Home' Technology Review : May June 2008 Contents ESSAY 77
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is less likely to be confronted during the early life of planets
and therefore, for us, more likely still to come.
If we discovered some very simple life-forms on Mars, in
its soil or under the ice at the polar caps, it would show that
the Great Filter must come somewhere after that period in
evolution. This would be disturbing, but we might still hope
that the Great Filter was located in our past. If we discovered
a more advanced life-form, such as some kind of multicellular
organism, that would eliminate a much larger set of evolution-
ary transitions from consideration as the Great Filter. The
e ect would be to shift the probability more strongly against
the hypothesis that the Great Filter is behind us. And if we
discovered the fossils of some very complex life-form, such
as a vertebrate-like creature, we would have to conclude that
this hypothesis is very improbable indeed. It would be by far
the worst news ever printed.
Yet most people reading about the discovery would be
thrilled. They would not understand the implications. For if
the Great Filter is not behind us, it is ahead of us. And that's
a terrifying prospect.
So this is why I'm hoping that our space probes will dis-
cover dead rocks and lifeless sands on Mars, on Jupiter's moon
Europa, and everywhere else our astronomers look. It would
keep alive the hope of a great future for humanity.
Now, it might be thought an amazing coincidence if Earth
were the only planet in the galaxy on which intelligent life
evolved. If it happened here, the one planet we have studied
closely, surely one would expect it to have happened on a lot
of other planets in the galaxy---planets we have not yet had the
chance to examine. This objection, however, rests on a fallacy:
it overlooks what is known as an "observation selection e ect."
Whether intelligent life is common or rare, every observer is
guaranteed to originate from a place where intelligent life did,
in fact, arise. Since only the successes give rise to observers
who can wonder about their existence, it would be a mistake
to regard our planet as a randomly selected sample from all
planets. (It would be closer to the mark to regard our planet
as a random sample from the subset of planets that did engen-
der intelligent life, this being a crude formulation of one of
the saner ideas extractable from the motley ore referred to as
the "anthropic principle.")
Since this point confuses many, it is worth expanding on it
slightly. Consider two di erent hypotheses. One says that the
evolution of intelligent life is a fairly straightforward process
that happens on a significant fraction of all suitable planets.
The other hypothesis says that the evolution of intelligent life
is extremely complicated and happens perhaps on only one
out of a million billion planets. To evaluate their plausibility in
light of your evidence, you must ask yourself, "What do these
hypotheses predict I should observe?" If you think about it,
both hypotheses clearly predict that you should observe that
your civilization originated in places where intelligent life
evolved. All observers will share that observation, whether the
evolution of intelligent life happened on a large or a small frac-
tion of all planets. An observation-selection e ect guarantees
that whatever planet we call "ours" was a success story. And
as long as the total number of planets in the universe is large
enough to compensate for the low probability of any given
one of them giving rise to intelligent life, it is not a surprise
that a few success stories exist.
If---as I hope is the case---we are the only intelligent species
that has ever evolved in our galaxy, and perhaps in the entire
observable universe, it does not follow that our survival is
not in danger. Nothing in the preceding reasoning precludes
there being steps in the Great Filter both behind us and ahead
of us. It might be extremely improbable both that intelligent
life should arise on any given planet and that intelligent life,
once evolved, should succeed in becoming advanced enough
to colonize space.
But we would have some grounds for hope that all or most
of the Great Filter is in our past if Mars is found to be barren.
In that case, we may have a significant chance of one day grow-
ing into something greater than we are now.
In this scenario, the entire history of humankind to date is
a mere instant compared with the eons that still lie before us.
All the triumphs and tribulations of the millions of people who
have walked the Earth since the ancient civilization of Meso-
potamia would be like mere birth pangs in the delivery of a kind
of life that hasn't yet begun. For surely it would be the height
of naïveté to think that with the transformative technologies
already in sight---genetics, nanotechnology, and so on---and
with thousands of millennia still ahead of us in which to perfect
and apply these technologies and others of which we haven't
yet conceived, human nature and the human condition will
remain unchanged. Instead, if we survive and prosper, we will
presumably develop some kind of posthuman existence.
None of this means that we ought to cancel our plans to
have a closer look at Mars. If the Red Planet ever harbored life,
we might as well find out about it. It might be bad news, but it
would tell us something about our place in the universe, our
future technological prospects, the existential risks confront-
ing us, and the possibilities for human transformation---issues
of considerable importance.
But in the absence of any such evidence, I conclude that
the silence of the night sky is golden, and that in the search
for extraterrestrial life, no news is good news.
NICK BOSTROM IS THE DIRECTOR OF THE FUTURE OF HU MANITY INSTITUTE AT
THE UNIVERSITY OF OXFORD.
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