NetNews Usenet Archive 1992 #16/NN_1992_16.iso/spool/sci/space/11048

Text File | 1992-07-31 | 8.2 KB | 161 lines

Newsgroups: sci.space
Path: sparky!uunet!cs.utexas.edu!torn!utzoo!henry
From: [e-mail] (Henry Spencer)
Subject: Re: ETs and Radio
Message-ID: <[e-mail]>
Date: Fri, 31 Jul 1992 17:39:58 GMT
References: <[e-mail]> <[e-mail]> <[e-mail]> <[e-mail]> <[e-mail]>
Organization: U of Toronto Zoology
Lines: 150

In article <[e-mail]> [e-mail] (Gary Coffman) writes:
>>>Now subtract out all Population II stars, no heavy elements like iron...
>
>I mentioned iron for two reasons. First, it's necessary for oxygen
>transport via the blood.

Tell it to the crustaceans, which use copper instead.

>Second, and this is opinion, it's needed
>for the planet to have a significant magnetic field to redirect energetic
>radiation from the planetary surface...

The bulk of the radiation shielding on Earth is from the atmosphere, not
the magnetic field.  Unquestionably the field is useful, but it's far
from essential.  (Note that life on Earth has survived many field
reversals, during which the field is more or less absent temporarily.)

>>>and subtract out all multiple star systems, no stable planetary orbits,
>>I'm told this doesn't look like as big a problem as was once thought.
>
>??? Let me put it another way, stable orbits in the liquid water zone.

Old opinions about the stability of planetary orbits in multi-star systems
appear to have been too pessimistic.  It's not that difficult, especially
if the stars are fairly widely separated.

>>>subtract out all systems that don't have a planet in the liquid water zone,
>The potential liquid water zone of a G star like ours is roughly from
>near Venus orbit to somewhat beyond Mars orbit given specially designed
>planets in the various places. Now on the solar system scale, that's a
>small range of distances.

Two planets inside it and one a near miss is "small"?  Sounds adequate
to me.  There is no reason to believe that our solar system is unusually
heavily populated in its inner regions, and indeed considerable reason
to believe that it is fairly typical.  Based on what we know now, most
any planetary system around a reasonable star should have at least one
planet in the liquid-water zone.  The major uncertainty is whether
that planet will be the right size for its location.  The odds of that
aren't 100% by any means (Earth hit, Mars missed), but they aren't one
in a million either.  One in two?  Maybe one in ten if you're a pessimist.

>... the accretion process, the remelt, the second
>cooling, and finally the out gassing of water vapor and methane that
>formed the primordial atmosphere took about 1/3 the life of the solar
>system to occur. (At least according to popular theories of planetary
>formation)

Please cite references.  My recollection is that the system is dated
at 4.5-5 GYr old, and life on Earth is dated back to 4GYr at least.
Life -- simple life -- appeared quickly on Earth, probably as soon as
it was cool enough.  (This is another encouraging sign, although it
might have been a fluke.)

>The great extinction, due to oxygen liberation, took another
>4 billion years, before oxygen based life became dominant. I think we
>can safely assume that intelligent life would require the energetics
>only possible to oxygen breathers.

I concur that you *probably* need a high-energy chemistry such as that
of oxygen, and there are no other good bets evident.  (The halogens are
comparatively rare elements for fairly fundamental reasons, oxygen is
quite abundant for the same reasons.)

However, again I'd like to see references for the time scale.  Animal
life based on oxygen has been extant and probably dominant for most of
the history of life on Earth, I believe.  What took a long time was
the transition to vertebrates, not to oxygen.  Superfically this doesn't
change the line of argument much, since intelligent invertebrates don't
seem very likely either.  The important difference is that we're now
dealing with a transition whose timing doesn't seem to be constrained
by fundamentals of physics.

>... One of the greatest unknowns
>is whether life bearing planets naturally evolve intelligent creatures,
>or whether that's a rare accident. We don't have any data to support
>either position very well.

Agreed.  It took a while for vertebrates to become intelligent here.
An unusually long while?  An unusually short while?  Open question.
However, note that it happened to some degree on three parallel lines
of development:  apes, whales, and elephants.  You have to really
stretch the point to claim that it's inordinately rare.

>>>who've thought about the question say that there are perhaps 50 systems
>>>in the galaxy that may have life as we know it.
>>References, please...
>One of Sagan's books quoted this figure...

If memory serves, that number was for communicating civilizations at roughly
our current level of development, not just for life.  Whether this is the
total set of interest depends on whether you assume a relatively short
lifetime for such civilizations, as Sagan did.  More to the point, he
assumed that interstellar distances were an impassable barrier, and that
communication was the only issue.  Wrong.

>>The only argument against extraterrestrial life/intelligence that strikes
>>*me* as being particularly telling is the Fermi Paradox:  if they're out
>>there, why didn't they colonize this planet long before we evolved?
>
>Assume Einstein is right. Interstellar flight takes longer than the
>time available since the Universe began to visit all the stars...

Fermi unquestionably assumed Einstein was right.  Colonizing the galaxy
at speeds we should be able to achieve within a century or two takes
millions of years, not billions.  Intergalactic flight is more problematic,
but shouldn't be necessary unless intelligent life is extremely rare.
Starflight is expensive, but not especially difficult.

>Or, we are the *first* technological civilization to evolve in the galaxy.

But, why?  Again, unless you cook the books to make intelligent life
extremely rare, there should be a reasonable number of them appearing
around now, even if you discount the previous generation of stars
(which didn't have a lot of heavy metals, but had some -- the Sun is
not a second-generation star, more like third or fourth).  Possibly
we are the very first, but it's not very likely unless the numbers
are really small.  The way to bet is that at least one of them would
have come about, say, 1% faster... which would be enough.

>Or, technological civilizations self destruct.

With 100% probability?  This seems implausible.  Once they no longer have
all their eggs in one basket, complete self-destruction should be hard.
We're close to that stage.


To try and shorten this a bit... there are something like fifty proposed
explanations for the Fermi Paradox.  (I did a series of articles on this
a few years ago for the Canadian Space Society newsletter.)  Most of them
face the same problem:  it's hard to make them work 100%, and one lucky
escape would have been enough.  You can do it with combinations, but it
starts to require a very strained argument, making an awful lot of key
assumptions with very little evidence.  A single convincing explanation
would be a lot more satisfying, especially if it treated our own case
as reasonably typical rather than as a one-in-a-billion fluke.  You can
get away with almost anything by asserting that we're a rare exception.

Exactly two of the proposed explanations seemed to me to have potential
for being satisfactory:  Berserkers, and Failure Of Extrapolation.  It's
hard to make any natural mechanism obliterate 100% of new civilizations,
but if you assume intelligent malice backed by high technology, it's not
so difficult... although you might have to stipulate robotic intelligence
to get constancy of purpose and will over the time frame involved.  My
own feeling is that Failure Of Extrapolation is more likely, though:
there simply are important facts -- about the universe, the development
of intelligence, or galactic civilization -- that we don't yet know.
--
There is nothing wrong with making      | Henry Spencer @ U of Toronto Zoology
mistakes, but... make *new* ones. -D.Sim|  [e-mail]  utzoo!henry