

The Great Filter (1998) - stavrianos
http://hanson.gmu.edu/greatfilter.html

======
goodside
There's good reason to believe that the Great Filter is not only behind us,
it's way behind us.

A Great Filter in the future would have to apply to the vast space of possible
intelligent lifeforms. This means that no matter what the psychology of the
intelligences that evolve--no matter what they value or attempt to create--
they all get themselves killed somehow. Every intelligence would need its own
unique story of how it died so quickly, and these are burdensome details on
our theory.

In contrast, the pre-biological past is simple and homogenized. Any Great
Filter would be a regularity of physics and chemistry, which we know apply
universally. If physics dictates that it's really hard for anything self-
replicating to form (excluding trivial things like crystals and fire) then
we've explained the lack of life everywhere. The theory is simpler, and
Occam's razor tells us the simpler theory wins.

~~~
stavrianos
The future answers don't need to be unique and convoluted. Offhand, there's
Baxter's suggestion in "Manifold: Space" that epic-scale collapses of multi-
star systems might happen more often than we think, with sterilizing effects
on big chunks of space. Egan used a variation of this in "Diaspora".

~~~
jeremyawon
the scenarios Baxter and Egan write about only push the Great Filter up to the
moment of being able to create self-replicating probes (the most basic form of
explosive colonization).

i think that's actually the point after which filters have to contend with a
diversity of "the psychology of the intelligences that evolve" (rather than at
the emergence of biology, since biology is localized and subject to
regularities in astrophysics such as supernovae).

how far are we from being able to produce such probes, and could our
psychology become so idiosyncratic between now and then that we wouldn't
deploy them?

~~~
eru
I guess if we'd throw all our resources at it, the probes would not be far
off.

Hey, if we transported enough garbage up to Mars, now, some Microbes would
probably survive, adapt and flourish.

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greendestiny
It's an interesting topic, but talking about probability when you're
extrapolating the extremely limited information we have is dubious. There
could be so many other explanations for why we haven't detected alien
lifeforms, and we don't know how to assign probabilities.

~~~
jeremyawon
i don't follow what you're suggesting between 'other explanations' and
'assigning probabilities' - are you saying there are filters we haven't
thought of, which we (obviously) aren't assigning probability to?

~~~
greendestiny
"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"

I think that statement is completely unfounded. We don't know how dead the
space is, except that we haven't detected anything so far. The author goes on
about us a single data point like that makes us incredibly rare, all it makes
us is incredibly ill informed.

~~~
jeremyawon
I think that quotation by itself refers just to the Great Silence, and there
seem to be two essential possibilities concerning this silence, both of which
the author addresses:

Either there is a sterilizing filter which lies ahead of planet bound species
such as ours, preventing explosive colonization;

Or, there is a psychological filter which "prevents" higher civilizations from
making their existence visible - the author addresses this when discussing the
Common Zoo hypothesis.

Within a single galaxy you might entertain the possibility that we are just
the first occurrence of intelligence to emerge. However:

"Consider, for example, that the energy of a single star might power an
intermittent very narrow-band signal detectable to pre-explosive life like
ours across the entire universe [Gott 82]."

So we would have to be a cosmically nascent intelligence. That makes the
absence of both a biological and psychological filter exceedingly improbable.

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donaldc
I think it's possible (although not a foregone conclusion) that in fact much
of the filter is from what he terms 'discrete' events. That is, having the
right sort of sun, the right-sized planet at the ideal distance from this sun,
with a substantial moon to act as a gyroscope, active tectonics to recycle
elements that would otherwise get locked up, etc.

I've not, for example, seen any conclusive estimates on how likely it is that
an otherwise earth-like planet would get a moon to act as a stabilizer for
axial tilt (which itself is probably important for complex life).

~~~
donaldc
Also, his assumption that life could spread at nearly the speed of light may
be overly optimistic. If, by contrast, one assumes that light can spread at
only 1/10th the speed of light, and that the earliest star-faring life could
have arisen was a billion years ago (due to as the time needed for a second-
generation yellow star to brighten enough to keep an earth-like planet warm
after its conversion to an oxygen/nitrogen atmosphere), then one only needs to
explain why star-faring life has not yet arisen in relatively nearby galaxies,
rather than why it has not yet arisen anywhere in the visible universe. This
is a much less daunting hurdle.

