  
...So deep is the conviction that there must be life out there
beyond the dark, one thinks that if they are more advanced
than ourselves they may come across space at any moment,
perhaps in our generation. Later, contemplating the infinity
of time, one wonders if perchance their messages came long
ago, hurtling into the swamp muck of the steaming coal forests,
the bright projectile clambered over by hissing reptiles, and
the delicate instruments running mindlessly down with no report. Loren Eiseley, The Immense Journey (1957) There are hundreds of billions of stars out there, and many of them are like our sun. Surely there must also be planets with climates hospitable to life. Is there a way to calculate, with any measure of certainty, just exactly how likely the existance of such planets is? How likely is it that there are beings out there with whom we could communicate...who are perhaps attempting to communicate with us! An equation has been proposed which attempts to measure the number of technical civilizations that exist in just our own galaxy. The equation:          'N' is the number of existing technical civilizations in our galaxy that are interested in, and capable of, communicating over interstellar distances. Click on each term in the equation above to see an explanation of what it represents. This value is a number which represents the average rate of
star formation in our galaxy. In order to explain how this
number will be estimated, it is necessary to understand a
little about how stars are formed. The value of 'R' has
changed since our galaxy first formed; it is now less than it was originally,
since there is far less dust, gas, and other interstellar matter in the galaxy
now, for stars to form from. 'R' will thus be an average over the lifetime
of the galaxy.
It's value can be put at 10 stars per year.
This variable represents the fraction of all stars that have planets.
Our theories about how stars form, and, indeed, some recent direct
evidence of planets observed orbiting nearby stars, indicates that
almost all stars will in fact have planets orbiting them. So we
can fairly confidently make this value in the equation 1.
This is the average number of planets in each star system that are
hospitable to life. Our solar system has at least one, and possibly
one or two more. Recent evidence shows that there was once freely
flowing water on Mars, and possibly even life of some sort. Very
hot stars will have a larger 'habitable zone' for planets; cooler ones will
have smaller such regions. A reasonable estimate for this number is 1.
This is the fraction of habitable planets on which life develops.
Much research into the origins of life on our planet suggests very
strongly that, given the initial conditions of warmth, liquid water,
and abundant chemicals, life will arise spontaneously.
Experiments attempting to duplicate conditions on the primordial
earth have, indeed, led to the formation, in the laboratory, of the
chemical compounds necessary.
More recent evidence from astronomical studies of interstellar
matter also show that life's 'building blocks', such as amino acids,
can and are being formed there.
The existance of a primitive but independent beginning of life on Mars
greatly strengthens the hypothesis that where the conditions necessary
for life are present, life will arise.
So we adopt a value for this variable of 1.
This is
the fraction of life-bearing planets where intelligent,
tool-using life develops, within the lifetime of the local sun.
The question of the evolution of intelligence is not an easy one
to answer, as it does not readily lend itself to experimentation,
and the number of intelligent species on the earth is limited.
Our own tool-using civilization has been present for just a few
million years, (since the time of the earliest known 'proto-human'),
and this is insignificant when compared to the lifetime of the solar
system (over 10 billion years).
However, our civilization has arisen only half-way into this time
period. Also, evolutionary pressures do tend to favour the selection
for intelligence...it has a high survival value.
We will make a guess at this value,... and suppose that only
one in ten planets where there is life will contain life evolved
to the level of intelligent tool-users.
Thus the value for this variable will be set at 0.1.
With this variable we are attempting to estimate the fraction
of intelligent tool-using life-forms that will go on to create a
technical civilization such as our own, which is capable of
communicating with us.
Once again, WE are our only evidence, and our fledgling human
technical civilization has only been around for a tiny amount of time.
There is not enough data to decide what it is that motivates a
civilization to seek to understand scientific principles, or to pursue
technological solutions. Do curiosity and intelligence go together?
Are we 'curious' because of our simian evolutionary prehistory?
Would we have developed a scientific, technological society if we
had evolved from feline, or canine, or ursine ancestors?
No-one knows the answers to these questions. The most we can say
is that curiosity, which surely is a prime prerequisite for a scientifically
oriented society, seems to have had, for us, a definite survival
value. Whether this means that curiosity (and hence, technical
civilizations) will always evolve, is not decided. So we will be
somewhat conservative, and again allocate the value 0.1.
This variable is the most difficult one to estimate a value for.
It represents the lifetime in years of a technological society.
In this case we have not even one example to guide us...we
have no idea how long our own technical civilization (which
is only about 100 years old) will last. Before the cold war ended, many
pessimists might have put a figure of mere decades here. Recent
events have allowed for more optimism. Might we expect another
hundred years? A thousand? Perhaps our society will last out the
lifetime of our star (another 5 billion years).
We will have to take two values here...a lower limit (the pessimistic
one), and an upper limit. So we make the value of 'L' a number
between 100 and 100,000,000.
Now we are ready to make an estimate of 'N', the number of
civilizations that exist in just our galaxy that are ready and
willing to send us a message. This value is determined by
multiplying all the other numbers. Since L had a low and a high estimate, we will do the multiplying first with the low value, and then with the upper one. This will give a range of expected values for 'N'. Our answer? In our galaxy, there are either 10 or fewer intelligent species, or more than 10 million Either way, it's remarkable when you think about it, isn't it? Is the 'right answer' somewhere in between? What effect will the value of this number have on the likelihood that we will ever make contact with any of them? (These values would place interstellar civilizations at average distances of either 10-100 light years, or as far as many thousands of light years.) More difficult questions, but maybe ones we can answer... |