[ExI] Bostrom on the Fermi Paradox in Technology Review

[Jeff Davis]

I've been getting a little behind in my reading, so if someone else
has posted about this, well, I missed it.

Here's Nick's piece, full text:

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 "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;
super­intelligent 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.
Copyright Technology Review 2008.