mumbo jumbo and devil worship, and less so for promoting the new
astronomy. If they wanted to, New Age mystics or satanists could claim
him as a martyr with at least as much veracity as scientists do today.
Bruno was one of the first to advocate that each star is a sun with its
own retinue of orbiting planets inhabited by intelligent creatures. Yet,
Spirits from the Vasty Deep
17
he based this conviction on metaphysical principles and mystical
visions rather than observation or physical theory. Bruno couldn’t have
cared less about evidence, measurement, or the intricacies of planetary
motions. His adaptation of Copernicanism was a convenient co-opting
of a recently published theory to support his belief in an infinite num-
ber of inhabited worlds—a belief derived from a spiritualistic faith in
the unity of the cosmos. If anything, Bruno did harm to the progress of
science (and certainly to poor Galileo) by encouraging the Church
authorities to associate Copernicanism with flagrant anti-Christian agi-
tation. Surely some of the wrath that the Church vented on Galileo was
really meant for Bruno, who refused to recant, reaffirming his beliefs
and taunting his persecutors with his dying breaths as flames engulfed
his body and freed his soul to travel among his infinite worlds.
Personality and timing aside, Galileo’s biggest problem was simply
that he had found the goods. The stark reality of his evidence suddenly
made Copernican beliefs much more threatening. Before Galileo’s tele-
scope opened a window to a new reality, cosmological questions were
all hypothetical. Discussions of other worlds seemed as abstract and
immune from verification as arguments about how many angels could
dance on the head of a pin. Now, there were actual planets that you
could see in the sky, and their existence implied that the Earth itself is in
motion, contrary to the received truth found in Scripture. Telescopes
and planets are mentioned in the Bible no more than particle accelera-
tors and quarks, but with a little digging and creative interpretation,
those who shrank with horror from the new, less human-centered uni-
verse could find scriptural objections to back up their fears.
The difficulty of the transition from biblically received knowledge
to observational cosmology is well represented in a scene in Bertolt
Brecht’s play Galileo. A group of learned astronomers have called upon
Galileo to express their concern over his claims of finding new worlds.
He invites his skeptical visitors to simply have a look through the tele-
scope and see the new worlds for themselves. He is confident that, once
they have seen with their own eyes, they will drop all objections.
Fearing trickery or sorcery, they refuse to look.
Forced to take a stand by Galileo’s observational successes and rhetor-
ical excesses, the Church decided to put the kibosh on Copernicanism,
but it was too late. Word was out. Telescopes are easy to manufacture.
Soon observers all over Europe were marveling at the moons of Jupiter
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and mapping the mountains of the Moon. Cusa and Copernicus had laid
the dry timber and Galileo had provided the spark. A wildfire of ram-
pant Copernicanism ripped through seventeenth-century Europe, and
though the Church leaders spread fear to douse the flame, they could not
stamp it out.
Plurality of Worlds
2
Every great scientific truth goes through three states:
first, people say it conflicts with the Bible;
next they say it has been discovered before;
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lastly they say they always believed it.
electronic edition
—LOUIS AGASSIZ
Where the telescope ends, the microscope begins.
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Which of the two has the grander view?
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—VICTOR HUGO, Les Misérables
A F T E R T H E R E V O L U T I O N
The Copernican revolution opened the floodgates, and the modern debate
over life on other planets began in earnest. The original Copernican revo-
lutionaries had approached ET life cautiously, but their followers went
wild. Advocacy of the Copernican solar system became identified with a
universe filled with planets and intelligent life, a “plurality of inhabited
worlds.” Throughout the seventeenth, eighteenth, and nineteenth cen-
turies, the phrase plurality of worlds was used to describe the idea of a densely inhabited cosmos, and “pluralists” believed in it.
Many of Galileo’s followers and defenders confused the issues of
Copernicanism and plurality. In the minds of the most ardent propo-
nents of each, life on other worlds validated the Copernican system and
vice versa. They would stand or fall together. The logic must have been
irresistible: if Earth is just another planet, and other planets are like
Earth (as far as could be told with seventeenth-century telescopes), then
why shouldn’t the planets be Earth-like in every way, including inhabi-
tants? This is called the argument by analogy, and it has reappeared
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in various forms right up to the present as a rationale for belief in
extraterrestrials.
When we knew next to nothing about the other planets, the scant
observations that did exist were interpreted as implying the existence of
extraterrestrial life. The facility with which seventeenth-century natural
philosophers put such a hopeful spin on their interpretations should
serve as a cautionary tale to modern scientists itching to find evidence
of alien life.
Kepler’s belief in an advanced civilization on the Moon was based, at
least in part, on careful observations. Recognizing the importance of
air and water for life, he found evidence for both on our Moon. He
thought that the dark areas were water and the bright areas dry land. In
support of this he noted that the bright areas have rugged surfaces.
Kepler decided that the Moon was not only habitable but densely
inhabited. Because the lunar “spots” (he did not call them craters) are
perfectly circular, he judged they must be cities: “When things are in
order, if the cause of the orderliness cannot be deduced from the motion
of the elements or from the composition of matter, it is quite probably a
cause possessing a mind.”
Kepler couldn’t imagine a natural process that created such perfectly
circular forms, so he concluded that rational creatures were responsi-
ble. Nearly four hundred years later, Arthur C. Clarke wrote 2001: A
Space Odyssey, in which astronauts on the Moon dig up a monolith
underneath the crater Tycho (named after Kepler’s despotic boss) and
come to the same conclusion: this doesn’t look natural, so someone
must have built it and put it there.
Now we know that the lunar craters are not cities. We’ve been to the
Moon. We understand how the energetic explosions caused by high-
velocity cosmic collisions produce beautifully circular impact crater
s on
all planets with solid surfaces.
If we find ordered structures without a known “natural” cause, is this
an indication of extraterrestrial intelligence? Kepler thought so, and
modern theorists of SETI (the Search for ExtraTerrestrial Intelligence)
agree.* Yet, as Kepler’s inference of lunar cities illustrates, failing to
deduce the “cause of the orderliness” may be due to the ignorance of
those attempting the deduction. Looking for surprising order in nature
*Proponents of “intelligent design” creationism make a similar argument, replacing ET
with God.
Plurality of Worlds
21
is not a bad approach for seeking extraterrestrial intelligence. But
Kepler’s mistake should remind us to beware lest the limitations of our
own intelligence cause us to find erroneous evidence of intelligence
elsewhere.
At the time of the Copernican revolution, the discovery of the “new
world” by European explorers was recent history. This strengthened
pluralist arguments by analogy. Vast realms of our own planet, previ-
ously unknown to Europeans, were found to be densely inhabited by
diverse creatures both familiar and exotic and, most importantly, by
other humans.* In the wake of these findings, widespread life on the
other “new worlds” of the post-Copernican solar system seemed just as
likely.
In addition to showing the planets to be worlds, the telescope also
revealed a seeming infinity of unknown stars, which many observers
believed to be suns. Teleological reasoning—the logic that things must
be created for a purpose—was rampant in the seventeenth century. It
was widely believed that the Earth, Sun, and Moon were created for
our habitation, comfort, and pleasure. All of those innumerable suns
must have been created for someone.
C U R I O S I T Y A N D P O O R E Y E S I G H T : F O N T E N E L L E
S P R E A D S T H E W O R D
In 1686, Bernard le Bovier de Fontenelle, a poet, novelist, and natural
philosopher who would later become secretary of the French Academy of
Sciences, wrote Entretiens sur la Pluralité des Mondes or Conversations on the Plurality of Worlds. This book was an instant best-seller and international sensation. Excitement about pluralism was building, and
Fontenelle both rode and helped to propagate that wave. Writing in a
playful, whimsical style, he produced what has been described as the first
popular-science book. To this day it is still a good read, and not just for a
peek into the mind of a seventeenth-century pluralist and popularizer. It is
a work of sweeping imagination written in provocative, witty prose with
a bit of an edge. Fontenelle predicts spaceflight, discusses the habitability
of other planets in our solar system and beyond, and offers vivid descrip-
*Whether the disastrous consequences of this “discovery” for those human inhabitants augurs potential dangers of contact with extraterrestrials is a question I’ll return to in later chapters.
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tions, centuries before the Apollo project, of the Earth as seen from space.
He even urges his readers to consider what the inhabitants of Jupiter
might think about Earth: “Even if they saw our Earth on Jupiter and
knew about it there, still they wouldn’t have the faintest suspicion that it
could be inhabited. If anyone were to think of it, heaven knows how all
Jupiter would laugh at him. It’s possible we’re the cause of philosophers
being prosecuted there who have tried to insist that we exist.”
Conversations consists of a sequence of five dialogs, on five consecu-
tive moonlit evenings, between a learned philosopher and an unedu-
cated but sharp-minded marquise. The philosopher is convinced that
the heavens are full of inhabited worlds. The marquise initially doubts
this pluralist vision, and as she is gradually won over, Fontenelle
skillfully defuses the doubts in the minds of his readers. By using a
female character, Fontenelle implicitly advocates the notion that women
can handle physics and philosophy—a radical notion in seventeenth-
century France—while infusing Conversations with a delightful flirta-
tious quality.
Placing his wild conjectures in the voices of his characters, Fontenelle
leaves the reader uncertain if the author himself actually believes what his
philosopher argues. This frees him to offer dangerous speculations, as in
his hints that the planets might be where they are just by chance instead
of a creator’s design. Aware of the Church’s continuing objections to
pluralist ideas, Fontenelle used a standard disclaimer, asserting that the
aliens of his speculations were assuredly not men, did not descend from
Adam, and were therefore irrelevant to biblical concerns. Nevertheless,
Conversations was placed on the Catholic index of banned books one
year after publication. But it was a hit with the public.
Passionate about his beliefs but never dogmatic, Fontenelle pokes fun
at everything, including himself and his profession: “All philosophy is
based on two things only: curiosity and poor eyesight; if you had better
eyesight, you could see perfectly well whether or not these stars are
solar systems, and if you were less curious, you wouldn’t care about
knowing, which amounts to the same thing. The trouble is, we want to
know more than we can see.”
It is remarkable how many modern scientific arguments about alien
life were anticipated by Fontenelle. Sure, we have a lot more data now
from astronomy, biology, paleontology, and geology. Yet, at its core,
scientific belief in aliens rests on most of the same leaps of faith now as
in the seventeenth century.
Plurality of Worlds
23
After convincing the marquise of the correctness of the Copernican
system, the philosopher proposes that there is intelligent life on the
Moon. He likens our view of the lunar surface to a Parisian in the tow-
ers of Notre Dame gazing at Saint-Denis in the distance: “Everything
one can see of Saint-Denis strongly resembles Paris; Saint-Denis has
steeples, houses, walls, and it might resemble Paris in that it’s inhabited
as well. All this will make no impression on my townsman; he will
obstinately maintain forever that Saint-Denis is uninhabited because he
has seen nobody there. Our Saint-Denis is the Moon, and each of us is
a Parisian who has never gone outside his city.”
“We’ll never know the people on the Moon,” the marquise declares,
“and that’s heartbreaking.” The philosopher, unafraid to speculate
about spaceflight, responds, “The art of flying has only just been born;
it will be perfected, and someday we’ll go to the Moon. Do we presume
to have discovered all things, or to have taken them to the point where
we can add nothing? For goodness’ sake, let’s admit that there’ll still be
something left for future centuries to do.”
The witty marquise counters this by asking, if such spaceflight is pos-
sible, why haven’t the people on the Moon already come to Earth? The
philosopher responds, “The Eur
opeans weren’t in America until after
six thousand years. It took that much time for them to perfect naviga-
tion to the point where they could cross the ocean. Perhaps the people
on the Moon already know how to make little trips through the air;
right now they’re practicing. When they’re more experienced and skill-
ful we’ll see them, with God knows what surprise.”
This last exchange presages the modern scientific debate about
extraterrestrial visitation. The marquise’s question has been rediscov-
ered as Fermi’s Paradox: if spaceflight between inhabited worlds is pos-
sible, surely we should have been visited by now. One popular modern
answer to this is identical to the philosopher’s response: it takes a long
time to develop spaceflight and travel between worlds. They’re just not
here yet. Note that in Fontenelle’s day, six thousand years was consid-
ered a good estimate for the age of the universe. The numbers have
changed, but the arguments have not.*
By the third evening, having convinced the marquise that the Moon
is inhabited, the philosopher retreats to a careful agnosticism on the
question: “You should never give more than half your mind to beliefs
*I’ll come back to Fermi’s Paradox in a later chapter.
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of this sort, and keep the other half free so that the contrary can be
admitted if it’s necessary.” He then gives physical arguments, based on
telescopic observations, suggesting the Moon may be uninhabitable.
Noting that observers on the Moon could easily deduce the existence of
water on the Earth by watching the motion of clouds, he points out
that, by contrast, the features of the moon are fixed and motionless:
“By this reasoning, the Sun doesn’t raise any vapors or mists above the
Moon. So then she’s a body infinitely more solid and hard than our
Earth, whose most volatile elements separate easily from the rest and
rise up as soon as they’re stirred into motion by heat. The Moon must
be some mass of rock and marble where there’s no evaporation, and
furthermore, evaporation is so natural and so necessary where there are
waters, that there can’t be any waters if none is taking place. Who then
are the inhabitants of these rocks which can produce nothing, and of
this land which hasn’t any water?”
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