Universal

2. HOW OLD ARE THINGS

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.

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.

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.