Seeing Further: The Story of Science, Discovery, and the Genius of the Royal Society - Bill Bryson (2010)

20. GREGORY BENFORD

TIME: THE WINGED CHARIOT

Gregory Benford is a Professor of Physics at the University of California, Irvine, and author of over thirty books, mainly science fiction. His novels include Timescape, Cosm, Beyond Infinity, What Might Have Been, The Sunborn and the six-volume Galactic Centre series. His non-fiction includes Deep Time and Beyond Human.

THE ROYAL SOCIETY IS 350 YEARS OLD, AND STILL GOING. SCIENCE, TOO, WILL GO ON. HOW LONG FOR? WELL, ANSWERING THAT QUESTION NEEDS A PROPER UNDERSTANDING OF TIME - SOMETHING WHICH, AS GREGORY BENFORD EXPLAINS, REMAINS ELUSIVE AFTER ALL THESE YEARS.

But at my back I always hear
Time’s winged chariot hurrying near;
And yonder all before us lie
Deserts of vast eternity.

- Andrew Marvell, To His Coy Mistress, 1652

When the Royal Society began, time seemed a simple, obvious subject, understood since ancient ages. To Isaac Newton and his colleagues, two long-standing traditions pervaded the idea of time.

The Greeks, like most ancient cultures, saw their world as not completely chaotic, though it could be capricious. Faith in a definite order in nature promised that it could be understood by human reasoning. To them, some physical processes, at least, had a hidden mathematical basis, and they sought to build a model of reality based on arithmetical and geometrical principles.

Adding to this Western tradition was the Judaic worldview, which had a timeline. God created the universe at some definite moment, arriving fresh and with a fixed set of laws. The Jews thought that the universe unfolds in a sequence running forward, which we now call linear time. Creation enabled evolution, which led forward in linear time to a future we could quite possibly change. This differed greatly from most other ancient cultures, which favoured cosmic cycles, probably by generalising from the march of the year’s seasons. In cyclic time everything ends, but eventually returns, so there is eternal recurrence.

These two ideas, time’s arrow vs. time’s cycle, persist today in physics and also emerge in our art and literature. Physics has constrained time, ordering the music, but the dance between these linear and cyclic views continues.

Four hundred years ago, Europeans assumed a God-created universe that unfolded in orderly ways, in linear time, but that did not mean that the universe always had to be as we see it now. Change was possible, but constrained by physical laws. Einstein once remarked that what most interested him was whether God had any choice in his creation. The Abrahamic religious tradition answered with a resounding yes.Further, they insisted on nature’s rationality, aided by mathematical principles. These were the only cultures to do so. This driving idea eventually altered the concept of time itself, as the cultural agenda played out in modern science.

EVOLVING TIME

Time has two faces.

First, our sense of it passing seems inevitable, an automatic intuition. Unlike space, in which we can move back and forth, time hammers on relentlessly. This is Intuitive Time.

Second, we frame our position in time, our historical era, by looking at our slowly changing landscapes, and our societies. These alter on the scale that we ourselves see as we age. This is Historical Time.

Both these faces appeal, but they deceive us.

In the 1700s, the philosopher Immanuel Kant saw space and time as elements of a systematic mental framework, structuring our experiences. Spatial measurements tell us how far apart objects are, and temporal measurements show how far apart events occur. This eventually intersected Charles Darwin’s idea that many abilities of organisms emerge from evolution by natural selection. Then it follows that time and space are the concepts we and other animals evolved to make the best use of the natural world. In this sense, they emerged from the primordial world where our minds evolved.

But that was not enough. Modern science reveals that time is supple, changeable, and even enigmatic. Further, we stand in a small slice of it, anchored in a moving moment that is an infinitesimal wedge compared with what has gone before, or will come after us. Our telescopes tell us of immensities of space, but other sciences - geology, biology, cosmology - speak of even grander scales of time.

Space and time are so familiar that we forget that they underlie the entire intricate and beautiful structure of scientific theory and philosophy. Perhaps it is not surprising that our first powerful theories built on assumed bedrock, metaphysical intuitions, came to be questioned only later. Clocks in Newton’s universe ran everywhere the same. He invoked ‘absolute, true and mathematical time’ saying that it ‘of itself, and from its own nature, flows equably without relation to anything external, and by another name is called duration’. Of the immense expanse of past time Newton had no true idea, for he took as gospel the Genesis story. Space was similarly absolute. Newton avoided the colossal scale of space by supposing that God had fixed up the cosmos so that gravity, the force he was the first to quantify, had not made it collapse - at least, so far.

This view held up well until the nineteenth century. By then even atheist scientists had faith in a lawlike order of nature - not from philosophy, but because it worked. Though this assumption springs from an essentially theological worldview, it gave useful predictions without a god attached. Still, few saw the full implications of regarding time as a subject of study, not belief.

The first collision between religious views and the study of the far past, which we now call Deep Time, came with the newborn science of geology. In 1830, the geologist Charles Lyell proposed that the features of Earth perpetually changed, eroding and re-forming continuously, at a roughly constant rate. This challenged traditional views of a static Earth with rare, intermittent catastrophes. In the eighteenth and nineteenth centuries the vast depth of the eras before humans arose became apparent, through development in geology and evolution’s grand perspective. These still had to be licensed by physics, the more secure and quantitative science which sets the stage for the events and processes probed by the other sciences.

When William Smith and Sir Charles Lyell first recognised that rock strata represented successive long eras, they could estimate timescales only very imprecisely, since rates of geologic change varied greatly. Even these early attempts got the sciences into trouble. Creationists, reasoning from the Bible, had been proposing dates of around six or seven thousand years for the age of the Earth based on the Bible. Early geologists suggested millions of years for geologic periods, with some even suggesting a virtually infinite age for the Earth. Geologists and palaeontologists constructed geologic history based on the relative positions of different strata and fossils, estimating the timescales based on studying rates of various kinds of weathering, erosion, sedimentation and lithification. The ages of assorted rock strata and the age of the Earth were hotly debated. In 1862, the physicist William Thomson, whose authority endured - as Lord Kelvin and President of the Royal Society - until the end of the century, set the age of Earth at between 24 million and 400 million years. He assumed that Earth began as a completely molten ball of rock, then calculated how long it took to cool to its present temperature. He did not know of the ongoing heat source from radioactive decay.

Physicists had more prestige, but even then, geologists doubted such a short age for Earth. Biologists could accept that Earth might have a finite age, but even 100 million years seemed much too short for evolution to have yielded such complex plenty. Charles Darwin argued that even 400 million years did not seem long enough.

Until the discovery of radioactivity in 1896, and the development of its geological applications through radiometric dating during the first half of the twentieth century (pioneered by geologists), there were no precise absolute datings of rocks.

Radioactivity introduced another measuring clock. Geologists quickly realised this upset the assumptions used before. They re-examined their estimates. This moved the age into the billions (thousands of millions) of years, sweeping away Archbishop Ussher’s biblically inspired dating of Creation to 4004 BC.

Much public ferment paralleled this scientific research and its clash with religion. But by the early twentieth century, opinion settled on an Earth older than a billion years.

Physics, meanwhile, was making hash of the simple view of time that underlay the other sciences. Geology, biology and astronomy would have been happy with Newtonian time, giving them a simple marker of change. The physicists, though, worried about more basic matters.

RELATIVE TIME

In physics, time is, like length, mass and charge, a fundamental quantity - intuitive, given by our basic perceptions. Newton used this view, holding that ‘I do not define time, space, place and motion, as being well known to all’ - i.e., obvious. But Einstein showed that it was not.

Nineteenth-century physicists felt that space was the most basic and irreducible of all things. It persisted while time changed, and points made up space - infinitesimal grains close-packed. Einstein’s fundamental insight was that space and time, which appear so different to us, are in fact linked. He argued this using gedanken (thought) experiments involving rulers and clocks. These were not just instruments to Einstein; he took them to generate space and time, since they represent it.

He took two basic assumptions. First, the speed of light seems the same to everyone in the universe, whether moving or sunk deep in a gravitational well. This may strike us as odd, but an earlier experiment had found it to be so. Not that Einstein cared; his intuition led him to the conclusion. He proved it valid by using the even deeper second assumption: that the laws of physics had to treat all states of motion on the same footing.

Combining these two assumptions generates the equations of his Special Theory of Relativity. It has astonishing consequences. Moving objects experience a slower passage of time. This is known as time dilation. These transformations are only valid for two frames at constant relative velocity. Naïvely applying them to other situations gives rise to such puzzles as the famous twin paradox.

This is a thought experiment in special relativity, in which a twin makes a journey into space in a fast rocket, returning home to find he has aged less than his identical twin, who stayed on Earth. This result appears puzzling because the laws of physics should exhibit symmetry. Since either twin sees the other twin as travelling, each should see the other ageing more slowly. How can an absolute effect (one twin really does age less) come from a relative motion? Hence it is called a paradox.

But there is no paradox, because there is no symmetry. Only one twin accelerates and decelerates, so this differentiates the two cases. Since we each experience minor accelerations, whether on horseback or in a jet plane, we each carry around our own personal scale of time. These are undetectable in ordinary life, but real.

When time stretches, space shrinks. When you rush to catch an aeroplane, the wall clock you see runs a tiny bit slower than your wristwatch. Compensating for the time, the distance to the aeroplane’s gate looks closer to you. Time is pricey, though - a second of time difference translates to 300,000 kilometres of space.

The stretching of space and time occurs because they are wired together. More fundamentally, Einstein’s work implied that time runs slower the stronger is the gravitational field (and hence the observer’s local acceleration). His general relativity theory sees gravity not as a force but as a distortion of space-time.

The rates of clocks on Earth then depend on whether they are on a mountain or in a valley; the valley clock runs slower. This is somewhat like the slowing of clocks as they move past us at high velocity. This gravitational effect is unlike that of the smoothly moving observers on, say, two trains moving by each other, each of whom thinks the other’s clock runs slowly. In a gravitational field, the clocks experience different accelerations if they are not at the same altitude. But observers both in the valley and on the mountain agree that the mountain clock runs faster. Experiments checked these results and found complete agreement. Further, particle acceleration experiments and cosmic ray evidence confirmed the predictions of time dilation, where moving particles decay slower than their less energetic counterparts. Gravitational time changes give rise to the phenomenon of gravitational ‘redshift’, which means that light loses energy as it rises against gravity. There are also well-documented delays in signal travel time near massive objects like the Sun. Today, the Global Positioning System must adjust signals to account for this effect, so the theory has even practical effects.

In empty space, the shortest distance between two points is a straight line. In space-time, this is called a ‘world line’ that forms the shortest curve between two events. If gravitation curves a space-time, then the straight line becomes a curve, which is the shortest space-time distance between two points. That curvature we see as the curve of a ball when thrown into the distance, a parabola.

This linked with a radical view, pushed by Hermann Minkowski, that neither space nor time is truly fundamental. In relativity, both are mere shadows, and only a union of the two exists in the underlying reality. Minkowski had called Einstein a ‘lazy dog’ when Einstein was his student. But while reading Einstein’s first paper on relativity, he had a brilliant idea, and so laid the foundations for the next great insight. Minkowski’s invention was space-time, a joint entity. Einstein later used his intuition to propose that mass curves space-time, and we sense this curvature as gravity.

The fundamental idea of space-time played out in many ways. Time runs faster in space than on a star, because gravity warps space-time. This leads to timewarps that can become severe, when a star implodes and time grinds to a halt. Stars a few times larger than our own can do this, capturing their own light and plunging into an infinitesimal speck we call a black hole. Its gravitation remains with us, though, a timewarp imprinted on empty space. Anyone falling along with the star will see the external world pass through all of eternity, while gravity pulls him into a spaghetti strand. The singularity where all ends up is a ‘nowhen’ and ‘nowhere’, signifying that the physical universe as we understand it ceases.

Einstein’s singular geometric and kinematic intuition motivated his theory. He assumed that every point in the universe can be treated as a ‘centre’, whether it is deep in a gravitational well (such as where we live) or in empty space, far from curvatures in space-time induced by gravity. Correspondingly, he reasoned, physics must act the same in all reference frames. This simple and elegant assumption led, after much labour, to a theory showing that time is relative to both where you are and how you are moving. Newton’s laws hold well enough in a particular local geometry. They work in different circumstances, though they must be modified for the environment. Still, this fact can be expressed in the theory itself. This leads to the principle that there is no ‘universal clock’. To get things right, we must perform some act of synchronisation between two systems, at the very least.

There is another victim of his intuition. Not only is there not a universal present moment, but also there is no simple division between past, present and future in general - that is, everywhere in the universe. Locally, they do mean something, but not necessarily to those far from us, in a universe that continually expands.

Though you and I on Earth may agree about what ‘now’ means on the nearest star, Proxima Centauri, an astronaut moving quickly through the solar system who asks this same question when we do will refer to a different moment on Proxima Centauri.

Does this mean that only the present moment ‘really’ exists? But one person’s past can be another’s future, so past, present and future must exist in a physical sense, and so be equally real.

Einstein said of the death of an old friend, within months of his own death, ‘Now he has departed from this strange world a little ahead of me. That means nothing. People like us, who believe in physics, know that the distinction between past, present and future is only a stubbornly persistent illusion.’

In physics, time is not a sequence of happenings, but a chain that is just there, embedded in space-time. Our lives move along that chain, like a train on a track. Observers differ over whether a given event occurs at a particular time, but there is no universal Now. Instead, an event belongs to a multitude of Nows, depending on others’ states of motion or position. Time stretches away into past and future, as we see them, just as space extends away from any place. This is the interwoven thing we call space-time, and it is more fundamental than our particular sense of our local world.

Even more odd possibilities come from these ideas. General relativity allows time travel of a sort, in special circumstances. These may be disallowed by a more fundamental theory, but for now, some puzzling paradoxes emerge from our understanding of time. Presumably events may not happen before their cause, but proving this in general has so far eluded us.

TIME’S MOMENTUM

Time goes, you say? Ah no!
Alas, time stays, we go.

- Henry Austin Dobson, The Paradox of Time, 1877

Why do we think that time moves, instead of the fixed, eternal space-time that theory suggests? Because evolution has not selected us to see it that way. Time’s flow is a simple way to order the world effectively; that does not mean it is fundamental. Space-time is simple and elegant, but that does not mean it plays well in the rough scramble of life. During a seminar at Princeton University, Einstein remarked that the laws of physics should be simple. Someone asked, ‘What if they aren’t?’ Einstein replied that if so, he was not interested in them.

Yet simplicity may not be the best way to regard time. Time seems to flow because that flow is a holistic concept, not reducible to simple systems like a collision of atoms. In this sense, the paradox of time’s flow is an aspect of our minds. We can see time as moving, bringing events to us, or the reverse: we flow through time, sensing a moving moment.

This interlaces with the findings of Sadi Carnot in 1824, when he carefully analysed steam engines with his Carnot cycle, an abstract model of how an engine works. He and Rudolf Clausius noted that disorder, or entropy, steadily increases as machines operate. This means the amount of ‘free energy’ available continually decreases.

This is the second law of thermodynamics. The continual march of time then defines an arrow of time, defined by the growth of entropy. It is easy enough to observe the arrow by mixing a little milk into your coffee. Try as you might, you can’t reverse it. In the nineteenth century entropy’s increase took its place beside other definitions of time’s momentum. Another definition is the psychological arrow of time, whereby we see an inexorable flow, dominating our intuitions. The third view, a cosmological arrow of time, emerged when we discovered the expansion of the universe in the twentieth century.

This dramatic time asymmetry seems to offer a clue to something deeper, hinting at the ultimate workings of space-time. For example, suppose gravity acts on matter - what is the maximum entropy nature can pack into a volume? There is a clean answer: a black hole. In the 1970s Stephen Hawking of Cambridge University, holding the chair Newton had, showed that black holes fit neatly into the second law. Originally the second law described hot objects like steam engines. Applied to black holes, that can also emit radiation and have entropy, the second law shows that a three-million-solar-mass black hole, such as the one at the centre of our galaxy, has a hundred times the entropy of all the ordinary particles in the observable universe. This is astonishing. Collapsed objects are giant repositories of disorder, and thus sinks of the productions of time itself.

These ideas spread throughout science, with varying results. Entropy inevitably increases in thermodynamics, but that seems to fly in the face of our own world, which flourishes with new life forms and increasing order. In contrast to the physical view of time, biologists pointed out that life depends on a ‘negative entropy flow’ which is local, driven by a larger decrease elsewhere. For us, this ‘elsewhere’ is the Sun, which supports our entire natural world. The Sun will expand and engulf the Earth in about five billion years. By then we may have a fix for that problem, if we are still around as humans. But then the stars themselves will die out, having burned their core fuels, this will take several tens of billions of years more, and thereafter the universe will indeed cool and entropy will rise throughout.

Increasing entropy implies a ‘heat death’ as our universe expands. This means the end of time will be cold and dark.

So biological systems do not refute the arrow of time; they define it well during our present, early state of the universe. These realisations ran in parallel through the nineteenth and twentieth centuries, promoting fruitful scientific dialogue.

DEEP TIME REVISITED

The human perception of time has ramified through many sciences. Such fundamental changes in a basic view always echo through culture.

The enormous expansion of our perceptions of time has altered the way we think of ourselves, framed in nature. Palaeontologists track the extinction of whole genera, and in the random progressions of evolution feel the pace of change that looks beyond the level of mere species such as ours. Geologists had told them of vast spans of time, but even that did not seem to be enough to generate the order we see on Earth.

The Darwin-Wallace theory explains our Earthly order as arising from evolution through natural selection. As perhaps the greatest intellectual event of the nineteenth century, it invokes cumulative changes that add up. The fossil record showed that mammals, for example, can take millions of decades to alter significantly. Our own evolution has tuned our sense of probabilities to work within a narrow lifetime, blinding us to the slow sway of long biological time. (And to the fundamental physical space-time, as we discussed.)

This may well be why the theory of evolution came so recently, in an era when our horizons were already quickly expanding; it conjures up spans of time far beyond our intuition. On the creative scale of the great, slow and blunt Darwinnowings, such as we see in the fossil record, no human monument can endure. But our neophyte primate species can now bring extinction to many, and no matter what the clock, extinction is for ever. We live in hurrying times.

Yet we dwell among contrasts between our intuitions and the timescape of the sciences. In their careers, astronomers discern the grand gyre of worlds. But planning, building, flying and analysing a single mission to the outer solar system commands the better part of a professional life. Future technologies beyond the chemical rocket may change this, but there are vaster spaces beckoning beyond which can still consume a career. A mission scientist invests the kernel of his most productive life in a single gesture toward the infinite.

Those who study stars blithely discuss stellar lifetimes encompassing billions of years. In measuring the phases of stellar mortality they employ the many examples, young and old, that hang in the sky. We see suns in snapshot, a tiny sliver of their grand and gravid lives caught in our telescopes. Cosmologists peer at distant galaxies whose light is reddened by the universal expansion, and see them as they were before Earth existed. Observers measure the microwave emission that is relic radiation from the earliest detectable signal of the universe’s hot birth. Studying this energetic emergence of all that we can know surely imbues (and perhaps afflicts) astronomers with a perception of how like mayflies we are.

No human enterprise can stand well in the glare of such wild perspectives. Perhaps this is why for some science comes freighted with coldness, a foreboding implication that we are truly tiny and insignificant on the scale of such eternities. Yet as a species we are young, and promise much. We may yet come to be true denizens of Deep Time.

COSMOLOGICAL TIME

Through the twentieth century’s developing understanding of stellar evolution, astronomy outpaced even the growing expanses of biological time by dating the age of stars. These lifetimes were several billion years, a fact some found alarming. In the mid-twentieth century some globular clusters of stars even seemed to be older than the universe, a puzzle that better measurement resolved.

However, a still grander canvas awaited. Perhaps the most fundamental aspect of time lies in our description of how it all began, along with the universe itself: cosmology.

There were many ‘origin stories’ of earlier cultures, but these gave little thought to how the universe came to be, beyond simple stories. Ancient times, until the nineteenth century, preferred eternity to process. As the Bhagavad Gita says, ‘There never was a time when I was not … there will never be a time when I will cease to be.’ Since time and space began together - as both St Augustine and the big bang attest - the Bhagavad Gita has a point. The chicken and the egg arrived at the same time.

Yet Newton thought that the universe had to be eternally tuned by God’s hand, or else gravitation would cause it to collapse. This view held fairly well until a new theory of gravity and time arrived.

When Einstein developed his theory of general relativity in 1915, physicists believed in a perfectly static universe without beginning or end, like Newton. Though he had a theory of curved space-time, and so could consider all the universe, Einstein inherited this bias. He attempted the first true cosmology - that is, a complete description of the universe’s lifetime, from simple assumptions - under the influence of the ancients.

To make his early equations describe a universe unchanging in time, he added a cosmological constant to his theory to enforce a static universe. It had matter in it, which he knew meant that gravitation favoured collapse - but he demanded that it be a time-independent, eternal universe. Analysis soon showed that Einstein’s static universe is unstable. A small ripple in space-time or in the mass it contained would make the universe either expand or contract. Einstein had brought his own concepts of time to the issue, and so missed predicting the expanding universe. Soon enough, astronomers’ observations showed that our universe is expanding from an earlier, smaller event. After this era, cosmological ideas of time moved beyond him.

Modern cosmology developed along parallel observational and theoretical tracks in the twentieth century. Correct cosmological solutions of general relativity emerged, and astronomers found that distant galaxies were apparently moving away from us. This comes from the expansion of space-time itself, not because we are uniquely abhorrent. Tracking this expansion backward gave a time when space-time approached zero. St Augustine had proposed that God made both space and time, and the big bang told us when that was.

Through the twentieth century, observations of how fast distant galaxies seemed to rush away from us have pushed the age of the universe back to the currently accepted number of 13.7 billion years. By then relativity had altered and even negated our understanding of Intuitive Time, so cosmology’s enormous extension of Deep Time only added to the startling changes.

Now astronomers observe that the universal expansion is accelerating, perhaps because of the unknown effects represented by Einstein’s added cosmological constant. We now seem to occupy an unusual niche in the long history of this universe, living beyond the early, hot era, yet well before the accelerating expansion will isolate galaxies from each other, then stars, and finally may wrench apart all of matter as space-time stretches ever-faster. Time seems then like a judge, not a mere clock.

The essential dilemmas of being human - the contrast between the stellar near-immortalities we see in our night sky, and our own all-too-soon, solitary extinctions - are now even more dramatically the stuff of everyday experience. We now know what a small sliver we inhabit in the long parade of our universe. Who can glimpse these perspectives and not reflect on our mortality? We are mayflies. Yet we now know enough of time and our place in it to reflect upon truly immense issues.

Time is a fundamental, its nature slowly glimpsed. After all this time, we do not fully understand it.

Here, on the level sand
Between the sea and land,
What shall I build or write
Against the fall of night?

- A.E. Housman

EPILOGUE

It’s sometimes wrongly imagined that cosmologists and evolutionists must be serenely unconcerned about next year, next week and tomorrow. I conclude with a ‘cosmic perspective’ which actually strengthens my own concerns about the here and now.

The stupendous timespans of the evolutionary past are, through the work of Darwin and the geologists, now part of common culture. But most people still regard humans as necessarily the culmination of the evolutionary tree. That hardly seems credible to an astronomer. Our Sun formed 4.5 billion years ago, but it’s got 6 billion more before the fuel runs out. It will then flare up, engulfing the inner planets and vaporising whatever remains on Earth. And the expanding universe will continue - perhaps for ever - destined to become ever colder, ever emptier. As Woody Allen said, ‘eternity is very long, especially towards the end’.

Any creatures witnessing the Sun’s demise 6 billion years hence, here on Earth or far beyond, won’t be human - they’ll be as different from us as we are from bacteria. As Charles Darwin himself recognised, ‘not one living species will transmit its unaltered likeness to a distant futurity’. Post-human evolution - here on Earth and far beyond - could be as prolonged as the Darwinian evolution that’s led to us - and even more wonderful. Life from this planet could spread through the entire Galaxy, evolving into a teeming complexity beyond what we can conceive.

However, even in this ‘concertinaed’ timeline - extending billions of years into the future, as well as into the past - the present century may be a defining moment. It’s the first in our planet’s history where one species - ours - has Earth’s future in its hands, and could not only jeopardise itself but foreclose life’s immense potential.

Suppose some aliens had been watching our planet for its entire history, what would they have seen? Over nearly all that immense time, 4.5 billion years, Earth’s appearance would have altered very gradually. The continents drifted; the ice cover waxed and waned; successive species emerged, evolved and became extinct.

But in just a tiny sliver of the Earth’s history - the last one millionth part, a few thousand years - the patterns of vegetation altered much faster than before. This signalled the start of agriculture. The pace of change accelerated as human populations rose.

Then there were other changes, even more abrupt. Within fifty years, little more than one hundredth of a millionth of the Earth’s age, the carbon dioxide in the atmosphere began to rise anomalously fast. The planet became an intense emitter of radio waves (the total output from all TV, cell-phone and radar transmissions). And something else unprecedented happened: small projectiles launched from the planet’s surface and escaped the biosphere completely. Some were propelled into orbits around the Earth; some journeyed to the Moon and planets.

If they understood astrophysics, the aliens could confidently predict that the biosphere would face doom in a few billion years when the Sun flares up and dies. But could they have predicted this unprecedented ‘fever’ less than halfway through the Earth’s life?

If they continued to keep watch, what might these hypothetical aliens witness in the next hundred years? Will a runaway spasm be followed by silence? Or will the planet itself stabilise? And will some of the objects launched from the Earth spawn new oases of life elsewhere?

The outcome depends on us. Wise choices will require the idealistic and effective efforts of natural scientists, environmentalists, social scientists and humanists - aided by the insights that twenty-first-century science will surely bring.