Published by Gregory Benford on June 5th, 2015

As the most world’s most prominent astronomer, Martin Rees is singularly well qualified to outline the implications of a simple observation—that on the scale of our galaxy, our own species’ evolution is just beginning. This is a truly long range view, done with remarkable skill.


This essay he wrote expressly for STARSHIP CENTURY, edited by myself and my brother James. — GB



                       TO THE ENDS OF THE UNIVERSE


Martin  Rees



Astronomers like myself are  professionally engaged in thinking about huge expanses of space and time. But this doesn’t make us serene and relaxed about the future.   Most of us worry as much as anyone about what happens next year, next week, or tomorrow. Nonetheless, our subject does offer a special perspective. We view our home planet in a cosmic context.   We wonder whether there’s life elsewhere in the cosmos. But, more significantly, we’re mindful of the immense future that lies ahead.


The stupendous timespans of the evolutionary past are now part of common culture (but maybe not in the Bible Belt, nor in parts of the Islamic world).   We’re  at ease with the idea that our present biosphere is the outcome of four billion years of Darwinian evolution.  But the even longer time-horizons that stretch ahead — though familiar to every astronomer — haven’t permeated our culture to the same extent. Our Sun is less than half way through its life.  It 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 any life that might then remain on Earth.  But even after the Sun’s demise, the expanding universe will continue — perhaps for ever —  destined to become ever colder, ever emptier.  To quote Woody Allen, ‘eternity is very long, especially towards the end’.   That, at least, is the best long range forecast that cosmologists can offer.


Any creatures witnessing the Sun’s demise 6 billion years hence won’t be human — they’ll be as different from us as we are from a bug.      Posthuman evolution — here on Earth and far beyond — could   be as prolonged as the Darwinian evolution that’s led to us, and even more wonderful.  Indeed this conclusion is strengthened when we realise that future evolution will proceed not on the million-year timescale characteristic of Darwinian selection, but at the much accelerated rate  allowed  by genetic modification and the advance of machine intelligence (and forced by the drastic environmental pressures that would confront  any humans who  were to construct habitats beyond the Earth). Natural selection may have slowed: its rigours are  tempered in civilised countries. But it will be replaced by ‘directed’ evolution.  Already, performance enhancing drugs, genetic modification, cyborg technology, are changing human nature, and these are just precursors of more drastic changes.


Darwin himself realised that  “No living species will preserve its unaltered likeness into a distant futurity”. We now know that ‘futurity’ extends far further, and alterations can occur far faster than Darwin  envisioned.  And we know that the cosmos, through which life could spread, offers a far more extensive and varied habitat  than he ever imagined.  So humans are surely  not the terminal  branch of an evolutionary tree, but  a species that emerged early in the overall roll-call of species, with  special promise  for diverse evolution — and perhaps of cosmic significance for jump-starting the transition to silicon-based (and potentially immortal) entities that can  more readily transcend human limitations.


We humans are entitled to feel uniquely significant, as the first known species with the power to mould its own future. And we live at a crucial time. Our Earth has existed for 45 million centuries, and still more lie ahead. But this 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 jeopardise the  immense potential stretching for billions of years. And, more central to the theme of this book, it’s the century when  the spread of life  beyond Earth could begin.


A famous picture in the  English edition of  Newton’s ‘Principia’  shows cannon balls being fired from the top of a mountain, If they go fast enough, their trajectory curves downward no more steeply than the Earth curved away underneath it – they  go into orbit. This picture is  still the neatest way to explain orbital flight.  Newton calculated that, for a cannon-ball to achieve an orbital trajectory, its speed must be 18000 miles/hour –far beyond  what was then achievable.


Indeed, this speed wasn’t achieved until 1957, when the Soviet Sputnik was launched. Four years later Yuri Gagarin was the first human to go into orbit. Eight years after that, and  only 66 years after the Wright Brothers’ first flight, Neil Armstrong made his ‘one small step’. The Apollo programme was a heroic episode. And it was a long time ago – ancient history to today’s young people. Those in England know that the Americans landed on the Moon, just as they know the Egyptians built pyramids — but both enterprises seem driven by equally arcane goals.


Since 1972,  humans have done no more than circle the Earth in low orbit – more recently,  in the International Space Station. This has proved neither very useful nor very inspiring.  But space technology has burgeoned — for communication, environmental monitoring, satnav and so forth. We depend on it every day. And unmanned probes   to other planets have beamed back pictures of  varied and distinctive worlds.

But it has been pictures of the Earth itself, showing how its delicate biosphere contrasts with the sterile moonscape where the astronauts left their footprint, that have  become iconic, especially  for environmentalists. We’ve had these images for 45 years. But suppose some aliens had been viewing such an image for our planet’s 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. Humanity’s ‘footprint’ got larger because   our species became  more demanding of resources — and also because of population growth.


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.  And  something else unprecedented  happened:  rockets launched  from the planet’s surface  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 sudden ‘fever’  half way through the  Earth’s life  — these human-induced alterations occupying, overall, less than a millionth of the Earth’s elapsed lifetime and seemingly occurring with runaway speed?


If they continued to keep watch, what might they witness  in the next hundred years?  Will the spasm be followed by silence? Will the planet make a transition to sustainability?  And, most important of all for the log-term future, will an armada of rockets leaving Earth have led to new communities elsewhere — on Mars and its moons, on asteroids, or freely floating in space?




Scientific forecasters have a dismal record.  One of my predecessors as Astronomer Royal said, as late as the 1950s, that  space travel was “utter bilge”.  Few in the mid-20th century envisaged the transformative impact of the silicon chip or the double helix.  The iPhone would have seemed magical even 20 years ago.  So, looking even a century ahead we must keep our minds open, or at least ajar, to what may now seem science fiction.   Indeed some proponents of the ‘singularity’ — the takeover of  humanity by intelligent machines — claim this transition could happen within 50 years.


Had the momentum  of the 1960s been maintained  over the next 40 years, there would be footprints on Mars by now. But after Apollo  the political impetus for manned spaceflight was lost. This was one of many instances of the widening  gap between what could be achieved technologically, and what is actually done. As with many technical forecasts, we can be more confident of what  could happen than of how soon it will happen. Development of supersonic airliners, for instance, has languished (Concorde having gone the way of the dinosaurs); in contrast, the sophistication and worldwide penetration of internet and smart-phones advanced much faster  than  most forecasters predicted.


I’d venture a confident forecast that  during  this  century,  the entire solar system — planets, moons and asteroids —  will  be explored and mapped by flotillas of tiny robotic craft.  The next step would be space mining and fabrication. (And fabrication in space will be a better use of materials mined from asteroids than bringing them back to Earth).  The Hubble Telescope’s successors, with huge gossamer-thin mirrors assembled under zero gravity,  will further expand our vision of stars, galaxies and the wider cosmos.


But what role will humans play? There’s no denying that NASA’s  ‘Curiosity’, now trundling across Martian craters, may miss startling discoveries that no human geologist could  overlook. But robotic techniques are advancing fast, allowing ever more sophisticated unmanned probes;  whereas  the cost gap between manned and unmanned missions remains  huge.  The  practical case   for manned spaceflight gets ever-weaker with each advance in robots and miniaturisation — indeed as a scientist or practical man I see little  purpose in  sending people into space at all.  But as a human being, I’m an enthusiast for manned missions. I hope some people now living will walk on Mars – as an adventure, and as a step towards the stars. They may be Chinese: China has the resources, the dirigiste government, and maybe the willingness to undertake an Apollo-style programme.  And China would need to aim at Mars, not just at the Moon, if it wanted to assert its super-power status by a ‘space spectacular’:  a re-run of what the US achieved 50 years earlier would not proclaim parity.


NASA’s manned programme, ever since Apollo, has  been impeded by public and political pressure into being too risk-averse. The Space Shuttle failed  twice in 135 launches.  Astronauts or test pilots would willingly accept this risk level,  but  the Shuttle had, unwisely, been  promoted as a safe vehicle for civilians. So each failure caused a national trauma and was followed by a hiatus while costly efforts were made (with very limited effect) to reduce the risk still further.


Unless motivated by pure prestige and bankrolled by superpowers, manned missions beyond the Moon will  need perforce to be cut-price ventures, accepting   high risks – perhaps even ‘one-way tickets’.  These missions will be privately funded; no Western governmental agency would expose civilians to such  hazards. There would, despite the risks, be many volunteers —  driven by the same motives as early explorers, mountaineers, and the like.  Private companies already offer orbital flights.  Maybe within a decade adventurers will be able to sign up for a week-long trip round the far side of the Moon – voyaging further from Earth than anyone has been before (but avoiding the greater challenge of a Moon landing and blast-off). And by mid-century the most intrepid (and wealthy)  will be going further.


(The phrase ‘space tourism’ should however be avoided. It lulls people into believing that such ventures are routine and low-risk. And if that’s the perception, the  inevitable  accidents will be as traumatic as those of the Space Shuttle were.  Instead,  these cut-price ventures must be ‘sold’   as dangerous sports,  or intrepid exploration. )


But don’t ever expect mass emigration.  Nowhere in our Solar system offers an environment even as clement as the Antarctic or the top of Everest. Space doesn’t offer an escape from Earth’s problems. Nonetheless, a century or two from now, there may be small groups of pioneers living independent from the Earth – on Mars or on asteroids. Whatever ethical constraints we impose here on the ground, we should surely wish these adventurers good luck in genetically modifying their progeny to adapt to alien environments. This might be the first step towards divergence into a new species: the beginning of the post-human era. And machines of human intelligence could spread still further. Whether the long-range future lies with organic post-humans or with intelligent machines is a matter for debate. Either way, dramatic  cultural and technological evolution will continue not only here on Earth but far beyond.


The most crucial impediment to space flight, even in Earth’s orbit  and  still more for those venturing further, stems from the intrinsic inefficiency of chemical fuel,  and the consequent requirement to carry a weight of fuel far exceeding that of the payload. (It’s interesting to note, incidentally  that this is a generic constrain, based on fundamental chemistry, on any organic intelligence that had evolved on another planet. If a planet’s gravity is strong enough to retain an atmosphere, at a temperature where water doesn’t freeze and metabolic reactions aren’t to slow, the energy required to lift   a  molecule from it will require more than one molecule of chemical fuel.)


Launchers will get cheaper when they can be designed to be more fully reusable.  It will then  be feasible to assemble, in orbit,  even larger artifacts than the International Space Station.  But so long as we depend on chemical fuels, interplanetary travel will remain a challenge.  Nuclear power could be transformative. By allowing much higher in-course speeds, it would drastically cut the  transit times to Mars or the asteroids (reducing  not only  astronauts’ boredom, but their exposure to damaging radiation).  And it could transform manned spaceflight from high-precision to an almost unskilled operation. Driving a car would be a difficult enterprise if, as at present for space voyages, one had to program the entire journey beforehand, with minimal opportunities for steering on the way.  If there were an abundance of fuel for mid-course corrections (and to brake and accelerate at will), then  interplanetary navigation would be a doddle — indeed simpler than driving a car or ship, in that the destination is always in clear view.


But even with nuclear fuel, the transit time to nearby stars exceeds a human lifetime. Interstellar travel is therefore, in my view,  an enterprise for post-humans, evolved from our species not via natural selection but by design. They could be silicon-based. Or they could be organic creatures who had won the battle with death, or perfected the techniques of hibernation or suspended animation.  Even those of us who don’t buy the idea of a singularity by mid-century would expect sustained, if not enhanced, rate of innovation in biotech, nanotech and in information science.  I think there will be entities  with superhuman intellect within a few centuries. The first voyagers to the stars will not be human, and maybe not even organic. They will be creatures whose life-cycle is matched to the voyage the aeons involved in  traversing the Galaxy are not daunting to immortal beings.





By the end of the  third millennium, travel to other stars  could  be technically feasible.  Before setting out from Earth the  voyagers would know  what   to expect at journey’s end. Most importantly of all, they will know whether their destination is lifeless or inhabited, and robotic probes will have sought out already-existing biospheres, or planets that could be terraformed to render them habitable.


It could happen, but would there be sufficient motive?  Would even the most intrepid leave the solar system? We can’t predict what inscrutible goals might drive post-humans. But the motive would surely be weaker if it turned out that biospheres were  rare. The European  explorers in earlier centuries who ventured across the Pacific were going into the unknown to a far greater extent than any future explorers would be (and facing more terrifying dangers)  — there were no precursor expeditions to make maps, as there surely would be for space ventures. Future space-farers would always be able to communicate with Earth (albeit with a timelag).  If precursor probes have revealed that there are indeed  wonders to explore, there will be a compelling motive —  just  as Captain Cook was motivated by the biodiversity and  beauty of the Pacific islands. But if there is nothing but sterility  out there, the motive will be simply expansionist — in resources and energy — and that might be better left to robotic fabricators.


How bright are the prospects that there is life out there already? There may be simple organisms on Mars, or remnants of creatures that lived early in the planet’s history; and there could be life, too, in the ice-covered oceans of Jupiter’s moons Europa and Ganymede. But few would bet on it; and certainly nobody  expects a complex biosphere in such locations.  For that, we must look to the distant stars – far beyond the range of any probe we can now construct.


In the last twenty years (and especially in the last five) the night sky has become far more interesting, and far more enticing to explorers, than it was to our forbears.    Astronomers have discovered that many stars — perhaps even most  — are orbited by retinues of planets, just as the Sun is.  These planets are  not detected directly. Instead, they reveal their presence by effects on their parent star that can be detected by precise measurements: small  periodic motions in the star induced by an orbiting planet’s gravity, and slight recurrent dimmings  in a star’s brightness when a planet transits in front of it, blocking out a small fraction of its light.


Data are accumulating fast, especially from NASA’s Kepler spacecraft, which is monitoring the brightness of 150000 stars with high enough precision to detect transits of planets no bigger than the Earth.  Some stars are known to be orbited by as many as seven planets, and it’s already clear that  planetary systems display a surprising  variety — our own Solar System may be far from typical. In some systems, planets as big as Jupiter are orbiting  so close to their star that their ‘year’ lasts only a few days.  Some  planets are on very eccentric orbits.   One is orbiting a binary star which in turn orbits another binary star:  it would have four ‘suns’ in its sky.  But there is special interest  in  possible ‘twins’ of our Earth — planets the same size as ours, orbiting other Sun-like stars, on orbits with temperatures such that water neither boils nor stays frozen. NASA’s Kepler spacecraft has identified  of hundreds of these.



But we’d really like to see these planets  directly — not just their shadows. And that’s hard. To realise just how hard,  suppose an alien astronomer with a powerful telescope  was viewing the Earth  from (say) 30 light years away — the distance of a nearby star. Our planet  would seem,  in Carl Sagan’s phrase, a ‘pale blue dot’, very close to a star (our Sun) that outshines it by many billions: a firefly next to a searchlight.  But if the aliens could detect the Earth at all, they could learn quite a bit about it. The shade of blue  would be slightly different, depending on whether the Pacific ocean or the  Eurasian land mass was facing them.  They could infer the length of the ‘day’,  the seasons, whether their are oceans, the  gross topography, and the   climate.  By analysing the faint light,  they  could infer that  the Earth had  a biosphere.


Within  20 years, the unimaginatively named ELT (‘Extremely Large Telescope’) planned by European astronomers, with a mosaic mirror 39 meters across, will be able to draw such inferences about  planets the same size as our Earth, orbiting other Sun-like stars. (And there are two somewhat smaller US telescopes in gestation too)


But do we expect alien  life on these extra-solar planets? We know too little about how life began on Earth to lay confident odds. What triggered the transition from complex molecules to entities that can metabolise and reproduce?  It might have involved a fluke so rare that it happened only once in the entire Galaxy – like shuffling a whole pack of cards into a perfect order. On the other hand, this crucial transition  might have been almost inevitable given the ‘right’ environment.  We just don’t know — nor do we know if the  DNA/RNA chemistry of terrestrial life the only possibility, or just one chemical basis among many options that could be realized elsewhere.  Even if simple life is common, it is of course a separate  question whether it’s likely to evolve into anything we might recognize as intelligent or complex — whether Darwin’s writ runs through the wider cosmos.  Perhaps the cosmos teems with life; on the other hand, our Earth could be unique among the billions of planets that surely exist.


And it might be too anthropocentric to limit attention to Earth-like planets.  Science fiction writers have other ideas — balloon-like creatures floating  in the dense atmospheres of Jupiter-like planets,  swarms of intelligent insects, nanoscale robots etc. Perhaps life can flourish even on a planet flung into the frozen darkness of interstellar space, whose main warmth comes from internal radioactivity (the process that heats the Earth’s core).  There could be diffuse living structures,  freely-floating in interstellar clouds; such entities  would live  (and,  if intelligent,  think)  in slow motion, but nonetheless may come into their own in the long-range future.


No life will survive around on a planet whose central Sun-like star became a giant and blew off its outer layers.  Such  considerations remind us of the transience of  inhabited worlds (and life’s imperative to escape their bonds eventually). We should also be mindful that seemingly artificial signals could come from super-intelligent (though not necessarily conscious) computers, created by a race of alien beings that had already died out.  Maybe we will one day find ET.  On the other hand,  SETI searches may fail; Earth’s intricate  biosphere may be unique.  But that would not render life  a cosmic sideshow. Evolution is just beginning.     Our Solar System is barely middle aged and if  humans avoid self-destruction, the post-human era beckons.    Life from Earth  could spread through the   entire Galaxy, evolving into a   teeming  complexity  far beyond what we can even conceive. If so, our tiny planet  — this pale blue dot floating in space — could be the most important place in the entire Galaxy. The first interstellar voyagers from Earth would have a mission that would resonate through the entire Galaxy and perhaps beyond.




In cosmological terms (or indeed in a  Darwinian timeframe) a millennium is but an instant. So let us ‘fast forward’ not for a few centuries, nor even for a few millennia, but  for an ‘astronomical’ timescale millions of times longer than that.  The  ‘ecology’ of stellar births and deaths in our Galaxy will proceed gradually more slowly, until jolted by the ‘environmental shock’ of an impact with Andromeda, maybe four billion years hence.   The debris of our Galaxy, Andromeda and their smaller companions within the local group will thereafter aggregate into one amorphous galaxy. If the cosmic acceleration continues, then, as Freeman Dyson and others have noted, the observable unlverse gets emptier and more lonely. Distant galaxies will not only move further away, but  recede faster and faster until they disappear — rather as objects falling onto a black hole encounter a horizon, beyond which they are lost from view and causal contact.


But the remnants of our Local Group could  continue for far longer — time enough, perhaps for Kardashev Type III phenomenon  to emerge as the culmination of the long-term trend for  living systems to gain complexity and ‘negative entropy’. All the atoms that were once in stars and gas could be transformed into   structures as intricate as a living organism or a silicon chip but on a cosmic scale.


But even these speculations are in a sense conservative.  I have assumed that the universe itself will expand, at a rate that no future entities have power to alter. And that  everything is in principle understandable as a manifestation of the basic laws governing particles, space and time that have been partly disclosed by 20th century science. Other chapters in this book envisage stellar-scale engineering to create black holes and wormholes — concepts far beyond any technological capability that we can envisage, but not in violation of these basic physical laws.  But are there new ‘laws’  awaiting discovery? And will the present ‘laws’ be immutable, even to a Type III intelligence able to draw on galaxtic-scale resources?


We are well aware that our knowledge of space and time is incomplete. Einstein’s relativity and the quantum principle are the two pillars of 20th century physics, but a theory that unifies them is unfinished business for 21st century physicists.  Current ideas suggest that there are mysteries even in what might seem the simplest entity of all — ‘mere’ empty space.  Space may have a rich structure, but  on scales a trillion trillion times smaller than an atom. According to string theory, each ‘point’ in our ordinary space, if viewed with this magnification, would be revealed as a tightly-wound origami in several extra dimensions. Such a theory will perhaps tell us why empty space can exert the  ‘push’ that causes the cosmic expansion to accelerate; and whether that ‘push’ will indeed continue for ever or could be reversed.  It will also allow us to model the very beginning – an epoch where densities are so extreme that quantum fluctuations can shake the entire universe — and learn whether our big bang was the only one.


The  same fundamental  laws  apply throughout the entire domain we can survey with our telescopes. Atoms in the most distant observable galaxies seem, from spectral evidence, identical to atoms studied in laboratories on Earth.  But what we’ve traditionally called ‘the universe’  —  the aftermath of ‘our’ big bang — may be just one island, just one patch of space of time,  in a perhaps-infinite archipelago.  There may  have been an infinity of big bangs, not just one.  Each constituent of this ‘multiverse’ cooled down differently, ending up governed by different laws. Just as Earth is a very special planet among zillions  of others, so– on a far grander scale — our big bang was also a very  special one.   In this hugely expanded cosmic perspective, the laws of Einstein and the quantum could be mere parochial  bylaws governing  our cosmic patch.  Space and time may have a structure as intricate  as the fauna of a rich ecosystem, but on a scale far larger than the horizon of our observations.  Our current concept of physical reality  could be as constricted, in relation to the whole, as the perspective of the Earth  available to a planckton  whose ‘universe’ is  a spoonful of water.


And that’s not all – there is a final disconcerting twist.

Post-human intelligence (whether in organic form, or in autonomously-evolving artefacts) will develop  hypercomputers with the processing power to  simulate living things — even entire worlds. Perhaps advanced beings could use hypercomputers to   surpass the  best ‘special effects’ in movies or computer games so vastly that they could simulate a universe fully  as complex as  the one we perceive ourselves to be in. Maybe these kinds of super-intelligences already exist elsewhere in the multiverse –  in universes that are older than ours, or better tuned  for the evolution of intelligence. What would these super-intelligences do with their hyper-computers? They  could create virtual universes  vastly outnumbering the ‘real’ ones. So perhaps we are ‘artificial life’ in a virtual universe. This concept opens up the possibility of a new kind of ‘virtual time travel’, because the advanced beings creating the simulation can, in effect, rerun the past. It’s not a time-loop in a traditional sense: it’s a reconstruction of the past, allowing advanced beings to explore their history.



Possibilities once in the realms of science fiction have shifted into serious scientific debate. From the very first moments of the big bang to the mind-blowing possibilities for alien life, parallel universes and beyond, scientists are led to worlds even weirder than most fiction writers envisage. We have intimations of deeper links between life, consciousness and physical reality. It is remarkable that our brains, which have changed little since our ancestors roamed the African savannah, have allowed us to understand the counterintuitive worlds of the quantum and the cosmos. But there is no reason to think that our comprehension is matched to an understanding of all key features of reality. Scientific frontiers are advancing fast, but we may sometime ‘hit the buffers’. Some of these insights may have to await post-human intelligence. There may be phenomena, crucial to our long-term destiny, that we are not aware of, any more than  a monkey comprehends the nature of stars and galaxies.


If our remote descendents reach the stars, they will surely  far surpass us in insight as well as technology.





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