THE PROBABILITY OF THE EXISTENCE OF EXTRA TERRESTRIAL INTELLIGENCE.

INTRODUCTION:

We seem at the moment to be in the grip of PMT, pre-millenium tension. In recent years there seems to have been an explosion of news stories about life on Mars, UFO's etc. and a number of Hollywood films on the theme of Extra Terrestrial Intelligence. Personally I think that this is all wishful thinking, and that the probability of ever encountering other intelligence is extremely remote. I would like to be proved wrong, but these are the reasons for my skepticism.

For me the answering of the question of the probability of finding intelligent life elsewhere in the universe is a means rather than an end in itself. There are many eminent astronomers and cosmologists who have tried to answer this question, but it appears to me that they all start off with the answer that they would like to find, and look for the evidence to support their belief. Some believe that the galaxy is packed with intelligent life, others believe that we are alone in the galaxy, and most take a position somewhere in between. In the final analysis, it makes no difference unless there is some possibility of making contact with other worlds, so I will be looking at the probability of being able to make contact with other intelligent beings, but the main benefit of tackling this question is that we have to examine the process by which we all got here, and marvel at its sheer improbability.

It is all too easy to get caught up in the daily drudgery of shopping, working, eating and sleeping and fill one's mind with petty differences of opinion, and forget how insignificant our little lives are. Human consciousness is a rare and possibly unique phenomena in this vast universe, and we should be careful not to squander it.

First, a word about numbers. I shall be using a billion to mean a thousand million. We often hear these numbers bandied around, and lose perspective on how big they are. A million seconds is less than two weeks, a billion seconds is more than 31 years.

It would perhaps be wise to begin with a brief overview of just how big the universe is, in order that we can understand just what the difficulties of looking for other signs of intelligence are.

Current theory suggests that the universe began in a single instant 15 thousand million years ago. If we look out into the night sky we can see galaxies like our own flying away from us at a speed proportional to their distance as the matter in the universe flies apart. Our own solar system is one third the age of the universe. It takes light, travelling at 186,000 miles a second, about 16 hours to cross the solar system. It takes 300,000 years to cross the galaxy. There are about 100 billion stars in our galaxy, and there are as many galaxies in the universe as there are stars in our galaxy. The nearest galaxy to us is the Andromeda Galaxy, 2.5 million light years away. The light reaching us from Andromeda left when our early ancestors were roaming the Seregetti plain.

In 1977 NASA launched two Voyager space probes to examine the outer planets. It took Voyager 2 12 years to reach Neptune, currently the outermost planet. It used the gravitational fields of Jupiter, Saturn and Uranus to accelerate to a speed of 60,000 mph. Without this help the journey would have taken 30 years.

In 60,000 years time it will leave the outer reaches of the solar system, and in 180,000 years' will approach the nearest star 2.5 light years away.

Despite several attempts to search for signs of Extra Terrestrial Intelligence (ETI), so far there is no evidence of life elsewhere in the universe. This is somewhat worrying for many astronomers who believe in the principle of mediocrity, that the earth is no different from anywhere else in the universe, therefore intelligent life must have sprung up all over the place, because if this were the case then we would have found something by now.

One of the first attempts to look for ETI was Project Ozma, initiated by Frank Drake. Drake is responsible for the most popular tool for calculating the probability of ETI, known as the Drake Equation. Drake made the assumption that in order for us to be able to contact other civilisations, certain conditions would have to be met. These conditions can be grouped into three categories;

l. Astrophysical and geological; An ETI would need to have a suitable physical environment for ,development, not too hot, not too cold, and not too unstable.

2.Biological and psychological; It must be the case that life must arise wherever conditions are suitable, and it must be the case that evolutionary pressure forces intelligence to emerge.

3. Sociocultural; Intelligent life must persist for a sufficiently long time to develop into a technological civ lisation that has the means and desire to engage in interstellar communication.

Drake developed his equation in order to attempt to quantify how many such civilisations might exist in our own galaxy.The equation is:

N = R* x fp x ne x fl x fi x fc x L

------------------ ------- --------

physical biological cultural

The definition of terms are:

R* = the rate at which stars are formed in our galaxy per year.

fp = the fraction of stars, once formed, that will have planetary systems

ne - The number of planets in each system that will have an environment sui table for life.

fl = The probability that life will develop on a suitable planet.

fi - The probability that life will develop to an intelligent state.

fe - the probability that intelligent life will develop a technology and culture capable of communicating over interstellar distances.

L - The number of years that such a culture will spend actually trying to communicate.

Many famous astronomers have attempted to come up with a value for N. The answers vary between 1 and 10,000,000. In order to come to our own assessment of the value of N we must examine each of the variables of the equation in turn.

Before we start, though, we have to recognise the preconceptions with which we approach the question. Rather like a puddle that suddenly gained consciousness might marvel at how lucky it was to be exactly the same shape as the hole in which it lived, and reflect (sic) that it is unlikely that there are any other puddles as they would have to have holes exactly the same, so we start from a position of living in a part of the universe where conditions are exactly right for us to live. This is of course because we have evolved to fit the conditions, and if the conditions were not right we would not be here to worry about it. There may be all.sorts of exotic life forms out there, but unless they are reasonably similar to ourselves we would be completely unable to communicate, and would probably not even realise that they are there if the evidence were before us. It would therefore make sense to look for the probability of similar life forms existing. There are also various constraints as to what sort of life forms are likely to develop intelligence. I shall discuss th.ese issues in greater depth later, but first let us examine the Drake Equation.

R* - The Rate of Galactic Star Formation:

Of all the terms of the Drake Equation this one is probably best understood as the mechanics of star formation have been extensively studied.

Stars form from vast clouds of interstellar dust. It is thought that they begin to form sometimes from chance concentrations of dust in a particular area, and sometimes as a result of shock waves from a supernova explosion. If dust begins to become more dense in a particular area of space, then it will have a greater mass on average than the surrounding dust cloud, and therefore has a stronger gravitational attraction, so it will attract more dust, and over a period of millions of years, as the mass of the dust cloud increases, it will attract more dust, gain greater mass and contract towards its centre. As the gas cloud contracts, two things happen. The first is that rather like water going down a plug hole in the bath, it will begin to spin, and as it spins and contracts, like an ice skater pulling their arms in in order to spin faster, so the conservation of angular momentum will make the gas cloud spin faster and faster. The centrafugal force of the spin will counteract the gravitational force in the plane of the spin resulting in a rapidly spinning disk.The disk is too unstable to form a star, so the disk must rid itself of nearly all its angular momentum in order to be able to form a star. It appears that stars do this in one of two ways . The most popular is that perturbations in the disk lead to the gas cloud splitting in two so that a binary star system is formed, i.e. two stars orbiting around a common centre of gravity, so that the angular momentum of the system is locked up in the orbital motion of the stars. If we look out into the universe, we can see that about 60% of stars are in binary systems.

The second method by which the spinning disk of gas is able to rid itself of sufficient angular momentum to form a star is to form planets, and this process will be discussed in greater detail later.

The second thing that happens as the gas cloud contracts is that it begins to heat up. Interstellar space is not completely cold, the heat of the Big Bang has been spread through the universe, so now interstellar space is at about 3 degrees Kelvin, 270°C below zero. The concentration of matter in interstellar space is about five to ten atoms per cubic meter. The concentration of atoms inside a star is many billions of atoms per cubic millimetre, so the star must have condensed from an area many billions of miles across to an area a few thousand miles across, concentrating the remaining heat in the universe as. it goes. The heat is a result of the atoms bouncing off each other, but once the temperature and pressure reach a certain point, they can no longer bounce off, and begin to fuse, becoming a giant, controlled hydrogen bomb.

At this point the star stops contracting and begins to give out light and heat. Once this process begins, there is a short settling down period as the outward force of energy and particles that become the stellar wind counteract and balance the gravitational force of the mass of the star. This is rather like a balloon, in which there is a balance between the pressure of the air inside which is trying to make it expand, and the tension of the rubber which is trying to make it contract. Some stars never settle down to an equilibrium m state, and oscillate in brightness all through their time on the main sequence. These stars, known as Cepheid Variables, vary their output of energy by as much as 40%, which would make life around them impossible. They have been invaluable to earthlings, however, as the period of oscillation is proportional to their absolute magnitude, so by calculating the absolute magnitude and comparing this with the apparent magnitude, we can calculate how far away the star is. It was by this method that the first attempts at calculating the distance to other galaxies was made.

The lifetime of a star depends on its mass, as the gravitational force of the star determines the internal pressure and temperature, and therefore the rate at which it burns its fuel. Large stars burn hot, but are short-lived, small stars burn cool, but for a long time.

Stars of greater than 1.4 times solar mass would not burn long enough for any living systems to emerge. Stars that formed long before our own, and there are some that are almost as old as the universe itself; could not support life for the following reasons:

Scientific investigation tells us that when matter formed after the big bang, 85% of matter by mass was hydrogen, almost all of the rest being made up of helium with a trace of lithium. We live on a planet with a core of iron, and the surface that we inhabit consists mainly of oxygen, nitrogen, carbon, calcium and a variety of metals and heavy elements. So where did all these things come from? To understand this we need to look at how stars die.

Our sun has been burning steadily for about 4.6 thousand million years, and is about half way through its life. More massive stars can burn out in a hundred million years or so. Once a star has burned up all its hydrogen it contracts and heats up under gravity as the outward force of the nuclear reaction no longer balances the inward force of gravity. Once the temperature has risen sufficiently, the helium that was produced by the hydrogen fusion would itself fuse to form heavier elements such as carbon and oxygen. These reactions do not produce much energy, and the star reaches crisis point as the heavier elements begin to combine to produce iron, as this reaction absorbs energy rather than releasing it. A G type star such as the sun will eventually contract quietly to form a white dwarf or neutron star, but stars much more massive than our sun collapse catastrophically in a fraction of a second as the nuclear fire goes out and gravity takes over. The result is an enormous explosion called a supernova, when, for a few seconds, the temperature and pressure are sufficient to fuse elements heavier than iron such as plutonium, gold etc. These heavier elements are blasted off in to the universe at immense speed, where they eventually form interstellar gas clouds that condense to form new stars seeded with the heavier elements.

We can see from this that it was not possible for life to exist long before the formation of our own solar system, as there was not sufficient time for the heavier stars, which are quite rare, to go through their cycle of short life and violent death to seed the universe with the heavier elements that are necessary for life to exist.

From detailed studies of star formation astronomers can calculate the about ten stars per year are formed in our galaxy, but few of these are capable of supporting life. As we have seen the majority of stars are in binary systems, where planets are unlikely to form, and if they did then the gravitational perturbations and temperature variation would make life impossible. The stars that are a little older than the sun contain too few heavier elements to support life, those that are a little bigger do not live long enough, and many of the remaining stars exhibit too great a variation in energy output to be able to support life. Many stars that are similar to the sun are in the wrong place, too near the galactic centre, where exotic events that would be fatal to most conceivable forms of life regularly occur. The sun has burned steadily for five billion years with a variation in output of less than one percent, we orbit a very rare star.

fp - The fraction of stars, once formed, that will have planetary systems.

There is little direct evidence of planets orbiting other stars. Planets are so small in relation to stars that there is little hope of ever being able to observe them directly. Our best bet is to look for stars that wobble as Jupiter size planets orbit around them. The best bet for this appears to be the star 36 Ursae Maj oris A, though the evidence is a little tenuous. Over the last few years astronomers have anounced the discovery of a number of planets orbiting stars, all dicsoverd by observing the way stars wobble slightly and extrapolating what kind of planet would be needed to cause that sort of wobble. There are many reasons why a star might wobble, and personally I find the explanation that the cause is a planet 1.5 times the size of Jupiter in an orbit closer than Mercury is to the Sun rather unconvincing. I think that largely these announcements are the result of wishful thinking rather than anything else. As the physical evidence is rather inconclusive, we must turn to theoretical issues to determine the proportion of stars that have planetary systems.

If we look at the solar system, we see that 99% of the angular momentum of the system is tied up in the orbital and axial rotation of the planets, most of it in Saturn and Jupiter, though the planets only account for 1% of the mass of the system. The sun itself rotates on its axis only about once every 25 days. This is an important observation, because, as mentioned above, a spinning disk of hot gas cannot form a star until it has rid itself of most of its angular momentum, and it seems that this can only be done by either forming a binary star system or forming planets to absorb the angular momentum. It would therefore seem that stars that are not part of binary systems must have had to form planets as they condensed from a spinning disk in order to be able to form a stable star. The prevailing opinion amongst astronomers is that planets must be common around single stars, though single stars appear to be in the minority.

ne - The number of planets having an environment suitable for life.

Computer simulations of planetary formation show that planetary systems will tend to end up looking .much like our own, having a number of very large gas giants in the outer orbits and smaller and more dense planets in the inner orbits. This is because planetary formation is rather like the fractional distillation of crude oil, where the heavier parts of the oil distil closest to the heat source while the lighter parts condense progressively as they cool.

Although it seems that most single stars are likely to have Earth like planets in similar orbits, this by no means indicated that these planets would be suitable for supporting life. It also makes it rather unlikely that massive gas giants would exist in very close orbits to their star as the astronomers would have us believe.

Computer simulations by Michael Hart have shown that had the earth's orbit been 5% closer to the sun then the primordial water vapour outgased from volcanoes would not have condensed to form the oceans but would have remained in a gaseous state. This would have created a runaway greenhouse effect that would have turned the earth into a copy of Venus with a surface temperature of 800°C and a permanent cloud cover of sulphuric acid.

Michael Hart has also calculated that the Earth's biosphere is viable for another 450 million years, about as long as there has been life on land. This means that 90% of Earth's evolutionary window has already passed. This will be a vital factor in later arguments, as it means that there is very little spare time for life to evolve. Had the sun had a slightly shorter life span, or life been a little later starting off, or if evolution had been a little slower developing then there would not have been time for a technological culture to develop on Earth before the conditions suitable for life came to an end.

The Earth's climate is a complex system that many scientists have attempted to model on computer. These simulations always ended with the same result, with the Earth at a uniform temperature of -17°C. At this temperature the earth's climate is stable and unchanging as the albedo, or reflective property of the surface is such that all the radiation received from the sun is reflected back. If the orbit of the earth had been 1% greater then the reduced radiation from the sun would have left the earth in exactly this state, permanently frozen.

The range of orbits around a star in which a planet can avoid runaway greenhouse and glacier effects is known as the Continuously Habitable Zone(CHZ) and depends on the mass of the star. The greater the mass, the larger the CHZ, but as we saw earlier stars of greater mass than the sun do not live long enough for life to develop. Stars smaller than the sun may have a CHZ that is too small for a planetary orbit to remain within.

If the mass of the earth had been 10% greater than it is, then the greater amount of outgassing when the planet was formed would have resulted in an enhance greenhouse effect such that there would be no orbit in which the earth could retain liquid oceans.

If the radius of the earth had been 6% smaller then the escape velocity of earth would be such that most of the outgassed material would have escaped into space, and the earth's gravity would not be strong enough to hang on to the ozone molecules that are vital to land based life on this planet. There was no life on land until the formation of the ozone layer 500,000 years ago.

The earth has an extremely strong magnetic field in relation to its mass and angular momentum compared to the other planets. This magnetic field is vital for maintaining the ozone layer and protecting us from other forms of radiation from the sun, which are drawn by the magnetic field of the earth into the Van Allen belts. It is the interaction of the Van Allen belts after a burst of radiation from a solar flare interacting with the upper atmosphere that creates the Aurora Borealis, the northern lights.

The earth has a very active molten core which results in large tectonic plate movement which isolate gene pools and speeds up evolution.

These effects may well be due to our moon, which is extremely large in relation to the size of the planet. The gravitational effect on the earth's core may well explain the earth's strong magnetic field and rapid tectonic plate movement. The moon affects us in other ways, as we will see later life probably started in tidal pools, and the colonisation of land would have been much slower without tidal forces that allowed aquatic creatures to take advantage of tidal areas and evolve into land based creatures.

It was originally thought that the moon was not formed with the earth, but captured by it later, though there is now some evidence that the moon formed as a result of a massive colission between the Earth and an Asteroid that ejected large amounts of material into Earth orbit soon after the Earth was formed. Since encounters between two bodies in orbit almost always result in either a flyby or collision and destruction, such double planet systems as the earth/moon may be very rare indeed.

Putting these factors together, it appears that the value of ne must be very small indeed

fl - The probability that life will develop on a habitable planet.

The process by which life started on this planet is not terribly well understood, though there are several theories as to how it might have happened.

Life on earth is formed out of a few organic compounds that have been created from material present at the formation of the earth. These are primarily amino acids, mono nucleotides, and sugars. These are thought to have formed from elements in plentiful supply on the early earth such as water, ammonia, hydrogen and methane, with energy input from ultraviolet rays from the sun, volcanic heating and electrical discharges in the form of lightening.

There are 5 basic steps through which life as we know it today emerged on Earth:

l. Small organic molecules had to form from the earth's original material

2. These small molecules had to combine to form long chains or polymers required for life.

3.In some way the polymers had to produce isolated, self replicating systems

4.Cells and multi cellular organisms had to form from the self-reproducing systems.

5. Evolution had to. act to produce the great variety of life that we see on Earth today.

In 1953 Stanley Millar, in a now famous experiment, showed that if an electrical discharge is passed through a mixture of the gasses that are supposed to have been present in the atmosphere of the early earth over a period of a week then many of the compounds necessary for life, including amino acids, would form providing that there was no oxygen present, so it appears that the first step on the road to life is a fair bet. In fact there is some speculation that life on earth was seeded from space; as some of the basic chemical building blocks have been identified in clouds of interstellar dust.

The second step is a little more problematic, as the long polymer chains necessary for life tend to be broken apart by the same energy sources that created them. Thus for the polymer chains to remain formed they must be protected from the ultra violet radiation , which was much stronger during the early part of the Earth's history as there was no oxygen in the atmosphere, and therefore no ozone layer to shield the earth from harmful radiation. The polymer chains would have been protected had they been a few feet under the surface of the ocean, but unfortunately water tends to act to break these polymer chain apart.

Experiments have shown that polymerisation will take place by one of the following processes:

l.Evaporation from water in tidal pools shaded from the sun.

2..Partial freezing in which water is removed as crystals.

3. Volcanic heating to drive off the water.

4. Attachment of the molecules to the surface of clays.

Each of these processes is common and has been shown to produce polymer chains of up to 200 amino acids, so polymerisation can arise by means of natural processes.

An alternate theory is that life originally started close to hydrothermal vents deep in the ocean, where some isolated colonies of previousl;y unknown creatures have been duscovered in recent years.

The means by which the third stage in which self replication is instigated is less clear, with estimates ranging from a one in a million fluke to the assumption that self replication will arise wherever the conditions are favourable.

Both Richard Dawkins and Stuart Kauffman have come up with plausible theories about how this might happen if crystals give rise to autocatalytic sets, where an inert crystaline substance such as clay could allow self replicating structures to evolve.

The process of biological evolution is much better understood. Oparin showed that water droplets suspended in an oily. solution can absorb enzymes and sugars and eventually split and continue to divide and grow, and by this process it is possible for self replicating organisms to form, while the well-known processes of natural selection and Darwinian evolution provide a tested mechanism by which the many species of plants and animals found today have arisen over the subsequent millennia.

In the case of our own planet, fossil records show that the first single celled organisms began to appear relatively soon after conditions became suitable. When the earth was formed it was an extremely hot and hostile place, and as it cooled and outgassed the materials that became the atmosphere and the oceans there was extremely violent volcanic activity and meteor strikes.

It seems that once the early solar system settled down the first signs of life appeared very quickly, which seems to indicate that life is quite likely to evolve if conditions are "suitable."

fi- The probability of the emergence of intelligence

If life is going to develop to the point of being able to communicate with us then it is obviously going to have to have some form of tool making capability. For a level of intelligence capable of producing the equipment necessary for interstellar communication or travel there are several steps that appear necessary .

l. development of an atmosphere containing free oxygen

2. Movement of life from the sea to the land.

3. Emergence of hands and eyes

4. Appearance of social structures.

Oxygen is a very reactive substance, as anyone who owns a car can verify. It doesn't take long for the free oxygen in the atmosphere to reduce your shiny new car into a heap of rust. If all life on earth were to cease then the free oxygen in the atmosphere would soon disappear as it combined with other elements to form oxides.

As there was no life on earth when it first formed, there was probably no oxygen. The single celled organisms that first appeared produced oxygen as a waste product from photosynthesis. Some organisms called stromatolites adapted to take advantage of the increased energy available by using chemical reactions using oxygen, while the rest perished in what was for them a poisonous oxygen rich atmosphere. This was the first of the great extinctions that I will discuss shortly. Stromatolite fossils show that they survived from 2.5 billion to 600 million years ago, when they were wiped out by the next evolutionary stage. ( Bear in mind that this is nearly half of the time the planet has existed) Organisms that used oxygen were better able to make use of the food available and embarked on the long evolutionary road that eventually led to human beings.

Eventually the oxygen high in the atmosphere formed into the molecule ozone which shielded the earth from ultraviolet radiation from the sun and made the colonisation of the land possible, though it took over two billion years for the oxygen levels in the atmosphere to reach a sufficient concentration for this to happen.

To put this in perspective, the solar system formed 4.6 billion years ago. If we think of the earth as a 46 year old
man, then he would be 10 - 15 years old before the first life appeared, in his late thirties before multicelled animals appeared, 40 before the first amphibians began to crawl over the land, and the whole history of human civilisation would fit into the last 2 weeks.

For 90% of the earth's history there has been no animal life on land, as it took this long for a sufficiently thick layer of ozone to form. The great diversity of life we see around us has evolves over the last 500 million years. It seems unlikely that a technological culture capable of interstellar communication could evolve in water. It is unlikelythat they would evolve the hands and eyes that are needed to manipulate tools, and the evolution of our complex brain is very heavily tied up with the evolution of our hands and eyes. Even if an aquatic technological civilisation evolved, they would be unlikely to be able to harness electrical energy in a conducting medium, and we have to wonder whether they would even think of communicating with other cultures if they could never see the sky. It might not occur to them that there could be any one else out there.

It seems most likely that an ETI culture would be land based, but even on land there is a big question over whether intelligence would naturally evolve. A large brain has certain survival advantages, but it also has many drawbacks. It is a voracious consumer of energy, requiring a much greater food intake and larger heart and lungs than a smaller brain. For example it would not be possible for a bird to evolve a large brain as the support system necessary would render it too heavy to fly. Human infants are totally helpless at birth and remain
very vulnerable and take many years to grow to independence, a process that requires a great deal of input from the parents so the rate of reproduction is relatively slow. The advantages ofa large brain would have to be very considerable to overcome its disadvantages, and the conditions in which this is likely to happen may not arise.

.

Sharks are perfect killing machines, many species have not evolved in any way for over 6 million years because there is no need. They are perfectly adapted to their environment, and as the environment has remained stable, there is no need to change. Although their nervous system is extremely complex, they are much less intelligent than man. It is possible to conceive of a world that is sufficiently stable that life develops, but intelligence does not evole sufficiently or fast enough to form an ETI culture. As we we have seen ,there is a definite time limit on evolution.

One of the problems with evolution is that if animals evolve to suite the environment, and the environment remains stable, then a point is quickly reached where all evolutionary niches are filled, and further evolution becomes a very slow process. If we take the hyena as an example, if it wants to evolve to be more successful at hunting, it could become leaner and faster, but then it would be competing directly with the leopard which is already leaner and faster, so it can't compete. If it evolves to become heavier and more powerful, it comes into competition with the lion, and will fail for the same reason. It is stuck in its evolutionary position, and can only make minor adjustments to its position in the ecosystem.

If this is the case, and evolution grinds almost to a halt as the population fills all the evolutionary niches available, then how did such a wide variety of life evolve on land in just 500 million years?

Darwin thought of evolution as a long, slow but steady process. The fossil records did not support this, as they tended to show that evolution goes in fits and starts. At first it was assumed that this was because the fossil record was incomplete, but as more and more fossils were discovered, this became less and less plausible. One of the great puzzles was that it seemed that the dinosaurs disappeared and were replaced by mammals very suddenly.

In 1978 geologists discovered a very thin layer of iridium rich soil in the boundary layer between the Cretaceous and Tertiary periods, exactly when the dinosaurs disappeared. Below this layer there were dinosaur fossils, above it there were none. Iridium is a very rare metal on Earth, but very common in meteors. This suggested that the dinosaurs were wiped out by an enormous meteor strike. Initially controversial, this theory has steadily gained ground as more evidence has appeared to support it. What is more it is evident that there have been a series of meteor strikes that have caused a series of mass extinctions going back till fossil records began 800 million years ago. There have been twelve mass extinctions in this time, and many smaller ones. The most severe, the Permian extinction, wiped out 96% of species on earth, and it is estimated that 90% of the forests burned. Most of these extinctions are accompanied by an iridium layer, and some can be matched with the many huge craters around the world. The force of a lOkm diameter meteor hitting the Earth is equivalent to 10,000 times the power of all the nuclear weapons on Earth put together, so the results are truly catastrophic. As the meteor travels at about 30km per second the kinetic energy released as it is slowed down by the Earth is truly enormous, sending a fireball many thousands of degrees centrigrade round the planet and setting fire to all the vegetation. Next to the iridium layer in the K-T boundary is a layer of soot from the global firestorm that engulfed the planet. Those creatures that weren't barbecued in the firestorm would have starved to death.

It is not necessarily the most adaptable of species that survive, often it is pure luck. Mammals and dinosaurs originated at about the same time, but the dinosaurs ruled the Earth for 140 million years, while the mammals remained on the sideline, small furry nocturnal creatures rather like a squirrel. In the aftermath of the K-T boundary meteor strike dust in the atmosphere would have made the Earth very cold and dark for a considerable period. This did not suit the dinosaurs, who died out. The warm blooded, furry, nocturnal mammals exploded across the planet. After 140 million years of very slow evolution, they achieved immense diversity within 100,000 years as the evolutionary niches previously occupied by the dinosaurs were opened up to them.

These periodic mass extinctions have immeasurably speeded up the rate of evolution on this planet. At each extinction a few percent of the most suitable or most adaptable survived and repopulated the planet until the next extinction, when the most adaptable few percent were able to pass their genes on the next wave of life to proliferate across the planet.

The important question for us is whether mass extinctions are a feature of all evolutionary systems or are peculiar to us. There is a growing body of evidence to suggest that these mass extinctions occur every 26 million years rather than at random, so there must be some process that sets them off. One explanation of these regular extinctions is that they are due to a combination of the Earth's orbital motion and tectonic plate movement, but the presence of iridium strongly suggests that they are caused by wider cosmic events. A possible explanation. is that they are caused by a companion star to the sun which orbits us every 26 million years. If it was quite small with an average distance of 2.5 light years then on its closest part of the orbit it would disturb the Oort cloud of cometary nucleii that surround the planets and send a shower of meteors or cometary nucleii speeding towards Earth over a period of about a million years. This fits in well with recent fossil evidence which shows that extinctions happen in pulses over a period of about a million years rather than all of a sudden.

If this theory is correct, then the earth may be a very special place indeed. Without the enormous acceleration in evolution brought about by this process, then it is unlikely that intelligence would have time to develop within the lifetime of a normal star.

fe - The probability of emergence of an ETI culture.

If intelligent life were to develop on another planet, would they necessarily want to communicate with other cultures?

If we look at the early civilisations on the Earth we can see that the remains of these cultures are the great edifices such as Stonehenge and the pyramids of Gizeh which were early cosmic observatories. All over the planet enormous time and energy was invested in erecting huge structures to study the heavens and determine the dates of the winter and summer solstice and the equinoxes. The undertaking of these projects must have added immeasurably to our understanding of astronomy and mathematics and accelerated the development of a technological society. As with aquatic life, if the composition of the atmosphere was such that there was constant cloud cover or the sky was constantly obscured in some other way, then intelligent cultures might develop too slowly, and would probably not even think that there was anything else in the universe other than themselves.

Since the beginning of civilisation until relatively recently the prevailing opinion has been that the Earth is at the centre of the universe and that God created the universe for man's benefit. Given the evidence available this is a reasonable conclusion, and it is likely that other cultures would have similar beliefs. In a geocentric, God created universe the possibility of other life forms may be inconceivable. It is possible to imagine a world where intelligence and technology develop, but cultural beliefs do not allow the possibility that other life forms exist.

L The lifetime of a communicating civilisation

If there is an ETI culture out there valiantly trying to communicate with us, how long will they continue to do so?

There are many reasons why they may not. It has taken one third of all the time that there has ever been to evolve intelligent life on earth, yet within 50 years there are many reasons why we may not continue to survive. Firstly we must survive the perils of our own success, such as nuclear war, overpopulation, degradation of the environment and genetic deterioration that threaten to bring our technological civilisation to an end. Our technology is based on the burning of fossil fuels which will run out sometime in the next century. In the future we may simply not have enough spare resources' to undertake projects with little chance of success and little benefit if they do succeed.

Even now the Hubble Space Telescope is often criticised on the grounds that the money could have been better spent on more direct benefits to people, despite the fact that the whole project cost less than a nuclear submarine, and nobody seems to be complaining about the cost of them. We have put very little effort into looking for signs of life elsewhere in the universe despite the overabundance of cheap energy. NASA recently withdrew funding for the most recent SETI project as it was considered not worthwhile. Other cultures without this abundance of energy may not bother at all. Most SETI projects are now carried out by amateurs with very little funding.

In 1974 modifications were made to the largest radio telescope in the world at Aracebo in Puerto Rico to allow it to transmit a message. This radio telescope is built into the crater of an extinct volcano, and is so large that it is bigger than all the other radio telescopes in the world put together. The message had a power output of 20 trillion watts, and was aimed at the globular cluster Messier 13. If there is an ETI culture in the path of the signal, and they have a receiver of the right type and of sufficient power to receive the signal, and it is pointed in exactly the right direction and tuned into the correct frequency at exactly the right time as the signal, which only lasts for a few seconds, and they are able to decipher the message and send one back, and we have our receiver tuned in and pointed in the right direction at the right time, which is exceedingly unlikely, then we may expect a reply in about 50,000 years, by which time we will have forgotten what we said in the first place. It is easy to see how NASA came to the conclusion that these projects are a waste of money.

We are, of course, sending out less powerful markers of our presence in the universe, a constant stream of energy spreading in all directions at the speed of light. Unfortunately as most of it is in the form of American and Australian TV soap operas it is unlikely that anyone receiving it would ever want to reply even if they could understand it. I often wonder what aliens would make of life on Earth if all they had to go on was TV transmissions. Perhaps they would conclude that the dominant life form is the motorcar, and that they keep humans as slaves.They would certainly wonder how we excrete our bodily waste. In any case would we make efforts to visit a planet that appeared to be inhabited by the characters in Nescafe and Wash'n'Go adverts? So far these transmissions have only reached 100 stars, none of which are suitable candidates for supporting life.

Joking aside, the real point is that if it takes around 5 billion years to evolve a communicating culture that only survives for a few tens or hundreds of years, then the probability of two cultures reaching the point of being capable of interstellar communication at the same time and are sufficiently close to each other to be able to communicate is vanishingly small unless the galaxy is packed full of intelligent life, and as we have seen, this is exceedingly unlikely.

That brings us to the end of our consideration of the elements of the Drake equation. A criticism of this approach is that it assumes that intelligent life must be like ours in order to be able to develop. Apart from the fact that if an ETI life form was radically different from us we would be completely unable to communicate, there are certain limitations on the sort of creature that can evolve to an intelligent state.

To evolve intelligence a life form must be capable of passing on information through the generations through a chemical structure containing information. There are only 2 elements capable of forming long enough chains to contain this information, Carbon and Silicon. Unfortunately silicon only forms these chains at temperatures below -200°C. There may be silicon based life forms swimming in a sea of liquid nitrogen on some far off planet, but at these temperatures chemical reactions are so slow that they would not evolve very far before their solar system came to an end. It would seem that a warm planet with carbon based life forms using oxygen based chemical reactions is the only way that life can evolve sufficiently quickly for intelligence to develop in the time available.

I have discussed the various factors affecting the probability of being able to communicate with other beings in the galaxy, and my conclusion is that it is exceedingly unlikely. In conclusion I would like to look at the possible consequences if we did receive a message from outer space.

The first consideration is that the difference in physiology and culture make it exceedingly unlikely that we would ever be able to gain any useful information from it.. Communications sent from Earth are based on the assumption that the rules of mathematics are woven into the structure of the universe, so any other culture is likely to recognise prime numbers on which the communications are based, but there is no way of knowing that this is true.

One of the hopes for contact with another culture is that we will gain new scientific information and new technologies. Apart from the fact that any scientific information from a more advanced culture is likely to be as useful to us as a wiring diagram for an IBM computer is to an aboriginal tribesman, technology and culture evolve together, and the former is very dangerous without the latter. If Ghengis Khan had had access to a nuclear arsenal he would have been much more inclined to use it as the technology and culture of international negotiation that have prevented nuclear war in the past would not be available.

We do enough damage with the technology that we have developed ourselves, and there is little evidence that it is wisely or humanely used. The consequences of having advanced alien technology or limitless free energy available could be catastrophic, especially as it is unlikely that it would be shared between nations.

Any extraterrestrials that did reach Earth would have a far more advanced technology than our own. In 1992, during the celebrations of the 500th aniversary of the 'discovery' of America, two films were shown one after the other. One was a superman film is which three characters who were the embodiment of all evil, and had far superior technology to Humans, took over the United States, captured the President and tried to turn Humans into their slaves. Luckily Superman saved the day. The second film was about the discovery of America, a situation in which a few men with superior technology took the land for their own, stole all the wealth, captured the native king, and wiped out most of the native population. Unfortunately the native Americans did not have Superman on their side. I'm not sure if the programming schedule was meant to be ironic, but I found it so.

There are few examples on Earth of humans with superior technology sharing their technological advantage with more primitive cultures, they usually engage in mass genocide or slavery. It is my impression of history that the more a culture invests in colonisation of other cultures, the less interested they are in preserving those cultures. Extrapolate this to the effort involved in colonising a culture fom another galaxy, and the results for us could be rather nasty.

My conclusion, then, is that there are probably many planets that harbour life, but they do not survive long enough or evolve fast enough to develop intelligent life. 0ther intelligent civilisations in this galaxy are already long dead, or more likely will not develop until long after our civilisation is over. The chances of an ETI culture existing at the same time as us and near enough to communicate are virtually zero. Even if they do exist and are trying to contact us the chances of hearing them and understanding what they are saying are virtually zero, and if we did manage to receive and understand a message, or were visited by aliens, the consequences might be quite unpleasant.

BIBLIOGRAPHY
NATIONAL GEOGRAPHIC Jan 94 New Eyes on the Universe
NATIONAL GEOGRAPHIC Aug 90 Neptune, Voyagers last picture show.
Voyage of the century.
Jun 89 The March Toward Extinction.
MILNER,R. 1990 The Encyclopedia of Evolution.
COHEN,N. 1988 Gravity's Lens
WALDROP,M.M. 1992 Complexity.
CASTI,J.L. 1989 Paradigms Lost.
HAWKING,S.W. 1988 A Brief History of Time.

Numerous articles from New Scientist i.e.

Are we all Martians 12/9/98

Bolts from the Blue 12/9/98