

Why I Hope the Search for Extraterrestrial Life Finds Nothing - mhb
http://www.nickbostrom.com/extraterrestrial.pdf

======
tptacek
Summary: "Because it means life is not incredibly unlikely (after all, it
occurred on two planets in our own solar system); ergo what must instead be
incredibly unlikely is the ability for life to progress to the point where it
can spread across the galaxy (after all, no other life form has contacted us);
ergo we are doomed."

~~~
amichail
We need a tldr service where summaries can be put in one place and embedded in
other sites such as HN.

~~~
dunk010
The author had a good point to be made but took too long to get it across.

~~~
gwern
He had a good point and took so long to make it so carefully because he knows
that if he doesn't, people will make all sorts of dumb comments & objections.

This HN submission actually isn't even all that bad as far as it goes; check
the comments on some of Bostrom's submissions to other, more popular, fora and
you'll be routinely smacking your forehead.

------
Androsynth
The problem with saying 'there is no extraterrestrial life because we haven't
observed it and it would probably be here by now' is that there is always the
first wave of lifeforms who would come to the same conclusion.

You could make a good case that we are part of this first wave: -The stars
created after the big bang would not have had any heavy elements and therefore
wouldn't have had the resources to create life. -These stars lasted ~10Bn
years before they started to go nova (which creates the heavy elements for the
next generation of stars) -Based on our experience, it takes ~4bn years for
life to evolve in a planetary system. -Therefore the first lifeforms wouldn't
appear until roughly 14bn years after the big bang. -The universe is currently
14bn years old.

~~~
obfuscate
_The problem with saying 'there is no extraterrestrial life because we haven't
observed it and it would probably be here by now' is that there is always the
first wave of lifeforms who would come to the same conclusion._

True, but that a form of reasoning inevitably gives a wrong conclusion under
rare circumstances is only a weak argument against it.

 _-Based on our experience, it takes ~4bn years for life to evolve in a
planetary system._

I don't think we have a strong reason to believe that there's something
special about that length of time. Not much change happened on Earth for most
of that time, suggesting dependence either on stochastic evolution of a few
things like photosynthesis (so 4bn probably isn't much more likely than 3bn)
or on something like increasing solar output crossing a threshold (it's
doubtful such a threshold would be crossed at ≥ 4bn years at every location).
That aside, even a few tens of millions of years is plenty of time for life to
cover a galaxy.

------
araneae
Wait. So this guy doesn't want us to find life, which would be super cool and
advance scientific knowledge significantly... because it might mean that human
civilization might not last that long in the timescale of the universe?

I mean, I think we knew that already. The vast majority of species that have
lived on Earth have gone extinct, and almost all of the rest have evolved to
be completely unrecognizable. It's unlikely we'll be an exception to that
rule, and I'm okay with that.

~~~
gort
Bostrom is untroubled by the idea that humans will evolve to be
unrecognisable; but he hopes we will indeed have those descendants.

~~~
araneae
Yeah, but for all he knows they'll have the intelligence of the common house
fly. And not only is that incredibly unlikely anyway, it's essentially no
different from dying out altogether. And who really cares? None of us will
live that long anyway.

------
dasht
Here is one possible (plausible) answer to Fermi, I think:

Some various assumptions:

Space travel is limited by the speed of light and the faster you are going,
the more energy it takes to go a little bit faster. Relatively is more or less
correct and absolute that way. No Star Trek "hyperdrive" is possible.

Nope, you can't extract energy from a quantum vaccuum or any other such
tricks.

Nope, you can't actually scoop up all that much fuel in inter-stellar space.
Most viable solar systems don't have all that much practically accessible fuel
for inter-stellar voyages. Transmutation of matter into arbitrary elements is
possible, but not practical at scale.

You can't, therefore, have (material) trade routes between any but perhaps the
closest stars. Even then, the total mass of material you are trading in a unit
of time is doomed to be disappointingly low - there is hardly any point to
bothering.

With assumptions along those lines, "colonization" per se would not exactly be
possible -- if by "colonization" we mean building any kind of empire.

At the very best, a species could accomplish a kind of "panspermia" -- sending
out "seeds" to whatever stars are in reach, but once a seed is launched, its
subsequent evolution is pretty much on its own. In a few cases, if one lucky
civilization is close by a second, resource-rich solar system, _maybe_ you get
get a little bit of mining going. But such opportunities would be
cosmologically rare and even more rarely economically viable.

Worse, sending out seeds to a near-ish solar system and letting them do what
they will would be dangerous to the life back in the solar system of origin.
Right now, it doesn't appear that their is a hostile, alien intelligence
anywhere in the galactic neighborhood. If we did a good job of launching
"seeds", we'd increase the odds considerably.

In short, there will never be a point at which it makes any kind of economic
or existential sense for us to begin a colonization project - and the
situation doesn't look like it would be all that different for other
imaginable forms of life.

So why not signal? Well, we already do a bit. Perhaps in a few years we'll
have found some not _too_ far off (but far enough!) solar systems that look
like they might host life -- and we'll point some lasers at them and start
sending "pings" to see if anyone is home. If other solar systems are already
doing that, it would be very hard to detect from here on Earth.

In other words, one answer to Fermi, but not quite the "Great Filter" that
Bostrom posits, is simply that galactic colonization is a practical
impossibility and, to the extent there is intelligent signaling that spans
solar systems, it's a lot more directional and harder to detect than anything
you could easily find with a few radio telescopes.

~~~
weavejester
That seems unlikely to me. There's easily enough energy and matter around in
our solar system to make a journey to another, and as far as we know, there's
plenty of energy and matter in other solar systems also.

So a sufficiently advanced civilization could construct a self-replicating
probe that heads to each solar system, replicates and recharges itself, then
heads on to the next stars until the whole galaxy is populated.

From the perspective of the civilization sending the probes, this is
relatively cheap; just the price of the materials necessary to build a single
ship capable of carrying itself outside the solar system, even if it takes
hundreds of years to get anywhere.

~~~
dasht
A "self-replicating probe" is either so weak in capability that it would
hardly amount to colonization, or so strong in capability that it would amount
to a rival life form. What functionality would you build into such a probe and
by what means would you prevent a "boomerang" effect wherein it comes back and
causes problems in the home system?

Additionally, if all you are talking about building is a device that would,
encountering our solar system, take up residence in orbit, take pictures, and
report "back" - how do you know there aren't already several of them present?
It would be rather hard to tell!

Colonization on Earth always had economic incentive. Populations migrated for
economic opportunities, even if the selection of who did and did not migrate
was heavily political. Colonization, as opposed to mere migration, was pretty
much always, more or less by definition, for resource extraction. The
colonized pay taxes and/or their raw materials and unique finished goods are
exported back to the empire, and/or their local economy is converted to the
coin of the empire's realm and taxes are extracted. It doesn't seem like any
such program of empire building could work at inter-stellar scales unless
there is some surprising physics that we've yet to discover that allows super-
luminal communication and transport.

Fermi is "wrong" if intelligent, space-faring life only "works out" given the
rules of physics at roughly the scale of our higher organisms and if, for
organisms at that scale, colonization is not a viable option. There's no need
for any "Great[er] Wall" than that.

~~~
weavejester
> A "self-replicating probe" is either so weak in capability that it would
> hardly amount to colonization, or so strong in capability that it would
> amount to a rival life form.

Why do you assume that? There are plenty of self-replicating machines on earth
that have been extremely successful at colonizing their environment, but we
would not typically view them as "rival life forms".

> What functionality would you build into such a probe and by what means would
> you prevent a "boomerang" effect wherein it comes back and causes problems
> in the home system?

The simplest method would be to have them accept any order signed by a
standard public key. I'm not sure why people tend to assume that a self-
replicating machine cannot be controlled. There are plenty of self-replicating
programs that are controlled very effectively.

> Additionally, if all you are talking about building is a device that would,
> encountering our solar system, take up residence in orbit, take pictures,
> and report "back" - how do you know there aren't already several of them
> present?

Because if have been many intelligent species in our galaxy over the past
billion years, it seems unlikely that all would be careful to remain
undetected. Certainly our species has little interest in hiding its
activities.

> Colonization, as opposed to mere migration, was pretty much always, more or
> less by definition, for resource extraction.

Plenty of species on Earth colonize new environments without bothering to send
resources back to their origins. And there have been plenty of human
colonizations not linked to a controlling empire, especially when we were just
hunter-gathers.

------
bioweek
<http://xkcd.com/638/>

~~~
obfuscate
Do floor tiles occur naturally?

Even if we can't intercept their communications, we should be able to see
their works. The universe looks far too natural to contain much
technologically advanced intelligence.

~~~
InclinedPlane
How much of the universe have you searched so far?

I think that's the point of the xkcd strip.

~~~
Semiapies
No, that we may be looking for the wrong _things_. The ants could search every
floor tile on Earth, but they wouldn't find pheromone trail messages from us.
The modest scale of SETI is just a secondary point.

On the other hand, how exactly do you find things without looking for them -
wait to get stepped on? :)

~~~
InclinedPlane
Perhaps I was too terse. The point of the xkcd strip is that the ants
conducted a very shallow test (looking for life that was most similar to
theirs) and then gave up. The post I responded to made a similar comment
generalizing about the nature of "the Universe", which is indicative of the
same fallacy.

Which is the point of the xkcd strip (the alt-text makes it even more clear).
It isn't about ants, ants don't read xkcd. The point is that despite all of
what we've done we have barely scratched the surface of knowing or
understanding the universe. We are as ignorant of the nature of intelligent
life in the universe, even whether it exists outside of Earth or not, as the
depicted ants are of human civilization. Far more so, in fact.

------
ggchappell
This is an interesting article, well worth posting.

However, to all those saying it needs a "tl;dr": I agree with you. In fact,
academic writing has included this concept for decades; it's called an
_abstract_. And this article could use one.

------
Kliment
Well, the consideration fails unless the possibility of a common origin is
excluded. Just because we find life nearby doesn't mean that the great filter
is not already in our past. On Mars specifically, it would be near enough to
allow for the possibility that life has somehow migrated from Earth to Mars,
or vice versa, or to both from some other source. Finding life far enough away
that intelligently developed spaceflight would be the only way for it to
migrate would indeed be a depressing thought, but the possibility of migration
must be ruled out first if we find life nearby.

------
radu_floricica
It could be something more mundane. The signs we're looking for are stuff like
radio signals, which we'll probably be using for how long? 500 years out of
the Sun's 10 billion lifespan. That's a pretty big filter already.

Plus, if light speed is really impossible to overcome, then distant stars get
less interesting as a civilization progresses. 100 years didn't mean so much
for a given population 5000 years ago (not for its individuals, of course),
but already are about 3 paradigm shifts these days.

------
josefresco
If we evolve into some sort of 'post-human' form, maybe the assertion that we
'have to' colonize everywhere breaks down. I think it's a stretch to assume
that every potential life form eventually wants to colonize space. If we as
humans or post-humans figure out how to balance our need for resources (by not
having to mine space) I think that could change things dramatically.

However I didn't go to Oxford/MIT so I really don't know jack.

~~~
jerf
The problem with this line of argumentation, and a number of other lines of
similar argumentation, is that you've explained why a _given_ entity might not
want to colonize space. What you haven't explained is why _every_ entity ever
in existence anywhere will not want to colonize space. It's not enough to
hypothesize the existence of a single future "space hippy" who just wants to
live in harmony with nature, man, you have to explain why every post-
human(-equivalent) from every species in all the observable universe (or at
least all our galactic history) has not chosen to colonize space.

All our current technology says that all future civilizations may have
radically greater resources than us, but still finite. Finite resources means
there will still be some sort of internal competition. Competition implies
that one solution to the local competition is to pack up and head for the next
star system to exploit. This logic then applies again from that system once it
has been tapped out. It borders on the inconceivable that it is physically
possible to colonize the stars, that intelligent life is common, and it is
easy to get to "post-human-equivalent", but absolutely nobody has ever chosen
to colonize the galaxy. One of those axioms is wrong (including, potentially,
the finiteness of future civilization's resources).

This argument is mentioned in the paper, I'm just re-drawing your attention to
it. It's a big one that is very important when talking or arguing about this;
the answer must not cover 10% or even 99.9% of the cases because if
intelligent life is that easy the galaxy still ends up colonized long before
humans are sentient. It must cover 100% of the cases across an unknowably wide
variety of intelligent life forms in an unknowably wide variety of initial
conditions.

It's a tall order and most, if not all, of the self-loathing answers like
"intelligent life like humans just suck so hard that they all die" really
don't hold up under scrutiny very well; _all_ life forms pollute themselves to
death? Any problem we can _see_ , some other life form somewhere will have
solved. All life forms develop weapons technology and war themselves to death?
Even hyperintelligent bees who early in their human-class civilization's
history figured out how to share one mind such that war is impossible? All
life forms ever decide to just stay home because they learn to live in
harmony? Even on Earth you can see that while some humans may make that
choice, other humans quite rationally never will.

And the whole "harmony with nature" thing isn't even a very rational idea if
you get down to it, it is a _very_ human form of romanticism that I seriously
doubt will be a universal among the stars. Not destroying that which you need
to live is rational, but posthuman technology generally implies that you no
longer need your original natural environment to survive; I seriously doubt
that _all_ civilizations and individuals will fetishize their ecosystems the
way some humans do, an ecosystem now distinctly more primitive and choatic
than their own technology.

(One reason why it matters if a species has infinite resources is that it may
mean that it is possible for a species to create it's own universe and migrate
there, which would make sense for a lot of reasons, especially if they can
guarantee that no other species can ever find it and muck with it. Be it a
real universe or an advanced simulation, doesn't matter. "All civilizations,
before advancing to the point they can colonize the stars, discover a method
of creating a new sub-universe or performing infinite computation in finite
time, and instead colonize that new infinite realm since it is so much safer"
is an appealing answer since it solves the Fermi paradox in a way that implies
a happy future possibility for humanity, but we're running out of physics that
might contain such a possibility. It gets to the point where even if we figure
out how to trick a string into performing infinite computation it wouldn't
matter anyhow since there may be no way to manipulate a string anyhow, for
example. This also falls afoul of my own argument above... _every_ member of
the species must choose this, _no_ member should have doubts about it and
decide to just safely colonize space anyhow and perhaps choose that answer in
the indefinite future.)

------
bioweek
He doesn't address the possibility that the start of life is "the great
filter", but after that point, microbes were shared between planets in our
solar system via metoerites, impacts, etc.

In which case it wouldn't matter what we found on Mars.

~~~
gort
He does use the word "independently", presumably to take this into account.

~~~
dmfdmf
Right. But that is a big assumption of his position. If there is life on Mars
and it is DNA or RNA based I would say the independence assumption is less
certain. Then the author can go on happily assuming that the "filter" is
probably behind us. Not sure why that is such a big thing though.

------
felixge
Maybe I'm missing something, but finding live on Mars would just be a single
data point. It would in no way provide a meaningful forecast to our own fate.

~~~
Androsynth
It would be a second data point. If we the Earth was our only data point, we
couldn't say whether life was a common trend or an anomaly. But if Mars had
life, then we would at least have proof of the former.

------
varjag
Nice, but there is also of course that old line of thinking that expansionist
civilization did exist, and Earth, in fact, was colonized.

~~~
eru
There's an alternative explaination in `Burning the Cosmic Commons:
Evolutionary Strategies for Interstellar Colonization`
(<http://hanson.gmu.edu/filluniv.pdf>):

"Attempts to model interstellar colonization may seem hopelessly compromised
by uncertainties regarding the technologies and preferences of advanced
civilizations. If light speed limits travel speeds, however, then a selection
effect may eventually determine frontier behavior. Making weak assumptions
about colonization technology, we use this selection effect to predict
colonists' behavior, including which oases they colonize, how long they stay
there, how many seeds they then launch, how fast and far those seeds fly, and
how behavior changes with increasing congestion. This colonization model
explains several astrophysical puzzles, predicting lone oases like ours, amid
large quiet regions with vast unused resources."

The point of the submitted article seems to be already mentioned in:

"The Great Filter - Are We Almost Past It?" (first version August 1996)
(<http://hanson.gmu.edu/greatfilter.html>)

"Humanity seems to have a bright future, i.e., a non-trivial chance of
expanding to fill the universe with lasting life. But the fact that space near
us seems dead now tells us that any given piece of dead matter faces an
astronomically low chance of begating such a future. There thus exists a great
filter between death and expanding lasting life, and humanity faces the
ominous question: how far along this filter are we?

Combining standard stories of biologists, astronomers, physicists, and social
scientists would lead us to expect a much smaller filter than we observe. Thus
one of these stories must be wrong. To find out who is wrong, and to inform
our choices, we should study and reconsider all these areas. In particular we
should seek evidence of extraterrestrials, such as via radio signals, Mars
fossils, or dark matter astronomy. But contrary to common expectations,
evidence of extraterrestrials is likely bad (though valuable) news. The easier
it was for life to evolve to our stage, the bleaker our future chances
probably are."

Both links are from the collection `The Economics of Science Fiction'
(<http://hanson.gmu.edu/econofsf.html>)

------
dogprez
a great filter in the future could also mean a movement to a new kind of
existence, not necessarily an extinction. like an exodus to a virtual world.

------
J3L2404
A few thoughts:

1\. The current rate of technological advancement is so steep as to make even
100 years hence, a nanosecond in the grand scheme, absolutely unknowable.

2\. Sufficiently advanced technologies would be indistinguishable,
intentionally, a la Star Trek's noncontact per Federation agreement.

3\. In terms of the author's "Great Filter" idea, maybe some fraction of the
supernovas observed by astronomers are really interstellar industrial
accidents.

~~~
gwern
> 2\. Sufficiently advanced technologies would be indistinguishable,
> intentionally, a la Star Trek's noncontact per Federation agreement.

 _would_ be? Why on earth would they inevitably - every single one of them -
develop such a civilization? We most certainly haven't; we'll set aside a few
areas as parks, but even those are heavily contaminated by our civilization.

Why wouldn't they make Dyson spheres around all their stars - spheres which
ought to have shown up on our infrared surveys - and use that energy? Think of
all the computations and matter they'd be wasting by just letting the star
blaze away. A few parks, sure, but an entire universe of them?

That point sounds like wishful thinking. 'They'll be, like, cosmic hippies man
- they won't _want_ to go around harshing Mother Nature's groove.'

> 3\. In terms of the author's "Great Filter" idea, maybe some fraction of the
> supernovas observed by astronomers are really interstellar industrial
> accidents.

Then why aren't the astronomers & astrophysicists in a tizzy about how all
these supernovas simply don't mesh with the well-thought-out models?
Supernovas are the outcomes of processes which take millions upon millions (or
billions) of years to finish. I doubt their models of include a random fudge
factor to explain away alien Homers.

~~~
J3L2404
2\. "...cosmic hippies man -" We are not hippies (there are some) but we have
to struggle to get permission to remove a beaver dam even if it is flooding
our home, and we are far from ADVANCED! The reasons for noncontact may be as
beyond our comprehension as a sump pump is to a beaver.

3\. Don't get too attached to current favorite theories-Dark Matter-Dark
Energy-etc(massive fudge factor) because they are always subject to
reinterpretation when new data is uncovered. "Difficult to see, Always in
motion the future is."

~~~
thaumaturgy
Ridiculous Yoda quotes aside...

Which "we" are you referring to? Your idealized American conservationist,
Canadians, Mexicans, Brazilians? Tibetans? Chinese? North Koreans?
Australians? Indians? Nigerians?

There are many human cultures on our planet which haven't created bureaucracy
preventing them from taking advantage of natural resources, and some of them
are as technologically advanced as any other culture on our planet.

~~~
J3L2404
I'm sorry you missed the point. I think I offended your sensibilities, but I
was referring to all mankind and how we do care about other forms of life and
propose that more advanced civilizations would even more so.

