
Relative Likelihood for Life as a Function of Cosmic Time - privong
http://arxiv.org/abs/1606.08448
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rm_-rf_slash
One of the unfortunate parts of our being the only intelligent species we know
of is that while we can mathematically guesstimate life's appearance
throughout the cosmos, we have no data on when intelligent civilizations die
out.

In the final episode of Cosmos, Carl Sagan describes a nightmare in which he
is traveling the universe and decides to return to earth. As he gets closer,
his spaceship intercepts broadcasts that have been traversing space since the
invention of radio, from the earliest radio messages to television signals and
then eventually...the signals stop.

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rwallace
> We find that unless habitability around low mass stars is suppressed, life
> is most likely to exist near 0.1 solar-mass stars ten trillion years from
> now.

Microbial life perhaps, but as far as complex life is concerned, I claim
habitability on those timescales will indeed be suppressed. Complex life needs
plate tectonics, which depends on a planet's internal heat. A red dwarf star
may shine for ten trillion years, but terrestrial planets will be stone cold
dead long before then.

~~~
dnsco
Why would complex life depend on plate tectonics?

~~~
rwallace
Partial summary: [http://hyperphysics.phy-
astr.gsu.edu/hbase/geophys/platelif....](http://hyperphysics.phy-
astr.gsu.edu/hbase/geophys/platelif.html)

~~~
jessriedel
These are speculative and rather weak. Like most theory in this area, it takes
the form "Look at these features of the development of life as we know at; if
they weren't there, it couldn't exist!". But of course, they are simply unable
to imagine, much less predict, what happens in dramatically different
conditions.

Also, talk about correlation does not equal causation at your link:

> The basic structure of the Earth, aided by plate tectonics, makes possible
> the Earth's magnetic field. Neither Venus nor Mars has such a magnetic
> field. Venus has core and molten regions like the Earth, but a very slow
> rate of rotation. It produces neither plate tectonics or a magnetic field.
> Mars has volcanism, but limited to a small number of spectacularly large
> volcanoes like Olympus Mons. It's magnetic field is weak despite a rotation
> period similar to that of the Earth. This suggests that the molten nature of
> the Earth's mantle that facilitates plate tectonics is also essential for
> the operation of the dynamo that produces the magnetic field.

I mean, N=3 !

~~~
rwallace
Okay, but which of the following two positions are you arguing for?

1\. We don't have enough data about other solar systems, so we don't know
where complex life might exist.

2\. We don't have enough data about other solar systems, so complex life
probably exists all over the place.

If you're arguing for the first position, fair enough; I might not entirely
agree, but it is at least reasonable.

But I see too many people arguing for the second position, taking scarcity of
data as a license to believe what they want to believe.

~~~
jessriedel
The first. I think the most likely resolution to the fermi paradox, which is
not that paradoxical, is just that abiogenesis is a galactically rare event.
Other proposed filters, like the advent of sexual reproduction, eukaryotes, or
intelligence, seem much less robust to a failure of imagination.

~~~
rwallace
Fair enough, that's a reasonable position. My opinion, for what it's worth, is
that every step is a hard step, because it's not just a matter of time elapsed
to the next one, but of each step having to be taken in just such a way as to
set the right preconditions for all the subsequent ones.

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themadcreator
_There is scope for considerable refinement in the choice of the second factor
p(life|HZ). [...] In our simplified treatment, this constant value has no
effect on dP(t) /dt since its contribution is also cancelled by the
normalization factor N._

Though this paper does provide some needed formalism around the Drake
Equation, I don't understand how its results can be meaningful if it "cancels
out" one of the largest sources of uncertainty.

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baking
If we were exceptional, we would expect the sun to be one tenth the size it
is. Therefore we are not exceptional.

Fine, but that same argument says that most life in the universe will be
coming along in trillions of years and a fat lot of good that will do for us.

