The
Drake Equation attempts to take into account all of the variables involved in
our encountering intelligent life. By inputting estimates of the values of
the various variables, we can come up with an estimate for the likelihood of us
having already been contacted by intelligent life. Alternatively, since we
have apparently not been contacted, we can use this information to obtain a
better estimate of one of the variables - such as the expected lifespan of a
technological civilization. As a disclaimer before we start, this equation
relies upon a lot of data for which we only have poor estimates. There are
many possibilities other than those presented here. This page is only
intended to provide some interesting material for further thought by laying out
a reasonable argument based on reasonable estimates - it is not intended to be a
comprehensive proof of anything.
The equation is N=S*P*n*d*I*C*L,
where N = the number of civilizations with whom we could theoretically
communicate. S = the rate of star formation in our galaxy. P = the
fraction of stars that have planets. n = the number of planets that could
potentially bear life around each star that has planets. d = the
fraction of planets that could bear life that actually do so. I = the
fraction of life-bearing planets that produce intelligent life. C = the
fraction of intelligent species that are capable of and interested in
communication. L = the lifespan of such technological
civilizations.
Estimates on the rate of star
formation range from 1 to 10 per year in the Milky Way. For our purposes
we'll use 5 as the number of stars that form each year. This number
may be high compared to modern star formation rates, but it is probably low compared
to the rate when the stars that may now shine on intelligent life were
formed. .
We don't know what fraction of stars have planets around
them. That said, using very crude techniques that are only applicable in
only a small percentage of possible situations, we've been able to show that
several other stars in our galaxy do have planets. Therefore it is quite
reasonable to estimate that the fraction of stars with planetary systems is at
least 0.5.
Obviously we
don't know what fraction of planets could potentially bear life.
After all, we only can prove that life developed on Earth -
and Earth can't provide statistical evidence regarding this sort of
question since if life had not developed here, or had developed at a later
point, we wouldn't be here asking the question. But, based on what we
know to be the necessary chemical and environmental precursors to our sort
of life, it seems quite possible that bacterial life may have developed
on Mars, and, potentially, several of the moons of our gas giants. So, just
in our own system there are several possible planets (or satellites)
that may support or have once supported some form of life. And,
of course, it's certainly possible that life can exist that's very different from
our own, in which case life might potentially be almost anywhere in the solar
system. So, while we can't claim a very high degree of certainty about
number of planets (and moons) in each planetary system that could potentially
bear life, 2 is an estimate that is well in line with the limited information we
have.
We also don't know what fraction of the planets (and
satellites) capable of bearing life actually do so. If the Martian
meteorite that made news a few years ago actually did contain fossilized life
forms, then that would suggest that it's reasonably easy for simple life to form
(since we may have discovered evidence of non-Earth life almost as soon as we
began looking for it). That said, our solar system might have uncommon properties
that are particularly conducive to the formation of life - so it
certainly wouldn't be strong proof of anything. Furthermore, some doubt has been cast
on the evidence of the Martian meteors. So, let's say that 1/10
of the planets capable of bearing life actually do at some point. This
estimate isn't a lot better than a guess, of course, but not
an unreasonable guess based on what we do know.
Of
course many planets probably bear life without bearing intelligent life.
So, to be safe let's say that only 1 in a million life bearing planets produces
intelligent life.
Then, there's no guarantee that an intelligent life form
would also be technological and both interested in and capable of communicating
with us. That said, there's some evidence that the evolution of
intelligence and technological ability may be somewhat dependent upon each other.
Our opposable thumbs created an evolutionary benefit to having a
brain capable of designing simple tools, which, in turn created an evolutionary
benefit to especially agile fingers. Likewise, our ability with
languages lead to us passing information between generations, which lead to technological
improvements, which helped to create the necessity for an expanded vocabulary.
So it's quite reasonable to expect that many intelligent life
forms would have the potential to be technological. That said,
dolphins also exhibit many signs of intelligence, yet they are certainly not
capable of communicating through outer space. And you can easily imagine other
similarly limited intelligences - intelligent oceanic species, intelligent
species on planets with a dearth of important elements (without metals, for
example), or intelligent species in a deep gravity well - are just some
examples of possible intelligent lifeforms that might not develop the technological
capacity to communicate with us. So, let's say that 1/10
intelligent species are technologically capable of communication at some point
during their existence. Furthermore, we'll also assume for now that a
species that is technologically capable of communicating would choose to do so
(I'll address this assumption later).
If we put
those numbers into the equation, we get N =
5*0.5*2*0.1*0.000001*0.1*L, or N =
1/(20 million)*L. So, if, for the time being we
accept the estimates above, the number of intelligent species in our galaxy available
for us to communicate with should depend on the
lifespan of technological civilizations. Since our current knowledge of science is
nearly enough for us to expand off-world if we are willing to devote sufficient
resources to the effort, it's clear that a technological society with enough time
and interest in exploration and growth would be able to settle
new planets. Furthermore, exponential growth would allow a species to colonize
virtually every habitable region of the galaxy within tens of millions of years. We have
not seen any evidence of such colonization, however, and it seems likely that a
colonizing species would have visited our own solar system at some point.
Furthermore, there is a good chance such a species would have settled regions of
space close to our own, yet we've received no attempt at communication from
them.
This implies one of a few possibilities. First, it's possible
that advanced technological species exist, but they are hiding from us.
Perhaps they are afraid of contaminating a developing civilization.
Second, an older technological species might have advanced to
the point that it can't reasonably communicate with us - in this scenario we might
still have seen evidence of its existence, however. Third, that technological
civilizations may lose interest in exploring and expanding. This
is a very real possibility given the unpredictable effects of rapidly increasing
computing power and the possibility of bio-engineering our own species.
Fourth, technological civilizations may be very short lived. We've been
able to commit suicide as a species for about half a century now. During
the next century, advances in bio-weaponry and nano-technology will make this
sort of power available to individuals. If technological civilizations
commit suicide, however, then we are likely to do so within the next century or
two. After that, we will probably have expanded off world and it will be
much much harder when outposts of our civilization are protected by large
buffers of empty space. If technological civilizations survive their
birthing processes and expand off-world, they probably can survive
indefinitely.
The other possibility is that one of the other variables
was wrong, and we are unique in our galaxy - other technological species do not,
and have not existed. In that case, all we learn from the Drake Equation
is that our sort of life is very rare indeed.
The scary thing about the Drake Equation is that many of the
possible implied futures are bad, or at least very uncertain. If we're
alone in the galaxy, ok. Or, if we're being left alone while we
develop, and will eventually be allowed to join the galactic community,
fine. But, one likely possibility is L is very small, and we're
about to face the collapse of modern civilization, or worse, we're about
to go extinct. And, should our civilization continue, we may
face a scary future where genetic engineering and ultra-intelligent computers
end our interest in exploration and expansion.