MadSci Network: Astronomy

Re: Why havent we heard from ET?

Date: Fri Jan 19 15:09:38 2001
Posted By: Erika Gibb, Grad student, Physics & Astronomy/Origins of Life, RPI
Area of science: Astronomy
ID: 979714492.As


The question about life in the galaxy (I'll restrict the discussion to just
this galaxy since any signals from a civilization in even the closest
galaxy outside our own would be too weak to detect) is a hotly debated
one.  So far we have no evidence from SETI or any other study that there
are any other life forms but us.  There are several possible explanations
for this.  First, it may be that advanced life is not all that common. 
Life started very early in the history of the solar system, which is 4.6
billion years old.  The earliest evidence for life on Earth is 3.8 billion
years old and life could have arisen earlier than that.  We can't be sure
because there are very few rocks on Earth that are that old and those that
are have mostly undergone metamorphism which would destroy most evidence
for life.  However, multicellular organisms did not arise until about 700
million years ago and humans with the ability to communicate or send
signals into space have only come on the scene in the last century.  That
would indicate that it is very time consuming and difficult to reach a
point of intelligence and technological advancement.  Any number of things,
such as an asteroid or comet impact, disease or predation, or a natural
disaster, could have wiped out the precursor to humanoids before the
population rose, preventing or further delaying the rise of an intelligent

Many scientists believe that microbial life may be common in the universe,
but that animal life may be very rare (a good popular science book which
addresses this is "Rare Earth" by Peter Ward and Donald Brownlee). 
Microbial life has adapted to every niche on earth that has been
investigated.  They are in hot springs, at ocean bottoms, volcanic vents,
the rocks of Antarctica, and buried deep in crustal rock.  They have even
been resuscitated after flying on spacecraft in high radiation, frigid
temperature conditions.  Animal life, on the other hand, has more
restrictive requirements for life.  We require liquid water and much more
stringent temperature restrictions.  This must be the case throughout the
evolution of animal life.  Venus and Mars may have had these conditions
early in the solar system's history, but have since lost them, preventing
the rise of complex animal life.  That leads to the question of how many
planets can sustain conditions conducive to animal life for the time needed
to evolve a technological species?  That is unknown.

It may very well be that intelligent, technological societies are very rare
in the galaxy.  It may be that we just haven't looked in the right place
yet (there are billions of stars and a huge wavelength range to
investigate).  It may be that another species would not wish to
communicate.  We just don't know the answer to that question.  My personal
speculation is that advanced life is rare and that if it is out there and
trying to communicate, it may take a while to find it.  After all, we've
only been looking for about 1/100,000,000 of our planet's history!  That
means any civilization looking for us would have to be within 70 light
years or so (since we've only been sending substantial radio signals for
about that long).  The galaxy is about 100,000 light years across--that
means only a very small percentage of the galaxy would even have the
ability to tell that we are here to communicate with.  The galaxy is a very
large place!


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