The article excerpted here is from Scientific American, July 2000,
pps. 38-43, entitled “Where Are They? Maybe we are alone in the galaxy
after all” by Ian Crawford, with commentary by Dr. Jonathan Burch.
Dr. Crawford is an astronomer in the Department of Physics and Astronomy
at University College London.
How common are other civilizations in the universe? This
question has fascinated humanity for centuries, and although we still have
no definitive answer, a number of recent developments have brought it once
again to the fore...the speed with which life became established on this
planet. The oldest direct evidence we have for life on Earth consists
of fossilized bacteria in 3.5-billion-year-old rocks from Western Australia,
announced in 1993 by J. William Schopf of the University of California
at Los Angeles. These organisms were already quite advanced and must
themselves have had a long evolutionary history. Thus the actual
origin of life, assuming it to be indigenous to Earth, must have occurred
closer to four billion years ago.
Earth itself is only 4.6 billion years old, and the fact that
life appeared so quickly in geologic time - probably as soon as conditions
had stabilized sufficiently to make it possible - suggests that this step
was relatively easy for nature to achieve. Nobel prize-winning biochemist
Christian De Duve has gone so far as to conclude, “ ”Life is almost bound
to arise... wherever physical conditions are similar to those that prevailed
on our planet some four billion years ago.” So there is every reason
to believe that the galaxy is teeming with living things.
Does it follow that technological civilizations are abundant
as well? Many people have argued that once primitive life has evolved,
natural selection will inevitably cause it to advance toward intelligence
and technology. But is this necessarily so? That there might
be something wrong with this argument was famously articulated by nuclear
physicist Enrico Fermi in 1950. If extraterrestrials are common place,
he asked, where are they? Should their presence not be obvious?
This question has become known as the Fermi Paradox.
This problem really has two aspects: the failure of search for
extraterrestrial intelligence (SETI) programs to detect radio transmissions
from other civilizations, and the lack of evidence that extraterrestrials
have ever visited Earth... In spite of all this activity [SETI programs
since proposed in 1959 by physicists Giuseppe Cocconi and Philip Morrison
in Nature], however, researchers have made no positive detections of extraterrestrials.
Of course, we are still in the early days of SETI, and the lack
of success to date cannot be used to infer that ET civilizations do not
exist... Nevertheless, initial results are already putting some interesting
limits on the prevalence of radio transmitting civilizations in the galaxy.
The Fermi Paradox becomes evident when one examines some of the
assumptions underlying SETI, especially the total number of galactic civilizations,
both extant and extinct, that it implicitly assumes. Paul Horowitz
of Harvard University, has stated that he expects at least one radio-transmitting
civilization to reside within 1,000 light-years of the sun, a volume of
space that contains roughly a million solar-type stars. If so, something
like 1,000 civilizations should inhabit the galaxy as a whole.
This is rather a large number, and unless these civilizations
are very long-lived, it implies that a truly enormous number must have
risen and fallen over the course of galactic history. (If they are
indeed long-lived - if they manage to avoid natural and self-induced catastrophes
and to remain detectable with our instruments - that raises other problems,
as discussed below.) Statistically, the number of civilizations present
at any one time is equal to their rate of formation multiplied by their
mean lifetime. One can approximate the formation rate as the total
number that have ever appeared divided by the age of the galaxy, roughly
12 billion years. If civilizations form at a constant rate, and live
an average of 1,000 years each, a total of 12 billion or so technological
civilizations must have existed over the history of the galaxy for 1,000
to be extant today. Different assumptions for the rate and average
lifetime yield different estimates of the number of civilizations, but
all are very large numbers. This is what makes the Fermi Paradox
so poignant. Would none of these billions of civilizations, not even
a single one have left any evidence of their existence?
Extraterrestrial Migration
This problem was first discussed in detail by astronomer Michael
H. Hart and engineer David viewing in independent papers published in 1975.
It was later extended by various researchers. Most notably physicist Frank
J. Tipler and radio astronomer Ronald N. Bracewell. All have taken
as their starting point the lack of clear evidence for extraterrestrial
visits to Earth. Whatever one thinks about UFO’s we can be sure that
Earth has not been taken over by an extraterrestrial civilization, as this
would have put an end to our own evolution and we would not be here today.
There are only four conceivable ways of reconciling the absence
of ET’s with the widely held view that advanced civilizations are common.
[1] Perhaps interstellar spaceflight is infeasible, in which case ET’s
would never have come here even if they had wanted to. [2] Perhaps ET civilizations
are indeed actively exploring the galaxy but have not reached us yet. [3]
Perhaps interstellar travel is feasible, but ET’s choose not to undertake
it. [4] Or, perhaps ET’s have been, or still are, active in our vicinity
but have decided not to interfere with us. If we can eliminate each
of these explanations of the Fermi Paradox, we will have to face the possibility
that we are the most advanced life-forms in the galaxy.
The first explanation [1] clearly fails. No known principle
of physics or engineering rules out interstellar spaceflight. Even
in these early days of the space age, engineers have envisaged propulsion
strategies that might reach 10 to 20 percent of the speed of light, thereby
permitting travel to nearby stars in a matter of decades. (See “Reaching
for the Stars,” by Stephanie D. Leifer; SCIENTIFIC AMERICAN, February,
1999).
For the same reason, [2] the second explanation is problematic
as well. Any civilization with advanced rocket technology would be
able to colonize the entire galaxy on a cosmically short time scale.
For example consider a civilization that sends colonists to a few of the
planetary systems closest to it. After those colonies have established
themselves,, they send out secondary colonies of their own, and so on.
The number of colonies grows exponentially. A colonization wave front
will move outward with a speed determined by the speed of starships and
by the time required by each colony to establish itself. New settlements
will quickly fill the volume of space behind this wave front....
Assuming a typical colony spacing of 10 light years, a ship speed
of 10 percent of that of light, and a period of 400 years between the foundation
of a colony and its sending out colonies of its own, the colonization wave
front will expand at an average speed of 0.02 light-year per year.
AS the galaxy is 100,000 light years across, it takes no more than about
five million years to colonize it completely. Though a long time
in human terms, this is only 0.05 percent of the age of the galaxy.
Compared with other relevant astronomical and biological time scales, it
is essentially instantaneous. The greatest uncertainty is the time
required for a colony to establish itself and spawn new settlements.
A reasonable upper limit might be 5,000 years, the time it has taken human
civilization to develop from the earliest cities to spaceflight.
In that case, full galactic colonization would take about 50 million years.
The implication is clear: the first technological civilization
with the ability and the inclination to colonize the galaxy could
have done so before any competitors even had a chance to evolve.
In principle, this could have happened billions of years ago, when earth
was inhabited solely by microorganisms and was wide open to interference
from outside. Yet no physical artifact, no chemical traces, no obvious
biological influence indicates that it has ever been intruded upon.
Even if earth was deliberately seeded with life, as some scientists have
speculated, it has been left alone since then.
It follows that any attempt to resolve the Fermi paradox must
rely on assumptions about the behavior of other civilizations. For
example, they might destroy themselves first, they might have no interest
in colonizing the galaxy, or they might have strong ethical codes against
interfering with primitive life-forms. Many SETI researchers, as
well as others who are convinced that ET civilizations must be common,
tend to dismiss the implications of the Fermi paradox by an uncritical
appeal to one or more of these sociological considerations.
But they face a fundamental problem. These attempted explanations
are plausible only if the number of extraterrestrial civilizations is small.
If the galaxy has contained millions or billions of technological civilizations,
it seems very unlikely that they would all destroy themselves, be content
with a sedentary existence, or agree on the same set of ethical rules for
treatment of less developed forms of life. It would take only one
technological civilization to embark, for what ever reason, on a program
of galactic colonization. Indeed the only technological civilization
we actually know anything about - namely our own, has yet to self destruct,
shows every sign of being expansionist, and is not especially reticent
about interfering with other living things.
Despite the vastness of the endeavor, I think we can identify
a number of reasons why a program of interstellar colonization is actually
quite likely. For one, a species with a propensity to colonize would
enjoy evolutionary advantages on its home planet, and it is not difficult
to imagine this biological inheritance being carried over into a space-age
culture. Moreover, colonization might be undertaken for political,
religious or scientific reasons. The last seems especially probable
if we consider the first civilization to evolve would by definition be
alone in the galaxy. All its SETI searches would prove to be negative,
and it might initiate a program of systematic interstellar exploration
to find out why...
The apparent rarity of technological civilizations begs for an
explanation... Thus the chemical evolution of the galaxy is almost
certainly not able to fully account for the Fermi Paradox.
To my mind, the history of life on Earth suggests a more convincing
explanation. Living things have existed here almost from the beginning,
but multicellular animal life did not appear until about 700 million years
ago. For more than three billion years, Earth was inhabited solely
by single-celled microorganisms. This time lag seems to imply that
evolution of anything more complicated than a single cell is unlikely.
Thus, the transition to multicelled animals might occur on only a tiny
fraction of the millions of planets that are inhabited by single-celled
organisms.... But even if multicelled life-forms do eventually arise
on all life-bearing planets, it still does not follow that these will inevitably
lead to intelligent creatures, still less to technological civilizations.
As pointed out by Stephen Jay Gould in his book Wonderful Life, the evolution
of intelligent life depends on a host of essential random environmental
influences.... The evolution of intelligent life on Earth has rested
on a large number of chance events [such as the meteor collision with the
earth that made the dinosaurs extinct, after 140 million years of dominating
the Earth, and allowed mammals to fill in the vacant environmental niches,
and prosper and evolve into primates, humans and the global knowledge civilization
- so far as we know, the First Flower of the Universe], at least some of
which had a very low probability. In 1983 physicist Brandon Carter
concluded that “civilizations comparable with our own are likely to be
exceedingly rare, even if locations as favorable as our own are of common
occurrence in the galaxy.
[I call this debate the Great Car Race. In the planet car,
Carl Sagan is driving. The more possible worlds he can find in space,
the faster his car goes and the farther ahead he gets in the race.
In the evolution car, Stephen Jay Gould is driving. The more critical
and unlikely steps in evolution he can find, the faster his car goes and
the farther ahead he gets in the race. In this article Ian Crawford
sees the evolution car pulling ahead, both by the number of unlikely critical
events in the history of evolution on earth to a technological civilization,
and by reducing the likelihood of more planets producing more such civilizations,
especially since we see no evidence of any.]
Of course all these arguments, though in my view persuasive,
may turn out to be wide of the mark. In 1853 William Whewell, a prominent
protagonist in the extraterrestrial-life debate, observed, “ The discussions
in which we are engaged belong to the very boundary of regions of science,
to the frontier where knowledge ... ends and ignorance begins.” In
spite of all the advances since Whewell’s day, we are in basically the
same position today.
[This leaves us where we began. So far as we know, the global knowledge civilization on Earth is the First Flower of the Universe, and we evolved humans have the sole duty to nurture it and see to it that it grows, thrives and reproduces, so that in a few million years, the galaxy and thereafter the universe will be populated by the new complexity engines of the stage of knowledge evolution, self-sustaining knowledge civilizations, each producing a continuous explosion of new knowledge and existence at ever higher levels of organization, the Flowers of the Universe blossoming so that the universe is a garden blossoming with existence in full bloom.]