Kermit wrote:

On Aug 13, 5:12 pm, "K_h" <KHol...@SX729.com> wrote:

Fermi's paradox suggests that there are little or no other intelligent
civilizations within the Milky Way galaxy.  On the other hand, intelligent
life should exist on a substantial fraction of planets with life because
natural selection broadly increases intelligence with time.  Here on the
Earth, for example, numerous mammals have a high degree of intelligence and
many of them could reach human intelligence with a few more million years of
evolution.

This contradiction can be resolved if the origin of life is far harder than
commonly believed.  That is, in the Drake equation, f_L should be far
smaller than most people think it is.  Even on planets that are life
friendly the formation of life should be extremely rare for the below
reasons.

For life to start, a molecule must arise that can make approximate copies of
itself.  Once that happens then natural selection can work its magic.  But a
molecule that can make approximate copies of itself must be a fairly
sophisticated nano-machine being comprised of dozens, if not hundreds, of
molecules and it must arise via inorganic and non-evolutionary processes.

From the study of DNA and genes, it is known that all life on the Earth has
a common origin (undoubtedly from a molecule of the aforementioned kind).
Since Earth is a life friendly planet, why hasn't another molecule (of the
aforementioned kind) arisen?  If it had, then life on the Earth would have
organisms with two different molecules for genetic codes: DNA and something
else.

Since all Earthly life is based on DNA, this suggests that, over the four
billion years of life on Earth, this has never happened again.  That is,
over the last four billion years, no other molecule has arisen by inorganic
and non-evolutionary processes that can make approximate copies of itself.
And Earth is a life-friendly planet so chances are optimal that such a
molecule should arise.

This suggests that the formation of such a molecule is a very rare event.
In other words, the reaction rate of inorganic chemistry per square meter
times the surface area of the Earth, times the average depth such reactions
take place, times four billion years is <<, much less, than the number of
such reactions needed before an approximately self reproducing molecule
arises by chance.

If that first molecule had not arisen here on the Earth then the Earth would
probably have been lifeless ever since.  This same reasoning applies if life
first started somewhere else in the solar system and then migrated to Earth
(for example from Mars).  If life rose independently on Mars once, over the
past four billion years, then that suggests that the reaction rate of
inorganic chemistry per square meter, times the surface area of a Mars sized
world, times the average depth such reactions take place, times four billion
years is about the number needed so that an approximately self reproducing
molecule arises by chance once, ~ 1.

It seems too much of a coincidence that the laws of chemistry work out in
such a way that life arises, on average, once per terrestrial world per
several billion years.  Rather, for such cases, it seems much more likely
that life arises multiple times or almost never.  The latter possibility
makes sense from a combinatorial perspective.  A self reproducing molecule
will be composed of dozens to hundreds of other molecules.  But the total
number of permutations for such a molecule's components will far exceed the
total number of inorganic chemical interactions that take place per
terrestrial world per several billion years.

A simple combinatorial thought experiment explains why.  The number of ways
of stacking a deck of playing cards is so huge that if 67.8 billion solar
masses were converted entirely into protons then each proton stands for a
different way of stacking the deck.  But there are 92 naturally occurring
chemical elements and a self reproducing molecule will probably be composed
of hundreds of atoms from the set of 92 different kinds (there only 52 cards
in a playing deck).

So, in the Drake equation, f_L could be something really small like 10^-90.
In this case the fact that life exists on the Earth simply shows that the
universe is super huge and its true size far exceeds the visible universe.

General relativity says that the universe sits on top of an infinite amount
of gravitational potential energy.  During both cosmic inflation and dark
energy inflation the universe falls down its own gravity well converting
huge quantities of its gravitational potential energy into vacuum energy and
expansion energy.  This probably explains why the universe is so huge.

So the universe could contain 10^150 planets, for example.  If f_L is 10^-90
then the total number of planets in the universe that have life is around
10^60.  So there are a lot of planets with life out there but none of them
are close by.  So this is one possible explanation for why there is only one
example of life in the solar system.  And this explanation is consistent
with Fermi's paradox.  It also suggests that any other life in our solar
system got there via migration.

In light of all this, it cannot be concluded that water, oxygen, and
methane, for example, are indicators of extraterrestrial life.  The presence
of these simple gases in the atmospheres of other planets can easily be
explained by inorganic processes.

If Earth is the only planet in 10^150 with life then that suggests that the
universe is fine tuned for Earthly life.  If a substantial fraction of the
10^150 planets have life then that suggests the whole universe is finely
tuned for life.  If the universe if not fine-tuned for life then that
suggests the number of planets with life should be around the logarithmic
middle of 10^150 or around 10^75.

In conclusion, it seems there are lots of planets with life out there but
none of them will ever communicate with humans.

k

It seems like once multicelled life evolves, intelligence would be
almost inevitable given sufficient time.

Sure, with the important bit being "sufficient time".

It's not that NS has any
progressive trend, it's just that it's an attribute which would be a
possible path for a species, given the right variables. Just as larger
animals are inevitable, given that we started very small. Not that
larger is the trend so much as a common direction taken with a random
walk. Look at how many times camouflage, poison, flying, snaring
appendages, armor, and the like evolved. If a plague wiped out humans
this year, there would likely be intelligent tool makers within 20
million years: apes, otters, cephalopods, elephants, cetaceans,
monkeys, parrots all have species comparable to our recent ancestors
in intelligence. (To the degree that the term means *anything in such
disparate species).

I would be interested to know how you figured out that 20 million years would be "sufficient time". That's where you lose me. It seems to me that if that were the case, we would have seen additional intelligent species by now, since cephalopods etc. have been around for quite a bit longer than 20 million years. That would suggest that "sufficient time" is quite a bit longer too. As many others have pointed out, the other adaptations you mention have happened convergently many times (except flight, which has only been achieved 4 times that we know of). Yet there's only one intelligent species, a quite recent one, and the absolute minimum necessary for anyone to be there to count. And it took 4 billion years to get that one. By contrast (though only, apparently, by contrast), multicellularity is easy; it happened at least 5 times (animals, plants, fungi, red algae, brown algae), more if you're generous.

I can imagine *many reasons why we haven't seen any visitors, and
since we don' t know enough to assign probabilities to these, none of
us are offering anything more than idle speculation.

[snip idle speculation]