Wonders of the Universe - Brian Cox, Andrew Cohen (2011)

Chapter 4. DESTINY


This is the story of something so fundamental that it’s impossible to imagine a universe without it, yet it is a property of the Universe that modern science still struggles to explain. Time is something that feels very human; it regulates our days and its relentless and unavoidable passing drives our lives forward. It is why each one of us has a beginning and an end. But time isn’t a human creation; we evolve with its passing, but so does the rest of the Universe. Time is woven into the very fabric of the cosmos. Even with our incomplete understanding, our exploration of time has allowed us to do something remarkable: just by investigating the nature of time and the natural world as we find it here on Earth, we’ve been able to not only glimpse the beginning of the Universe, but to imagine how it might end.


The towers of the ruined temple of the ancient hilltop fortress at Chankillo are a remarkable sight, standing tall through the sand-laden skies of the Peruvian desert.

On the arid coastal plain of northwestern Peru lies one of South America’s greatest astronomical secrets. Few people know about the hilltop fortress at Chankillo, and even fewer visit it, but for archaeologist and astronomer alike it is both evocative and fascinating. Two and a half thousand years ago, a civilisation we know almost nothing about built a city in this inhospitable place. The grandest of the structures was a fortified temple with walls of brilliant white covered with red-painted figures. Commanding a sweeping view across the desert, the temple would have dominated the sand-laden skies, however, today all but the smallest fragments of the decorations are gone, dulled by passing centuries. The building’s location has puzzled archaeologists for many years because, while it is commanding, the hilltop site is not the best defensive position in the area, and it is unimaginable that the residents of Chankillo made a mistake when siting their fortress. Recent research has suggested that the key to understanding this place may lie not on the hilltop, but on the desert plain below.

Away from the ruined fortress and aligned north to south along the ridge of a nearby hill are thirteen towers. Recent excavations have uncovered further buildings to the east and west of the towers which archaeologists now believe to be intimately connected to this reptilian structure’s true purpose. To see why, you must stand at the western observation point at the end of a night, facing the brightening eastern horizon through the towers. I have seen many sunrises, but nothing as dramatic and evocative as a Chankillo dawn. The edge of the solar disc, reddened and distorted by air heavy with sand, suddenly flares between two of the towers on the hill, and for the briefest of moments the Sun emerges as a single sparkling diamond in the desert sky. Within seconds, the normally imperceptible rotation of our planet drags the star into full view, and you must avert your gaze as if to avoid staring into the face of a god.


The Thirteen Towers of Chankillo are more than a temple, however. It is thought that they are an ancient calendar, diligent timekeepers that have measured the passing of the days for thousands of years, outliving their creators by millennia. There is no clockwork here, no pendulums or cogs to keep the timepiece ticking; instead, time is measured using the most reliable pulse that the ancients had at their disposal – the Sun. In a beautiful piece of grand astronomical engineering, the thirteen towers are placed to mark the passing of time using the position of the sunrise on the eastern horizon. On 21 December, which in the Southern Hemisphere is the summer solstice – the longest day – the Sun rises just to the right of the most southerly tower, marking the beginning of a journey that will take it across the horizon as Earth orbits the Sun. As the year passes, the sunrise moves along the towers until, on 21 June – the shortest day – it rises just to the left of the northerly tower. So at any time of year, watching the sunrise at Chankillo would have allowed its inhabitants to determine the date within an accuracy of two or three days. I stood at the western observing point on 15 September, aware that the Sun has risen between the fifth and the sixth towers on this morning for the past two thousand years. Chankillo still works as a calendar because the Sun still rises and sets in very nearly the same places on the horizon today as it did when these stones were first set down.

The Thirteen Towers of Chankillo…stand testament to our ancestor’s instinct and desire to quantify and understand the ticking of the cosmic clock.

Even though I understand the true nature of the Sun, when confronted with such a magnificent sunrise in such a dramatic and quiet place, I understand why these people would have almost certainly deified it. The high status of this place is clear, in that the scale of Chankillo is far grander than is necessary simply for a calendar. It is part-clock, part-temple, part-observatory; a place where on sacred days the people of Chankillo would have been able to greet the appearance of their god, the rising Sun, in the most spectacular of settings.

Today, the Thirteen Towers of Chankillo continue to tell the time, having long outlived their creators; they stand testament to our ancestors’ instinct and desire to quantify and understand the ticking of the cosmic clock image





The Thirteen Towers of the temple at Chankillo are believed to serve a dual purpose: they are also an ancient calendar. The towers are carefully placed to use sunrise to mark the passing of the days.



Each day we awake to the rhythm of our planet as it spins at over 1,500 kilometres (932 miles) an hour, relentlessly rolling us in and out of the Sun’s glare. Earth’s ceaseless motion beats out the tempo of our lives with unerring repetition. A day is the twenty-four hours it takes Earth to rotate once on its axis; the 86,400 seconds it takes for anyone standing on the Equator to be whipped around the 40,074-kilometre (24,901-mile) circumference of our planet. This is the most obvious rhythm of the Earth, which comes about because of the spin rate of our rocky, ironed-cored ball that was laid down somewhere in Earth’s formation and 4.5-billion-year history.

Travelling at 108,000 kilometres (67,108 miles) an hour, we move through space in orbit around our star. Racing around the Sun at an average distance of 150 million kilometres (93 million miles), we complete one lap of our 970-million-kilometre (600-million-mile) journey in 365 days, five hours, 48 minutes and 46 seconds, returning regularly to an arbitrarily defined starting point. As we sweep through this place in space relative to the Sun, we mark the beginning and end of what we call a year.

Everywhere we look in the heavens we see celestial clocks marking the passage of time in rhythms. Our moon rotates around Earth every 27 days, seven hours and 43 minutes, and because it is tidally locked to Earth it also takes almost exactly the same amount of time to rotate on its own axis: 27 Earth days. This means that the Moon always presents the same face to Earth. Further out in the Solar System, a Martian day is very similar to our own, lasting one Earth day and an additional 37 minutes. But because Mars is further from the Sun, a Martian year lasts longer, with the red planet taking 687 Earth days to complete an orbit. In the farthest reaches of the Solar System, the length of a year gets progressively greater, with distant Neptune taking over 60,000 Earth days or 165 Earth years to make its way around its parent star. In September 2011, Neptune will have completed its first full orbit of the Sun since it was discovered in 1846.


Here on Earth our calendar is determined by the clockwork rhythm and movement of our planet as it rotates on its axis, working its way through space and along its annual orbit around the Sun.

As we look deep into space, the clockwork of the cosmos continues unabated, but as the distances extend, the cycles become grander, repeating on truly humbling timescales. Just as Earth and other planets mark out the passing of the years as they orbit the Sun, so our entire solar system traces out its own vast orbit. We are just one star system amongst at least 200 billion in our galaxy, and all these star systems are making their own individual journeys around the galactic centre. We are all in orbit around the super-massive black hole that lies at the heart of the Milky Way. It is estimated that it takes us about 225 million years, travelling at 792,000 kilometres (492, 125 miles) per hour to complete one circuit, a period of time known as a galactic year. Since Earth was formed four and a half billion years ago, our planet has made 20 trips around the galaxy, so Earth is 20 galactic years old. Since humans appeared on Earth a quarter of a million years ago, less than one-thousandth of a galactic year has slipped by. In Earth terms, that is the length of a summer’s afternoon.

This is an immense amount of time; difficult to comprehend when we speak of the entire history of our species as the blink of a galactic eye. We live our lives in minutes, days, months and years, and to extend our feel for history across a galactic year is almost impossible. Yet here on Earth there are creatures that have existed for lengths of time that span these grandest of rhythms image


Nothing stays still in the Universe, our galactic clock is forever ticking, moving everything on to a new chapter in the story of the Universe, marking out the days, weeks, months and years in each and every planet in our galaxy. Everywhere in the heavens, time moves on using its own rhythms; as you journey to the farthest reaches of the Solar System the length of a year gets progressively greater and the cycles become grander. Every solar system among the 200 billion that exist in our galaxy makes its own unique journey around the galactic centre, as we all orbit the supermassive black hole that lies at the heart of the Milky Way Galaxy.


Nathalie Lees © HarperCollins



Pregnant sea turtles return to the sands on the Pacific coast year after year in one of the oldest life cycles on Earth.

The Ostional wildlife refuge on the Pacific coast of Costa Rica is home to one of nature’s most spectacular sights. On many nights of the year, a small number of tropical beaches along this thin land bridge between North and South America are visited by prehistoric creatures. They emerge from the ocean to lay their eggs in the sand. We filmed on Playa Ostional, a tiny strip of sand which is adjacent to a friendly village clustered around a makeshift football pitch. It is one of the few beaches in the world where large numbers of sea turtles make their nests, and the events that occur here form part of one of the oldest life cycles on Earth.

We are here to film the turtles hauling themselves from the ocean as they have done year on year without interruption for over 120 million years – half a galactic year. As we wait for them with our night-vision camera equipment, it is hard not to reflect on the sheer size of the mismatch in the histories of these ancient creatures and the species that built the football pitch by the sea. We humans know our planet well. We know there is a landmass called Europe, separated from Africa by a thin strip of ocean. We know that if you journey east from northern Europe you cross the vast expanses of Siberia and arrive eventually in Japan. Carry on, and you’ll cross the Pacific Ocean and meet the Californian coast in the United States. The shape of our countries and continents is familiar and seemingly eternal, but the ancestors of the turtles I can see bobbing offshore were waiting for the right moment to crawl out onto the land when the shape of our continents was very different; they were waiting one hundred million years ago in the same ocean, but in those days the beaches marked out shorelines of continents that would be totally unrecognisable to our eyes. As the turtles patiently waited for their moment to give birth in the sand, the continents of Earth were slowly on the move. North America was close to Europe, South America was connected to Africa and Australia was joined with the Antarctic. It is moving to see the care with which these ancient creatures dig deep into the sand to protect their precious eggs, but equally powerful to reflect on the temporal mismatch between us and them. Collectively, they have witnessed the reshaping of our planet and the heavens above; the patterns of the stars must look very different from the other side of the Galaxy. I watch as one after another of these beautiful creatures covers its eggs and silently return to the ocean.


Humans have long been measuring time, and we’ve developed our skills from the bluntest of temporal measurements to the extreme accuracy with which we can measure time today. The first attempts in chronometry may have begun thirty thousand years ago, when Stone Age humans used the lunar cycle to mark time. To early humans, the Moon would have marked out the clearest rhythm in the night sky, and by following it through its phases they were able to create the first calendars. Giving structure to the year beyond the day– night cycle allowed them to name periods of time, and so our classification and division of the cycles of the cosmos began.

Beyond the naming of the morning, afternoon and evening, the fine division of the day required the invention of one of our most enduring pieces of technology, the influence of which has been incalculable.

The first clocks were simple pieces of technology employed throughout the ancient world. Using nothing more complicated than a stick known as a ‘gnomon’ to cast a shadow, many civilisations were able to use sundials to track the passing of time during the day by measuring the movement of the shadow across a calibrated surface. Sundials are surprisingly accurate, but they have limited use as timekeepers, not least because they are difficult to use on a cloudy day and impossible to use at night!

Ancient Egypt was the first civilisation we know of that took measuring time beyond the sundial. The technique of using the flow of water to measure time may date as far back as 6000 BC, but the oldest physical evidence of a water clock can be found in the reign of Pharaoh Amenhotep III in 1400 BC. These elegant devices were simply stone vessels that allowed water to escape at a near-constant rate from a hole in the base. Inside the clock were twelve markings by which time could be measured as the water level dropped. These primitive clocks gave accurate measurements both night and day so that priests could perform their rituals at the appointed hour.

Water clocks continued to be refined and used by cultures across the globe for many centuries, and hourglasses employing the flow of sand to measure time were also used extensively. The Portuguese explorer Ferdinand Magellan used 18 hourglasses as a navigation tool on his ship when he circumnavigated the globe in 1522.

Time keeping was elevated to a completely new level of accuracy with the invention of pendulum clocks. Galileo was the first scientist to investigate the physics of a swinging pendulum. The key property of the pendulum, which makes it useful as a timekeeping device, is that the period of the swing – the familiar tick-tock of the clock – depends only on the length of the pendulum and Earth’s gravitational pull. Perhaps counterintuitively, the period doesn’t depend on how high you lift the pendulum to start the swing, as long as it’s not too high. Physics students have the formula for the time period of a pendulum permanently etched in their minds. It is:


where T is the period, L is the length and g is the acceleration due to gravity – in other words, a measure of the strength of Earth’s gravitational field, which is almost the same wherever you are on Earth; approximately 9.81 metres (300 feet) per second squared. This means that all you need to do to make a clock that ticks accurately is get the length of the pendulum right. Most grandfather clocks have a pendulum that swings with a period of two seconds, which a little simple mathematics will tell you requires a pendulum approximately one metre long. The Dutch astronomer Christiaan Huygens invented the first pendulum clock in 1656, and it remained the most accurate way of telling the time until the 1930s.

Christiaan Huygens invented the first pendulum clock in 1656, and it remained the most accurate way of telling the time until the 1930s.


Galileo first investigated the physics of a swinging pendulum and how it could be used effectively for keeping time.

Today we rely on atomic clocks to measure time with extraordinary accuracy. Atomic clocks use the frequency of light emitted when electrons jump around in atoms (usually caesium) as the ‘pendulum’. This is highly accurate because the structure of atoms is unchanging, and therefore the light emitted from them always has the same frequency. This light can be used, with some clever engineering, to keep an oscillator ticking at a precise rate, allowing atomic clocks to tell the time with an accuracy of one-thousand-millionth of a second per day. The second itself has been defined since 1967 using the theory behind atomic clocks; one second is defined as the duration of 9,192,631,770 periods of the radiation corresponding to the transition between the two hyperfine levels of the ground state of the caesium 133 atom. In English, this means a second is the time it takes for 9,192,631,770 peaks in a wave of light, emitted when an electron makes a specific jump in an atom of caesium, to fly past you.

Atomic clocks allow us to measure incredibly small periods of time. Until now, the shortest period we have been able to measure is 12 attoseconds, or 12 quadrillionths of a second. This is how long it takes light to travel past 36 hydrogen atoms lined up together. That’s not far at all image

For all the accuracy and precision we have achieved in keeping time, we have never managed to do anything more than observe it. From the very earliest solar calendars to the electrons jumping around in caesium atoms, one thing about the nature of time is clear: we can measure its passing, but we cannot control it. It moves inexorably forward; it cannot be stopped. This tells us something profound about our universe.


The Perito Moreno glacier in Patagonia, Southern Argentina, is a stark but beautiful place where the passage of time moves progressively forward but so slowly that it almost goes unnoticed.


Few places on our planet are as spectacular as the Perito Moreno glacier in Patagonia, southern Argentina. This dense blue wall of frozen water in the Los Glaciares National Park is part of a system of hundreds of glaciers that sweep down the continent from the southern Patagonian ice fields. Together they form the third-largest icecap on our planet. The Perito Moreno glacier alone covers an area of 250 square kilometres (96 square miles) and in places it is 170 metres (560 feet) deep. The ice ends where solid meets liquid at Lake Argentino; a great wall of ice towers over the surface of the lake, and the few who make it to this bleak but utterly beautiful place have the chance to sail along its edge across one of the most dramatic expanses of water in the world.

At first sight the glacier appears static and unmoving; standing on the lake shore, this seems like a place where the passage of time goes as unnoticed as the laws of physics will allow. Yet there is a reason why boats don’t venture too close to the edge of the ice cliff. As we approached I didn’t only see the passage of time; I felt it. Tthis glacier is in constant motion; relentlessly carving its way down from the Andes as it has done for tens of thousands of years. At the glacier’s edge, the wall of ice is 70 metres (230 feet) high, and the whole face of the glacier is sliding into the lake at around 50 centimetres (20 inches) per day. That means that well over a quarter of a billion tonnes of ice cascades into the lake every year. You don’t often see it, but you can hear it; every now and then there is a tremendous cracking sound, followed by a deep rumbling. The surface of the lake comes alive as a turbulent wave powers beneath your boat. The pace of change in this place is anything but glacial. It is so vast and complex that you perceive it to be alive; an unpredictable, overwhelmingly powerful organism clawing the land in vain as it inevitably slides into the waters.

This is all part of a highly ordered sequence. As time passes, snow falls, ice forms, the glacier gradually inches down the valley, and when the ice meets the water, pieces break off and fall into the lake creating waves. In many ways this ordering of events into a sequence is the simplest way to think about time. The fact that sequences of events always happen in order is a fundamental part of our experience of the world. We expect to see ice fall from the glacier, splash into the water and create waves. If it happened in any other way we’d immediately know there was something wrong. Yet there is a legitimate question here about what we mean by events happening ‘in order’. However long we might stand on the edge of this beautiful lake we would never expect to see this dramatic sequence of events happen in reverse, even though there is nothing in the laws of nature that prevents this happening. There is no physical reason why all the water molecules moving around in the lake shouldn’t gather together on the surface, reduce their collective temperature such that they bind together to form ice, jump out of the water and glue themselves onto the surface of the glacier. We do, however, have a scientific explanation for why such a dramatic reversal never happens; we call it the ‘arrow of time’.

We expect to see ice fall from the glacier, splash into the water and create waves. If it happened in any other way we’d immediately know there was something wrong.


This phrase was first used by the British physicist Sir Arthur Eddington in the early twentieth century to describe this deceptively simple and yet profound quality of our universe: it always seems to run in a particular direction. Eddington was instrumental in bringing Einstein’s theory of relativity to the English-speaking world during the First World War, and also one of the first scientists to directly confirm the findings of relativity when he led an expedition to observe the total solar eclipse on 29 May 1919. In 1928 he published The Nature of the Physical World, in which he introduced two great ideas that have endured in popular scientific culture to this day. The first was the image of the infinite monkey theorem, which states that given an infinite amount of time, anything consistent with the laws of physics will happen: ‘If an army of monkeys were strumming on typewriters, they might write all the books in the British Museum’. This is related in a deep way to the arrow of time, which Eddington described as follows:



A great wall of ice towers over the surface of Lake Argentino, where this vast, seemingly immovable glacier is slowly and relentlessly sliding down into the icy waters below.

‘Let us draw an arrow arbitrarily. If as we follow the arrow we find more and more of the random element in the state of the world, then the arrow is pointing towards the future; if the random element decreases the arrow points towards the past. That is the only distinction known to physics. This follows at once if our fundamental contention is admitted that the introduction of randomness is the only thing that cannot be undone. I shall use the phase “time’s arrow” to express this one-way property of time which has no analogue in space.’

Eddington’s arrow vividly and economically expresses a key property of time; it only goes in one direction. But what does he mean by randomness? It seems obvious that the Universe is constantly evolving, but what drives this evolution? How should we quantify how random something is? Why is the past different from the future? Why is there an arrow of time? Time is something we all understand, and yet a plausible scientific reason as to why time marches inexorably forward wasn’t offered until the late nineteenth century, coming about as the solution to a practical problem on Earth image


In 1712 the English inventor Sir Thomas Newcomen created the first commercially successful steam engine, paving the way for the Industrial Revolution. This accolade is more usually awarded to the Scottish inventor James Watt. In 1763 Watt was asked to repair a Newcomen engine by the University of Glasgow, and in doing so he developed a new steam engine which, it is appropriate to say without hyperbole, transformed the landscape of modern life. Watt’s steam engine was more efficient and more flexible than its predecessor; it used far less coal than the Newcomen for a given power output, and was therefore much cheaper to run. More importantly still, Watt’s engine could do more than pump water out of the wet mines, it could also generate the rotary motion that was needed to power the machines on the factory floor. No longer did a factory have to be situated by a river to turn its equipment; with the help of Watt’s engine a factory could be sited anywhere, catalysing the emergence of the modern industrial landscape. Steam-powered machines changed the course of history, and yet despite their importance, the nineteenth-century engineers who followed Watt struggled to improve them. There seemed to be fundamental principles that restricted their efficiency, but with profit margins to maximise, even a small increase in their effectiveness would be highly valuable. So understanding how hot the fire should be or what substance should be boiled in the engine were problems that were not only interesting from a scientific perspective but were also critical for businesses. It was out of these questions of engineering design that the science of thermodynamics arose, and with it the concepts of heat, temperature and energy entered the scientific vocabulary in a precise way for the first time.


In a series of simple experiments, Joule demonstrated that mechanical work could be converted into heat. Using a paddle wheel turned by falling weights, he stirred water in an insulated barrel and observed how the temperature of the water rose by the amount that depended on how far the weights fell.

One of the scientists working on these problems was the German mathematician Rudolf Clausius. Clausius was interested in heat, which until the first half of the nineteenth century was thought to be a fluid that flowed from hot things to cold things. Clausius and others realised that this description was not able to explain the cycle of a steam engine. The foundation for Clausius’s theoretical advances was laid by one of his contemporaries, the English physicist and brewer James Joule, who was working to improve the efficiency of the steam engines in his brewery. What finer motivation for the advance of fundamental physics? The quest for cheaper beer motivated him to investigate the relationship between the work his steam engines could do, and heat. In doing so he managed to reduce the costs of beer production and lay one of the cornerstones of the science of thermodynamics.

Using a series of beautifully simple experiments, Joule was able to demonstrate that mechanical work could be converted into heat. One such experiment used a falling weight to spin a paddle within an insulated barrel of water. Joule knew how much work was done by the falling weight and so could measure the temperature rise of the water. He conducted similar experiments on compressed gases and flowing water, and each time he found that it took the same amount of work to raise the temperature of a fixed amount of water by one degree Fahrenheit. Inscribed on his tombstone in Brooklands cemetery near Manchester is the number 772.55 – his measurement of the amount of work done in foot-pounds force that is required to raise the temperature of one pound of water by one degree Fahrenheit.

The reason that Joule’s work was important is that it demonstrated that heat is not a thing that can be created or destroyed. It doesn’t literally flow between things or move around, it is in fact a measure of something else. Even today, this is perhaps not obvious because we still speak of the flow of heat from hot to cold things. Heat, we now understand, is simply a form of energy. Just as a ball resting on a table has energy which can be released by dropping it (known as gravitational potential energy), so a hot thing has energy that can be released, at least in part, by putting it next to a cold thing. To heat something up, you simply have to transfer energy to it by doing work on it, as Joule found by using a falling weight, and it doesn’t matter how that work is done. It can be a falling weight, a shining light or an electric current, but as long as you do the same amount of work, the temperature increase will be the same. This was all quantified, as a result of Joule’s work, into the First Law of Thermodynamics, which is a statement of the fact that energy cannot be created or destroyed; it can only be changed from one form into another. Rudolf Clausius made the first explicit statement of the law, and laid down the foundations of the science of thermodynamics, in his landmark 1850 publication ‘On the mechanical theory of heat’.


Newcomen’s engine, created in 1712, was the first commercially successful steam engine and laid the foundations for the work of other inventors, such as James Watt, which would power forward the industrial revolution in Britain. the Newcomen atmospheric engine was used to pump water out of coal mines, using a pivoted arm (top) to transfer power between the piston and the rod. the piston was driven down by the pressure of a partial vacuum in the cylinder, which drew the rod upwards. as steam in the cylinder condensed, the piston was forced up, and the rod down.

The first law can be written down mathematically as


which in words says that the increase in the internal energy of something (ΔU) is equal to the heat flow into it (Q) minus the work performed by it (W). If you performed work on it, the W would have a plus sign, and if you took heat out of it, the Q would have a minus sign.

Fifteen years after writing down the first law of thermodynamics, and far more importantly for our understanding of the arrow of time, Clausius introduced a new concept known as entropy, which lies at the heart of the Second Law of Thermodynamics. Clausius’s statement of the second law does not at first sight sound as if it has profound implications for the future of our universe. He simply stated that ‘No process is possible whose sole result is the transfer of heat from a body of lower temperature to a body of higher temperature’. This simple proposition occupies such a profound position in modern science that Arthur Eddington said of the second law:

‘If someone points out to you that your pet theory of the Universe is in disagreement with Maxwell’s equations, then so much the worse for Maxwell’s equations. If it is found to be contradicted by observation, well, these experimentalists do bungle things sometimes. But if your theory is found to be against the Second Law of Thermodynamics I can give you no hope; there is nothing for it but to collapse in deepest humiliation.’

The concept of entropy enters when the second law is written down in quantitative form. The change in entropy of a system, such as a tank of water, is simply the amount of heat added to it at a fixed temperature. In symbols,


where ΔS is the change in the entropy as a result of adding a small amount of heat, ΔQ, at a fixed temperature T. It may still be unclear what this has to do with the Universe, but here is the profound point discovered by Clausius. In any physical process at all, you find that entropy either stays the same or increases. It never decreases. Here is the thermodynamic arrow of time. Clausius had discovered a physical quantity that can be measured and quantified which only ever increases in practice, and never decreases even in theory, no matter how cleverly you design your experiment or piece of machinery. This is extremely useful information if you are designing a steam engine, because it puts a fundamental limit on the efficiency. It also prevents the construction of the so-called ‘perpetual motion machines’ so beloved of crackpot inventors to this day. You could say that the second law tells you that you can’t get something for nothing, but the second law is more profound than this, because it introduces a difference between the past and the future. In the future, entropy will be higher than it is in the present because it always increases. In the past, entropy was lower than it is now because it always increases.

Clausius introduced the concept of entropy because he found it useful, but what exactly is entropy, and what is the deep reason that it always increases? And what was the meaning of Eddington’s cryptic quote about randomness and the arrow of time? He seemed to be equating entropy with the amount of randomness in the world, and indeed he was. Understanding this will make it clear why the Second Law of Thermodynamics mandates that our entire universe must, one day, die image


In 1908 in the small town of Kolmanskop in southern Namibia, a railway worker by the name of Zacharias Lewala found a single diamond lying in the sand. He showed the precious stone to his manager – railway inspector August Stauch – who immediately realised its significance and set in motion a train of events that turned this desolate place into one of the most valuable diamond mines in the world. The colonial German government closed the entire area to outsiders; only German entrepreneurs were allowed to make their fortunes here. For 40 years, Kolmanskop was home to a thriving community as over a thousand people gathered, seeking to become millionaires by picking diamonds out of the desert. As the money rolled in, the residents built a town in the finest German tradition; grand houses stood beside a casino, a ballroom and the first X-ray station in the Southern Hemisphere. They led a champagne lifestyle in the desert, and created a little piece of opulent German architecture in the sand. Eventually, though, as with all cash cows, the diamonds could no longer be found and the town gradually lost its sparkle until it was abandoned in 1954. For half a century it has fallen into disrepair as the buildings are slowly reclaimed by the sands.

Today Kolmanskop is a ghost town, a place where our efforts to replace the geological grandeur of the desert with architectural grandeur of our own have been thwarted by the power of the winds.

Kolmanskop lies just outside the modern port town of Lüderitz, which sits in spectacular isolation on the southern Namibian coast. One of our guides told us that it takes a special kind of Namibian to set up home in Lüderitz – you have to really want to live there. The reason this place has a reputation for being particularly harsh, even by the standards of this part of the world, is the wind. This strip of the southern African coast is permanently assaulted by the untamed winds of the South Atlantic that whip up the fine-grained sands of the Namib Desert and hurl them unrelentingly into machinery, houses, camera equipment and eyes. I have never experienced anything like it. While filming, I found myself walking through the wind at Kolmanskop with my hands completely shielding my face. I didn’t do this for dramatic effect, I genuinely couldn’t look into the lacerating sand-laden wind. We also shot a scene showing a little sandcastle gradually blowing away; the camera we left in the desert for hours to film that had its lens sandblasted – the high-precision optics felt like sandpaper after a single spring afternoon in the vicinity of Lüderitz. If it wasn’t for the fact that it never rains here, and nothing rots, the ghost town of Kolmanskop would surely already have already disappeared back into the desert.



The opulent buildings of the once-glorious town of Kolmanskop are a shadow of their former selves as the desert sands blow across them and reclaim the landscape.

The little sandcastle slowly decaying in the desert wind vividly demonstrates the connection between decay, randomness and entropy. To understand why this is so, we’ll need a different and much more intuitive definition of entropy than that given by Clausius. Known as the statistical definition of entropy, it was developed by Ludwig Boltzmann in the 1870s.

A sandcastle is made of lots of little grains of sand, arranged into a distinctive shape – a castle. Let’s say there are a million sand grains in our little castle. We could take those million grains and, instead of carefully ordering them into a castle, we could just drop them onto the ground. They would then form a pile of sand. We would be surprised, to say the least, if we dropped our sand grains onto the floor and they assembled themselves into a castle, but why does this not happen? What is the difference between a pile of sand and a sandcastle? They both have the same number of sand grains, and both shapes are obviously possible arrangements of the grains. Boltzmann’s definition of entropy is essentially a mathematical description of the difference between a sandcastle and a sand pile. It says that the entropy of something is the number of ways in which you can rearrange its constituent parts and not notice that you’ve done so. For a sandcastle, the number of ways in which you can arrange the grains and still keep the highly-ordered shape of the castle is quite low, so it therefore has low entropy. For a sand pile, on the other hand, pretty much anything you do to it will still result in there being a pile of sand in the desert, indistinguishable from any other pile of sand. The sand pile therefore has a higher entropy than the sandcastle, simply because there are many more ways of arranging the grains of sand such that they form a pile of sand than arranging them into a castle. Boltzmann wrote this down in a simple equation, which is written on his gravestone:


S is the entropy, W is the number of ways in which you can arrange the component bits of something such that it is not changed, and kB is a number known as Boltzmann’s constant. For the more mathematically adventurous, ln stands for ‘natural logarithm’. If you don’t know what that means, don’t worry; the equation simply relates to the number of ways in which you can arrange things to the entropy.

As long as each particular arrangement of the sand grains is equally likely, then if you start moving sand grains around at random they are overwhelmingly more likely to form a shapeless pile of sand than a sandcastle.

That may seem a bit complicated, and not entirely illuminating yet, but here is the key point: as long as each particular arrangement of the sand grains is equally likely, then if you start moving sand grains around at random they are overwhelmingly more likely to form a shapeless pile of sand than a sandcastle. This is because most of the arrangements you create at random look like a formless pile, and very few look like a sandcastle.

This is common sense, of course, but now think about what this looks like at a microscopic level – the level of individual sand grains. There is nothing at all in the laws of nature to stop the wind blowing a grain of sand off one of the turrets of our castle and then picking up another grain from the desert and blowing it back onto the turret again, leaving our castle perfectly unchanged. Nothing at all, that is, other than pure chance. It is much more likely that the grains of sand blown off the castle are not replaced with others from the desert, and so our castle gradually disintegrates, which is to say it gradually changes into a formless sand pile. In Boltzmann’s language, this is simply the statement that the entropy of the castle will increase over time; the castle will become more and more like a sand pile. Why? Because there are many more ways of arranging the grains of sand into a pile than there are into a castle, so if you just randomly blow grains around they will tend to form piles more often than castles. Here is the deep reason that entropy always increases: it’s simply more likely that it will! Notice that there is nothing in the laws of nature that prevent it from decreasing; it’s possible that the wind will build a sandcastle, but the chances are akin to tossing a coin billions of times and each one coming up heads. It’s simply not going to happen.

Boltzmann’s statistical definition of entropy is the key to understanding Eddington’s arrow of time. This is such a key concept with such profound consequences that it is worth repeating it once more in a slightly different way. If there are a million different ways of arranging a handful of sand grains, with 999,999 of the ways producing disordered sand piles but only 1 producing a beautifully ordered castle, then if you keep throwing the sand grains up in the air they will usually land in the form of a disordered pile. So, over time, if there is a force like the wind that acts to rearrange things, things will get more messy or disordered simply because there are more ways of being disordered than ordered. This means that there is a difference between the past and the future: the past was more ordered and the future will be less ordered, because this is the most likely way for things to play out. This is what Eddington meant by his statement that the future is more random than the past, and his description of the arrow of time as the thing that points in the direction of increasing randomness. And this is why entropy always increases.


Watching my carefully constructed sandcastle gradually disintegrate in the strong desert winds perfectly demonstrates how entropy increases, and the idea that the past is always more ordered than the future.




For the purposes of our story, this is sufficient; if you take a university physics degree, this is what you will learn about entropy and the arrow of time. But there is still a great deal of debate and research surrounding entropy, and it centres on something we have dodged slightly. We have only spoken about entropy differences; the past had a lower entropy than the future; ordered things become disordered as time ticks by, but one might legitimately ask where all the order in the Universe came from in the first place. In the case of our sandcastle, it’s obvious – I made it – but how did I get here? I’m very ordered. How did Earth get here? It’s very ordered too. And how did the Milky Way appear if it is composed of billions of ordered worlds orbiting around billions of ordered stars? There must have been some reason why the Universe began in such a highly ordered state, such that it can gradually fall to bits. The answer is that we don’t know why the Universe began with sufficient order in the bank to allow planets, stars and galaxies to appear. We understand how gravity can create local order in the form of solar systems and stars, but this must be at the expense of creating more disorder somewhere else. So there must have been a lot of order to begin with. In other words, the Universe was born in a highly ordered state, and there should be a reason for that. It is unlikely to have been chance, because by definition a highly ordered state is less likely to pop into existence than a less ordered one; a sandcastle is less likely to be formed by the desert winds than a pile of sand. Since the Universe is far less ordered today than it was 13.75 billion years ago, this means it is far more likely that our universe popped into existence a billionth of a second ago, fully formed with planets, stars, galaxies and people, than it is that the Universe popped into existence at the Big Bang in a highly ordered state. There is clearly something fascinating about the entropy of the early Universe that we have yet to understand image


The sands of time are slowly and literally overrunning Kolmanskop, dismantling the highly crafted town and returning it to dust once more.

The arrow of time has been playing out dramatically in Kolmanskop since the mining facility was abandoned in 1954. In every building you can see the gradual transition from order to disorder; every room that was once full of structure is slowly being returned to a less-ordered state. This is the march of the arrow of time on Earth, but it is nothing compared to the grand journey that time’s arrow forces our universe to make.


Our Universe follows the law of any living thing: it develops in stages from birth through life and ultimately to death. We understand the early stages of its life because observations by scientists have provided valuable information as to how the Universe was created, and also fill in the crucial facts about the history of the Universe thus far. We are living in an early phase of our universe, the Stelliferous Era, with many more stages of life and change still to come, and yet we can confidently make predictions about our Universe’s future. By observing the life cycles of the stars above us we can map out the remaining years of our universe’s life.


Nathalie Lees © HarperCollins


Just as human beings, planets and stars are born, live their lives and die, so the Universe also lives its life in distinct stages. It began 13.75 billion years ago with the Big Bang, and in this embryonic period, known as the Primordial Era, the Universe was a place without the light from the stars, although in its early years the swirling hot matter would have glowed as brightly as a sun. For the first 100 million years, the conditions were far too violent for stars to form. This changed when the Universe had expanded and cooled sufficiently for the weak force of gravity to begin to clump the primordial dust, gas and dark matter into galaxies. With this came the dawning of the second great epoch in the life of our universe: the Stelliferous Era, the age of stars.

The moment the first stars were born is one of the most evocative milestones in the evolution of the cosmos. It signals the end of an alien time when the Universe was without structure – a formless void. The beginning of the Stelliferous Era marks the beginning of the age of light, the moment when the Universe would have become recognisable to us. The sky would have become black, punctuated with the glowing mist of the galaxies and the sharp silver of the stars. This is our universe today – a place where starlight decorates our nights and illuminates our days.


The Sun is one of at least two hundred billion stars in our galaxy, and it, along with countless others, shine brightly over Earth, night and day, in an ever-changing, ever-evolving cosmos.

Our sun is one of at least two hundred billion stars in our galaxy; one of a hundred billion galaxies in the observable universe. We live in a cosmos of countless islands of countless stars which bathe the Universe in light. Yet despite the fact that the Universe is over 13 billion years old, we are still just at the beginning. Although the cosmos is awash with stars, is populated with vast nebulae and systems of planets and countless billions of worlds that we’ve yet to explore, we are living close to the beginning of the Stelliferous Era, an era of astonishing beauty and complexity. But the cosmos isn’t static and unchanging; it won’t always be this way because as the arrow of time plays out, it produces a cosmos that is as dynamic as it is beautiful.

The moment the first stars were born is one of the most evocative milestones in the evolution of the cosmos…it marks the beginning of the age of light, the moment when the Universe would have become recognisable to us.


In our age of stars, the Milky Way Galaxy is filled with stars igniting and scattering their light across the night sky.



A gamma-ray burst is one of the Universe’s most spectacular and luminous explosions. As the core of a dying star collapses into a black hole, gas jets blast out from it into space.


This dramatic image shows the gamma-ray burst from GRB 090423, combining data from the Ultraviolet/Optical (blue, green) and X-ray (orange, red) telescopes of NASA’s Swift satellite.


On 23 April 2009 at 07.55 GMT, NASA’s Swift detected one of the most distant cosmic explosions ever seen – a gamma-ray burst that lasted ten seconds. The Swift satellite was designed and built with the intention that it would aid the study of a rare type of event known as a gamma-ray burst. These events, which last only a few seconds, are the most energetic and powerful emitters of radiation in the known universe. It is thought that gamma-ray bursts occur in supernova explosions – as the dying act of the most massive stars as they collapse to form black holes. By 08.16 GMT, minutes after the burst had faded away, the UK’s Infrared Telescope (UKIRT) in Hawaii saw the glowing ember of the explosion. As the day wore on, the largest telescopes across the world focused on the event as it appeared above their horizon. The afterglow was observed for several hours, but by 28 April the event had faded completely from view.

When these stars run out of nuclear fuel…they die in a dramatic fashion, collapsing in an instant and releasing more energy in one second than our sun will produce in its entire 10-billion-year lifetime.

The picture shown here merges data from two of Swift’s telescopes, and the important feature of this composite image is the rather unremarkable-looking red blob at the centre. This blob is the fading remains of GRB 090423 – once one of the brightest stars in the Universe. The poetically named GRB 090423 was once a Wolf-Rayet star. Named after the two French astronomers who discovered the first one in 1867, Wolf-Rayet stars are massive – over twenty times the mass of our sun – and because they are so massive, and burn so brightly, they are also extremely short-lived. When these stars run out of nuclear fuel after only a few hundred thousand years, they die in a dramatic fashion, collapsing in an instant and releasing more energy in one second than our sun will produce in its entire 10-billion-year lifetime.

GRB 090423 was a big Wolf-Rayet star – perhaps 40 or 50 times the mass of the Sun – however, this is not the only thing that is interesting about it. It’s not just the story of the death of this star, revealed by the brief appearance of the pale red dot, that has captivated astronomers, it’s the age of it. The light from this dot has travelled a very long way across the Universe to reach us, and has taken a very long time to do it. When we look at the afterglow of this explosion, we are looking at an event that happened a long time ago, in a galaxy far, far away. In fact, this light has been travelling towards us for almost the entire history of the Universe. GRB 090423 died over thirteen billion years ago, just over 600 million years after the Universe began. This is incredibly early in the Universe’s history. At the time of filming Wonders of the Universe, in autumn 2010, GRB 090423 was the oldest single object ever seen, although just after filming a galaxy was discovered in the Hubble Space Telescope’s Ultra Deep Field Image (see Chapter 3) that is slightly older than GRB 090423. Even more poetically named UDFy-38135539, this galaxy currently holds the distance and age record with a light travel time of slightly over 13 billion years. Allowing for the expansion of the Universe, the (so-called co-moving) distance of UDFy-38135539 is currently 30 billion light years away from Earth.

However, it is the discovery of GRB 090423, this ghostly pale red dot, and the sight of the explosive death of one of the first stars in the Universe, that gives us a glimpse of the grandest timescale of them all image


The arrow of time has been playing out in every corner of the Universe since the beginning of time. It dictates the destiny of everything; our civilisation, our planet, the Solar System, and all that lies beyond. The entropic march is inevitable and relentless. Nothing can resist the arrow of time, nothing can last forever, no star can shine without end and no planet can continue to turn. The Universe, bound by the laws of nature, must decay towards a radically different tomorrow.


We take for granted the sight of the Sun rising and setting on our horizon, but we now know its presence is not eternal.

Today, 13.7 billion years after the Universe began, we are living through the most productive era that our universe will ever know. The Stelliferous Era is a time of life and death, with the constant dance between gravity and nuclear fusion creating a dynamic, ever-changing landscape in the heavens. For a human being, for whom a century is a lifetime, the changes may appear slow, but be in no doubt that you are part of the Universe at its most vibrant. As we’ve watched the stories of stars like GRB 090423 play themselves out in the night sky, we have seen at first hand that no star can last forever. Every one of those brightly burning lights has a destiny as defined and as certain as our own, and this of course includes the star at the centre of our solar system.

The Sun was formed 4.57 billion years ago from a collapsing cloud of hydrogen and helium and a sprinkling of heavier elements. For the tiniest fraction of this time, humans have marked the passing of the days as it rose and set, and surely considered it to be an eternal presence. It was only during the twentieth century that we discovered the Sun’s fires must one day dim image


The computer-generated image shows how dramatically different the Sun will look in our heavens as it dies and dims.


At the moment the Sun is in the middle of its life, fusing hydrogen into helium at a rate of around 600 million tonnes every second. It will continue to do this for another five billion years; but eventually, perhaps fittingly given the grandeur and beauty it has nurtured in its empire, it won’t simply fade away. As the stores of hydrogen run dry, the Sun’s core will collapse and momentarily, as helium begins to fuse into oxygen and carbon, a last release of energy will cause its outer layers to expand. Imperceptibly at first, the extra heat of the Sun will extend towards us as its diameter increases by around 250 times. The fiery surface of our star will move beyond Mercury, towards Venus and onwards to our fragile world.

The effects on our planet will be as catastrophic as they are certain. Gradually, the Earth will become hotter. In the distant future, if any of our descendants still remain, someone will experience the last perfect day on Earth. As the surface of the Sun encroaches, our oceans will boil away, the molecules in our atmosphere will be agitated off into space, and the memory of life on Earth will fade into someone’s history; or perhaps no one’s history if we have steadfastly remained at home.

Long after life has disappeared, the Sun will fill the horizon; it may extend beyond Earth itself. This swollen stage in a star’s life is known as the Red Giant phase, marked by the final release of energy and the beginning of a long, long decline. In six billion years’ time, in a most beautiful display of light and colour, our sun will shed its outer layers into space to form a planetary nebula. We know this because we have seen this sequence of events unfold in the final breath of distant stars – on someone else’s sun? Written across the night sky in filamentary patches of colour are the echoes of our future.

If in the far future, somewhere in the Universe, astronomers on a world not yet formed gaze through a telescope at our planetary nebula and reflect on its beauty, they may glimpse at its heart a faintly glowing ember; all that remains of a star we once thought of as magnificent. She will be smaller than the size of Earth, less than a millionth of her current volume and a fraction of her brightness. Our sun will have become a white dwarf – the destiny of almost all the stars in our galaxy – a fading, dense remnant, momentarily masked by a colourful cloud.

If our planet survives, little more than a scorched and barren rock will remain, silhouetted darkly against the fading embers of a star.

Sirius, the brightest star in our sky, sits at just over eight light years away, which makes it one of our nearest neighbours. It is so bright that on occasion it can be observed during bright twilight, partly because of its proximity but also because it is twice as big as our sun and twenty-five times as bright. It is therefore not surprising that observations of Sirius have been recorded in the oldest of astronomical records.

For thousands of years we looked up at this beacon and assumed it was a single star, but in 1862 American astronomer Alvan Graham Clark observed a sister star hidden in the glare of Sirius’s light. It took so long to notice Sirius’s companion because, as the photograph taken by the Hubble Space Telescope (bottom right) reveals, it is so much dimmer than its sibling. Shining faintly in the lower left-hand corner, the small dot of light is an image of the white dwarf star Sirius B. This is one of the larger white dwarf stars discovered by astronomers, with a mass similar to our sun that is packed into a sphere the size of Earth. With no fuel left to burn, white dwarfs like Sirius B glow faintly with the residual heat of their extinguished furnaces. Like most white dwarfs, Sirius B is made primarily of oxygen and carbon (the remnants of helium fusion) packed tightly with a density a million times that of a younger, living star. This is the future of our star; a vision of the Sun’s death. Slowly cooling in the freezing temperatures of deep space, it is estimated that our sun will reach this phase in around 6 billion years’ time. From Earth, if indeed there is an Earth at that time, our sun will shine no brighter than a full moon on a clear night.

Death must come to all stars. One day every light in the night sky will fade and the cosmos will be plunged into eternal night. This is the most profound consequence of the arrow of time; this structured Universe that we inhabit alongside all its wonders – the stars, the planets and the galaxies – cannot last forever. As we move through the age of stars, through the aeons ahead, countless billions of stars will live and die. Eventually, though, there will be only one type of star that will remain to illuminate the Universe in its old age image


An artist’s impression of Sirius A and its diminutive companion Sirius B in close-up. They are overlain on a real image of the night sky containing the three stars of the Summer Triangle: Vega, Deneb and Altair. As seen from Sirius, our sun would appear as a moderately bright star in this same area of sky. It is shown here just below right of Sirius A.


This image from NASA’s Hubble Space Telescope shows the Boomerang Nebula in early 2005 and the two lobes of matter that are being ejected from the star as it dies. The rapid expansion of the planetary nebula around this dying star has made it one of the coldest places found in the Universe so far.


A Hubble Space Telescope image of the dazzling Sirius A with the faint speck of Sirius B to its lower left. Sirius B is 10,000 times fainter than Sirius itself.


Although relatively young now, the Sun, like every other star in the Universe, must one day die. In around five billion years’ time, the Sun’s stores of hydrogen will run dry and the star will begin its long, dramatic swansong. During this lengthy goodbye, the last dying bursts of extra heat will extend towards us, passing Mercury and Venus on the way and leaving a trail of destruction in its wake. Long after life has disappeared on Earth, the Sun will continue to fill the horizon as it swells in the Red Giant phase until, in about six billion years’ time, our Sun will shed its outer layers of gas and dust into space, exposing its core which will fade into a white dwarf, living on in the heavens as a shadow of its former self.


Nathalie Lees © HarperCollins


The nearest star to our solar system is Proxima Centauri. Although only a mere 4.2 light years away, Proxima Centauri is not visible to the naked eye from Earth and doesn’t even stand out against more distant stars in many of the photographs that have been taken of it. The reason for this is that Proxima Centauri is small, very small when compared to our sun – having just 12 per cent of the Sun’s mass – so to our eyes this star would appear to shine 18,000 times less brightly than our sun.

Proxima Centauri is a red dwarf star – the most common type of star in our universe. Red dwarfs are diminutive and cold, with surface temperatures in the region of 4,000K, but they do have one advantage over their more luminous and magnificent stellar brethren: because they’re so small, they burn their nuclear fuel extremely slowly, and consequently they have life spans of trillions of years. This means that stars like Proxima Centauri will be the last living stars in the Universe.

If we do in fact survive into the far future of the Universe, it is possible to imagine our distant descendants building their civilisations around red dwarfs in order to capture the energy of those last fading embers of stars. Just as our ancestors crowded around campfires for warmth on cold winter nights, so some time long in the future humans may take their warmth from a red dwarf as the last available energy in the Universe.

The rate of the fusion reactions in the cores of these red dwarfs that is needed to provide the thermal pressure to resist the inward pull of their weak gravity is very low, which enables them to live longer. Even so, these are still active stars, and their surfaces are whipped up into turmoil by the turbulent convective currents that constantly churn their interiors. Amongst all this activity, explosive solar flares occur almost continually, blasting bursts of light and X-rays out into space.

Ultimately, though, the frugality of these stars is no defence against the arrow of time. Four trillion years from now, at 300 times the current age of the Universe, Proxima Centauri’s fuel reserves will finally run out and the star will slowly collapse into a white dwarf. After trillions of years of stellar life and death, only white dwarfs and black holes will remain in the Universe, and then, in around 100 trillion years’ time, this age of the stars will draw to a close and the cosmos will enter its next phase: The Degenerate Era. And yet, even after 100 trillion years of light, the vast majority of the Universe’s history still lies ahead. Bleak, lifeless and desolate, our universe will go on, as it enters the dark image




These computer-generated images reveal how Proxima Centauri will meet its end. Over the next four trillion years, this red dwarf will gradually collapse into a much dimmer white dwarf.

After trillions of years of stellar life and death, only white dwarfs and black holes will remain in the Universe, and then, in around 100 trillion years’ time, this age of the stars will draw to a close and the cosmos will enter its next phase: The Degenerate Era.


A white dwarf is visible amongst brighter, living stars in this enhanced image, taken by NASA’s Galaxy Evolution Explorer, of Z Camelopardalis, a binary star system.


On the northern coast of Namibia, where the cold waters of the South Atlantic meet the Namib Desert, lies one of the most inhospitable places on Earth. The Skeleton Coast has been feared for as long as sailors have travelled near its shores; seventeenth-century Portuguese mariners used to call this place ‘the gates to hell’, and the native Namib Bushmen named it ‘the land God made anger in’. Today, you can just about make it to the coast in a sturdy 4x4, or effortlessly cruise in from the port city of Walvis Bay in a helicopter. But even so, when you stand on the sands beside the South Atlantic, the gods still have anger left. Each morning a dense ocean fog rolls along the coastline, fed by the upwelling of the cold Benguela current. Coupled with the constantly shifting shape of the sandbanks in the intense Atlantic winds, this toxic navigational conspiracy has meant that over the years thousands of ships have been wrecked along the Skeleton Coast. The decaying carcasses of the rusting ships and the bleached bones of marine life swept ashore by the currents all add to the coast’s gothic feel. The name Skeleton Coast also reflects the large number of human lives lost here over the centuries; even if you made it ashore after a shipwreck, the onshore currents are so strong that there is no way of rowing back out to sea, and the only route to safety is through hundreds of miles of inhospitable desert. This genuinely was a place of no return: if you were shipwrecked here, this was the end of your universe.


Just as the ship’s iron will eventually rust and be carried away by the desert winds, so we think the last matter in the Universe will eventually be carried off into the void.

One of the ships to end her days here was the Eduard Bohlen, a 91-metre (300-foot), 2,272-tonne steamship that ran aground here on the 5 September 1909 on a journey from Germany to West Africa. A century’s shifting sands have carried her hundreds of metres inland and the Atlantic winds have attacked her carcass, leaving her rusting and skeletal. When we arrive she is guarded by a phalanx of jackals who are less wary of us than I expected. She forms an abstract backdrop to our story; the symbolism is immediate, brutal even, and for me surprisingly powerful. These wrecks, complex structures dismantled by the passage of time, are like our last stars.



The Skeleton Coast: one of the most inhospitable places on Earth, where humans have perished for centuries, and where only jackals and the strongest life forms remain.

In the far future of the cosmos, the last remaining beacons of light will no more be permitted to evade the second law of thermodynamics than the Eduard Bohlen. Even the white dwarfs must fade as the laws of physics methodically dismantle the Universe. Slowly, as the glowing embers of the last stars lose their warmth to space, they will cease to emit visible light. After trillions of years, the final beacons burning in the cosmic sky will turn cold and dark – their remnants are known as black dwarfs.

Black dwarfs are dark, dense, decaying balls of degenerate matter. Nothing more than the ashes of stars, they take so long to form that after almost 14 billion years, the Universe is currently too young to contain any at all. Yet despite never seeing one, our understanding of fundamental physics allows us to make concrete predictions about how they will end their days. Just as the iron that makes up the ships of the Skeleton Coast will eventually be carried away by the desert winds, so it is thought that the matter inside black dwarfs, the last matter in the Universe, will eventually evaporate away and be carried off into the void as radiation, leaving nothing behind. The processes by which matter might, given enough time, decay, are not understood. Physicists need a more advanced theory of the forces of nature, known as a Grand Unified Theory, to speak with certainty about the behaviour of protons, neutrons and electrons over trillion-year timescales. There are reasons to expect that such a theory may exist, and that a mechanism for even the most stable sub-atomic particles to decay into radiation might be present in nature. For this reason, experiments to measure the lifetime of protons are ongoing in laboratories around the world, but as yet nobody has observed proton decay, and we are therefore now in the realm of speculation. But here is one possible, and given our understanding of physics today, probable, story of how our universe will end.




This composite X-ray image from the Chandra X-ray Observatory shows gas blowing away from a central supermassive black hole in the active galaxy NGC 1068.

Once the last remnants of the last stars have decayed away to nothing…the story of our universe will finally come to an end.


In trillions of years, our universe will be littered with black dwarfs. From the ashes of stars, dark, dense and decaying balls of degenerate matter will form.

With the black dwarfs gone, there will not be a single atom of matter left in the Universe. All that will remain of our once-rich cosmos will be particles of light and black holes. After an unimaginable expanse of time, it is thought that even the black holes will evaporate away, and the Universe will consist of a sea of light; photons all tending to the same temperature as the expansion of the Universe cools them towards absolute zero. When I say unimaginable period of time, I really mean it: ten thousand trillion trillion trillion trillion trillion trillion trillion trillion years. In scientific notation, that’s 10100 years. That is a very big number indeed; if I were to start counting with a single atom representing one year, there wouldn’t be enough atoms in all the stars and planets in all the galaxies in the entire observable universe to get anywhere near that number.

Once the last remnants of the last stars have decayed away to nothing and everything reaches the same temperature, the story of our universe will finally come to an end. For the first time in its life the Universe will be permanent and unchanging. Entropy finally stops increasing because the cosmos cannot get any more disorganised. Nothing happens, and it keeps not happening forever.

This is known as the heat death of the Universe, an era when the cosmos will remain vast, cold, desolate and unchanging for the rest of time. There’s no way of measuring the passing of time, because nothing in the cosmos changes. Nothing changes because there are no temperature differences, and therefore no way of moving energy around to make anything happen. The arrow of time has simply ceased to exist. This is an inescapable fact, written into the fundamental laws of physics. The cosmos will die; every single one of the hundreds of billions of stars in the hundreds of billions of galaxies in the Universe will expire, and with them any possibility of life in the Universe will be extinguished image


The fact that the Sun will die, incinerating Earth and obliterating all life on our planet, and that eventually the rest of the stars in the Universe will follow suit to leave a vast, formless cosmos with no possibility of supporting any life or retaining any record of the living things that brought meaning to its past, might sound a bit depressing to you. You might legitimately ask questions about the way our universe is put together. Surely you could build a universe in a different way? Surely you build a universe such that it didn’t have to descend from order into chaos? Well, the answer is ‘no’, you couldn’t, if you wanted life to exist in it.

The arrow of time, the sequence of changes that will slowly but inexorably lead the Universe to its death, is the very thing that created the conditions for life in the first place. It took time for the Universe to cool sufficiently after the Big Bang and for matter to form; it took time for gravity to clump the matter together to form galaxies, stars and planets, and it took time for the matter on our planet to form the complex patterns that we call life. Each of these steps took place in perfect accord with the Second Law of Thermodynamics; each is a step on the long road from order to disorder.

The arrow of time has created a bright window in the Universe’s adolescence during which life is possible, but it’s a window that won’t stay open for long. As a fraction of the lifespan of the Universe, as measured from its beginning to the evaporation of the last black hole, life as we know it is only possible for one-thousandth of a billion billion billionth, billion billion billionth, billion billion billionth of a per cent.

And that’s why, for me, the most astonishing wonder of the Universe isn’t a star or a planet or a galaxy; it isn’t a thing at all – it’s a moment in time. And that time is now.

Around 3.8 billion years ago life first emerged on Earth; two hundred thousand years ago the first humans walked the plains of Africa; two and a half thousand years ago humans believed the Sun was a god and measured its orbit with stone towers built on the top of a hill. Today, our curiosity manifests itself not as sun gods but as science, and we have observatories – almost infinitely more sophisticated than the Thirteen Towers – that can gaze deep into the Universe. We have witnessed its past and now understand a significant amount about its present. Even more remarkably, using the twin disciplines of theoretical physics and mathematics, we can calculate what the Universe will look like in the distant future and make concrete predictions about its end.


This colour image of the Earth, named the ‘Pale Blue Dot’, is a part of the first-ever portrait of the Solar System taken by NASA’s Voyager 1. The spacecraft took 60 frames which could be used to create a mosaic image of the Solar System from a distance of over four billion miles from Earth.


This seemingly insignificant image of a pale blue dot is in fact one of the most important and beautiful images ever taken, revealing our planet at a distance of over six billion kilometres away.

I believe it is only by looking out to the heavens, by continuing our exploration of the cosmos and the rules that govern it, and by allowing our curiosity free reign to wander the limitless natural world, that we can understand ourselves and our true significance within this Universe of wonders.

In 1977, a space probe called Voyager 1 was launched on a ‘grand tour’ of the Solar System. It visited the great gas giant planets Jupiter and Saturn and made wonderful discoveries before heading off into interstellar space. Thirteen years later, after its mission was almost over, Voyager turned its cameras around and took one last picture of its home. This picture (left) is known as the Pale Blue Dot. The beautiful thing, perhaps the most beautiful thing ever photographed, is the single pixel of light at its centre; because that pixel, that point, is our planet, Earth. At a distance of over six billion kilometres (3.7 billion miles) away, this is the most distant picture of our planet that has ever been taken.

The powerful and moving thing about this tiny, tiny point of light is that every living thing that we know of that has ever existed in the history of the Universe has lived out its life on that pixel, on a pale blue dot hanging against the blackness of space.

As the great astronomer Carl Sagan wrote:

‘It has been said that astronomy is a humbling and character-building experience. There is perhaps no better demonstration of the folly of human conceits than this distant image of our tiny world. To me, it underscores our responsibility to deal more kindly with one another, and to preserve and cherish the pale blue dot, the only home we’ve ever known.’

Just as we, and all life on Earth, stand on this tiny speck adrift in infinite space, so life in the Universe will only exist for a fleeting, dazzling instant in infinite time, because life, just like the stars, is a temporary structure on the long road from order to disorder.

But that doesn’t make us insignificant, because life is the means by which the Universe can understand itself, if only for an instant. This is what we’ve done in our brief moments on Earth: we have sent space probes to the edge of our solar system and beyond; we have built telescopes that can glimpse the oldest and most distant stars, and we have discovered and understood at least some of the natural laws that govern the cosmos. This, ultimately, is why I believe we are important. Our true significance lies in our continuing desire to understand and explore this beautiful Universe – our magnificent, beautiful, fleeting home image


Our time on Earth is precious and fleeting. The most important use of this time that we can make is to ask questions about our wonderful universe, so that perhaps one day one of our descendants will truly understand the natural laws that govern our cosmos.

‘Somewhere, something incredible is waiting to be known’

—Carl Sagan, 1934–1996





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