to date, we count the number of 87Rb atoms and divide by the number of 86Sr atoms. We also count the number of 87Sr atoms and divide that by the number of 86Sr atoms. We mark these two ratios as a point on a graph, as shown in Figure 2.6878687