Read an article
recently on the Great
Filter, the notion that we may not come across any evidence of
advanced civilizations beyond our own because something eventually
rubs them out. We have been sending out electro-magnetic signals for
over a hundred years and have been listening for almost as long. We
have by now discovered almost 1800 exoplanets. An estimated 22% of
sun-like stars in our galaxy may have earth-like planets orbiting in
their habitable zones. That would mean 20 billion candidates for
life such as ours. Four of such earth-like exoplanets planets have
been identified within 50 light years of us, another two within 500
LYs.
There is no reason
to assume that life would have to be similar to our carbon-based form
or would require conditions similar to ours. Life
on our planet sprung up quickly and the physics and chemistry of
our universe seem to favor self-organizing processes. Life forms
could be quite varied and perhaps universal.
Enrico Fermi
suggested in 1950 that if any advanced civilization developed the
ability to travel beyond its solar system, even at less than light
speed, in ten million years it should be able to colonize the whole
Milky Way (100,000 LYs in diameter). So why don't we see them? Why
haven't we even heard anyone else? The Great Filter suggests various
possibilities.
The first would be
that advanced life is rare. The conditions for it to develop are
quite special. While life on earth arose quickly, in just 400
million years after earth formed a solid crust, it took another
almost two billion years for complex single cells to evolve. Add
another billion years – about 550 million years ago – for
multi-cellular creatures. Most of the history of life on earth is
this long prelude to the development of us. Humans arose only in the
last two million years of the earth's 4,500 million years. Along the
way, life went through several mass extinction events. The last one,
65 million years ago, took out the dinosaurs leaving the ground clear
for the development of mammals. The combination of events and
circumstances that led to us may be so rare as to make us one of the
very few – or only – lucky ones.
But with some
probable 20 billion earth-like exoplanets and some 100 billion likely
planets in all, chances are that however rare, odds would favor
the development of a considerable number of advanced life forms in
our galaxy. Some might have arose millions of years ago. Any
signals they sent would have had plenty of time to reach us. Any
earth-like planet with advanced life within 500 LYs would presumably
have been heard by now. So far, the SETI project has found none.
Perhaps our
listening capabilities are still not sensitive enough to pick up any
signals. But clearly we are now able to tease out the existence of
exoplanets themselves out some two thousand light years.
Maybe cosmic natural
disasters – nearby super-novas, meteor strikes, etc – occur
frequently enough to set back life and knock out civilizations before
they can get very far? But we've gone 65 million years without one
and there is no reason to expect any such for at least the next few
hundred years.
Maybe someone is out
there, able to hide themselves and/or tracking down and destroying
any potential competitors before they get too far? This is a common
science fiction trope. But it assumes that advanced civilizations
would either be very modest – and thus hide themselves, perhaps
quietly visiting and making crop circles or waiting for us to rise to
the level where we could join their Federation – or especially
vicious and aggressive. Based upon the only advanced civilization we
know of – ourselves – one could not rule out the second
possibility.
Finally, there is the
possibility that there is something about advanced technologies that
operates to cut short the civilization that develops them:
industrial civilization leading to run-away climate change;
biotechnology leading to – or failing to keep up with –
disruptions in the present web of life; failure of critical
management systems to handle increasingly complex and changing
political, social, economic and ecological dynamics.
Bottom line, so far
we have no evidence that we have company anywhere out there. We may
be special. Question is, are we doomed to be filtered out and will
we have ourselves to blame?
