Are we alone? This course introduces core concepts in astronomy, biology, and planetary science that enable the student to speculate scientifically about this profound question and invent their own solar systems.
Interview with Freeman Dyson
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From the course by Princeton University
Imagining Other Earths
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From the lesson
Kepler’s Law and Search for Extrasolar Planets
This lecture shows how Kepler’s Law, the relation between a planet’s Period and the radius of its orbit, can be understood in terms of the physics of gravity. We then see how we can use observations of star’s motions (and Kepler’s Law) to detect extrasolar planets and determine their basic properties.
Meet the Instructors
David SpergelCharles Young Professor of Astronomy on the Class of 1897 Foundation and Chair
Department of Astrophysics
>> Welcome.
Today, we have the pleasure of talking to Freeman
Dyson, a theoretical physicist at the Institute for Advanced Study.
The institute's one of our neighbors here at Princeton.
And Freeman graciously has come over and we're
going to talk about a number of topics
in what, of, with life outside Earth and
places where it might not just survive, but thrive.
Much of the discussion about life usually focuses on the habitable
zone, the region that is most like Earth, and most like
the Earth's surface yet our imaginations might
not be might be limited by our experience, and what's out there might be much richer.
And one of the many things Freeman has done, is
thought about some of the possible places where life might thrive.
I know one idea you've thought about is comets, and the
idea that life might survive in the outer solar system and thrive.
Do you want to say a bit about some of those ideas?
>> Yes.
I think the main thing to be said at the beginning is, life has to be
somewhere exposed to a source of energy. And the convenient source of energy is the
sun or a star. So that means here on the surface outside.
And if you look at the universe or the solar system, by
far the biggest amount of surface is on small objects not big ones.
So the small objects even around the sun have far more area than
the planets.
So I've always considered the planets as not the
main place to look for life, because planets are
interesting for all sorts of reasons, but the small
objects are just as interesting and probably even more varied.
So let's look at some of those.
Well, of course, there are two big classes of small objects, the asteroids, which
are close in between Mars and Jupiter and then there are the comets,
which are a big swarm.
Much farther out, beyond Neptune and either
of those would be good places for life.
The point is that,
what life needs is first of all liquid water and secondly a source
of energy and minerals, but all those are available on the asteroids, on the comets,
just as they are on planets, and the huge the advantage of being outside
on one of this small objects, is it becomes then so easy to spread.
A planet is a sort of a trap.
If you're sitting on a planet like
the Earth, it's extremely hard for life to escape.
We don't know whether life ever has escaped from the earth.
There's no evidence that it ever has.
It's extremely difficult to imagine.
Whereas, if you happen to be on the surface of a small object, like a
rock in space or a snowball floating around in space.
Then you
get, have a collision with another small object, which goes on all the time.
You can easily transfer from one to the other, without changing your way of life.
So, once established on these small objects, life could
spread and be all over the universe, that's why I
consider it more likely that we'll find life out there
than it will, than on the limited environment
we have available on the planets.
>> So comets are what I guess Fred Whipple called dirty snowballs.
>> Right.
>> Right so that there.
We should really think of them as, you know, filled with water and some organics.
So they really are.
>> Well, there may be a lot of rock as well.
But, we don't know.
But I mean, some of them certainly have a lot of rock.
>> And one of the things we've learned from comet
fly-byes is that there there was liquid water on these comets
perhaps caused by comet-comet collisions.
There seems to be very clear evidence for melting.
>> Right.
>> I've wondered about comets, whether once you get life on one comet, that it
might very effectively pollute, if you like, with
life up the whole solar system around it.
If I think about what the young solar system probably looked like we had a young
sun, a disc of gas and dust further out comets which
are so the leftover remnants and today we see what was the early disc,
but the outer protoplanetary disk was probably
made entirely of comets, a material like that.
And the collision rates were probably pretty high.
I believe you get of order of collision or so every orbit or most.
>> Right. >> Between them.
So once your life form exists on one of these comets, it will spread through all
of the comets almost in the a, our early solar system.
>> Yes, I mean, the problem is it has to exist in a
vacuum and that's, that's difficult, I mean, all the life on earth of course
lives under normal atmospheric pressure, so it has air and
that means it's much easier for water to stay liquid.
Out in a vacuum, water evaporates
or freezes, liquid water can't sustain itself.
So if you're in a vacuum, you've got to have
some sort of a greenhouse around yourself as a habitat.
So these creatures, whatever they are that live on comets, have to be
at least as complicated as greenhouses.
So they have to have some sort of a transparent shell around them,
into which sunlight can penetrate, that contains the water and the air inside.
And so the question is whether that can ever originate outside in a vacuum.
That, that's difficult.
But once they exist, I'm sure that they'll flourish.
>> Now, you had speculated about a
kind of flower-like structure forming, just so that things would
be warm enough so you'd have liquid water in the center.
>> Right.
If you're far from the sun.
Of course then you have to have some sort of concentration for sunlight.
Either a lens or a mirror to concentrate sunlight, that could be outside in the
cold, and the sunlight is then concentrated on
your little greenhouse, where you keep warm.
And all that,
could be easily engineered and we could imagine doing that
ourselves, but whether nature can do it, that's another question.
>> Right, so I think, if I remember the idea you had sort of mirrors growing out,
if you like, concentrating the sunlight, so that there
would be a spot warm enough in the center.
>> Right.
And such things actually exist. We do have some plants that
are a little bit like that in the Arctic on the Earth,
with a flower with petals that concentrate sunlight onto the ovaries, which are
the living part of the plant so it does
exist in a rudimentary fashion in plants that we know.
>> If like was out there like this, is
there some way we might be able to discover it?
>> Yes.
That's another trick which makes this sort of an
easy thing to do, to look for life of this sort.
Which is called pit lamping.
A pit lamp is a light that miners use underground.
You have the light on your forehead and it
points forward so you can see where you're going.
And those a
form of hunting which is practiced in Canada, where people
go out at night with pit lamps on their forehead.
And then the animal staring at them at the dark reflect
the light, you see the light, the eyes of the animals
glowing in the dark, so, you, its an easy way of,
of shooting deer and other creatures that may come into your view.
So anyhow, that's illegal in Canada.
But it's still done. And,
and so that's what I would do in the solar system.
That these creatures living on comets are far away from the sun.
They'll have to have some sort of eyes of that kind.
Reflecting the light back toward the sun.
So we can actually see them by the reflective light, rather easily.
>> So we would look towards a cometary body that's, lines up.
So the Sun is here, the Earth is here and the cometary body's there.
>> Right. >> And.
>> As we are swinging around the Sun, then you'd see
this lights up and as we pass through the beam and.
>> You'd see a sudden flash.
>> Not a flash.
It would be slow. >> Right.
>> Because we're moving around at this.
>> Right. >> Only once a year.
>> Yeah.
It would, it would be a source of, on a comet of
variable brightness but it will be something interesting if it's not alive.
>> You've thought about the possibility
of life in a outside our solar system in a number of different ways.
I know one thing you've thought about is
the notion that the most efficient way for life
to survive is, or make good use of a star is to form a sphere around it.
These have been called Dyson spheres after you.
Do you want to say a little bit about those?
>> Yes, and I mean, that's of course a totally
different game when you have a technological society of creatures
who have learned how to use energy in large quantities.
And they have a highly evolved industry.
So, these are creatures either like us or more advanced.
Then, of course, they will tend to expand as we do
and use up more and more of the real estate.
So they'll come a time when they'll have run out of real estate.
They'll start building structures in space.
And you could imagine then, it doesn't take very long, if the rate of
growth is even in the order of 1% per year or something of that kind.
It only takes a few thousand years, and they've filled up
the whole area around the star with some sort of artificial structures.
And that's something interesting because we could detect it if it exists.
Iâm not saying that we shall
necessarily go that way ourselves, but certainly if
anybody has gone that way, we shall be
able to pick up the radiation that they're emitting
from their surfaces there if any
creature has have built these large structures in space.
They will be warm and they will
be radiating infrared radiation which we can detect.
So that's another way of looking for
life is looking for this artificial infrared sources.
>> And so one of the things I think is important to,
for us all to keep in mind, when we think about life, is time scale.
>> Yes, right.
>> So that The earth is 4 and half
billion years old, our universe about 13.7 billion years old.
So, any place where life has evolved, life
has probably started either a billion years before
Earth, it did on Earth, or a billion
years later, and any place where life, technologically advance
life exists, technologically advanced life will either be perhaps
100 million years more advanced than we are.
Well, we've only been technologically advanced for about 10,000 years, since we
started as agriculture, so I've always found this notion that we
should be thinking about not what life would, is like now,
but what life would be like when it became more advanced and
underwent this period of growth and try
to make the best use of energy out there.
>> Yes and, of course, the fact, the
sad truth is that, the universe appears to be totally dead.
That we've looked pretty hard and we haven't found anything.
And that's sort of sad, because it implies that maybe we are the first.
On the other hand, that's also something to be happy about.
If we are the first, then we have the whole universe
for ourselves, which is, in a way, also a happy prospect.
>> Yeah, I know that we've been
looking for life, certainly listening for radio communication.
But I've wondered whether we've been looking
too much for things like ourselves, or,
with radio communication, some ways like what we were in the 1950s and 60s.
>> Absolutely.
No, of course we've been very, very narrow minded in
our searches so far.
We've looked in the very narrow waveband that, for creatures very
much like ourselves, and because it's obvious that
what we will, what we will find when we do find
it, will probably be a total surprise, something which we'd never imagined.
It's always true in astronomy, is that
nature's imagination is much richer than ours.
And so I look
forward to discovering life on some weird place that nobody's ever expected.
>> And I think about the way we communicate and now most of our
information is not sent by radio waves by sent by, you know, optically over fibers.
Right.
>> And it's sent in an encoded way
right when we compress information to send it.
It looks statistically like noise. >> Right.
>> So even if someone were able to see
the optical signals we send they're the signals that
most people are picking up when they're watching this recording.
It's been encoded in such a way that statically it looks like noise.
>> Absolutely.
No, I think it's the celestial life isn't something separate.
Celestial life is just one part of exploring the universe.
And you should always explore.
So that even if you don't find life,
still you make discoveries and.
So that's happily what most people are doing.
>> I mean, the hope I think many of us have is,
as we explore, well, I'm confident, as we explore, we'll be surprised.
>> Right.
>> And that one of the surprises may be the
discovery of life and even the signs of technologically advanced life.
>> Yes.
And
one of the things that's fortunate about this search for infrared
source is it doesn't depend on the motivation of the aliens.
If they have an advanced society with massive industrial machinery.
They have to radiate away waste heat, there's no way they can avoid that.
>> Yeah, I mean this is in a sense just thermodynamics.
>> Right.
>> If you want to do something in
the universe, you need to take something that, from a higher energy state,
low entropy, to a low, until you create entropy, and you create heat.
Right.
>> And that will be seen.
>> So no matter what they want or what they're trying to do that they can't
help radiating this waste heat radiation which we can see, so it's,
I'm, the trouble with that is, we have searched the sky with infrared
satellites, with sky surveys in the infrared and what
we find is, the sky is crawling with infrared sources.
In fact, there are millions and millions
of these infrared sources which are obviously natural
objects, but there's no way to tell whether a few of them might be artificial.
It's very hard to tell the difference.
>> So let's turn to life in the future and how other
things that you've taught about is could life persist into the distant future?
And how efficient it could be?
>> Yes.
This is, of course, highly speculative but, but all we
know is, life is enormously versatile that it's able to adapt
to almost all environments, in particular, on
the, or what Mr. [UNKNOWN] was saying about.
The deep life in the crust of the Earth is just one example.
So, life has this capacity to adapt itself to the environment.
Well, what's going to happen in the remote future, very, very long
after the stars have died and the universe is cold and dark.
Still there are sources of energy and there are
still places where life could flourish. So I'm assuming that life is able to adapt
so that no matter how cold and dark it gets, life will still be there.
And then you can make then mathematical calculations
about what is possible and what is not.
And it turns out it's sort of marginal. We don't know for sure.
It's going to be hard for life to survive forever.
But it's not absurd.
And one of the things that
I find most interesting here is how life can process information.
There are, roughly we know two ways of processing information.
Which we call analog and digital.
Analog is the old fashioned way, where you use a slide rule, or some kind of
simple device measuring distances and digital is the
modern way of doing it, where you have
electronic machines that manipulate numbers, but life
could use either of those or both, we
know our genomes, in fact our digital genome
is a chemical alphabet that which codes information digitally.
And the brains, we don't know, but the brains code information in some
other fashion which may be analog, so in the long run the interesting
fact is that analog does better but you can sort of, you can prove mathematically,
there were two mathematicians who proved this in Richardson Marion Poorel who.
Proved a theorem that in some definite mathematical sense,
analog computing is more powerful than digital computing, so in the long run we
shall come back to analog, which I find interesting so you think of the.
>> And this is all just in the classical regime of computing.
>> No, it may be, it was classical, the mathematics applies to classical.
I think the theorem is probably true for
quantum computing also, that remains to be seen.
But anyhow.
But this is all
very poorly understood.
But I, it's an interesting subject to look at more carefully.
And of course, it may turn out to be relevant.
Because there are creatures, maybe in other parts
of the Universe which already made this step.
And for them the Universe may be already too cold.
>> Mm-hm.
>> And so they had to make the switch, we just don't know.
>> As we look out and think about environments where life might thrive and
searching and search for life what steps do you think we should be taking?
>> Well, I like to take a long view I
recently went to a space launch at Baikonur in Kazakhstan.
Where the Russians launched their manned missions.
>> hm.
>> And watched one of our friends, Charles Shimoni,
go up into the space station. And it was a joyful occasion.
What is interesting is the Russians take so
much a longer view of things than we do.
It's a very different culture, which I found very noteworthy, that in
some ways the Russians have a better understanding of things then we do.
They think centuries whereas we think decades
and so we're always talking about the
next mission to Mars and what we're going to
discover and what we should plan to do it, everything
is short range, in Russia it's quite different, this is,
it's a ceremonial procedure that they have this space launch.
The whole town comes out to celebrate.
There are speeches, the mayor of the city of Baikonur
makes a speech telling the astronauts how great they are.
And astronauts
make a speech saying, we are ready to fly, and there's an orthodox
priest bless, blessing the enterprise, so it
is a ceremonial procedure which everybody enjoys.
And it's not so much a question what you are
going to do this time, it's just we're on our way,
it'll maybe take a few hundred years but we're on our
way to the stars and this is the way to go.
It's a
very different way of thinking.
And I think more appropriate for a manned mission.
If you're talking unmanned, and of course a short time scale.
>> Mm-hm.
>> Is appropriate, unmanned missions go much faster and
we can plan over the next decade or so.
But from, a manned program, I think will run
much better if it has a long time scale.
That was the big
mistake of Apollo.
The Apollo missions to the moon were planned as a one decade enterprise.
After the decade was over, the whole thing came to an end.
And that was very sad.
And, I, if the Russians had been running it, they would have
done much less in ten years, but more in the long run.
>> Really?
For me, the model I think of is cathedral building.
>> Right.
>> Where there are cathedrals that took, you
know, 100 and sometimes 200 years to construct.
>> Right.
>> And, well, working on it, you knew that you weren't going to
finish it, but that you were, you know, but that progress was being made.
>> Exactly.
And I think that we ought to think that
way when we talk about manned missions to Mars.
It's not manned missions, it's actually life as a whole going to Mars that
won't be just us, it'll be a whole community of life adapting itself
to living on Mars and we shall just be a part of it.
That will take a long time, but it'll happen.
>> So, this was a wonderful discussion, thank you so much.
>> Thank you. >> And I thank you all for joining us.
>> Thank you.
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