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of Oz, “a place very far away, difficult to reach, and populated by
exotic beings.” Are there whisperings from more aged creatures, wait-
ing for us to grow up and talk to them, coursing through the airwaves
above our planetary crib? Ozma detected no messages, but the age of
experimental SETI had begun.
Project Ozma immediately provoked a full spectrum of strong reac-
tions. In a 1960 article in Physics Today, Drake’s boss, astronomer Otto Struve, wrote that Ozma “has divided the astronomers into two camps:
those who are all for it and those who regard it as the worst evil of our
generation. There are those who pity us for the publicity we have
received and those who accuse us of having invented the project for the
sake of publicity.”
Struve served as chairman of the first American meeting on SETI, a
now legendary gathering held in Green Bank during Halloween week,
1961. The ten participants included most of the early prime movers in
American SETI. Morrison, Struve, Drake, and Sagan were all there, as
were the astronomer Su-Shu Huang (who had invented the concept of
“habitable zones” around stars in 1959), the engineer and SETI theo-
rist Barnard Oliver, the biochemist Melvin Calvin (who was awarded a
Nobel Prize during the Green Bank meeting for his work on photosyn-
thesis), and the dolphin researcher John Lilly. Lilly had just published
Man and Dolphin, a book arguing that dolphins have a complex lan-
guage, and that we might learn to communicate with them. That Earth
might have evolved not one, but two intelligent communicative species
seemed encouraging for the prospect of intelligence evolving elsewhere.
The presence of all of these distinguished scientists discussing the num-
ber of intelligent civilizations in the galaxy, and working through the
practical challenges of communicating with them, helped to infuse the
dawning era of SETI experimentation with a new aura of respectability.
As Sagan later described it, “There was such a heady sense in the air that finally we’ve penetrated the ridicule barrier. . . . It was like a 180-degree flip of this dark secret, this embarrassment. It suddenly became
respectable.”
The Green Bank participants were so full of camaraderie and hope,
and so enthralled by Lilly’s reports of success in talking to dolphins, that
they formed a club called the Order of the Dolphin, with Sagan as sec-
retary. They even made membership pins with little dolphins on them.
But, the Dolphins never had a second meeting. John Lilly started
writing publicly about his lengthy conversations with ethereal, extrater-
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restrial beings, which occurred while he was tripping his brains out on
large doses of psychedelic drugs (acid and ketamine being his favorites
for interstellar discourse). The ridicule barrier may have been pene-
trated, but it was not left far behind. The SETI pioneers decided that
the association with Lilly would not help their credibility, and the
Order of the Dolphin was unceremoniously dissolved.*
Unfortunately, there are no published proceedings (and no group
photos) from the Green Bank meeting. Perhaps no one appreciated the
historical significance of the gathering, or they were still too embar-
rassed to go on record as taking the subject matter seriously. However,
SETI had arrived as a science.
F I G U R I N G T H E O D D S
Frank Drake began the Green Bank meeting by writing an equation on
the blackboard that summarized the major questions encountered
when trying to estimate the number of intelligent civilizations in the
galaxy. He intended only to suggest an agenda for the meeting, but his
formula was destined to become “the Drake Equation,” the most
famous equation in SETI research. Four decades later, it is still used in
nearly every SETI paper, book, and discussion.
The Drake Equation is not really supposed to provide an answer.
You can’t just plug in all the variables and determine that there are pre-
cisely 3,741 civilizations in the Milky Way. What it gives us is a clear
way of thinking through the problem and all of the factors that are
important in determining the answer.
Before I actually tell you the Drake Equation, I’ll give you an anal-
ogy: the date equation. Say you are a single person going to a large
dance party, and you would like to come away with a date for the fol-
lowing weekend. Arriving in front of the house, you can hear the music
pumping and feel the bass rattling your gut. You are excited, but ner-
vous as hell, so you decide to calm yourself with some math. Before
going inside, you try to calculate your chances of getting lucky. You
*Lilly kept “Order of the Dolphin” listed under “professional associations” on his curriculum vitae until his death in 2001. Shortly after Green Bank Lilly set his dolphins free and focused on self-experimentation with drugs and isolation tanks. The protagonist in the movie Altered States is loosely based on Lilly.
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start by guessing the total number of people at the party. You notice
that people are arriving at a rate of three per minute. We’ll call this rate
of arrival R. People are leaving at roughly the same rate, but you realize
that you can estimate the number of people inside if you know how
long they are staying. Let’s call this length of stay L. The number of
people inside will be roughly R times L. So, if people on average are
staying for, say, one hundred minutes, there will be about three hun-
dred inside.
But they are not all potential dates. After all, you have standards and
preferences, and some may not be available. So you multiply the total
number at the party by several factors, each expressing the probability
that the average partygoer will meet one of your requirements. Each of
these probability factors will have a value between zero and one. Zero
means that nobody measures up to a particular requirement. One
means that anyone will do. If half of them are okay, the probability is
one-half, or 0.5, and so on.
For instance, you might want to rule out potential dates because they
don’t fit your sexual p reference. We will call this factor fp (pronounced
“f-sub-p”) and assume that this is roughly 0.5, meaning that it rules out
half of the people there.* Then you are going to multiply that by the
fraction that you find yourself at tracted to. If you are being picky, we’ll say that fat = 0.1. In other words, one in ten meets this criterion. Again,
it cannot be higher than 1, even if you are drunk or desperate. Now,
some people are not going to be av ailable because they are already
hooked up and not interested in multiple partners. Let’s say optimisti-
cally that a quarter of the people (or 25 percent) you are interested in
are free. So fav = 0.25.
You also have to factor in your own behavior. Some are just so hot,
you can’t get up the n erve to talk to or dance with them. But all this math is making you feel pretty confident, so we’ll say you can deal with
approaching three-q
uarters of them: fn = 0.75. Then we have to multiply
again by the fraction who turn out to actually be i nterested in you.
Because you are fascinating and fun to dance with, and because you can
talk knowingly and winningly of probability (chicks and cats dig that),
no one can refuse you, so fi = 1. Assuming you have not forgotten any
*Woody Allen has pointed out that if you are bisexual, it doubles your chances for a date on a Saturday night. Put quantitatively, for these lucky people fp = 1.
SETI: The Sounds of Silence
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important factors, you can now estimate your chances of scoring at the
party. The total number of likely candidates, N, will follow the formula
N = R × fp × fat × fav × fn × fi × L
This is the “date equation.” Given the numbers we’ve estimated, N =
3 × .5 × .1 × .25 × .75 × 1 × 100. So, N = 2.8: 2.8 people at the party
will go out with you next weekend. Jackpot! Although this is just a
rough estimate, since we had to estimate the various f factors, your best
guesses lead to an N greater than one, so you figure your chances for
success are pretty good. Thus emboldened, you check your hair one last
time and enter the party.
Now back to the Drake Equation. The equation that Frank Drake
wrote on the board to start off the Green Bank meeting looked like this:
N = R* × fp × ne × fl × fi × fc × L
The Drake Equation is parallel to the date equation, but the party we
wish to crash is much larger, more frightening, and more enticing: N is
the number of communicating civilizations in the galaxy. Each star in
the Milky Way is a potential dance partner for a lucky biosphere, but
some won’t have the right chemistry. We have to narrow them down.
On the right side of the equation are all of the factors that we must esti-
mate in order to size up N.
Here’s what they all stand for, going from left to right:
The first three factors are astronomical: R represents the rate, per
year, at which stars are being born in the galaxy, fp is the fraction of
stars with orbiting p lanets, and ne is the average number of planets, per star, with e nvironments where life can evolve.
The next two factors are biological: fl is the fraction of suitable plan-
ets where l ife actually does develop, and fi is the fraction of these where i ntelligent life evolves.
The final two terms in the equation represent cultural or social fac-
tors: fc is the fraction of planetary cultures that are c ommunicating over interstellar distances, and L is the average l ongevity, or l ifetime, of these civilizations, in years.
Multiplied together, all of these terms give us N, our estimate of the
number of communicating civilizations in the galaxy. Obviously, most
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of these numbers are highly uncertain, if not totally unknown. If this
were a serious attempt to nail down an answer, it would be laughed out
of science. The value of the equation is that it allows us to examine the
consequences of our assumptions and explore how changing them
changes the answer we get about finding intelligence in the universe. It
helps us to identify the most crucial small questions we must answer to
crack the big one.
Drake had the Green Bank group take their best shot at estimating
each factor. Much of the discussion and literature of SETI in the forty-
two years since then has attempted to refine, refute, or improve these
estimates. Notice that the terms in the Drake Equation become less cer-
tain, and more highly subjective, as we read from left to right. R*, the
rate of star formation, has been pinned down by astronomical observa-
tions. The Dolphins knew that between one and ten new stars are born
per year.* In contrast, the fraction of stars with orbiting planets was
completely unknown in 1961. The Dolphins estimated that half of all
suitable stars had planets (fp = 0.5).
They could not agree on a single estimate for ne, the average number
of life-friendly planets per star. Some thought that a conservative esti-
mate was ne = 1. Sagan, believing that several planets in our own solar
system probably harbored life, argued for an average of five. They
agreed to disagree, settling on a range of ne from 1 to 5.
Since 1961, we’ve narrowed down the uncertainties in the astronom-
ical factors ever so slightly. The biological and cultural factors remain
pretty much where they were when Kennedy was in the White House,
although different beliefs have come in and out of fashion. The Green
Bank group reasoned that since life apparently arose quickly here on
Earth, any planet that could have life must have life. So they estimated
that fl = 1. This view is still quite common in astrobiology. They also
estimated that fi = 1, arguing that biological evolution would always
lead to intelligence, because of the great survival advantage it conveys.
This has proven to be one of their most controversial assumptions.
Finally, there are the “cultural factors.” How many intelligent civi-
lizations will build radio telescopes and choose to use them for interstel-
lar communication? In this innocuous term (fc) are contained a huge
host of thorny questions about the nature of intelligence, the universal
*In our galaxy, that is. This is not counting the massive stars that burn out in a few million years.
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attraction of technology, and the motivations and desires of alien beings.
The Green Bank Dolphins decided that one in ten such civilizations
would have the required radio technology and the will to use it (fc = 0.1).
The final term in the equation, L, the longevity of technical civiliza-
tions, proved to be the hardest to estimate. The best we can do is to
ponder our own future, estimate our longevity, and pray for rain. Then
we extrapolate recklessly to the rest of the galaxy. The Green Bank
group recognized that this number is totally unconstrained.* We might
destroy ourselves after less than a hundred years of radio listening,
which began in 1960. Or we might learn how to get along with one
another, control our technology, live sustainably, and achieve security
against natural disasters. (Well, anything is possible.) If we do all that,
we might last for billions of years. So there is a factor of 107, or ten mil-
lion, separating the optimistic from the pessimistic estimates. At Green Bank, recognizing that we can’t really pin down L, they settled on a
range of 103 to 108 (one thousand to 100 million) years for the average
lifetime of a communicating civilization.
Multiplying all of the factors together, the Dolphins calculated a huge
range of 103 to 109 for N, the number of civilizations in our galaxy. Take
a look at the numbers in the following table and notice a couple of
things. First, even in the most pessimistic estimate conceivable to all of
these brilliant scientists, the galaxy should contain, at present, a thou-
sand advanced civilizations. This is a modern, quantitative reaffirmation
of the ancient principle of plenitude: in a galaxy this vast, with so many
stars—each a potential home—we’ve sti
ll got plenty of company, even if
the odds are extraordinarily unfavorable to life and intelligence. And
that’s the pessimistic scenario.
*Which is the technical term for not having a freaking clue.
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On the other end of the range, the number of civilizations may
approach 1 billion, or more than one in every thousand stars. If this is
the true state of our galaxy, then several of the few thousand stars you
can see with your own eyes on a clear night probably have planets that
are home to advanced cultures. In this crowded galaxy, the average dis-
tance between civilizations would be only a few tens of light-years, and
we might hold two-way interstellar radio conversations within the
short span of a human life.
Also, note that the calculated range for N, the number of civiliza-
tions, comes out identical to the range estimated for L, their average
lifetime in years. That’s because in the (possibly somewhat optimistic)
Green Bank estimates, all the numbers to the left of L, multiplied
together, give a value around 1. Accepting this greatly simplifies the
Drake Equation, which becomes
N ≈ L*
In other words, the number of civilizations in the galaxy is equal to the
average lifetime, in years, of a civilization. This may or may not be
close to the truth of the matter, but it does highlight the most solid con-
clusion that we can draw from the Drake Equation: the number of
intelligences in the galaxy hinges most crucially on one factor, L, the
longevity of communicating civilizations.
If intelligence is self-limiting, if brainy races are often too smart for
their own good and survive, on average, only for a century or a millen-
nium, then unless we are lucky (always an annoying possibility when
trying to use probability on ourselves), even our nearest communicative
neighbors are probably thousands of light-years away. Conversation
would be impractical, to say the least, since every answer would take
millennia, and most civilizations wouldn’t last that long. If, on the
other hand, the average high-tech race lasts for millions of years or
more, then the galaxy must be full of wise and ancient races. We con-
clude that the nature of our galaxy, whether it is a quiet desert with an
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