The SETI Debate:
In Defense of the
Search for Extraterrestrial Intelligence
The
Abundance of Life-Bearing Planets
The best current estimates of the number and
spacing of
Earth-mass planets in newly forming planetary systems (as George
Wetherill
reported at the first international conference on circumstellar
habitable zones
[Doyle, 1995]) combined with the best current estimates of the
long-term
stability of oceans on a variety of planets (as James Kasting reported
at that
same meeting [Doyle, 1995]) suggest one to two blue worlds around every
Sun-like star. Stars much more massive than the Sun are comparatively
rare and
age quickly. Stars comparatively less massive than the Sun are expected
to have
Earth-like planets, but the planets that are warm enough for life are
probably
tidally locked so that one side always faces the local sun. However,
winds may
redistribute heat from one hemisphere to another on such worlds, and
there has
been very little work on their potential habitability.
Nevertheless, the bulk of the current evidence
suggests a
vast number of planets distributed through the Milky Way with abundant
liquid
water stable over lifetimes of billions of years. Some will be suitable
for
life--our kind of carbon and water life--for billions of years less
than Earth,
some for billions of years more. And, of course, the Milky Way is one
of an
enormous number, perhaps a hundred billion, other galaxies.
Need Intelligence
Evolve on an Inhabited World?
We know from lunar cratering statistics,
calibrated by
returned Apollo samples, that Earth was under hellish bombardment by
small and
large worlds from space until around 4 billion years ago. This
pummeling was
sufficiently severe to drive entire atmospheres and oceans into space.
Earlier,
the entire crust of Earth was a magma ocean. Clearly, this was no
breeding
ground for life.
Yet, shortly thereafter--Mayr adopts the number
3.8 billion
years ago--some early organisms arose (according to the fossil
evidence).
Presumably the origin of life had to have occupied some time before
that. As
soon as conditions were favorable, life began amazingly fast on our
planet. I
have used this fact (Sagan, 1974) to argue that the origin of life must
be a
highly probable circumstance; as soon as conditions permit, up it pops!
Now, I recognize that this is at best a
plausibility
argument and little more than an extrapolation from a single example.
But we
are data constrained; it's the best we can do.
Does a similar analysis apply to the evolution of
intelligence?
Here you have a planet burgeoning with life, profoundly changing the
physical
environment, generating an oxygen atmosphere 2 billion years ago, going
through
the elegant diversification that Mayr briefly summarized-- and not for
almost 4
billion years does anything remotely resembling a technical
civilization
emerge.
In the early days of such debates (for example,
G.G.
Simpson's "The Non-prevalence of Humanoids") writers argued that an
enormous number of individually unlikely steps were required to produce
something very like a human being, a "humanoid"; that the chances of
such a precise repetition occurring on another planet were nil; and
therefore
that the chance of extraterrestrial intelligence was nil. But clearly
when
we're talking about extraterrestrial intelligence, we are not
talking--despite
Star Trek--of humans or humanoids. We are talking about the functional
equivalent of humans-- say, any creatures able to build and operate
radio
telescopes. They may live on the land or in the sea or air. They may
have
unimaginable chemistries, shapes, sizes, colors, appendages and
opinions. We
are not requiring that they follow the particular route that led to the
evolution of humans. There may be many different evolutionary pathways,
each
unlikely, but the sum of the number of pathways to intelligence may
nevertheless be quite substantial.
In Mayr's current presentation, there is still an
echo of
"the non-prevalence of humanoids." But the basic argument is, I
think, acceptable to all of us. Evolution is opportunistic and not
foresighted.
It does not "plan" to develop intelligent life a few billion years
into the future. It responds to short-term contingencies. And yet,
other things
being equal, it is better to be smart than to be stupid, and an overall
trend
toward intelligence can be perceived in the fossil record. On some
worlds, the
selection pressure for intelligence may be higher; on others, lower.
If we consider the statistics of one, our own
case--and take
a typical time from the origin of a planetary system to the development
of a
technical civilization to be 4.6 billion years--what follows? We would
not
expect civilizations on different worlds to evolve in lock step. Some
would
reach technical intelligence more quickly, some more slowly, and--
doubtless--some never. But the Milky Way is filled with second- and
third-generation
stars (that is, those with heavy elements) as old as 10 billion years.
So let's imagine two curves: The first is the
probable
timescale to the evolution of technical intelligence. It starts out
very low;
by a few billion years it may have a noticeable value; by 5 billion
years, it's
something like 50 percent; by 10 billion years, maybe it's approaching
100
percent. The second curve is the ages of Sun-like stars, some of which
are very
young-- they're being born right now--some of which are as old as the
Sun, some
of which are 10 billion years old. If we convolve these two curves, we
find
there's a chance of technical civilizations on planets of stars of many
different ages--not much in the very young ones, more and more for the
older
ones. The most likely case is that we will hear from a civilization
considerably more advanced than ours. For each of those technical
civilizations, there have been tens of billions or more other species.
The
number of unlikely events that had to be concatenated to evolve the
technical
species is enormous, and perhaps there are members of each of those
species who
pride themselves on being uniquely intelligent in all the universe.
Need
Civilizations Develop the Technology for SETI?
It is perfectly possible to imagine civilizations
of poets
or (perhaps) Bronze Age warriors who never stumble on James Clerk
Maxwell's
equations and radio receivers. But they are removed by natural
selection. The
Earth is surrounded by a population of asteroids and comets, such that
occasionally
the planet is struck by one large enough to do substantial damage. The
most
famous is the K-T event (the massive near- Earth-object impact that
occurred at
the end of the Cretaceous period and start of the Tertiary) of 65
million years
ago that extinguished the dinosaurs and most other species of life on
Earth.
But the chance is something like one in 2,000 that a
civilization-destroying
impact will occur in the next century.
It is already clear that we need elaborate means
for
detecting and tracking near-Earth objects and the means for their
interception
and destruction. If we fail to do so, we will simply be destroyed. The
Radiotelemetry, radar monitoring of asteroids, and
the
entire concept of the electromagnetic spectrum is part and parcel of
any early
technology needed to deal with such a threat. Thus, any long-lived
civilization
will be forced by natural selection to develop the technology of SETI.
(And
there is no need to have sense organs that "see" in the radio region.
Physics is enough.)
Since perturbation and collision in the asteroid
and comet
belts is perpetual, the asteroid and comet threat is likewise
perpetual, and
there is no time when the technology can be retired. Also, SETI itself
is a
small fraction of the cost of dealing with the asteroid and comet
threat.
(Incidentally, it is by no means true that SETI is
"very limited, reaching only part of our galaxy." If there were
sufficiently powerful transmitters, we could use SETI to explore
distant
galaxies; because the most likely transmitters are ancient, we can
expect them
to be powerful. This is one of the strategies of the Megachannel
Extraterrestrial Assay [
Is SETI a Fantasy
of Physical Scientists?
Mayr has repeatedly suggested that proponents of
SETI are
almost exclusively physical scientists and that biologists know better.
Since
the relevant technologies involve the physical sciences, it is
reasonable that
astronomers, physicists and engineers play a leading role in SETI.
But in 1982, when I put together a petition
published in
Science urging the scientific respectability of SETI, I had no
difficulty
getting a range of distinguished biologists and biochemists to sign,
including
David Baltimore, Melvin Calvin, Francis Crick, Manfred Eigen, Thomas
Eisner,
Stephen Jay Gould, Matthew Meselson, Linus Pauling, David Raup, and
E.O.
Wilson. In my early speculations on these matters, I was much
encouraged by the
strong support from my mentor in biology, H.J. Muller, a Nobel laureate
in
genetics. The petition proposed that, instead of arguing the issue, we
look:
We are unanimous in our conviction that the only
significant
test of the existence of extraterrestrial intelligence is an
experimental one.
No a priori arguments on this subject can be compelling or should be
used as a
substitute for an observational program.
Answer to "The
Abundance of Life-Bearing Planets"
By Ernst Mayr
I fully appreciate that the nature of our subject
permits
only probabilistic estimates. There is no argument between Carl Sagan
and
myself as to the probability of life elsewhere in the universe and the
existence of large numbers of planets in our and other nearby galaxies.
The
issue, as correctly emphasized by Sagan, is the probability of the
evolution of
high intelligence and an electronic civilization on an inhabited world.
Sagan adopts the principle "it is better to be
smart
than to be stupid," but life on Earth refutes this claim. Among all the
forms of life, neither the prokaryotes nor protists, fungi or plants
has
evolved smartness, as it should have if it were "better." In the 28
plus phyla of animals, intelligence evolved in only one (chordates) and
doubtfully also in the cephalopods. And in the thousands of
subdivisions of the
chordates, high intelligence developed in only one, the primates, and
even
there only in one small subdivision. So much for the putative
inevitability of
the development of high intelligence because "it is better to be
smart."
Sagan applies physicalist thinking to this
problem. He
constructs two linear curves, both based on strictly deterministic
thinking.
Such thinking is often quite legitimate for physical phenomena, but is
quite
inappropriate for evolutionary events or social processes such as the
origin of
civilizations. The argument that extraterrestrials, if belonging to a
long-lived civilization, will be forced by selection to develop an
electronic
know-how to meet the peril of asteroid impacts is totally unrealistic.
How
would the survivors of earlier impacts be selected to develop the
electronic
know-how? Also, the case of Earth shows how impossible the origin of
any
civilization is unless high intelligence develops first. Earth
furthermore
shows that civilizations inevitably are short-lived.
It is only a matter of common sense that the
existence of
extraterrestrial intelligence cannot be established by a priori
arguments. But
this does not justify SETI projects, since it can be shown that the
success of
an observational program is so totally improbable that it can, for all
practical purposes, be considered zero.
All in all, I do not have the impression that
Sagan's
rebuttal has weakened in any way the force of my arguments.
Is
Earth-Life Relevant? A Rebuttal
By Carl Sagan
The gist of Professor Mayr's argument is
essentially to run
through the various factors in the Drake equation (see Shklovskii and
Sagan,
1966) and attach qualitative values to each. He and I agree that the
probabilities concerning the abundance of planets and the origins of
life are
likely to be high. (I stress again that the latest results [Doyle,
1995]
suggest one or even two Earth-like planets with abundant surface liquid
water
in each planetary system. The conclusion is of course highly tentative,
but it
encourages optimism.) Where Mayr and I disagree is in the later factors
in the
Drake equation, especially those concerning the likelihood of the
evolution of
intelligence and technical civilizations.
Mayr argues that prokaryotes and protista have not
"evolved smartness." Despite the great respect in which I hold
Professor Mayr, I must demur: Prokaryotes and protista are our
ancestors. They
have evolved smartness, along with most of the rest of the gorgeous
diversity
of life on Earth.
On the one hand, when he notes the small fraction
of species
that have technological intelligence, Mayr argues for the relevance of
life on
Earth to the problem of extraterrestrial intelligence. But on the other
hand,
he neglects the example of life on Earth when he ignores the fact that
intelligence has arisen here when our planet has another five billion
years
more evolution ahead of it. If it were legitimate to extrapolate from
the one
example of planetary life we have before us, it would follow that
1. There are
enormous numbers of Earth-like planets, each stocked with enormous
numbers of
species, and
2. In much
less
than the stellar evolutionary lifetime of each planetary system, at
least one
of those species will develop high intelligence and technology.
Alternatively, we could argue that it is improper
to
extrapolate from a single example. But then Mayr's one-in-50 billion
argument
collapses. It seems to me he cannot have it both ways.
On the evolution of technology, I note that
chimpanzees and
bonobos have culture and technology. They not only use tools but also
purposely
manufacture them for future use (see Sagan and Druyan, 1992). In fact,
the
bonobo Kanzi has discovered how to manufacture stone tools.
It is true, as Mayr notes, that of the major human
civilizations, only one has developed radio technology. But this says
almost
nothing about the probability of a human civilization developing such
technology. That civilization with radio telescopes has also been at
the
forefront of weapons technology. If, for example, western European
civilization
had not utterly destroyed Aztec civilization, would the Aztecs
eventually--in
centuries or millennia--have developed radio telescopes? They already
had a
superior astronomical calendar to that of the conquistadores. Slightly
more
capable species and civilizations may be able to eliminate the
competition. But
this does not mean that the competition would not eventually have
developed
comparable capabilities if they had been left alone.
Mayr asserts that plants do not receive
"electronic" signals. By this I assume he means
"electromagnetic" signals. But plants do. Their fundamental existence
depends on receiving electromagnetic radiation from the Sun.
Photosynthesis and
phototropism can be found not only in the simplest plants but also in
protista.
All stars emit visible light, and Sun-like stars
emit most
of their electromagnetic radiation in the visible part of the spectrum.
Sensing
light is a much more effective way of understanding the environment at
some
distance; certainly much more powerful than olfactory cues. It's hard
to
imagine a competent technical civilization that does not devote major
attention
to its primary means of probing the outside world. Even if they were
mainly to
use visible, ultraviolet or infrared light, the physics is exactly the
same for
radio waves; the difference is merely a matter of wavelength.
I do not insist that the above arguments are
compelling, but
neither are the contrary ones. We have not witnessed the evolution of
biospheres on a wide range of planets. We have not observed many cases
of what
is possible and what is not. Until we have had such an experience--or
detected
extraterrestrial intelligence--we will of course be enveloped in
uncertainty.
The notion that we can, by a priori arguments,
exclude the
possibility of intelligent life on the possible planets of the 400
billion
stars in the Milky Way has to my ears an odd ring. It reminds me of the
long
series of human conceits that held us to be at the center of the
universe, or
different not just in degree but in kind from the rest of life on
Earth, or
even contended that the universe was made for our benefit (Sagan,
1994).
Beginning with Copernicus, every one of these conceits has been shown
to be
without merit.
In the case of extraterrestrial intelligence, let
us admit
our ignorance, put aside a priori arguments, and use the technology we
are
fortunate enough to have developed to try and actually find out the
answer.
That is, I think, what Charles Darwin--who was converted from orthodox
religion
to evolutionary biology by the weight of observational evidence--would
have
advocated.
References
L.R. Doyle, ed., Circumstellar Habitable Zones:
Proceedings
of the First International Conference, Travis House Publications,
Carl Sagan, "The Origin of Life in a Cosmic
Context," Origins of Life, vol. 5, 1974, pp. 497-505.
Carl Sagan and Ann Druyan, Shadows of Forgotten
Ancestors: A
Search for Who We Are, Random House,
Carl Sagan et al., "A Search for Life on Earth
from the
Galileo Spacecraft," Nature, vol. 365, 1993, pp. 715-721.
Carl Sagan, Pale Blue Dot: A Vision of the Human
Future in
Space, Random House,
I.S. Shklovskii and Carl Sagan, Intelligent Life
in the
Universe, Holden-Day,
G.G. Simpson, "The Non-prevalence of Humanoids,"
Science, vol. 143, 1964, pp. 769-775.
Carl Sagan died in 1997.
He was the David Duncan Professor of Astronomy and Space
Sciences and
Director , Laboratory for Planetary Studies , at