Download Professor`s commentary for Theme 12 Part 1

Theme 12.1
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SETI: First Considerations
Why a Search for “Intelligence”
In this unit, we'll focus our attention on SETI, the search for
extraterrestrial intelligence, as opposed to a more broad search for
extraterrestrial Life. Intelligence implies the ability to communicate with
us, which means that there will be a flow of information in both directions
after contact is made. We could hope to find evidence of extraterrestrial life
in a number of ways: for example, on an exoplanet that has free oxygen in its
atmosphere, as we described in an earlier unit. However, there may be planets
that are covered with life forms which are incapable of communicating with us:
ants, plants, dolphins, flamingos… They will be of less interest in terms of
addressing some of the communication questions that we would like to explore
with extraterrestrials. So here, we will describe SETI itself.
Before we design and launch any search, we should think about what an
extraterrestrial intelligence would need. Minimally, we imagine it would
require a platform that provides heat and nutrition: a basis on which a species
can evolve and develop. We would require stability over immense periods of
time, so that primitive life forms can evolve to become more complex. We
expect this location, given time, to lead to the emergence, evolution, and
persistence of life and eventually intelligence. Discovering ETI of course
requires that the ET's be communicative, which means the development of certain
technologies and interests, even if those communications are only inadvertent,
as we'll see.
The Physical Form of ET
It is very easy, as you may know from science fiction reading, to imagine life
forms of a great variety: for example, clouds of ionized gasses in interstellar
space that communicate telepathically. More logically, though, we should think
about the kind of biology in which we think the life forms on planets will form
and evolve. We will address the question from the point of view of Life As We
Know It. First of all, we're going to assume that life is probably going to be
carbon-based, given the great versatility of carbon and its many compounds.
We'll assume that it is dependent on water as a solvent: as we'll see in a
moment, water has certain special properties that make this likely. It may be
that the creatures that come into existence elsewhere utilize proteins of the
sort that we are familiar with, using amino acids; and it could be that
they're also DNA-based. These last two requirements are not fundamental
assumptions, but may be likely. That will be a matter for further exploration.
Water has certain special properties that make it particularly useful in
biological contexts. For one thing, it's a polar molecule. This means that it
is asymmetric: it has one oxygen and two hydrogens, but the asymmetry of the
molecule means that there's a net negative charge on the oxygen end, and net
positives on the hydrogen ends, because of the way the electrons are
distributed within the molecule. This means that water molecules bond together
to an extent, which gives water a cohesion, including the surface tension
effects. For reasons we needn't go into, this is important in biological
considerations. Secondly, water contracts as it gets colder, but as it
approaches the temperature at which it freezes, it expands considerably. This
is why ice floats in water, as shown on the picture here with an iceberg, much
of which is below the surface of the water. What this means is that in winter
on earth, ice forms over the tops of lakes, but the lakes do not freeze right
to the bottom because the warmth is trapped underneath the ice layer. This
allows fish and other aquatic animals to survive through the winter, an
important consideration. Finally, water has what is described as very high
heat capacity. It can absorb an enormous amount of heat, which it slowly gives
off. This explains for example why the warm water of the Gulf Stream keeps the
northern parts of Britain mild through the winter months, although they're at
the latitude of northern Alberta where you might expect it otherwise to be
tremendously cold. Water therefore has an important climatological and
biological effect on life.
However and whenever life first comes into existence on a planet, it will
probably develop fairly quickly into a primitive unicellular form that will
have many basic similarities, given the reliance on carbon as the building
block and water as the solvent. We expect many universal properties. On the
other hand, our understanding of biological evolution, and our experience on
earth, demonstrates that evolution drives to a great variety amongst species an
enormous differentiation, as is shown in some of the pictures in this panel.
It is an interesting and entertaining question to ask: what will the first ETs
look like, when we do make contact? Here are some examples, a few of which are
drawn from the cinema and not meant to be taken very seriously. There are of
course certain plausible arguments that can be presented: for example, there
are evolutionary benefits to having two eyes, which gives one binocular vision
and depth perception; to having something like a hand, with independent digits
that can be used to manipulate small things; to standing vertically, so that we
have some control over our surroundings, rather than lying on our stomachs as a
snake would; and so on. But really the question is open to enormous
speculation.
In other words, there is no clear-cut answer. On the other hand, it may
matter, at least psychologically. On this panel, we see on the left a drawing
from an early publication of ‘War of the Worlds,’ representing Wells'
description of the tentacled Martian. If our first contact with ET reveals a
creature like that, we will respond very differently than if it appears like
one of the creatures shown on the bottom right of this panel. Yet we will have
to get past any atavistic repulsion we may feel at the appearance of what could
be a hideous monster, from our perspective.
Estimating the Numbers of ET Civilizations
Let's now turn to the very important question of trying to roughly estimate how
many extraterrestrial intelligences there might be in the galaxy. For ETs to
be in existence now so that we can communicate with them, you make have to make
the following assumptions:
•
First of all that they are on a planet associated with a star, and that
the planet is in the right location for life to have emerged and
prospered, not too hot or cold.
•
The star will have to provide energy for a sufficiently long time for
biological evolution to take place after life itself comes into existence
on that planet.
•
That evolution will lead to increasing complexity, and we have to expect
that this will lead to the emergence of intelligence, and therefore the
ability to make contact to technology - and indeed the interest in doing
so.
•
Finally, we have to assume that the ET society will survive long enough
to be around for a sufficient time that we can indeed make contact.
We'll return in a moment to working our way through that string of factors, and
recognizing their different importance. But let's first of all apply similar
reasoning to a much more mundane question. Suppose someone asked you how many
left-handed female taxi drivers there are in New York City. Obviously, there
must be some, but how would you come up with even a rough estimate? Here's a
stepwise approach to addressing this question.
•
First of all, we assume that there are about ten million people in New
York City, and there could be perhaps one taxi driver for every thousand
people or so. (A comparison to my own city of Kingston makes that seem
like a reasonable sort of estimate.) That would imply 10,000 taxi
drivers in the New York City.
•
•
Some fraction of them will be female. It's unlikely to be 50/50, but
it's a profession where there may be fair equity between the genders. So
let's assume a third of them are female, say 3,000 or so.
Ten percent of the human population is left-handed, so that implies as a
guess that there will be about 300 left-handed female taxi drivers in New
York City.
That may seem like a silly or irrelevant exercise, but there are some important
things to note. First of all, our answer is probably good to within a factor
of ten. I expect there’s at least 30 such taxi drivers in New York City, but
not likely to be, say, 3000. So we’ve made a ‘ballpark’ estimation, an ‘order
of magnitude’ estimate. But the point of the analogy is not to show how to get
a precise answer, which is not the goal, but rather to get you to think in a
new way about how to identify all the important contingent factors, and also to
consider any interdependence between them. So, for example, if I'd asked about
the number of colorblind taxi drivers, you'd have to remember that colourblindness is actually gender-specific, more common in males than females.
The “Drake Equation”
So let's turn back to astronomy and ask the question about ETs in our own Milky
Way galaxy. There are of course billions of other galaxies, but they're very
remote, and to have any hope of real contact, will limit ourselves to our own
galaxy, which contains about 100 billion stars.
The chain of reasoning we'll now describe owes to Frank Drake, an astronomer
who in some ways can be thought of as the ‘father’ of SETI. It was he who
first asked us to consider the relevance of the various factors we'll now
enumerate, and encapsulated them in the so-called “Drake Equation,” which we'll
see presently. First of all, he pointed out that we have to consider some
fairly reliable astrophysical factors. For example, we have to know how many
stars are being formed each year in our galaxy, and how long they last. In
other words, what is the current population of stars in the galaxy? How many
planets are associated with each star? The more planets there are per star,
the more likelihood of finding life forms emerging on the planets. On the
other hand, we have to ask what fraction of those planets are suitability
located: not too close to or too far from the parent star. These are fairly
straightforward astrophysical factors.
Fairly quickly, though, we move away from the well-determined astrophysical
factors to consider issues that are less certain. For example, if we know of a
planet that is in a suitable location, around an appropriate star for a very
long time, what is the chance that life will come spontaneously into existence
on that planet? If it does so, what fraction of such life forms will evolve to
considerable complexity and the emergence of intelligence? Even if
intelligence arises, what fraction of these life forms will develop
technological skills and interests that will lead them to communicate with
other species in the galaxy? And how long will a typical technological ET
society last in its communicative phase, so that communication with others is
possible. As you can see, these factors are much less well-constrained.
Let's now consider these factors in turn: first, the number of stars in the
galaxy. The galaxy contains something like a hundred billion stars, and it has
been around for something like ten billion years, which means that over its
lifetime, it has produced, on average, about ten stars a year. Currently
though it seems to be forming about one new star a year on average, and an
average star like the sun might last about ten billion years. So in the steady
state, there will be at least several billion stars radiating away, living
their quiet lives and suitable perhaps for life forms on planets surrounding
them. Stars that are much more massive than the sun, by the way, burn up their
fuel very quickly, so even if life gets a foothold there, it won't evolve much
before the star is gone. But the vast majority of stars are the size of the
sun or smaller, and will last a very long time. Some stars are unsuitable
because they're in dense star clusters or very close binary star systems which
make planetary orbits quite unstable, but at least 50% are not. So, there's a
large number of stars that could provide suitable abodes for life.
The second factor is one about which we knew little until the last few decades,
and that is the number of planets that might be associated with a given star.
As we now know from radio-velocity measurements, as shown on the bottom left,
and from transit measurements, using the Kepler telescope as shown on the
right, planetary systems are very routinely formed along with stars. A typical
star could have as many as perhaps ten planets. (That's, roughly speaking, the
size of the solar system.)
Of course, not every planet will be appropriately placed. The planet has to be
in what we call the “habitable zone.” If it's too close to the parent star,
water will boil off, and life as we know it will not be possible. If it's too
far away, water will be completely frozen out, and again, life will not be
possible. In our own solar system, the Earth and Mars seem to be in
appropriate orbits, in the habitable zone of the sun; Venus however was
probably too close, even before the runway greenhouse effect.
Consideration in the habitable zone allows us to refine our understanding about
which stars are appropriate. Massive stars have very big habitable zones, but
don't live very long. A star like the sun has a fairly generous habitable
zone, and as we saw, two of the planets in the solar system are within that
zone. Small stars, of the type known as M-dwarfs for example, have very tiny
habitable zones, and only a planet that is very close to the star could be in a
habitable region at all. Unfortunately the M-stars are very common, but the Gstars, like the sun, are still around in large numbers, and they are still many
millions, tens of millions, of possible parent stars for ET in the galaxy.
So on the astronomical front, things look fairly promising: there seem to be
lots of stars around with planets in habitable zones. But now we have to
consider the uncertain factors relating to: the emergence of life; the
evolution and development of complexity and intelligence; the emergence of
technological interest and capabilities; and the longevity of any societies at
that stage.
Of these factors, evidently the most uncertain is that of the emergence of life
itself. Given a planet under a radiating sun with an appropriate mix of
chemicals in a water bath in a tidal pool, how likely is it that life will
emerge spontaneously? We know from earth that life emerged fairly soon after
the cooling of the planet, following the formation of the Solar System 4.6
billion years ago; but this does not guarantee that is always the case.
Various experiments give some reason for optimism. A famous experiment by
Miller and Urey, for example, in the 1950s tried to replicate the early Earth
atmosphere over a bath of water with heat and electric sparks to simulate
lightning. Very quickly, that bath of gases and water formed some complex
molecules, including simple amino acids and so on. This, however, is a far cry
from the development of true self-replicating molecules, which are important
for life. So although this is a positive result, it is in no sense conclusive.
However, it does remain that life appeared early in the history of earth, and
can survive under extreme circumstances. So there may be reason for optimism
elsewhere.
This panel reminds us of the existence of living creatures known as
extremophiles, simple creatures that can survive in circumstances which we
would not have thought possible: for example, extremes of temperature as shown
in the top panel; and extremes of acidity and alkalinity, as shown in the
bottom panel. Living creatures have been found, for example, next to the
‘black smokers’ in the mid-Atlantic Ridge, where hot magmatic material is
spewing out from the upper mantle of the earth.
One factor in which we can be reassured, however, is that if life does emerge
in a suitable situation, there can be plenty of time for further biological
evolution to take hold. Here we see a spiral that represents the passage of
time on Earth, back to the formation of the planet 4.6 billion years ago and
the emergence of the earliest fossil cells, a few billion years ago, followed
by the long and amazing record of biological evolution subsequently. Astronomy
provides platforms on which that passage of time is guaranteed.
The nature of biological evolution suggests that complexity and variation will
follow very naturally, should life itself come into existence. The emergence
of intelligence is perhaps a little less clear. However, it is obvious that
should intelligence emerge, it would bestow an enormous evolutionary advantage
on any creature within which it arises; so evolution would seem to point the
way towards the emergence of intelligence and its rapid domination of the
biosphere.
Even if intelligence emerges, does that having necessarily imply the
development of technological capabilities and interests? Here we see a
Maxfield Parrish painting in which a society has elected to lie about on grassy
banks, having philosophical discussions and playing music, rather than pursue
any technological endeavours. Could there be whole intelligent societies that
have that equivalent mindset?
Even if a technological society emerges, will it be around long enough to make
contact with other species -- other societies in the galaxy? There are reasons
to be concerned about the fragility of life on planets, as we’ve seen in the
course of our discussions in earlier units. For example, there's the threat of
asteroid or cometary bombardment on the Earth that could wipe out civilizations
as we know it. There's also the prospect that planets may migrate in their
orbits around the parent star; that has not happened in the solar system in a
way that affected life on Earth, but other solar systems may not be so
fortunate.
In addition, our long-term survivability on the planet may be determined by our
own actions to some extent, and indeed the development of technology may play
an important role in this. There's the threat of war; global catastrophes;
global warming caused by the development of an industrial society; uncontrolled
diseases which may sweep the planet because of the growth of rapid
communication and travel between societies; and so on. So we don't really
know, beyond our own example, how long a technological society might last
before succumbing to one of these catastrophes.
The last factor raises the interesting point that the possible longevity of the
society in a technological phase, may matter more for SETI than does the
longevity of the parent star. It is still, of course, the case that the star
has to be around for many millions or billions of years for life to form and
develop, but that matters not at all if the technological society that emerges
obliterates itself in very quick order. Can emerging societies survive their
troubled adolescence? That's an interesting question with respect to the human
population certainly.
Some Numerical Results
Having discussed these factors qualitatively, let's now combine them
numerically in what is known as the “Drake Equation.” (By the way, there are
variants of this, in which certain factors are considered in slightly different
ways.) What we see here is a multiplication of the various numbers and factors
that we've been discussing, out of which will emerge an estimate of the number
of civilizations in the galaxy whose electromagnetic radiation is detectable at
present -- in other words, the number of societies with which we could hope to
communicate.
We’re going to consider a couple of different cases here: one, optimistic; and
the other, less so. The optimistic assumption begins by
•
assuming that life spontaneously forms on essentially every planet that
has the potential for it. The fraction of planets producing life (fl) is
•
•
1.0; that is to say, all of them. We could justify this in part by
noting that life formed quickly and easily on planet Earth.
Secondly, we're going to assume that, once life forms, essentially all
the time it leads to the evolution into an intelligent species, and that
these intelligent species develop technological interests.
Finally, we're going to assume that these civilizations mature quickly:
they get through their troubled adolescence and find a way to survive for
quite a long time, say ten million years at least. (The human species,
by the way, may be able to do that if we can survive the next few
thousand years and propagate out into nearby star systems to other
planets.)
As noted, those are very optimistic assumptions. More conservatively, we could
recognize that perhaps the emergence of life takes some very flukey event, and
happens only once in every million or billion promising situations. The
question of the origin of the first self-replicating molecules is a very
important area of research, but an extremely challenging one. No one really
knows if the existence of a ‘primordial soup’ of the right chemicals and the
infusion of energy and the passage of time is sure to be enough.
Here's a warning by the way: beware what you might think of as ‘neutral’
assumptions. For example, you might say: “let's assume, to be on the safe
side, that the chance of life emerging is only 50:50” in the hope of being
unbiased. But that's rather like saying: “I have a 50/50 chance of winning the
lottery. Either I will or I won't.” The reality is that the formation of life
is probably extremely unlikely, with very rare exceptions, or else very close
to certain. The trouble is, we don't know which it is! If we were to find
evidence of even one extraterrestrial lifeform, it would support the view that
life spontaneously forms all over the place. Astronomers, by the way, tend to
think that it's very likely, because they see so much opportunity out there:
billions and billions of stars in a huge universe. Biologists, however, tend
to be much less optimistic, because they appreciate just how complex even the
very simplest life forms are.
Moreover, those with strong religious beliefs would argue that the infusion of
life and the development of a thinking mind may require divine origin. If
that's the case, of course, we have no idea of just how ubiquitous it is in the
universe or the Milky Way.
Here's a table in which we summarize in numerical form the various factors
we've discussed: in the middle column, the optimistic view; and in the righthand column, the conservative view. There's not much disagreement about the
star formation rate, the numbers of stars and planets, and so on. But the
differences arise when we consider the origin of life, how it evolves, the
development of technology, and how long it will last. The optimistic view
leads to a calculation of one million ETs at present; the conservative view
suggests that we are alone in the Milky Way at present.
But how do we interpret these different numbers? The optimistic view, which of
course is predicated on some numbers that are really just guesswork, suggest
that there might be a million communicative ET civilizations right now in the
Milky Way. Notice, however, that our working assumptions really mean that ET
springs up on every well-placed planet around every suitable star, but that it
will persist in communicative form for a million years. This implies that in
the past, there were many civilizations in the galaxy that have since
disappeared, and that there will be many more in the future. At the current
moment, though, if our numbers are correct, there will be one million such
civilizations in the Milky Way.
The conservative view should be likewise interpreted in the sense that we are
alone at present, but over the ten billion year history of the galaxy, there
may occasionally have been others that arise flukily, but then last only a
short time. Currently though, if the statistics are correct, it's very
unlikely that there will be another extraterrestrial intelligence for us to
talk to within our whole galaxy.
The optimistic assumptions led to the conclusion that there might be a million
communicative ET civilizations in our Milky Way galaxy at present. That sounds
very encouraging, but it must be remembered that our galaxy contains about one
hundred billion stars, which means that on average, there may be one
communicative ET civilization in every random sample of a hundred thousand
stars. In other words, even on these optimistic assumptions, the nearest
communicative society is likely to be very far away, probably hundreds of light
years or more.
It's also important to remember that the Drake equation provides a way of
thinking about the problem and addressing the potentialities, rather than
giving us a credible answer. It's important not to take numbers too seriously,
as I'm sure you realized as we went through the discussion. Some of the
factors are so uncertain that there's no realistic constraint at all.
Moreover, we're unlikely to make any progress on some of the issues: for
example, the question of the spontaneous emergence of life. That will probably
never be resolved by laboratory experiments on small scales. Finding evidence
of ET elsewhere would be a critical breakthrough.