Date: Wed May 07 2008 - 21:39:48 EDT
FYI... This is an interesting perspective....
Technology Review - Published by MIT
May/June 2008
Where Are They?
Why I hope the search for extraterrestrial life finds nothing.
By Nick Bostrom
People got very excited in 2004 when NASA's rover Opportunity discovered
evidence that Mars had once been wet. Where there is water, there may be
life. After more than 40 years of human exploration, culminating in the
ongoing Mars Exploration Rover mission, scientists are planning still more
missions to study the planet. The Phoenix, an interagency scientific probe
led by the Lunar and Planetary Laboratory at the University of Arizona, is
scheduled to land in late May on Mars's frigid northern arctic, where it
will search for soils and ice that might be suitable for microbial life (see
" <http://www.technologyreview.com/Energy/19502/?a=f> Mission to Mars,"
November/December 2007). The next decade might see a Mars Sample Return
mission, which would use robotic systems to collect samples of Martian
rocks, soils, and atmosphere and return them to Earth. We could then analyze
the samples to see if they contain any traces of life, whether extinct or
still active.
Such a discovery would be of tremendous scientific significance. What could
be more fascinating than discovering life that had evolved entirely
independently of life here on Earth? Many people would also find it
heartening to learn that we are not entirely alone in this vast, cold
cosmos.
But I hope that our Mars probes discover nothing. It would be good news if
we find Mars to be sterile. Dead rocks and lifeless sands would lift my
spirit.
Conversely, if we discovered traces of some simple, extinct life-form--some
bacteria, some algae--it would be bad news. If we found fossils of something
more advanced, perhaps something that looked like the remnants of a
trilobite or even the skeleton of a small mammal, it would be very bad news.
The more complex the life-form we found, the more depressing the news would
be. I would find it interesting, certainly--but a bad omen for the future of
the human race.
How do I arrive at this conclusion? I begin by reflecting on a well-known
fact. UFO spotters, Raëlian cultists, and self-certified alien abductees
notwithstanding, humans have, to date, seen no sign of any extraterrestrial
civilization. We have not received any visitors from space, nor have our
radio telescopes detected any signals transmitted by any extraterrestrial
civilization. The Search for Extra-Terrestrial Intelligence (SETI) has been
going for nearly half a century, employing increasingly powerful telescopes
and data-mining techniques; so far, it has consistently corroborated the
null hypothesis. As best we have been able to determine, the night sky is
empty and silent. The question "Where are they?" is thus at least as
pertinent today as it was when the physicist Enrico Fermi first posed it
during a lunch discussion with some of his colleagues at the Los Alamos
National Laboratory back in 1950.
Here is another fact: the observable universe contains on the order of 100
billion galaxies, and there are on the order of 100 billion stars in our
galaxy alone. In the last couple of decades, we have learned that many of
these stars have planets circling them; several hundred such "exoplanets"
have been discovered to date. Most of these are gigantic, since it is very
difficult to detect smaller exoplanets using current methods. (In most
cases, the planets cannot be directly observed. Their existence is inferred
from their gravitational influence on their parent suns, which wobble
slightly when pulled toward large orbiting planets, or from slight
fluctuations in luminosity when the planets partially eclipse their suns.)
We have every reason to believe that the observable universe contains vast
numbers of solar systems, including many with planets that are Earth-like,
at least in the sense of having masses and temperatures similar to those of
our own orb. We also know that many of these solar systems are older than
ours.
From these two facts it follows that the evolutionary path to life-forms
capable of space colonization leads through a "Great Filter," which can be
thought of as a probability barrier. (I borrow this term from Robin Hanson,
an economist at George Mason University.) The filter consists of one or more
evolutionary transitions or steps that must be traversed at great odds in
order for an Earth-like planet to produce a civilization capable of
exploring distant solar systems. You start with billions and billions of
potential germination points for life, and you end up with a sum total of
zero extraterrestrial civilizations that we can observe. The Great Filter
must therefore be sufficiently powerful--which is to say, passing the
critical points must be sufficiently improbable--that even with many
billions of rolls of the dice, one ends up with nothing: no aliens, no
spacecraft, no signals. At least, none that we can detect in our neck of the
woods.
Now, just where might this Great Filter be located? There are two
possibilities: It might be behind us, somewhere in our distant past. Or it
might be ahead of us, somewhere in the decades, centuries, or millennia to
come. Let us ponder these possibilities in turn.
If the filter is in our past, there must be some extremely improbable step
in the sequence of events whereby an Earth-like planet gives rise to an
intelligent species comparable in its technological sophistication to our
contemporary human civilization. Some people seem to take the evolution of
intelligent life on Earth for granted: a lengthy process, yes; complicated,
sure; yet ultimately inevitable, or nearly so. But this view might well be
completely mistaken. There is, at any rate, hardly any evidence to support
it. Evolutionary biology, at the moment, does not enable us to calculate
from first principles how probable or improbable the emergence of
intelligent life on Earth was. Moreover, if we look back at our evolutionary
history, we can identify a number of transitions any one of which could
plausibly be the Great Filter.
For example, perhaps it is very improbable that even simple self-replicators
should emerge on any Earth-like planet. Attempts to create life in the
laboratory by mixing water with gases believed to have been present in the
Earth's early atmosphere have failed to get much beyond the synthesis of a
few simple amino acids. No instance of abiogenesis (the spontaneous
emergence of life from nonlife) has ever been observed.
The oldest confirmed microfossils date from approximately 3.5 billion years
ago, and there is tentative evidence that life might have existed a few
hundred million years before that; but there is no evidence of life before
3.8 billion years ago. Life might have arisen considerably earlier than that
without leaving any traces: there are very few preserved rock formations
that old, and such as have survived have undergone major remolding over the
eons. Nevertheless, several hundred million years elapsed between the
formation of Earth and the appearance of the first known life-forms. The
evidence is thus consistent with the hypothesis that the emergence of life
required an extremely improbable set of coincidences, and that it took
hundreds of millions of years of trial and error, of molecules and surface
structures randomly interacting, before something capable of
self-replication happened to appear by a stroke of astronomical luck. For
aught we know, this first critical step could be a Great Filter.
Conclusively determining the probability of any given evolutionary
development is difficult, since we cannot rerun the history of life multiple
times. What we can do, however, is attempt to identify evolutionary
transitions that are at least good candidates for being a Great
Filter--transitions that are both extremely improbable and practically
necessary for the emergence of intelligent technological civilization. One
criterion for any likely candidate is that it should have occurred only
once. Flight, sight, photosynthesis, and limbs have all evolved several
times here on Earth and are thus ruled out. Another indication that an
evolutionary step was very improbable is that it took a very long time to
occur even after its prerequisites were in place. A long delay suggests that
vastly many random recombinations occurred before one worked. Perhaps
several improbable mutations had to occur all at once in order for an
organism to leap from one local fitness peak to another: individually
deleterious mutations might be fitness enhancing only when they occur
together. (The evolution of Homo sapiens from our recent hominid ancestors,
such as Homo erectus, happened rather quickly on the geological timescale,
so these steps would be relatively weak candidates for a Great Filter.)
The original emergence of life appears to meet these two criteria. As far as
we know, it might have occurred only once, and it might have taken hundreds
of millions of years for it to happen even after the planet had cooled down
enough for a wide range of organic molecules to be stable. Later
evolutionary history offers additional possible Great Filters. For example,
it took some 1.8 billion years for prokaryotes (the most basic type of
single-celled organism) to evolve into eukaryotes (a more complex kind of
cell with a membrane-enclosed nucleus). That is a long time, making this
transition an excellent candidate. Others include the emergence of
multicellular organisms and of sexual reproduction.
If the Great Filter is indeed behind us, meaning that the rise of
intelligent life on any one planet is extremely improbable, then it follows
that we are most likely the only technologically advanced civilization in
our galaxy, or even in the entire observable universe. (The observable
universe contains approximately 1022 stars. The universe might well extend
infinitely far beyond the part that is observable by us, and it may contain
infinitely many stars. If so, then it is virtually certain that an infinite
number of intelligent extraterrestrial species exist, no matter how
improbable their evolution on any given planet. However, cosmological theory
implies that because the universe is expanding, any living creatures outside
the observable universe are and will forever remain causally disconnected
from us: they can never visit us, communicate with us, or be seen by us or
our descendants.)
The other possibility is that the Great Filter is still ahead of us. This
would mean that some great improbability prevents almost all civilizations
at our current stage of technological development from progressing to the
point where they engage in large-scale space colonization. For example, it
might be that any sufficiently advanced civilization discovers some
technology--perhaps some very powerful weapons technology--that causes its
extinction.
I will return to this scenario shortly, but first I shall say a few words
about another theoretical possibility: that extraterrestrials are out there
in abundance but hidden from our view. I think that this is unlikely,
because if extraterrestrials do exist in any numbers, at least one species
would have already expanded throughout the galaxy, or beyond. Yet we have
met no one.
Various schemes have been proposed for how intelligent species might
colonize space. They might send out "manned" spaceships, which would
establish colonies and "terraform" new planets, beginning with worlds in
their own solar systems before moving on to more distant destinations. But
much more likely, in my view, would be colonization by means of so-called
von Neumann probes, named after the Hungarian-born prodigy John von Neumann,
among whose many mathematical and scientific achievements was the concept of
a "universal constructor," or a self-replicating machine. A von Neumann
probe would be an unmanned self-replicating spacecraft, controlled by
artificial intelligence and capable of interstellar travel. A probe would
land on a planet (or a moon or asteroid), where it would mine raw materials
to create multiple replicas of itself, perhaps using advanced forms of
nanotechnology. In a scenario proposed by Frank Tipler in 1981, replicas
would then be launched in various directions, setting in motion a
multiplying colonization wave. Our galaxy is about 100,000 light-years
across. If a probe were capable of traveling at one-tenth the speed of
light, every planet in the galaxy could thus be colonized within a couple of
million years (allowing some time for each probe that lands on a resource
site to set up the necessary infrastructure and produce daughter probes). If
travel speed were limited to 1 percent of light speed, colonization might
take 20 million years instead. The exact numbers do not matter much, because
the timescales are at any rate very short compared with the astronomical
ones on which the evolution of intelligent life occurs.
If building a von Neumann probe seems very difficult--well, surely it is,
but we are not talking about something we should begin work on today.
Rather, we are considering what would be accomplished with some very
advanced technology of the future. We might build von Neumann probes in
centuries or millennia--intervals that are mere blips compared with the life
span of a planet. Considering that space travel was science fiction a mere
half-century ago, we should, I think, be extremely reluctant to proclaim
something forever technologically infeasible unless it conflicts with some
hard physical constraint. Our early space probes are already out there:
Voyager 1, for example, is now at the edge of our solar system.
Even if an advanced technological civilization could spread throughout the
galaxy in a relatively short period of time (and thereafter spread to
neighboring galaxies), one might still wonder whether it would choose to do
so. Perhaps it would prefer to stay at home and live in harmony with nature.
However, a number of considerations make this explanation of the great
silence less than plausible. First, we observe that life has here on Earth
manifested a very strong tendency to spread wherever it can. It has
populated every nook and cranny that can sustain it: east, west, north, and
south; land, water, and air; desert, tropic, and arctic ice; underground
rocks, hydrothermal vents, and radioactive-waste dumps; there are even
living beings inside the bodies of other living beings. This empirical
finding is of course entirely consonant with what one would expect on the
basis of elementary evolutionary theory. Second, if we consider our own
species in particular, we find that it has spread to every part of the
planet, and we have even established a presence in space, at vast expense,
with the International Space Station. Third, if an advanced civilization has
the technology to go into space relatively cheaply, it has an obvious reason
to do so: namely, that's where most of the resources are. Land, minerals,
energy: all are abundant out there yet limited on any one home planet. These
resources could be used to support a growing population and to construct
giant temples or supercomputers or whatever structures a civilization
values. Fourth, even if most advanced civilizations chose to remain
nonexpansionist forever, it wouldn't make any difference as long as there
was one other civilization that opted to launch the colonization process:
that expansionary civilization would be the one whose probes, colonies, or
descendants would fill the galaxy. It takes but one match to start a fire,
only one expansionist civilization to begin colonizing the universe.
For all these reasons, it seems unlikely that the galaxy is teeming with
intelligent beings that voluntarily confine themselves to their home
planets. Now, it is possible to concoct scenarios in which the universe is
swarming with advanced civilizations every one of which chooses to keep
itself well hidden from our view. Maybe there is a secret society of
advanced civilizations that know about us but have decided not to contact us
until we're mature enough to be admitted into their club. Perhaps they're
observing us as if we were animals in a zoo. I don't see how we can
conclusively rule out this possibility. But I will set it aside in order to
concentrate on what to me appear more plausible answers to Fermi's question.
The more disconcerting hypothesis is that the Great Filter consists in some
destructive tendency common to virtually all sufficiently advanced
technological civilizations. Throughout history, great civilizations on
Earth have imploded--the Roman Empire, the Mayan civilization that once
flourished in Central America, and many others. However, the kind of
societal collapse that merely delays the eventual emergence of a
space-colonizing civilization by a few hundred or a few thousand years would
not explain why no such civilization has visited us from another planet. A
thousand years may seem a long time to an individual, but in this context
it's a sneeze. There are probably planets that are billions of years older
than Earth. Any intelligent species on those planets would have had ample
time to recover from repeated social or ecological collapses. Even if they
failed a thousand times before they succeeded, they still could have arrived
here hundreds of millions of years ago.
The Great Filter, then, would have to be something more dramatic than
run-of-the mill societal collapse: it would have to be a terminal global
cataclysm, an existential catastrophe. An existential risk is one that
threatens to annihilate intelligent life or permanently and drastically
curtail its potential for future development. In our own case, we can
identify a number of potential existential risks: a nuclear war fought with
arms stockpiles much larger than today's (perhaps resulting from future arms
races); a genetically engineered superbug; environmental disaster; an
asteroid impact; wars or terrorist acts committed with powerful future
weapons; super-intelligent general artificial intelligence with destructive
goals; or high-energy physics experiments. These are just some of the
existential risks that have been discussed in the literature, and
considering that many of these have been proposed only in recent decades, it
is plausible to assume that there are further existential risks we have not
yet thought of.
The study of existential risks is an extremely important, albeit rather
neglected, field of inquiry. But in order for an existential risk to
constitute a plausible Great Filter, it must be of a kind that could destroy
virtually any sufficiently advanced civilization. For instance, random
natural disasters such as asteroid hits and supervolcanic eruptions are poor
Great Filter candidates, because even if they destroyed a significant number
of civilizations, we would expect some civilizations to get lucky; and some
of these civilizations could then go on to colonize the universe. Perhaps
the existential risks that are most likely to constitute a Great Filter are
those that arise from technological discovery. It is not far-fetched to
imagine some possible technology such that, first, virtually all
sufficiently advanced civilizations eventually discover it, and second, its
discovery leads almost universally to existential disaster.
So where is the Great Filter? Behind us, or not behind us?
If the Great Filter is ahead of us, we have still to confront it. If it is
true that almost all intelligent species go extinct before they master the
technology for space colonization, then we must expect that our own species
will, too, since we have no reason to think that we will be any luckier than
other species. If the Great Filter is ahead of us, we must relinquish all
hope of ever colonizing the galaxy, and we must fear that our adventure will
end soon--or, at any rate, prematurely. Therefore, we had better hope that
the Great Filter is behind us.
What has all this got to do with finding life on Mars? Consider the
implications of discovering that life had evolved independently on Mars (or
some other planet in our solar system). That discovery would suggest that
the emergence of life is not very improbable. If it happened independently
twice here in our own backyard, it must surely have happened millions of
times across the galaxy. This would mean that the Great Filter 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 evolutionary transitions
from consideration as the Great Filter. The effect 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 discover 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 effect." 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 engender
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 different 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 fraction of all planets. An observation-selection
effect 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 growing 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 Mesopotamia 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 confronting 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 Humanity Institute at the
University of Oxford.
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