powerful, advanced, hopefully benign species, or a “galactic club” of
advanced societies that long ago developed a protocol for protecting
primitive societies from the shock and dislocation of premature contact.
Perhaps our part of the galaxy is being protected as a “wildlife pre-
serve.” Or perhaps they are monitoring us—waiting to see if we will
develop in a way that contributes to, rather than threatens, galactic soci-
ety. If that’s the case, they will decide when to make contact with us and
* Star Trek’s Prime Directive again.
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tell us the rules of galactic intercourse. This scenario has been well
explored in fiction. At the conclusion of the 1951 film The Day the Earth
Stood Still, Klaatu, the alien, warned: “It is no concern of ours how you run your planet. But if you threaten to extend your violence, this Earth of
yours will be reduced to a burned-out cinder. Your choice is simple: Join
us and live in peace or pursue your present course and face obliteration.
We shall be waiting for your answer. The decision rests with you.” If you
were in charge, would you give modern humans the keys to the galaxy?
When we consider such far-out notions as the zoo hypothesis (we are
being protected) or directed panspermia (we were seeded by advanced
aliens), we must keep this in mind: even before the Earth was born,
there likely were technological civilizations living for billions of years, possessing capabilities that we cannot even begin to contemplate. Our
Sun is not among the oldest stars. Even given the conservative assump-
tion that it always takes 4 billion years for a planet to progress to intel-
ligence (that is, if we assume that all living planets evolve as slowly as
ours has), the first civilizations should have begun appearing in our
galaxy at least 4 or 5 billion years ago. More likely, there is a spread in
the rates of evolution, and on some planets intelligence developed much
faster than on ours, so some civilizations should have started appearing
8 or 9 billion years ago, long before our Sun and planets were a gleam
in the eye of their parent molecular cloud.* We had just better hope
that they have our interests in mind.
Even the zoo hypothesis is human-centered, in that it assumes that
we are interesting or important enough to merit a cage in their zoo. A
less anthropocentric idea might be the “seedling hypothesis.” They may
be ignoring our planet because young seedling noospheres like ours are
a dime a dozen, carpeting the floor of the galactic forest where a few
magnificent trees and flowering vines are much more captivating.
T H E G R E A T S I L E N C E
Fermi’s Paradox has also sometimes been called the great silence.
Although Fermi’s original “Where are they?” was meant to refer to
*The galaxy is at least 10 billion years old. A few generations of star birth and death were needed to build up enough interesting heavy elements to make life likely. However, the massive stars that do most of this atom-making burn bright and die young, so after a billion years many generations of them had already added their ashes to the stellar compost heap.
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alien visitation, it can also be applied to the lack of signals seen, so far,
on the cosmic airwaves. It is hard to imagine a purely physical explana-
tion for this great silence. You can’t reasonably argue that radio com-
munication between the stars is impossible, since we routinely observe
radio phenomena at huge interstellar, and even intergalactic, distances.
The Arecibo radio dish in Puerto Rico could exchange messages with
an equivalent dish clear across the galaxy.
As SETI advocates are quick to point out, the silence is not all that
great. It may be that we simply haven’t yet searched the right star at the
right frequency. This could all change tomorrow. If not, we can enlist
many of the sociological explanations that work for “the great
absence” to explain “the great silence.” Let’s take the zoo hypothesis.
Maybe advanced aliens are careful not to reveal their presence with
radio signals because they know that primitive societies like ours are
likely to conduct radio searches.
I suppose this is as good a time as any to mention a chilling, but
impossible to refute, scenario of cosmic paranoia: What if there is no
one out there because they have all been destroyed by something pow-
erful and ruthless? A technically advanced civilization, obsessed with its
own safety, might wish to suppress all possible challengers.* Maybe the
universe is silent because something is out there that seeks out radio
broadcasts and destroys their sources. So, in sending out our first radio
messages, we may be like the first monkey who climbs down from the
tree, stands upright, and, overcome with pride, shouts out, “Hey, look
what I can do!”—awakening the hungry lions who are napping all
around. Only species that shut their yaps are the ones who survive.
This bleak possibility is, unfortunately, a perfectly logical answer to the
Fermi Paradox. Even if such predators do not exist, the fear that they
might could be a reason why other civilizations are not shouting their
names all over town.
If we regard this as a serious possibility, should we be more careful
about drawing attention to ourselves? Should we be banning signals
in the direction of nearby stars? Nah. Let’s not be wimps hiding in
the nursery. You can be too careful. I say let’s give a shout-out and see
*A fleet of Von Neumann machines could lie in wait throughout the galaxy, listening for the first radio signals of fledgling technological races. The deadly probes sweep down on these worlds and obliterate all life. Such killer probes are often called berserkers after a SF
novel by Fred Saberhagen that introduced the concept.
Fermi’s Paradox
323
who’s there. Onward to the stars. Maybe I’m just a sap, but I doubt
that alien civilizations would want to harm us. It’s much more likely
that they won’t be interested in us at all while we’re still in the “just lis-
tening” phase. Maybe the moment a civilization achieves the ability to
start broadcasting for real is when they become worth talking to. Like
a chat room that discourages “lurkers,” maybe galactic communica-
tion is done in such a way as to prevent the little seedlings from listen-
ing in.
The silence may say more about our own limitations in conceptualiz-
ing intelligence than the ability of the universe to produce it. On the
days that I don’t believe radio SETI will succeed (usually Sundays and
alternate Wednesdays), it is not because I doubt that lots of communi-
cating creatures are out there. I’m just not sure they’re using radio.
Radio aliens are appealing because they are well-behaved. We know
just what we are looking for. Although it makes just as much sense to
search vigilantly for strange, “unnatural” objects in our solar system,
this idea is problematic. It opens a floodgate that spills out in the super-
market checkout line: pretty soon we are mired in the National
Enquirer al
ongside the “face” on Mars, secret alien bases on the Moon,
and “Man gives birth to a pickup truck with the face of Elvis!” What
do we do when our scientific reasoning leads us perilously close to
beliefs that are widely associated with the dreaded pseudoscience?
One alternative to radio that is now being taken seriously is the idea
of looking for flashing lights in the sky. Like a ship at sea beaming mes-
sages in Morse code, someone out there might try to get our attention
with brief and powerful laser flashes. This idea was proposed, and
largely ignored, in the early 1960s, when radio SETI was just getting
going. Now, with the rapid development of laser technology on Earth,
it’s easier to conceive of using powerful lasers to flash signals between
the stars. Several “optical SETI” search programs have recently come
on-line. Most of these are in the United States, but one in Australia cov-
ers the southern hemisphere. Frank Drake, the father of radio SETI, is
now also doing optical SETI from the Lick Observatory, near San Jose
(although he still feels that radio has a better chance of success). Most
optical SETI programs are targeted searches, sifting one by one through
thousands of nearby stars, but an “all sky” optical SETI program is just
getting under way at Harvard. Nobody knows whether this is more or
less likely to succeed than a radio search, but rather than second-guess
the aliens, we search in any way we can think of.
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M E S S A G E I N A V I R U S ?
One problem with radio or light rays is that they just flash through a
system. The message does not stick around, so someone there has to be
paying attention. If Earth had been bombarded with a continuous radio
broadcast from a nearby star for 100 million years, ending in 1903, we
would have completely missed it.
What about a message that could be left on a planet to stick around
until someone evolved with the ability to recognize and decode it? It
has even been proposed that the genomes of living organisms might
have been designed with hidden messages, waiting for us to develop the
requisite biotechnology to decode them. There is something poetic in
this idea—the essential machinery of life containing a message from
one life-form to another.
An early research experience introduced me to this possibility. In the
summer of 1978, I was an eighteen-year-old college freshman, working
in Sagan’s lab at Cornell.* At the time Carl was the editor of ICARUS:
International Journal of Solar System Studies. As you can imagine,
ICARUS, a journal run by a flamboyant, telegenic editor with a reputa-
tion for publishing speculative papers about exobiology and SETI,
received some flaky submissions that did not merit serious attention.
Carl asked me to check out a paper entitled “Is Bacteriophage φX174
DNA a Message from an Extraterrestrial Intelligence?” to see if it was
legit. The paper, by Japanese biochemists Hiromitsu Yokoo and Tairo
Oshima, suggested that a message from extraterrestrials might be hid-
den in the DNA of a virus that infects the common intestinal bacteria
E. coli. As outlandish as this sounds, the genetic code contained some
bizarre and unexplained patterns that seemed as if they could conceiv-
ably represent a message of some kind.
A little background will help here. SETI theorists have devoted a lot
of thought to certain mathematical patterns that would indicate an
intelligent message. A favorite, along these lines, is to use prime num-
bers.† Prime numbers are those that cannot be made by multiplying
together any other whole numbers: 1, 3, 5, 7, 11, 13 and on up, as high
*The same summer I worked with Reid Thompson playing Microcosmic Gods (chapter 7).
†In 1941 Sir James Jeans proposed that we flash the prime numbers toward Mars with powerful searchlights—one of a series of ideas that have been advanced over the years to show the Martians that we are good at math.
Fermi’s Paradox
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as you want to go. No known formula or natural process generates
them. If you see primes, you know that mind is not far behind.
A widely accepted idea (on Earth at least) for interstellar message
construction is to send digital pulses that repeat with a number that is
the product of two primes multiplied together. This would suggest that
a two-dimensional picture is being sent. For example, if you received a
message that was repeating a sequence of 143 pulses, you, or your
machine, would say, “Oh, 143 is 11 times 13, and these are both prime
numbers. Let’s make an 11-by-13 array and see if there’s a picture
encoded here.” This technique—using primes to create easily decoded
2-D images—is a pillar of SETI theory.
Now back to the message in that virus, bacteriophage φX174. It’s the
first organism for which the entire genome was decoded. A remarkable
Image unavailable for
electronic edition
Image unavailable for
electronic edition
Fermi’s Paradox
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feature discovered was the presence of “overlapping genes.” These are
sequences of nucleotides that can be read in two different frames, to
encode for two completely different proteins.* In other words, the
DNA sequence CAATGGAACAACTCA, can be read as the three-letter
“words” CAA TGG AAC AAC TCA, and this will instruct a cell to
start building a protein by putting together the five amino acids speci-
fied by these triplets. However, starting at a different letter, the same
sequence can also be read as ATG GAA CAA CTC, and so on, which
will build a completely different protein. It is as if you could write an
English sentence criticizing a bickering couple, “CAN YOU TWO
NAG,” that also contains a message about the ninth inning of a base-
ball game, “ANY OUT WON,” except that to make proteins you
would have to continue in this overlapping mode for hundreds of
words, with both sentences making complete sense.
But wait, there’s more. This organism (φX174) contained not one,
but three pairs of these overlapping genes. And, if you count the num-
ber of letters in these overlapping sequences, you find that they are 121,
91, and 533. Each of these is the product of two prime numbers (11 ×
11, 7 × 13, and 13 × 41). How weird is that? Here was the widely
accepted signature of an intelligent message, turning up in the strangest
of places.
Yokoo and Oshima had done what anyone suspecting a message
would do: they tried to make two-dimensional pictures from these
sequences. Unfortunately, the pictures looked like random noise. I went
back to the original literature on the decoding of the virus to see
whether the claims about overlapping genes and prime numbers were
accurate. They were. Then, under Carl’s instructions, I set about trying
to decode the message. I made a series of 2-D pictures from the gene
sequence, trying all kinds of tricks, with the hope that a smiling alien
face, or the Pythagorean theorem, would pop out at me. With no alien
-
virus decoding manual, I just tried every system I could think of, but I
never did find a message. Being a pack rat, albeit a disorganized one, I
still have the “messages” I decoded that summer. Here’s what they look
like:
*Recall that the genetic code consists of a string of nucleotides, similar but nonidentical units, which we represent with the letters A, T, G, and C, and that every three-letter
“word” codes for one amino acid in a protein that the organism uses to build itself or run its chemical machinery (chapter 7).
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Fermi’s Paradox
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My pictures, like those of Yokoo and Oshima, always came out look-
ing like random noise. However, their paper makes two important
points: (1) that a biological message might have some inherent advan-
tages over a radio message for interstellar communication, and (2) that
structures in this particular genome seem anomalous and could, just
conceivably, be a message we had not yet successfully decoded.
Carl accepted the paper and it was published in ICARUS in 1979.
Since then, the genomes of many organisms have been decoded, includ-
ing, famously, our own.* Overlapping sequences are found to be com-
mon in the genomes of many species. Some of the overlaps seem to be
related to prime numbers, but many of them don’t.
As physicist George Marx wrote, in defense of the biological inter-
stellar message concept, “How does one send a letter to a faraway
planet, a letter that is light enough for easy transportation, that multi-
plies itself on arrival, that can correct misprints automatically, and that
will be read definitely by the intelligent race of the target planet after
they have reached scientific maturity?”
Biologically encoded messages actually seem somewhat less far-
fetched now than they did in the 1970s. We’re just beginning to figure
out how to read and write in the language of DNA. It is not at all diffi-
cult to imagine that less than five hundred years from now, let alone
millions of years, we may be able to do intelligent design of just about
any kind of organisms we want.
But the idea of a biological message also has large drawbacks. Most
importantly, you would have to know an awful lot about the biology of
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