- Posted
09.28.04
- NOVA
We
Earthlings have seen no credible signs that there are beings elsewhere in the
universe that match us in smarts. Yet are there?
Paleontologist Peter Ward,
coauthor of Rare Earth: Why Complex Life is Uncommon in the Universe,
points to evidence that intelligent life must be exceedingly rare.
Other
scientists, however, think that advanced life is common—Carl Sagan estimated a
million intelligent civilizations in our galaxy alone. So NOVA asked
astrophysicist Neil deGrasse Tyson to play devil's advocate in a conversation
with Ward.
Neil
deGrasse Tyson (left) and Peter Ward square off in a friendly debate. EnlargePhoto credit: © Daniel Deitch
(Tyson) / © WGBH Educational Foundation (Ward)
TIME
IS OF THE ESSENCE
Neil
deGrasse Tyson (NdGT): Peter, when you combine the fossil record on Earth with all
that we know of biology and evolution, what can we expect as we look for
intelligence elsewhere in the cosmos?
Peter
Ward (PW): There
are maybe 30 million species on the planet today—10 to 30 million. If we look
at the fossils, there are hundreds of millions of species in the past. And one
time on Earth has intelligence arisen to the point where we can build a radio
telescope (which is the definition of intelligence to a radio astronomer). One
time out of hundreds of millions of possibilities. That's an astronomically
small number of intelligences that have arisen—just one.
NdGT: At
last tally, there are several hundred billion stars in the Milky Way galaxy and
perhaps 100 billion galaxies in the universe. Yet according to your book, we're
just cold, dark, and alone.
PW: No,
we're not alone; we're lonely. There's a big difference between being alone and
being lonesome.
NT: But
in this big galaxy of ours—hundreds of billions of stars—surely Earth is
repeated many places, many times.
PW: We got
to intelligent organisms on this planet after 500 million years of animal life.
Now, that doesn't say you couldn't get it sooner at other places, but you still
need long periods of time. And to me that is the major argument against there
being intelligent civilizations. We are on a planet that's very safe, a planet
that has not been repeatedly bombarded by the thing that killed the dinosaurs—asteroids.
How many other planets are going to have such long periods of time? Not many, I
think.
NdGT: But
suppose life is common in our galaxy. We are now asking, of all the planets
with life, isn't intelligence surely going to show up?
I still
think it's a question of time, Neil. I agree with you, I think life is
pervasive in the galaxy. I think that the way the Earth has extremophiles
[organisms that thrive in extreme hot or cold habitats], the fact that we can
have life living in so many bad environments, suggests that you can have life
in really nasty, nasty places.
But it
takes time to evolve into a T. rex or an Allosaurus, whatever;
it takes a long period of time. Now, our planet has had a very low record of
large asteroid impacts. Jupiter is the major reason for that. One of the great
studies of the 20th century showed that if we did not have a Jupiter, the
impact rate on Earth would be 10,000 times higher than it is.
Jupiter
sucks in wayward asteroids and comets like a massive vacuum cleaner. This has
left Earth free to enjoy the long period of relative calm and stability that
has encouraged the evolution of intelligence, Ward argues. EnlargePhoto credit: © NASA
LOCATION
IS EVERYTHING
NdGT: It's
not hard for me to imagine a solar system that forms when the vacuuming up of
the debris in the early phases happened more efficiently than here. For you to
base a whole idea about the likelihood of intelligence in the galaxy on the
fact that we had some extra bombardment seems kind of thin.
PW: No,
that's only a piece of the puzzle. But let's stay on that one. "Oort
Cloud" is the name of the big bunch of comets around us. There are many
studies suggesting that Oort clouds will be bigger and denser the closer you
get to the center of the galaxy. So it could be that we are in a section of the
galaxy real estate-wise that's great for life, and that as you head more
centrally, where there's more and more stars that form, it gets worse. You know
the nature of galaxies' centers. They are very dangerous places.
NdGT: They're
nasty places.
And it's
not just asteroids. It's gamma ray bursts, it's supernovae. You've got black
holes, you've got energetics, you've got sterilization events. So the centers
of galaxies aren't going to work. And perhaps half the stars in the galaxy are
in the center. So now we have to move off to the suburbs, where things start
getting spread out. If we go too far out, we run out of the heavy elements
necessary to make a habitable planet.
"I
think life is almost everywhere in the universe. But you're asking about
intelligent life, and that's a whole different ball game."—Peter Ward
PW: So we've
got a portion of the galaxy that is no good, and another portion that is no
good. We're looking at narrow real estate areas. How narrow that is is
debatable. But this debate we're having would not have taken place 10 years
ago. We have new information just coming in that seems to be narrowing the
areas where habitability can take place.
Ward
thinks that astronomers, who have been the primary experts debating the issue
of E.T. intelligence, have failed to appreciate the nature of their own planet.
EnlargePhoto credit: © NASA
ENTER
GEOLOGISTS
NdGT: Alright,
so you're an intelligence pessimist. But clearly people who professionally look
for intelligence in the galaxy—the SETI people—don't share that pessimism. What
gives them the confidence that you don't have?
PW: They're
not geologists. I think what astronomers have missed is the record of our own
planet. Geology tells us a couple of very important things. First of all, you
need a long period of temperature stability. That's a geological phenomenon. A
very finite set of conditions on this planet has allowed us to have liquid
water for four billion years. We never went above 100°C. We never went below
0°C for long periods of time. That, for four billion years, is a rare thing, I
think.
Secondly,
the impact rate is also something that causes species to go extinct. Sixty-five
million years ago, millions of dinosaurs were suddenly wiped out by an impact.
NdGT: Yeah,
but that same incident enabled mammals to become something more ambitious than
a tree shrew.
PW: Sure,
maybe that was an instigator towards intelligence.
NdGT: But
you just said it was bad for intelligence.
PW: Well,
what if such a cataclysmic event happens 100 times, 1,000 times, or 10,000
times, and you're looking at things being knocked out? It took 65 million years
after the knockout to get to the level of intelligence that we have.
NdGT: But
as a geologist, is it possible that you're a little close to the problem? You
see Earth in such detail as a planet that sustains intelligent life, that it's
inconceivable to you how many different kinds of planets could give you
intelligence. You're kind of stuck in a geological paradigm. Maybe it's as much
a handcuff as a liberator of ideas.
PW: But you
astronomers never even considered it.
NdGT: No,
we didn't.
PW: You've
been so far out in space. I never heard the term "plate tectonics"
ever used; it's not even in your lexicon. And this is where I think a whole new
view comes from biologists and geologists and paleontologists to add to the
great work you astronomers are doing. It's not simply a function of how many
planets are out there.
There's
an enormous amount of new information coming from planetary geologists. We know
so much more now about how planets form and how habitable planets work. We have
this entire new science called "Earth System Science" that examines
how the life systems of Earth continue, the carbon cycle, how we have water,
how we have temperature changes, the weather patterns. But all of this in sum
brings together Earth as a planet, which over long periods of time is able to
stay alive. It's like the life of an individual. It's born, it grows, and it
has these systems that allow it to maintain life.
ANIMALS
ARE RARE
NdGT: I
don't hear you exploring exotic models for what a solar system or planet can
be. For instance, maybe there are planets where the water is frozen on the
surface, but deep down there's a place where it's liquid. And wherever it
happens to be liquid life goes to that.
PW: That's a
great idea, and I agree with you entirely. I think there are so many places in
the universe that allow life that I think life is almost everywhere in the
universe. But you're asking about intelligent life, and that's a whole
different ball game. I think that even animals are rare. The lowest animals on
this planet, worms or snails or whatever, are far from intelligent. Even they
are rare, I think, because it takes a long time for a planet to allow them to
evolve.
Even a
creature as humble as a flatworm only arose after billions of years of
evolution, Ward notes. EnlargePhoto credit: © Corbis Images
NdGT: It
took a long time for Earth to do it. Now, when you say "a planet,"
you just generalize this single case that you know for all possible cases that
could exist in the galaxy. That's kind of bold.
PW: It is
bold, but I think time is still the constant. You need a certain amount of time
for evolution to take things from inorganic to organic, from organic to a cell
that combines with another cell, to eventually intelligence.
NdGT: You're
accusing Earth of being rare. But history is filled with cases of people
asserting that our place in the cosmos is rare. And the march of science has
continuously demonstrated the opposite of that. So here you are now making
cogent arguments, but I see you as one in a long series of people who have come
before you trying to claim that we are the product of something special. So
what makes you the only one in that history of scientists who's correct?
PW: Luckily,
it's not just me. I'm really cutting short a lot of my colleagues. This has not
been just my work, of course.
NdGT: When
I look at the variation in the kinds of planets there might be, it's not hard
for me to imagine that you could have a planet where all the conditions of that
planet promote intelligence, rather than here on Earth where it's kind of a
special case, or a peculiar branch that happened to eke it out of the variation
in the biosphere. It might be that there are planets out there where
intelligence is rampant.
PW: There
might be. It's a big galaxy. I'll give you that.
INTELLIGENCE
IS RARE
NdGT: Suppose
that as we start looking for planets around stars in the galaxy we find out
that planets are common, which appears to be happening. Does that up your
estimates a little bit?
PW: No. Let
me use an analogy. Every once in a while a musical genius arises, and great
symphonies come out. Now, this is one in a million, one in ten million—a really
rare happenstance. But still it takes a long time, even for a prodigy, a
musical genius, to get to the point where he or she can write that symphony.
"So
in practice we're alone because they're not coming to us, and we're not going
to them?"—Neil deGrasse Tyson
PW: Now,
what happens if you change the rules, and all of these kids die at two years of
age, none gets even to 10 years. Maybe once in a while you've got one rare case.
You've cut down the possibilities to near zero. And our point is, it's probably
hard for a planet to exist long enough as a habitable planet for animals to
arise. A few do, and of those few that do, you then need another long period of
time for intelligence to arise. The numbers get so small.
And by
the way, let's get rid of the word "special." Special denotes that
there's some moral force or something. We don't need to think special; all we
need to think is numbers. And we talk about rarity. We're asking in this huge
Milky Way, how many intelligent civilizations are there? Carl Sagan estimated a
million. We know there's one.
NdGT: It
could be zero [laughs].
PW: So the
number is somewhere between one and a million; maybe it's more than a million.
I think the most important point is, if it's not one, is it two? Is it ten? Is
it a hundred? Even if it were a hundred, there are 500 billion stars. That's
pretty rare.
It took
four and a half billion years for intelligence to develop along just one branch
of the tree of life, the primate line. But, Tyson asks, couldn't planets that
offer better conditions for promoting intelligence exist?EnlargePhoto credit: Courtesy of UNWF
BEING
EARTH-LIKE
NdGT: Suppose
we come to learn that solar systems that look like ours are everywhere. Will
that up your confidence in finding intelligent life?
PW: Let's
examine the concept of an Earth-like planet. This is the most commonly used
phrase, "Earth-like planet." Well, Earth-like planet is a moment in
time. Earth-like planet is Earth as we know it now. If we went back four
billion years, our planet would be the most hostile-looking place you can
imagine; it would not be an Earth-like planet. An Earth-like planet is not only
geography, it is time, because planets start as uninhabitable. They become
habitable if they're lucky. And they go uninhabitable again. So we are looking
again not just at the number out there. It's where they are in the whole time
frame of things.
NdGT: In
principle then, Mars might have had its time in the sun as an Earth-like
planet, when now it's clearly completely hostile to life as we know it, as is
Venus.
PW: Mars is
one of the best examples. Mars probably had water to begin with. Mars surely
had early plate tectonics. It had everything necessary for life as we know it.
And it only lasted maybe 100 million years or less. Where are the intelligent
martians? They never had a chance. Mars maybe had bacteria and then they died
out. I think that's the fate of most planets. Mars' fate is the fate of most
Earth-like planets. They don't stay Earth-like very long; they die.
NdGT: If
you look at when the dinosaurs went extinct, 65 million years ago, out of the
four and a half billion years of Earth, that's not very long. Yet in that
period of time, intelligence as we know it rose up from whatever existed;
animals went from tree shrews to modern humans.
PW: But that's
not right, Neil, because you needed all the time that came before. You have to
go all the way back to trilobites and flatworms and everything that came
before. You just can't start 65 million years ago and say, "We're starting
from scratch. Here's 65 million years to intelligence." What about all
that genomic construction that was necessary over the hundreds of millions of
years prior?
THE
RIGHT RECIPE
NdGT: What,
then, is your recipe for a planet that will breed intelligence? Just go down
the list, what do you need?
PW: Let me
answer by saying, What do I need to breed animals, which is our complex life;
intelligence is a whole other ball game. Obviously, you have to have a sun
that's large enough to give a constant amount of energy over time, and yet a
sun that is not so large that it burns out quickly. As you know, our sun is
getting brighter, brighter, brighter. A billion years from now, this planet
will no longer be habitable for intelligence, because our sun will be too
bright. So you have to have something that gives you enough time. Now, the
small stars, of course, let you go for a long time—20 or 30 billion years.
NdGT: They
live longer.
PW: They
live longer, but as a planet you would have to be so close to them as to become
totally locked—just like our moon, the same face all the time. Well, you've got
one side that's really hot, and one side that's really cold. That's not going
to work either.
NdGT: Well,
then everyone lives along the border between the hot and the cold.
PW: I don't
think so. People are studying that right now. It does not seem to be an option.
Now, you know and I know that three-fifths of the stars in this galaxy—correct
me if I'm wrong—are those tiny stars.
NdGT: Yeah,
the majority.
So we've
just thrown out more than half of all the stars in the galaxy. The big stars,
they're gone. The double stars, they're probably gone because the orbits of the
planets get so irregular that you start throwing things around. You throw out
all those. Now you're really cutting down the number. The centers of galaxies,
they're gone. So we have really cut down to maybe 10 percent of the stars in
the galaxy. That's still a huge number, so let's think about planets.
"I
stand by the great Enrico Fermi's question, 'Where are they?'"—Peter Ward
Earth
maintains constant temperature because of the strangest circumstance. It turns
out that plate tectonics causes carbon to recirculate in such a way that when
we get too hot, we pull greenhouse gases out of the atmosphere and turn them
into coral reefs. When we get too cold, the coral reefs die. Volcanoes put more
carbon dioxide in the atmosphere. This only works because of plate tectonics.
One of the most interesting aspects of studying the geology of planets is
asking the question, How common will plate tectonics be? We don't know, but all
indications are it's rare. It takes very specific geological conditions to
allow it.
Coral
reefs and all other living things on Earth owe their existence to plate
tectonics, which may be rare on other planets, Ward says. EnlargePhoto
credit: Courtesy of NOAA
WHERE ARE THEY?
NdGT: Getting
back to intelligence, if we're not alone, you're telling me there's other
intelligence out there in the cosmos?
PW: I cannot
believe that we're the single intelligence in the universe. What I can believe
is maybe there's one per galaxy. We are really scattered, lonesome, very
lonesome.
NdGT: So
what you're saying is, if we're going to try to find other intelligence...
PW: Good luck!
NdGT: ...we
need to broaden that search to go well beyond the Milky Way to the rest of the
galaxies in the cosmos.
PW: There
are probably so few intelligences in our galaxy that to find any of them could
be a remote, remote chance.
NdGT: So we
should just give up.
PW: I don't
think we should give up. SETI is an interesting intellectual endeavor. Why not
continue it? I just don't think they'll ever find anything. Or if they do, the
separation is so great, you could never signal back and forth.
NdGT: So in
practice we're alone because they're not coming to us, and we're not going to
them?
PW: I stand
by the great physicist Enrico Fermi's question, "Where are they?" If
there were so many, we would have had a signal, we would have seen somebody,
there would have been a visitation. That's not happened.
Interview
of Peter Ward conducted on June 10, 2003, by Neil deGrasse Tyson, host of the
"Origins" miniseries, and edited by Peter Tyson, editor in chief of
NOVA Online.
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