IN 1969, after graduating from Harvard but before starting further study in
astrophysics at Princeton University, I took a summer holiday in Europe and
visited the Berlin Wall. It was the height of the Cold War, and the wall was
then eight years old. Standing in its ominous shadow, I began to wonder how long
it would last. Having no special knowledge of East-West relations, I hadn鈥檛 much
to go on. But I hit on a curious way to estimate the wall鈥檚 likely lifetime
knowing only its age.
I reasoned, first of all, that there was nothing special about my visit. That
is, I didn鈥檛 come to see the wall being erected or demolished鈥擨 just
happened to have a holiday, and came to stand there at some random moment during
the wall鈥檚 existence. So, I thought, there was 50 per cent chance that I was
seeing the wall during the middle two quarters of its lifetime
(see Diagram).
If I was at the beginning of this interval, then one-quarter of the
wall鈥檚 life had passed and three-quarters remained. On the other hand, if I was
at the end of this interval, then three-quarters had passed and only one-quarter
lay in the future. In this way I reckoned that there was a 50 per cent chance
the wall would last from 1/3 to 3 times as long as it had already.
Before leaving the wall, I predicted to a friend that it would, with 50 per
cent likelihood, last more than two and two-third years but less than 24. I then
returned from holiday and went on to other things. But my prediction, and the
peculiar line of reasoning that lay behind it, stayed with me. Twenty years
later, in November 1989, the Berlin Wall came down鈥攗nexpectedly, but in
line with my prediction.
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Intrigued that the approach seemed to work, I eventually set out its logic in
a paper that appeared in Nature (vol 363, p 315, 1993). There, instead
of using the 50 per cent mark, I adopted the more standard scientific criterion
that predictions should have at least a 95 per cent chance of being correct.
This makes the numbers in the formula come out a bit different, but the argument
remains the same. If there is nothing special about your observation of
something, then there is a 95 per cent chance that you are seeing it during the
middle 95 per cent of its observable lifetime, rather than during the first or
last 2.5 per cent
(see Diagram).
At one extreme, the future is only 1/39
as long as the past. At the other, it is 39 times as long. With 95 per cent
certainty, this fixes the future longevity of whatever you observe as being
between 1/39 and 39 times as long as its past.
This formula can be used to amusing effect. Mathematicians Peter Landsberg,
Jeff Dewynne and Colin Please of the University of Southampton used it in 1993
to predict how long Britain鈥檚 Conservative government would stay in power (
Nature, vol 365, p 384, 1993). The Conservative Party had then been in
power for 14 years, and they estimated with 95 per cent confidence that it would
remain in power for at least 4.3 more months but less than 546 more years. Sure
enough, the Conservative Party was ousted just 3.5 years later.
Living proof
As another test, I used my formula on the day my Nature paper was
published to predict the future longevities of the 44 Broadway and off-Broadway
plays and musicals then running in New York; 36 have now closed鈥攁ll in
agreement with the predictions. The Will Rogers Follies, which had been
open for 757 days, closed after another 101 days, and the Kiss of the Spider
Woman, open for 24 days, closed after another 765 days. In each case the
future longevity was within a factor of 39 of the past longevity, as
predicted.
This is all good fun. You can predict approximately how long something will
last without knowing anything other than its current age. But in the past few
months, in the light of the spectacular success of the NASA鈥檚 Mars Pathfinder
mission, I鈥檝e been reminded of a far more serious implication of this way of
thinking. Applying it to the human race forces me to conclude that our
extinction as a species is a very real possibility, and that we had better take
steps to improve our survival prospects before it鈥檚 too late. Let me explain why
I have such a sense of urgency, and why we had better begin colonising
space鈥攁nd very soon.
In the 16th century, Nicolaus Copernicus pointed out that the Earth revolved
about the Sun, rather than vice versa, and in one swift move, displaced humanity
from its privileged place at the very centre of the Universe. We now see the
Earth as circling an unexceptional star among thousands of millions of others in
our unexceptional Galaxy. This perspective is summed up more generally in the
鈥淐opernican principle鈥�, which is the supposition that one鈥檚 location is unlikely
to be special.
Early this century, when astronomer Edwin Hubble observed approximately the
same number of galaxies receding from Earth in all directions, it looked as if
our Galaxy was at the exact centre of a great explosion. But reasoning with the
Copernican principle, scientists concluded instead that the Universe must look
that way to observers in every galaxy鈥攊t would be presumptuous to think
that our galaxy is special. As a working hypothesis, the Copernican principle
has been enormously successful because, out of all the places intelligent
observers could be, there are only a few special places and many nonspecial
places. A person is simply more likely to be in one of the many nonspecial
places. But the Copernican principle doesn鈥檛 apply only to placement of galaxies
in space鈥攊t works for the placement of moments in time as well.
What does it imply for Homo sapiens?We have been around for about
200 000 years. If there is nothing special about the present moment, then it is
95 per cent certain that the future duration of our species is between 1/39 and
39 times 200 000 years. That is, we should last for at least another 5100 years
but less than 7.8 million years.
Since we have no actuarial data on other intelligent species, this Copernican
estimate may be the best we can find. It gives our species a likely total
longevity of between 0.205 million and 8 million years, which is quite in line
with those for other hominids and mammals. The Earth is littered with the bones
of extinct species and it doesn鈥檛 take much to see that we could meet the same
fate. Our ancestor H. erectus lasted 1.6 million years, while H.
neanderthalensis lasted 0.3 million years. The mean duration of mammal
species is 2 million years, and even the great Tyrannosaurus rex lasted
only 2.5 million years.
For us, the end might come from a drastic climate change, nuclear war, a
wandering asteroid or comet, or some other catastrophe that catches us by
surprise, such as a bad epidemic. If we remain a one-planet species, we are
exposed to the same risks as other species, and are likely to perish on a
similar timescale.
Some people might think that the discoveries of our age鈥攕pace travel,
genetic engineering and electronic computers鈥攑lace us in a special
position. These breakthroughs, they might say, could lead us to spawn new
intelligent species, including intelligent machine species, enhancing our
chances of survival. But such thinking may raise false hopes. For, according to
the Copernican principle, you are likely to be living in a century when the
population is high because most people will be born during such periods. And
since it is people who make discoveries, it is not surprising that you will live
in a century when many interesting discoveries are being made. But your chance
of being born 200 000 years after the beginning of your intelligent lineage, in
the very century when a discovery is made that guarantees it a billion-year
future, is very small, because a billion years of intelligent observers would be
born after such a discovery, and you would be more likely to be one of them. If
you believe that any current discovery will dramatically increase our longevity,
you must ask yourself: why am I not already one of its products? Why am I not an
intelligent machine or genetically engineered?
Act now
This is a pretty grim reckoning, but we can see where our best chances lie.
If we plant self-supporting colonies in the Solar System, we will have an
insurance policy against catastrophe. If something happens to us on Earth, some
colonists might even eventually return and repopulate it. But we had better move
quickly because the Copernican principle also implies that we may not have the
capacity for space travel for very long.
In my 1993 Nature paper I estimated how long the human space
programme, then 32 years old, would last. Since my paper was not likely to fall
in either the first 2.5 per cent or the last 2.5 per cent of this programme, I
predicted with 95 per cent confidence that its future duration would be more
than 10 months but less than 1248 years. The upper limit of 1248 years is the
total number of future years of human space flight, regardless of how many
periods of inactivity occur.
You might argue that there is no hurry to colonise space within the next
century. Why not wait a few centuries until technology has become so advanced
that colonising becomes easy? But if we lose the capacity for spaceflight before
we鈥檝e colonised鈥攂y the collapse of civilisation, loss of technology or
diminished economic ability鈥攖hen we鈥檝e missed our chance. It鈥檚 good that
we went to the Moon in the 1960s. If we鈥檇 waited another 30 years hoping for an
easier time of it, we might never have made it, as we now seem to have less
money for such ventures.
Unfortunately, I believe that we are likely to make precisely this kind of
mistake. In 1969, Wernher von Braun had plans to send astronauts to Mars by
1982; in 1989 President George Bush proposed sending people there by 2019. This
is an unfortunate trend, and I鈥檓 worried that the day may come when there is no
one left alive who can say 鈥淚 walked on the Moon鈥�. People who realise that
colonising the Galaxy would be very beneficial to our survival have generally
regarded such a move as inevitable. But it is not.
Since you are still on your home planet, the Copernican principle tells you
that a significant fraction of all intelligent observers must also still be
sitting on their home planets (otherwise you would be special). This explains
why we have not been colonised by extraterrestrials鈥攁 significant fraction
of them are still sitting at home.
I would be more confident about the future if we were members of a
billion-year-old civilisation which had already colonised its galaxy. But our
chance of colonising the entire Galaxy, increasing our current population by
about a factor of a billion, is about one in a billion. Why? Because it would
mean that you were born within the first billionth of all humans, which is
exceedingly unlikely (my colleagues Brandon Carter, John Leslie and Holgar
Nielson have reached similar conclusions). The fraction of all civilisations
that achieve galactic colonisation is likely to be small鈥攐therwise, you
would likely be living in such a civilisation now.
But that doesn鈥檛 mean that we can鈥檛 at least get off the planet and plant
some colonies that will greatly enhance our survival chances. Colonisation,
starting with Mars, should be our first goal. Space experiments could be geared
towards making this possible. And if colonisation were the goal, you would not
have to bring astronauts back from Mars鈥攁fter all, that is where we want
them. Instead, we could equip them to stay
(鈥淓scape from Mars鈥�, New Scientist, 28 June, p 24)
and establish a colony at the outset鈥攁 good
strategy if one is worried that funding for the space programme may not
last.
So, we should be asking ourselves: what is the cheapest way to establish a
permanent, self-sustaining colony on Mars? How many Saturn V-class launches
would it take to land eight people on Mars with enough supplies to last them 30
years, with a reasonable recycling strategy? Could those people in 30 years
build a habitat big enough for them and their 16 children using only indigenous
materials? And in another 30 years, without further supplies from Earth, could
those 16 children enlarge the habitat to include 32 grandchildren, and so on?
The original mission could even include a large sample of frozen egg and sperm
cells to supply additional genetic diversity.
What a bargain colonising space is鈥� we send out a few astronauts, and
their descendants do most of the work. Shouldn鈥檛 we be trying, before it鈥檚 too
late?
* * *
YOU can use the Copernican principle today to forecast the future duration of
your current relationship, the company you work for, or the country you live in.
Why? Because the arrival of today鈥檚 issue of New Scientist has no
special relationship to the history of these things鈥攊t comes at some
random moment during their existence.
But at a friend鈥檚 wedding, you couldn鈥檛 use the formula to forecast the
marriage鈥檚 future. You are at the wedding precisely to witness its beginning..
Neither can you use it to predict the future of the Universe itself鈥攆or
intelligent observers emerged only long after the Big Bang, and so witness only
a subset of its timeline. Also, Earth-derived intelligent life is unlikely to
last forever. For if it did, your observation that its age is smaller than the
Universe鈥檚 age by a large factor, would be very special.
My Copernican formula is most useful when examining the longevity of
something, like the human race, for which there is no actuarial data available.
We know only one human race. In predicting your lifetime, you can do better by
using statistics on the life spans of people who have died, again assuming that
you are not special.