The science of eternity |
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January 2002 |
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This century may be a defining moment for the cosmos. If humans do not destroy
themselves they may spread beyond the earth into a universe that could last
almost forever. Life would have tunnelled through its moment of maximum jeopardy |
Martin Rees |
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Over the past few centuries, the earth has aged
spectacularly. Its creation has been moved back from 6pm on Saturday, 22nd
October, 4004 BC, as calculated by the 17th-century scholar and Archbishop
of Armagh, James Ussher, to a time and date some 4.5 billion years earlier.
The story of life has been stretched back almost as far, and the story of
complex, multicellular life-forms-relative newcomers-has itself been almost
a billion years in the making. As a result, the way we see the world has
changed profoundly. Not only can we now have some sense of the millions of
years it takes to raise and then level mountains, or to open and then close
oceans, we also have the clearest evidence of humanity's absence throughout
those ages. To Ussher's mind, the creation of the world and the creation
of humanity were within a week of one another; to our modern minds, the two
events are unimaginably far apart. There was a vast absence before us, a
physical and biological world untouched by introspection, and its record
stares out at us from every rock.
If the earth's past has been stretched,
what of its future? To those of Ussher's faith, the end of the world was
a certainty and to some of his contemporaries history was already nearing
its close. Sir Thomas Browne wrote, "the world itself seems in the wane.
A greater part of Time is spun than is to come."
To look forward,
we must turn from geology to cosmology. Current cosmology suggests a future
that, if not infinite, dwarfs the past as much as the depths of time we now
accept dwarf Ussher's exquisite estimates. What it cannot tell us, though,
is whether these vast expanses of time will be filled with life, or as empty
as the earth's first sterile seas. In the aeons that lie ahead, life could
spread through the entire galaxy, even beyond it-and outlast it too. But
life could also snuff itself out, leaving an eternity as empty as the space
between the stars.
getting very hot
To begin at the
beginning, we can, with some confidence, trace cosmic history back to its
first few seconds, some 12 billion years ago. But in response to the fundamental
question, "what happened before the beginning?" we cannot do much better
than St Augustine in the 5th century, who sidestepped the issue by arguing
that time itself was created with the universe. The origin of the "big bang"
is, in some ways, as mysterious to us as it was to St Augustine. Cosmologists
who study it are forced to jettison commonsense ideas, invoking extra spatial
dimensions and postulating that space and time may have an inherently "grainy"
structure on very tiny scales.
The sun-and the earth with it-came
into being when the universe was approximately two thirds its current age.
Minuscule though they may be in terms of their size, on a cosmic scale, they
are thus of a respectable age. The sun will continue to shine for five billion
years-longer than it has taken for our earth to evolve from a lump of molten
rock to its present state. It will then swell up into a red giant, engulfing
the inner planets and vaporising any life that remains on earth.
It
is hard to say what life might remain on the earth at that point. The sun
has been getting hotter since its birth and will continue to do so. This
heating has been balanced on earth, to some extent, by trends that have tended
to cool the planet; the most clear cut of which, over geological timescales,
has been a long-term drop in carbon dioxide and a resultant weakening of
the earth's natural greenhouse effect. This trend, however, cannot continue
for long; carbon dioxide, once more plentiful in the atmosphere than oxygen,
is now present at a mere 300 parts per million. Even a drop to zero would
not cool the planet a great deal in the face of increased solar luminosity-but
it would kill off the plants which rely on carbon dioxide as the raw material
for their photosynthetic growth, and thus remove oxygen from the atmosphere.
Without the cushion provided by ever-lower carbon dioxide levels, over the
next billion years the earth will start to feel the warming of the sun much
more than it has in the past, and when the surface reaches about 50ºC, the
increased levels of water vapour will permit a new form of "runaway" greenhouse
effect that will quickly raise the temperature high enough to boil the remaining
oceans. Current estimates suggest that the biosphere cannot survive much
beyond a billion years.
So by the time the sun finally licks the earth's
face clean, life on earth will either be extinct, spread beyond its original
planet or in a form impervious to extremes of temperature. Just as the universe
is still young, so it seems that the emergence of intelligence and complexity
is near its cosmic beginnings; we are far from the culmination of evolution.
Intelligently-controlled modifications could lead to faster and more dramatic
changes than Darwinian natural selection allows. The future may lie in artefacts
created by us and in some way descended from us, that develop via their own
directed intelligence. Such entities might see the death of the sun as a
minor or sentimental matter.
In the 1960s, Arthur C Clarke imagined
the "Long Twilight" after the death of the sun and today's other hot stars
as a realm at once majestic and slightly wistful. "It will be a history illuminated
only by the reds and infrareds of dully glowing stars that would be almost
invisible to our eyes; yet the sombre hues of that all-but-eternal universe
may be full of colour and beauty to whatever strange beings have adapted
to it. They will know that before them lie, not the... billions of years
that span the past lives of the stars, but years to be counted literally
in trillions. They will have time enough, in those endless aeons, to attempt
all things and to gather all knowledge. But for all that, they may envy us,
basking in the bright afterglow of creation; for we knew the universe when
it was young."
from big crunch to eternal expansion
But even after the longest twilight, night will fall. There will come a time
when the dimmest, slowest-burning stars are done. While academic cosmologists
publish, month after month, hundreds of scientific papers discussing the
ultra-early universe, they have written little about this long-range future.
But it is an area ripe for speculation. I can claim to have made one of the
first scientific contributions to "cosmic futurology" in a short 1968 paper
entitled "The Collapse of the Universe: an Eschatological Study." Many cosmologists
suspected then that the expansion that currently characterises our universe
would cease and reverse itself. Galaxies would then fall towards each other,
eventually crashing together into a "Big Crunch." I described how, as galaxies
merged together in the countdown to the crunch, individual stars would accelerate
to almost the speed of light (rather as the atoms speed up in a gas that
is compressed). Eventually these stars would be destroyed as the blue-shifted
radiation from other stars rushing towards them made the sky above them hotter
than the fires within.
Currently, though, the big crunch is out of
favour; more recent long-range cosmic forecasts have predicted that the expansion
of the universe will continue for ever, with its contents becoming ever colder
and more diffuse. Ten years after my paper, the Princeton theorist Freeman
Dyson-who would not countenance the Big Crunch because it "gave him a feeling
of claustrophobia"-made scientific eschatology more respectable in an influential
article called "Time without End: Physics and Biology in an Open Universe,"
published in the austere scholarly journal Reviews of Modern Physics. "The
study of the remote future," Dyson wrote, having noted that the handful of
papers on the subject were written in an apologetic or jocular style, "still
seems to be as disreputable today as the study of the remote past was 30
years ago." He set out to change the state of affairs with a rigorous study
of the physics of the far future and the prospects for some sort of life
persisting there.
Dyson charted the processes that would take place
after the suns had burned out, and their timescales; the loss of planets
from dead stars, the slow evaporation of galaxies, the decay of black holes
(a process that takes 1060 years-that is one with 66 zeros behind it for
a stellar-mass hole, and longer for a giant one) and the eventual transmutation
of all remaining matter into iron. That last alchemical endgame is spun out
for such a large number of years that to write it down would need as many
zeros as there are atoms in all the galaxies we can see. That is how long
you would have to wait before a giant quantum fluctuation caused an entire
star to "tunnel" into a black hole. Throughout it all, Dyson imagined, life
might persist in some form.
The universe's usable energy reserves
are finite, and at first sight this might seem to be a basic restriction
on everlasting life. But Dyson showed that this constraint was not fatal.
Just as an infinite series can have a finite sum (for instance 1 + H + G
+ ... = 2) so there is no limit to the amount of information processing that
could be achieved with a finite expenditure of energy. And the nature of
mathematics ensures that there could be infinite novelty generated in that
infinite information processing. It might be an odd, stripped down sort of
"life," a life abstracted to such an extent that only the most mathematical
of minds could conceive it as life-but it would not have to be repetitive,
and thus might not have to be dull. The rate at which the calculations were
made would get slower and slower and enforced periods of hibernation longer
and longer-as Woody Allen said, "eternity is very long, especially toward
the end"-but life in some form could persist.
what freeman dyson didn't know
Recent developments in physics have modified Dyson's picture in two ways.
First, we now suspect that atoms themselves live for "only" 1036 years-not
for ever. In consequence, the cold remnants of stars and planets (and any
complex entities made of atoms) will erode away as the atoms within them
decay. Thoughts and memories would only survive beyond this stage if downloaded
into circuits and magnetic fields in clouds of electrons and positrons-maybe
something that would resemble the alien intelligence in The Black Cloud,
the most imaginative of Fred Hoyle's science fiction novels of the 1950s.
Second, Dyson saw no limit to the scale of artefacts that could some day
be constructed. He envisioned the observable universe getting ever vaster,
as the expansion of space slowed down and galaxies whose light has not yet
had time to reach us (galaxies further away in space than the big bang is
in time) slowly came within range of possible communication and "networking,"
offering scope for ever larger cosmic construction projects. But within the
past few years, cosmologists have discovered to their surprise that the expansion
of the universe is not slowing down at all: some repulsive force or antigravity
seems to be pushing galaxies apart at an accelerating rate. This means that,
rather than more and more galaxies coming into contact with us, we will lose
contact with most of the galaxies we can now see: these galaxies will accelerate
towards the speed of light, eventually receding "over the horizon" into the
unobservable. In the very distant future, our descendants (if any) would
have lost contact with everything except the small group of galaxies that
contains our own milky way (by then bereft of the starlight that makes it
milky). There would therefore be a permanent limit on how large and complex
anything can get. Some theorists suggest that this limit on the energy and
complexity available would preclude infinite novelty and that eternity would
become very dull.
The implications of accelerating expansion, though,
need to be put in the context of the other uncertainties. First, we cannot
be sure that the regions beyond our present horizon are like the parts of
the universe we see, and this might influence future expansion. For another
thing, there is a theoretical possibility that the nature of the universe,
and the laws that hold sway in it, might be subject to radical and unexpected
change. Very pure water can stay liquid below 0ºC-a state known as supercooling-only
to freeze in a flash when a snowflake falls on it. It has been suggested
that our present "vacuum"-the fundamental stuff of space time-may also be
"supercooled," and thus that some comparatively minor event could trigger
a change to a new state of being, governed by quite different laws. There
are occasional scares that this sort of catastrophe could be induced artificially
by experiments that crash particles together with high energy. Those in charge
of the Brookhaven national laboratory in the US were cautious enough to commission
an expert assessment on whether any of their planned experiments could end
the universe this way or trigger some other global catastrophe. (Another
scare was that a new kind of atom called "strange matter" could be created
in accelerators, which would by contagion gradually transform the entire
earth into this new form-a scenario reminiscent of Kurt Vonnegut's Ice Nine,
where the oceans are transformed into a new kind of ice with a high freezing
point.) It is reassuring to note that "cosmic ray" particles, hurtling through
space with far more energy than any terrestrial experiment can generate,
have been hitting each other, on and off, for billions of years, without
tearing the fabric of space or destroying the possibility of matter.
the future of time travel
To see any of these wonders far ahead in time would require a time machine-something
like that contraption of "black and brass" imagined by HG Wells, just over
a century ago. As Wells's chrononaut gently eased the throttle forward, "night
came like the turning out of a light, and in another moment came tomorrow."
As he sped up "the palpitations of night and day merged into one continuous
greyness." Eventually, after some unpleasantness with the Eloi and the Morlocks,
he ends up 30m years hence, in a world where all familiar forms of life have
become extinct. He then returns to the present, bringing strange plants as
evidence of his trip.
Time travel clashes less with current scientific
concepts than it did with those of Wells' readers at the end of the 19th
century. Einstein has taught us that time is not "absolute." If you move
almost as fast as light, or if you are exposed to strong gravity near a black
hole, time is stretched. This may seem counter-intuitive. But our intuitions
are based on a constricted range of experience. Few of us have travelled
faster than a millionth of the speed of light (the speed of a jet airliner)
and we live on a planet where gravity is far weaker than it is near black
holes. But time travel still takes place, if only on a small scale. Richard
Gott, another Princeton theorist, calculates that the American astronaut
Story Musgrave is a millisecond younger than he would be if he had not travelled
repeatedly into space. The designers of the extremely accurate clocks at
the heart of the GPS satellite navigation system have had to take into account
the fact that the clocks on their satellites measure time at a slightly different
rate to clocks on the earth.
Time travel into the far future violates
no fundamental physical laws. A spaceship that could travel at 99.99 per
cent of the speed of light would allow its crew to "fast forward" into the
future. An astronaut who managed to navigate into the closest possible orbit
around a rapidly-spinning black hole without falling in could, in a subjectively
short period, view an immensely long future time span in the external universe.
Such adventures may be technically challenging, but they are not impossible.
Nor, rather more surprisingly, is travel into the past-as far as we know.
Time, as Wells and his chrononaut knew, is a fourth dimension, but it is
different from the three dimensions that define space. In three-dimensional
space we can move to the left as easily as to the right, backward as well
as forward, up as well as down. But we seem to be dragged forward in time
willy-nilly. In some ways this seems like a good thing, since returning to
the past involves the risk of changing it in such a way that makes history,
or the universe itself, internally inconsistent. Science fiction writers
like Isaac Asimov and Poul Anderson have imagined busy time- police trying
to keep reality straight, stopping people from murdering their grandmother,
or engineering the defeat of Charles Martel at the battle of Tours, or whatever.
Physicists, less in need of plot lines, have preferred to have such things
banned by the laws of nature rather than the laws of man. But insisting that
time travel cannot change the past is not the same as saying that time travel
cannot happen.
More than 50 years ago, the great logician Kurt Gödel
(Princeton again) invented a bizarre hypothetical universe, consistent with
Einstein's theory, that allowed "time loops," in which events in the future
"cause" events in the past that then "cause" their own causes, introducing
a lot of weirdness to the world but no contradictions. (The film, The Terminator,
in which a son sends his father back in time to save-and inseminate-his mother,
wonderfully combines the insights of the greatest Austrian-American mind,
Gödel, with the talents of the greatest Austrian-American body, Arnold Schwarzenegger.)
Several later theorists have used Einstein's general theory of relativity
(which tells you how to change the shape of spacetime) to design "time machines"
that might create temporal loops. But these machines are not things you can
build in a Victorian basement. Some of them need to be of effectively infinite
length; others need vast amounts of energy.
Despite the remoteness
of these concepts from realistic technology, Gödel's discovery raised a fundamental
question: is time-travel into the past precluded by a "chronology protection
law?" The science-fiction writer Larry Niven believes that time travel would
create a chronology protection law; if time travel is allowed, time travellers
will endlessly change the universe until one of them changes something so
basic that time travel becomes impossible, at which point the universe stabilises
with a new set of laws that rule out time travel. One piece of evidence for
the protection of chronology sometimes put forward is that time travel is
not, in fact, observed: "tourists from the future" don't swell the crowds
at historical events, as far as we can tell. But this may tell us only that
no time machine has yet been made, not that it is impossible. Even theories
that allow time machines suggest that they have limitations: a time machine
could not enable its users to travel back prior to the date of its construction.
If there is no absolute chronological protection, perhaps there is some subtler
means whereby, while time travel is possible, it never changes what has already
happened. This would restrict the time traveller's free will, but that is
nothing new. Physics already constrains us: we cannot exercise our free will
by walking on the ceiling. Another option is that time-travellers would shift
into a parallel universe, where events played out differently rather than
repeating themselves exactly, as in the film Groundhog Day. Perhaps in the
sunless future, as galaxies fade over the cosmic horizon like astronauts
sinking into black holes, it will be through time travel to parallel universes
that our descendants stave off the ennui of eternity. As the long day wanes
and the slow moons climb, they will make off for newer worlds, endlessly
seeking a perfect past.
A dangerous moment for life
A hackneyed anecdote among astronomy lecturers describes a worried questioner
asking: "how long did you say it would be before the sun burnt the earth
to a crisp?" On receiving the answer, "five billion years," the questioner
responds with relief: "thank God for that, I thought you said five million."
What happens in far-future aeons may seem blazingly irrelevant to the practicalities
of our lives. But I don't think it is. It is widely acknowledged that the
Apollo programme's pictures of the island earth, its fragile beauty contrasted
with the stark moonscape, changed the way we see ourselves in space-strengthening
the collective ties that bind us to our environment. No new facts were added
to the debate; just a new perspective. A new perspective on how we see ourselves
in time might do something similar. (That is the hope of the Long Now Foundation,
an organisation devoted to the construction of clocks and other instruments
that can last thousands of years.)
Taking the truly cosmic long view,
the view in which billions of years pass as hours, would require pictures
of the earth, not from the neighbouring moon, but from light years away,
from some far-off star. Within a few decades, new generations of space telescopes
should provide the technology to take just such pictures-to produce images
of the planets around other stars. Will they have biospheres? Will they harbour
intelligent life? We know too little about how life began, and how it evolves,
to be able to say whether alien intelligence is likely or not. The cosmos
could already be teeming with life: if so, nothing that happens on earth
would make much difference to life's long-range cosmic future. On the other
hand, advanced life may be rare-so rare, perhaps, that it is unique to our
earth. The emergence of intelligence may require such an improbable chain
of events that it has not occurred anywhere else-not around even one of the
trillion billion other stars within range of our telescopes. Claims that
advanced life is widespread must confront the famous question first posed
by the physicist, Enrico Fermi: "why aren't the aliens here?" Why haven't
they visited earth already, or at least manifested their existence in some
way? This argument gains extra weight when we realise that some stars similar
in many ways to our sun are billions of years older: if life were common,
its emergence should have had a head start on planets around these ancient
stars.
But if earth is the unique abode of intelligence, the fate
of our planet could have an importance that is truly cosmic: it might conceivably
be the difference between a near eternity filled with ever more complex
and subtle forms of life and one filled with nothing but base matter. Yet
this fate is finely balanced. One does not need to believe that the Brookhaven
national laboratory is a threat to believe that technological civilisation
carries risks that get greater with every century. The 20th century brought
us the bomb; the 21st offers the more far-reaching threats of catastrophically
replicating rogue nano-machines and engineered diseases. The latter are the
biggest worry. Within a few years, thousands-even millions-of individuals
may acquire the capability to make and disseminate weapons that could cause
widespread epidemics. An organised network of al Qaida type terrorists would
not be required: just one fanatic or weirdo with the mindset of those who
now design computer viruses. In a few decades, such individuals may be able
to trigger truly global catastrophes. Even if all nations applied a strict
precautionary principle to dangerous procedures, the chances of effective
worldwide enforcement are small. There is also, of course, the possibility
of accidents in the most respectable of institutions.
Pessimism on
these matters seems the rational stance. But technology also offers hope.
Perhaps, by the end of the century, self-sustaining communities will have
been established away from the earth-on the moon, on Mars, or freely floating
in space. Although it will be little consolation to those on earth, life's
long-term cosmic potential could thereafter not be quenched by any terrestrial
catastrophe: life would have "tunnelled through" its era of maximum jeopardy.
This is the best reason for prioritising programmes of manned, rather than
unmanned, spaceflight. (New technologies may offer another option too: downloading
our blueprint into inorganic memories that could be launched into space.)
The most crucial location in space and time (apart from the big bang itself)
could be here and now. This new century, on this planet, may be a defining
moment for the cosmos. Our actions could initiate the irreversible spread
of life beyond the solar system. Or, in contrast, through malign intent,
or through misadventure, 21st-century technology could jeopardise life's
cosmic potential when its evolution has still barely begun. |
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The author is the astronomer royal and fellow of King's College, Cambridge |
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