The Search for Extraterrestrial Life (Lectures #31-32)
Astronomers have been making all sorts of mind-boggling discoveries over the past
century – black holes, quasars, the leftover radiation from the Big Bang, just to name a
What we have
yet found with any certainty is alien life that evolved somewhere
other than the Earth.
And yet clearly this would be the most stunning discovery the
human race could ever make.
The public may be fascinated with black holes and the Big
Bang in an academic sense, but the discovery that we are not alone in the universe would
social, political, and possibly economic
implications for our entire planet.
The possibility of extraterrestrial life is the subject of practically all science fiction, and it
is hard to imagine a universe with only one inhabited planet out of trillions.
Given the number of planets that must exist in our galaxy (the Milky Way) alone, it
seems a virtual certainty that life must have evolved somewhere else – possibly even
within our own solar system.
If this is so, why haven’t we yet detected it?
this question, and the question of whether there might be
civilizations out there, we need to delve in some detail into the nature of life on
Only once we understand how life came about on our own planet will we be
able to speculate meaningfully about the possibility that it might have arisen elsewhere,
and about the forms that life might take in other environments.
What is life?
There is no single, widely-accepted answer to the question “What is life?”.
How do we
distinguish between animate and inanimate matter?
Does it have to do with the chemical
composition, or perhaps with the complexity of the physical structures and chemical
reactions that take place in a particular lump of material?
Or is there some mysterious
“essence” that lies outside the physical realm that renders matter “alive”?
Obviously, people have been trying to answer this question for a long time.
As far as
are concerned, we’re going to take a pragmatic, descriptive approach.
For our purposes,
for something to be considered alive, it must satisfy two characteristics:
The ability to reproduce
The ability to evolve into other forms
Note that a flame on a matchstick satisfies the first criterion – it can reproduce itself by
starting other fires and creating “copies” of itself. But it fails on the second criterion –
fire by itself cannot change form into something essentially different.
All it can do is
flicker and change its shape.
On the other hand, since Darwin we have realized that a distinguishing feature of living
organisms on Earth is that they not only reproduce from generation to generation, but
over time, with one species changing slowly into another through the
joint processes of genetic mutation and natural selection.
It is clear from the fossil record