Where is everybody?

Professor John D. Barrow

Our title today is a question.  It is a rather famous one for everyone working in astronomy, and it?s one that really as yet has no very definite answer, but I hope by the end of today?s talk, you will see something new and something more about the whole issue of life in the universe than perhaps you have thought about before.

This question was first posed by a famous physicist called Enrico Fermi.  He was an Italian.  He was probably the last all-round physicist there has been, someone who is a world leader in theoretical physics ? he discovered the first theory of the weak interaction and much of what we know about nuclear physics.  He was one of the people behind the Manhattan Project in the Second World War, so he was a skilled experimentalist as well as a theorist.

In 1950, he asked this question while at a meeting where people were thinking about the possibility of life on other worlds and even communicating or listening to signals from other worlds.  He recognised that really there is a very embarrassing silence in the universe, that we did not receive any signals, and it seemed to him very easy to imagine how signals could be sent to us and very easy to imagine how life could evolve and spread through the universe.

In 1951, you have to remember that there is something of a fantastic context to these deliberations.  They did not just come out of nowhere.  The first flying saucer was seen in June 1947 by a Mr Kenneth Arnold, who was an American businessman flying his private plane between Oregon and Idaho and, en route, he had this strange experience where he saw what he described as a saucer-like pattern of light flashing through the sky.  He followed it for a little time, and then when he landed, and talked to the press, he kept describing it as ?like a flying saucer?.  That is where this term came from.  Ever after, anything in the sky that was mysterious or unidentified was a flying saucer.  The whole concept of a flying saucer and pictures of flying saucers then came to dominate the whole image of extraterrestrials and life in the universe.

Also, at about the same time, something else happened in New York which just added to the whole momentum of the idea.  In New York, a rather strange problem had arisen.  It seemed that what Americans call garbage cans, what we call dustbins, were disappearing from the City, so everybody was finding that their garbage can was missing.  In the end, this turned out to be some people from Idaho and Nebraska who were simply driving them off in trucks and selling them there! 

But a famous cartoon in the New Yorker brought together the two ideas, with these gentlemen gradually loading the garbage cans into their flying saucer and taking them off to the other side of the universe for reasons best known only to themselves!

Later, in the 1950s, we begin to see fantastic science fiction comics, the great era of science fiction.  Everything is dominated by flying saucers, by extraterrestrials arriving.  Even the military get involved.

This climate provoked some astronomers, notably the radio astronomer Frank Drake, to try to think more seriously, systematically, about the likelihood of extraterrestrial life and the possibility of detecting evidence for it.  Drake came up with a famous formula that became known as the Drake Equation.  It is rather fanciful but it is interesting as a guide to one?s thinking.  It was an attempt to estimate how many civilisations there might be willing and able to get in contact with us, and so he was thinking about civilisations in our galaxy.  He thinks of lots of different factors that could influence this.  He assumes they are independent, which they undoubtedly aren?t, but if you assume they are independent, then you can multiply the probabilities together to get the overall probability, and you multiply by a lifetime the number of civilisations who might contact you.  So what does he pick?  It depends on the rate of forming stars in our galaxy ? obviously more stars, more potential sites.  It depends on the fraction of stars that have planets around them.  At this time, people had no idea whether there were any planets around other stars at all, so that factor could have been zero, or it could have been one, if every star has a planet round it.  And then the number of those planets which are life-supporting, so which have atmospheres, which have a good enough gravitational strength, or perhaps have water on them; and then the fraction where life actually happens to evolve.  Next, having got life, what fraction of those do you actually get intelligence evolving as well?  And of those, how many are willing and able to engage in interstellar signalling?  And finally, how long are they on the air for, so what is the lifetime of sending signals?

So you can see how this formula, that Tommy Gold once called the Nonsense Formula when I asked him about it came about.  Almost every factor was hugely uncertain and you could get answer from zero to 100 billion.  So it really was not very useful as a predicted object, but it focused people?s attention on what really were the uncertain links in the chain.  Drake became a key individual in this whole business, eventually being instrumental in creating the SETI, the Search for Extra Terrestrial Intelligence.

If we move on to more specific aspects of the factors in that equation, a key concept is what astronomers call the ?habitable zone?.  So if you have a star then there is a comfortable, temperate region around the star where a planet is able to be life-supporting.  So there are three zones shown here around the central star. 

If you are too close to the centre, you will be in what we might call a hot zone, and we will end up like Venus, with surface temperatures of 700 or 800 degrees, runaway greenhouse effect, so hot all the carbon dioxide has been released and all the new radiated heat is trapped.  This is an inferno; it is not a place to first evolve any interesting form of life.

If you are too far away, for example you are out in the cool zone, somewhere like Mars, you are very far away from the central Sun.  You are not really receiving significant amounts of heat from the central Sun.  In the case of Mars, you actually don?t have a magnetic field and so the wind of particles that have blown out from the surface of the Sun gradually strip off the atmosphere that you once had.  Mars has no atmosphere any more. 

In the case of the Earth, it sits in this sort of Goldilocks region, where things are not too hot and not too cold.  It is possible for water to be liquid over most of the surface.  The complex atmospheric behaviour, plate tectonics, and the is a magnetic field whereby that wind of charged particles blown out from the Sun gets steered around the Earth by the magnetic field, play a role in ensuring that the atmosphere does not get stripped away.

I said back in the early days of the Drake Equation, people had no idea whether the formation of planets was a fluke that was triggered around the Sun by some vast supernova explosion and therefore unpredictable or whether it was something generic and systematic in the universe.  We now know the answer to that question.  Modern technology enables us to see planets around other stars, and almost every few days, you can hope to discover a new one.  So even while I was preparing this slide, things changed.  So there are 212 planets discovered around other stars, and 21 systems of planets, so where there is more than one planet around a single star.  So there is really no doubt that planet formation is something that is part and parcel of the formation of particular types of star. 

What happens in effect is when the star forms, there is lots of debris left around ? the sort of thing that you see in the lanes of dust around the rings of Saturn ? and that debris gradually accumulates and sweeps up everything on its orbit, getting bigger and bigger and bigger until eventually, it is all swept up and you have got a planet then sitting on that orbit.  The material going round in a different orbit will accumulate and make a different sized planet. 

These planets that are now so commonly found are unusual in some respects because, so far, we can only see rather big ones.  The way they are detected is by looking at the effects of the planet?s motion on the position and motion of the central star, so a little perturbation, a little wobble, is created by the planet moving around it.  So this means that we can only really see the effects of big planets, with just one or two exceptions.

So here is a picture of the range of masses of these planets that have been found, the number along here, and they are units of the mass of Jupiter.  So Jupiter is about 1,000 times more massive than the Earth.  It is a great ball of liquid and gaseous hydrogen.  It is not solid.  So are most of the planets around that size.  Some are much bigger, up to ten times bigger.  Going down here, you are getting ones that are maybe just about 200 times the mass of the Earth.  One has been inferred to be present with a mass of only seven or eight times the mass of the Earth in a well-studied system.   So these are solid planets.  In a few years? time, gradually there will be the capability to see and detect Earth-sized planets, and also to analyse what is present in their atmosphere.

So at first it sounds as though these are all potential sites for life, but all of these planets, except perhaps just one that has been seen in this programme, are very unusual in one other respect: that is, they don?t orbit around their star in almost circular orbits, like the objects in the Solar System do.  So the Earth sits in its habitable zone, and because it goes round in a circle, it stays in the habitable zone.  If it were to have a highly eccentric orbit, it would freeze for half the year and then fry for the other half, and it might precess around, gathering up more and more material as it went, becoming bigger and bigger and bigger.  But all these planets that are found by these extra-solar planet searches have orbits which are extremely elliptical and eccentric. So these planetary systems are different to our Solar System in one crucial respect, and it could well be that in order to give life a good chance of beginning to evolve, to give those stable early conditions, we do need an almost circular orbit.

Well, why haven?t we heard from anyone if there are people on other planets in our galaxy?  There is a whole range of responses to this question.  One type of response you might say is that they have already called but we were not listening.  So, before 1961, people were trying desperately to contact us, but they did not even get an answering machine message, and so they just gave up and moved elsewhere.  Some people, rather fancifully, proposed in Fermi?s day that in fact extraterrestrials were already here and that they were in disguise ? that they were Hungarians!  This was meant as a compliment because some of Fermi?s colleagues where Hungarian ? the rather remarkable John von Neumann for example, for all intents and purposes, appeared to be from another world, such were his mathematical and computational abilities.

But a more serious possibility, as we will see in a moment, is that they are not perhaps quite as big, energetic and obvious as we might have imagined.  We have seen the way modern technology is moving on Earth ? things are becoming smaller and smaller, more nanotechnological ? and it could well be that an extremely advanced technological civilisation would have moved very much in that direction and become imperceptibly small.  Space probes may be the size of pinheads rather than the size of the probes that we launch.

But if extraterrestrials were parking their spacecraft in the neighbourhood of the Earth, however small they might be, where would they do it?  There is an interesting answer to this.  If you consider the Earth and the Moon system, then the two gravitational pulls of these objects in the gravitational field of the Sun means that there are four special places, called Lagrange points, where the net gravitational force cancels out.  Therefore, if you want to park a spacecraft, as we do with our satellites and astronomical observatories, these are the places you should do so because they will be least susceptible to tidal forces and wobble and thus they will be more stable.

Not all Lagrange points are the same. Some will be unstable, in that if you are just displaced very slightly from them, you will move steadily away. In contrast, others will have a range in their vicinity which is stable. This means that if you perturb yourself around them, you will only oscillate and return to that point.  So if you are looking for extraterrestrials, these are the places to look.  These are the types of places where they would be parking, and a sort of congestion charge would be appropriate in these regions.

I mentioned von Neumann just now.  There are all sorts of mad stories about von Neumann but I will resist the urge to tell any of them.  Von Neumann was one of the people responsible for the formalisation of quantum mechanics and the invention of computers as we know them.  But he also, as a result of that work on computers, had the idea of self-reproducing machines, whereby you could write a programme which would create a replica of the object that was reading the programme.  By gathering raw materials from its environment, it makes a copy of itself, and pastes on it a copy of the programme to make yet another copy of itself.  It is the sort of thing that we do in a biological way, but he showed how this could be done by computing machines.  We see it also in the mineral world ? things like crystals are very good at making replicas of each other.  So if you have one copper sulphate crystal and you dissolve it in water, you make many replicas of that crystal.  Von Neumann had this idea of creating machines which would make replicas of themselves. 

Back in the1980s people began to think about this as a possible gloss on Fermi?s argument.  Once a civilisation has the ability to make such machines, it can very easily seed the galaxy with self-reproducing probes.   All the raw materials are around in the galaxy, in dust, gas, rocks and so forth, and such machines can be programmed to keep making copies of themselves and explore.  Since we do not see these machines everywhere, they could be imperceptibly small. But one imagines that perhaps they are not there at all, so perhaps there is some deep reason this has not been done.  An extremely wise civilisation might have foreseen a future disaster of having begun such a process, that these start to behave like a new living organism and are perhaps ultimately uncontrollable.  Alternatively, there may be simply operational problems, as the airlines say, that it may be that the failure rate for probes is really very high and the chance of a critical number of them surviving a long time is rather low.

A lot of people used to think, and many people still do, that the way to detect the visible forms of intelligence in the universe was to try and categorise civilisations by the amount of mass, garbage and waste energy that they are going to produce.  A famous classification from the 1950s, late 1950s, envisaged just four types of civilisation.  The first would be a civilisation which has a technology which is sufficient to manipulate their planet.  So we are in this position ? we can reclaim the sea, we can inadvertently or advertently change the atmosphere, we can bring about melting of ice caps ? so we can do something to our planet.  The amount of energy you need to expend to manipulate your planet determines the amount of signal or the amount of raised heat and light that your civilisation will be emitting.  A good deal of the information that our civilisations boom into space are television signals.  The first ones began in the 1930s, and so if you are far away and that light front of television signals is racing out through the universe, then the first ones to be received would be Hitler?s broadcast from the mid-1930s.

The second level of civilisation is more sophisticated and energetically capable.  You would be able to manipulate the solar system. Going up further, you might have a civilisation that is able to manipulate stars.  It might live in a situation where there are two binary stars and it might be able to manipulate the behaviour of the stars by changing their mass in some way.  More fancifully, you could imagine civilisations that are able in some way to manipulate the structure of their galaxy, and ultimately, to do something that affects the dynamics of the universe.  So you can see, as you move up the scale, from type one to type four, to the end of the scale, you have civilisations that apparently leave more and more trace of their activities.  Thus it has been argued that the more advanced a civilisation, the more waste heat and energy it leaves.

But others felt that it was much more likely that civilisations that become highly advanced would actually become more and more capable of engineering and manipulating things that are much smaller than before.  You could imagine a type one civilisation ? this is rather like Neolithic man.  He can manipulate and engineer things that are about his own size, so he can build houses, files, change the landscape, catch animals, and so forth.  If we move up the scale, we are in a situation where you can start engineering yourself, so you can do transplant surgery, carry out various types of genetic engineering and replace non-working organs. 

If you further along scale beyond the genetic, you then reach the stage of being able to manipulate molecules and atomic bonds ? design materials, design medicines, design chemistry for your own use.  If you go smaller still, from molecules to single atoms, you start to reach the scale of nanotechnology and other forms of artificial life.  We are just on the edge of that technology, so a way of engineering machines, motors, devices that do things, down on the scale of single or small numbers of atoms.  Imagine a small machine that swims through your bloodstream, and if it detects the presence of blockages in arteries or veins, then it just excavates the material out to keeps things clear and running smoothly.

Extrapolating further, down to type five, you would have a civilisation that is able to do engineering with nuclei of atoms.  To some extent, we can do that ? nuclear energy, nuclear bonding, explosions and so forth.  The next step that we have not been able to do is to make complex structures from elementary particles.  We only make elementary particles and we watch them colliding, but we do not really have an engineering that is based upon elementary particles.  Ultimately, the acetate for this type of civilisation is to be able to manipulate the structure of space and the passage of time in a way that is advantageous to their existence.  What is curious about this advance from type one to type omega is that such civilisations, as they become more sophisticated leave less trace.  They use less energy and less raw materials.

Going back to the more psychological aspects of things, we might ask, supposing that there are these advanced civilisations, why don?t they signal?  One answer is that we may be too boring, that there are hundreds of thousands of civilisations approximately developing and evolving along the lines of our own on Earth, and for an extraterrestrial civilisation that has advanced to discover us it is a bit like discovering yet another species of beetle in the Amazon jungle ? we are in the catalogue, number 697143/A.  Perhaps some graduate student will eventually do a PhD thesis on it in a thousand years time, but there is not a matter of great urgency to study us, let alone get in contact with us.  So we may, unfortunately, just be too boring, that we are entirely typical, we are following a sequence of events that is perhaps going to lead to obvious disaster in a relatively short timescale, and it is not really cost effective to invest in studying yet another example.

On the other hand, you may be pleased to know, it could be that we are too interesting.  We are so interesting that there is a policy of non-interference, that we are watched by extraterrestrials rather like birdwatchers in a perfect hide.  They want to see and learn what is happening; they do not want to interfere; they do not want to effect what is happening, so we are a sort of laboratory for the study of what can happen over very long periods of time.

On the other hand, we may be so primitive that either we are not in a position to actually receive or access the type of signals and information that is being sent.  That may be a type of self-selection process, whereby there might be a sort of club, call it a Galactic Club, in the galaxy and these are very advanced, relatively sophisticated societies who talk to one another, but they do not want any old people to join the club, so you will be pleased to know that this psychology has transferred on the galactic scale.  The reason is because we might turn out to be irresponsible in some way, we have not sort of paid our dues, we have not shown that we would be responsible members of the club, and so the self-selection process for the club is that we have to reach a certain stage of development, a type of very high technological stage of development, that shows that we can pass through various predictable crises without disaster.  Those crises, you could well imagine, things like having the ability to create nuclear weapons but not destroying most of your planet with them, having the ability to change the climate in a catastrophic way but not actually doing it.  If you pass these tests, you will necessarily have a particular, very high level of technological capability, and so the club members ensure that you can only hear other members of the club by using a technology that has that level of sophistication.  You must have passed the tests of Hercules, as it were, in order to participate in the conversation.

What is the conversation likely to be like?  Well, there is always an interesting speculation that if there are people with some knowledge of physics ? and if you do not know any physics, you are not going to be sending or receiving any signals ? then there is a prime candidate for both signalling and listening, and it is the 21 centimetre wavelength line of hydrogen, so this is at about 1420 megahertz.  This is the wavelength of radio wave radiation that is given off by electron and hydrogen when it changes its spin orientation, so it is sometimes called the spin-flip transition.  It is a very slow transition which is not seen in a laboratory.  It was predicted by Ort and colleagues in Holland during the Second World War, that this should be seen astronomically.  The probability of a transition occurring is incredibly low but in an astronomical situation, there are so many hydrogen atoms together that a very large number of them would undergo this transition.  Therefore, if you know anything about astronomy, you know about 21 centimetre radiation, as it is called. 

21 centimetre radiation is very interesting.  There are various sorts of lines and distortion from the Earth?s atmosphere, background radiation from the universe, noise from other sources of radiation in our galaxy, and detector noise cut out a lot of regions as being not very good ones to listen in. However, regions exist where hydrogen and the hydroxyl combination give tantalising radiation signals.  This is a region that astronomers came to call the Water Hole, because this is the place where H and OH emit.  If you are an extraterrestrial wanting to signal or listen, this is the prime place where you would listen in the radio spectrum.

Remarkably, in 1977, a famous event took place.  At Ohio State University in America a search had been set up.  People were scanning for signals from anywhere in the universe that showed evidence of some type of intelligent origin.  On the 15th of August in 1977, Jerry Ehman running the faculty there made a famous detection, which became known as the ?WOW! Event?, or the ?WOW! Signal?.  This is a rather faint trace of data that he was looking at, and next to which he wrote ?WOW!?.  Well, what he saw was a linear reading of the intensity of radiation that was being detected by the radio telescope.  It was a rather primitive data recorder in those days ? the lowest signal level was one, and then it goes 2, 3, 4, all the way up to 9, and once you reach level 9, you then swap to letters, A, and up to the very highest of all, which would be U.  The detections were all low numbers, with just background noise, and all of a sudden, there was this fantastically high signal ? there were some Us in there and otherwise all letters.  This is the strongest signal ever detected by this telescope.  It was at 1420 megahertz, give or take a few decimal places, almost right on the 21 centimetre line.  It lasts for 72 seconds which is crucial because 72 seconds was the maximum time which a signal could be received by this beam.  Because of the rotation of the Earth, you rotate away from the source after 72 seconds and you cannot see the same source any longer.

Unfortunately, when the second beam came round 3 minutes later, this was not seen again.  A massive search was carried out with many telescopes, both immediately afterwards and long afterwards, to try to find the source of this signal and see it again.  There were 50 or more attempts, and it was never seen again.  It is unexplained as nobody knows what it was.  Perhaps there was some scintillation of signals in the atmosphere that produced this high burst.  The only place you hear about it now I think is in things like the X Files.  In one of their stories, Little Green Men, the story revolves around this particular signal.  But this is the only really interesting signal that was ever seen.

We do not only listen, we also send things.  Pioneer 10 and 11 were space missions that were launched in 1972 and 1973.  Their mission was to observe the outer planets of Jupiter and Saturn, but when they finished that mission, they were programmed to go off towards the nearest stars.  10 is now heading towards Taurus ? it will get there in about 2 million years ? and 11 is heading towards Aquila, quite close to Sagittarius, and it will get there in 4 million years, unless it is intercepted by debris and hits something on the way.  At the last minute, Carl Sagan and Frank Drake thought that it would be a good idea to put a message on these space probes, and so in a space of two weeks, they conceived the idea of this plaque, which was designed by Linda Sagan.  This is put on the side on some nice unscratchable, high quality, metallic material.

The message had various things on it. There were two human figures ? there was a great deal of controversy about this in United States at the time about the rightness of showing these naked figures on a spacecraft.  The poses are not just incidental.  The man was displaying his limb movement, and he was also showing his opposed thumb.  In the background was an image of the spacecraft itself, so that you can get an idea of the size of the people relative to the size of the spacecraft.  Down the bottom, there was a little child-like drawing of the Solar System and the trajectory of the spacecraft.  The distances are in a sort of binary arithmetic, in units of one-tenth of the distance from the Sun to Mercury, so it was a scale picture.  From the Sun came lines emanating out pointing towards various pulsars.  These are 14 pulsars and the length of the line that went to the pulsar gives you a measure of the period of the pulsar, so that it uniquely identified them.  The direction told you the direction in space.  The length of the line told you the period.  There is a little block on the end of each line which was a measure of how far above the galactic plane these pulsars are located.  Knowing just three of these pulsars would be enough to triangulate and locate the Earth, but you cannot assume that people know all of these pulsars, or even that all of them will still be around when the message is read, so 14 was a big redundancy factor to give many candidates to locate where it comes from.  There was also a line that runs all the way across to show the distance and the direction to the centre of our galaxy on the same scale.  Finally there was the 21 centimetre spin-flip line and a measure of its frequency.

Again, sending messages like this is very controversial.  There are people who object that there should be two people, Drake and Sagan, and NASA, who should choose to speak for the whole of the Earth.  The next mission that was sent by NASA sent a huge compilation of human music, culture, language, picture, so it was like a vast long-playing record with instructions as to how to play it and so forth.

Another type of message was sent in 1974 by the Arecibo Radio Telescope.  This was to mark the refurbishment and reopening, of the Telescope.  What was given was 1679 bits.  1679 is the product of two primes, so you can express it as 73 times 23 or 23 times 73.  You could arrange it to have 73 rows or you could have 32 rows.  If you arranged it with 32 rows the message is designed just to be gibberish, with no interesting structure.  If you have it with 73 rows, then it appears structured which is a hint.  What you had at the top, first of all, were the numbers 1 to 10, in binary, so the last light block is always just a full stop.  So the first one was, dark-dark-light, which is ?001?, that is the number one; then ?10?, which is two; ?011?; ?100?; ?101?; ?110? and so on.  We work up in binary from one up to ten.  After having done that, you have got some particular numbers you can compare to the binary number code and work out to be six, seven, eight and 15.  These are the atomic numbers of the elements that form DNA, which are hydrogen, carbon, nitrogen, oxygen and phosphorus. 

Then there was a set of green images were the chemical formulae, in effect, of the nucleotides of DNA made out of these elements.  Then there was a message that gives you a DNA double helix, and there was a numerical estimate of the number of nucleotides in the human genome.  Unfortunately, this was badly wrong; wrong by about a billion.  Down here, you will recognise yourself, there in the middle.  Using the binary from the beginning, there was an indication of typical human height and an estimate of the human population.

There was a map of our Solar System, starting with the Sun and, moving out, Mercury, Venus, Earth, Mars, and so forth, giving a measure of the sizes. Next was the Arecibo Telescope, and, finally, an indication of the size of the telescope and other things in units of 126 nanometres. 

So this is the challenge that our extraterrestrial friends have here of trying to eventually decode that type of information.   Maybe that is why they keep quiet!  Perhaps they are not really into Sudokus and all this sort of stuff, and they think that we are a sort of planet of puzzlers.

On another another, more morbid, tack, let us begin to look at perhaps more realistic reasons.  It could be that we do not hear because they only listen, they do not send, and there might be a serious reason for that.  We know from our own experience on Earth what tends to happen historically when advanced civilisations encounter primitive ones ? primitive ones tend to get wiped out, either for hostile reasons or maybe through germs and disease which they have no resistance to.  There is certainly a body of opinion amongst astronomers that says that we should not be signalling, we should not be revealing our presence, because it is pure fantasy to think that high intelligence and high levels of technological sophistication go hand in hand with high ethical principles.  We have seen a lot of movies in recent years that really brought out that type of picture of the hostile extraterrestrial rather than the sweet little ET-like figure.   So there may also be people who are too scared to broadcast, because they really do not want to reveal their presence, and do not want to reveal what they have. 

It could be that they are simply not extroverts, but they may also not be curious.  You would imagine that they were curious in order to develop a certain high level of technology, but certain things die, and maybe that curiosity is lost. 

Maybe they just do not do astronomy. That is easy to understand if you live on a planet in a star system where visibility is poor.  It is like living in Manchester all the time ? you just cannot see the sky.  You are not going to do a lot of astronomy.  You will do a lot of meteorology and gain a lot of understanding of murk and ?cloudy with scattered showers? sort of things, but you do not do any astronomy ? at first.  Eventually, when you have the capability to build radio telescopes, you can do radio astronomy, and that is why they do radio astronomy at Jodrell Bank in Manchester!  So if you live in an environment where the cloud cover in the local environment does not allow you to see very easily, you will not take much of an interest in astronomy.

It could also be that you are in a dusty environment in the plane of the galaxy, and it is difficult to see any great distance.  So there can be perfectly good reasons why your science being highly developed but very much focused on other things.  Your planet may be extremely magnetic, rather radioactive in places, and you have a very high level of understanding of radioactivity, electricity and magnetism very early on.  There may be an awful lot of water on your planet and you may have highly advanced understanding of hydrodynamics and weather systems.

They may also, as we can appreciate, have much bigger problems to worry about.  If you are a much more advanced civilisation than us, you could imagine that the sort of problems that we foresee, with our environment, with health, with stability and so forth, might have become much more acute and much more magnified.  Thus, thinking about sending signals to people 25,000 light years away will not be a pressing electoral issue to the politicians of the day.  It requires massive amounts of forward planning, far more so even than climatic action today, and it is quite easy to understand why it is never regarded as a pressing problem.  As resources become scarcer and more greatly under pressure, medical, health, environmental problems become far more important and everything is focused in a completely different direction. So we may find that we on Earth are actually living in a period where we have the luxury of thinking about and investing money in these sorts of activities, but our descendents may not have.

The other and perhaps more sinister possibility to explain the great silence is really that technological civilisations do not survive very long.  Therefore, that period of signalling in Drake?s Equation, that air that is such a dominant factor in getting the large answer if you want one in the Equation, is not as long as people imagined.  Again, we can appreciate how that might come about; that it is so easy to destroy yourselves from within.  We have already appreciated how things like pollution, nuclear war, nuclear accidents, diseases out of control, the pressures of overpopulation, food, exhaustion of natural resources, technological disasters ? any of these things, let alone some large combination of them, could very easily, if not eradicate life on Earth, certainly set it back by a huge period of time.  There might well be a very significant internal barrier to technologically advanced civilisations living and developing for a very long time.  The Industrial Revolution in the 19th Century is really not very long ago.  The ability to create nuclear devises only appeared in the last Century.  We have not been very long in a highly technological environment, and already we are creating and appreciating very serious consequences and problems.

However, I think the most serious problem of all if you want to survive, be advanced and send signals everywhere, is that the universe itself conspires against you. Even if you avoid destroying yourself from within, the chances of being destroyed from outside are really overwhelmingly large.  We have begun to appreciate in the last 50 years or so the frequency and impact that objects like asteroids and comets can have on the direction and scope of evolution on Earth.  We now appreciate that one great phase of evolution on Earth that featured the dinosaurs was brought to a catastrophic end by a major impact in the Yucatan long ago.  We also know that in 1905 there was a pretty large impact in Siberia, and in the 1920s, another one in South America.  There are no doubt others that we see no evidence of because so much of our planet is covered with water.  But the impact of comets and asteroids set the clock of evolution back quite catastrophically.  It is very difficult to keep evolution moving steadily in the face of these dramatic external perturbations.  It is not only impacts by comets and asteroids than can trouble you.  Supernovae, gamma ray bursts, other sources of radiation nearby, can be equally devastating to simple early forms of life.  To survive these sorts of catastrophes you have to be sufficiently developed to be able to deflect incoming objects and you have to be able to shield yourself from rapid outbursts of solar radiation such as giant solar flares, and supernovae, gamma ray bursts, and so forth.

Wit hthese types of things, we need to remember that the energy that is released by such projectiles hitting the earth is huge because kinetic energy depends on the square of the incoming velocity.  The rather small objects, just a few metres across, we do not have to worry about too much because they burn up in the atmosphere.  They do not reach the surface and they do not yield a crater. 

But objects 50 metres or so across, leave a one kilometre crater, but the effects are not confined to one kilometre.  The energy release and the amount of dust and shockwaves that are produced by that impact are sufficient to devastate the climate and behaviour of an entire continent.  Craters of just about 10 kilometres in size, coming from an object one kilometre in size ? a small asteroid ? is sufficient to kill off all living things, all agriculture on Earth.  This would be the size of the object that wiped out the dinosaurs and pretty much everything else above the surface of the Earth long ago.  If you get to the real extreme of something far bigger it would in effect wipe out everything.

We are rather fortunate in that our Solar System is set up in such a way that we are shielded from receiving as many hits from these objects as we should.  In the outer Solar System, we have a giant protective planet, Jupiter, which is so big that its gravitational field captures many objects that enter the Solar System heading towards us.  Just a few years ago, we were able to watch a comet captured by Jupiter as it entered the Solar System.  Had Jupiter not been there, it would have entered the inner Solar System, and encountered ourselves, maybe many times, as it also went around the Sun.  If an object gets past Jupiter, it still has to get past the Moon.  The Moon is really rather large compared to the Earth, and no other planet in the Solar System has got a moon that is, relatively speaking, so large.  You can see, long ago, from the surface of the Moon, how many things have hit the Moon that would have hit the Earth.  Thus the Moon acts as a shield by its gravitational field and its simple physical face protecting us from another type of close encounter for objects that get into the inner Solar System.

Perhaps for a civilisation without these peculiarities, the sheer frequency of impacts from outer space on planetary surfaces can set back and wipe out life long before it ever reaches a sophistication that can deflect these objects.  In most cases, I suspect, this is the most suggestive reason for the lack of highly sophisticated signalling forms of life around our galaxy.  The galaxy is simply a rather dangerous place.

Lastly, another idea you can apply to the spread of life and communication in the galaxy is percolation.  Imagine that you have got an orchard, you plant all the trees fairly close together, and a disease breaks out on one tree, then that disease spreads and blights the entire orchard.  What could determine that?  If you have spaced the trees rather closely together, it is easy for the infection to jump from one tree to another.  Also, it depends upon the number of neighbours that each tree has ? you would think that the more neighbours there were, the bigger the chance of the disease spreading.  This is a percolation problem.  It is rather like the spread of a disease.  If you have a cold, you will spread it to other people, with some probability, and that will be determined by how close you get to them and so forth.  This type of reasoning applies to the spread of all sorts of thing, and it might, indeed, apply to the spread of life within our galaxy.  Percolation problems have a special property, that this probability of spreading something to your neighbour has a critical structure, that there is always a particular probability of spread to your neighbour such that when they are below that value, when you spread it, it does not spread everywhere.  It remains patchy or it dies out.  But once the chance of spreading to a neighbour crosses some threshold, the blight spreads everywhere very quickly, and everything becomes blighted. 

If the probability of spread of the disease, or life through the galaxy is below that critical threshold what happens is that you get only patches of it in different places but it is not blighted everywhere.

But when you have twice the probability of spreading, you are then in a situation where you can link up everywhere, going horizontally or vertically, through the structure, and that is said to be percolated or critical.  It could be that the spread of life within our galaxy is not like this; we may not have a spread taking place that quite reaches the condition for criticality.  So life remains rather patchy; in some places.  If the probability had been higher, as Fermi imagined perhaps, it would go critical and spread almost everywhere, and it would be easy to find evidence for it.

It is easy to assume that the things that we prize very much, like consciousness and free will and so forth, are somehow intimately linked with intelligence and higher forms of evolution.  It could be that consciousness and hence this sort of desire to signal is just a passing phase in evolution, it plays an important role, but super-advanced forms of life may not be conscious at all.  At first that sounds very odd ? how can this be?  Many of the most complicated things that we do, we do not do consciously.  So if I throw this into the audience and it comes towards you, you will automatically just pick out your hand from your pocket and catch it.  When you cross the road, you do not do many detailed calculations of mechanics in order to avoid oncoming cars and judge just the right moment to step off the kerb.  If a rock suddenly falls from a cliff while you are looking up at it, you automatically jump out of the way.  These are all reactions: they are highly complex calculations, but they are programmed, in effect, into an automatic pilot part of your brain.  They are not things that you consciously process.  One might imagine a super-advanced civilisation containing beings whose minds do all sorts of fantastically complex things, scientifically speaking, in the same, subconscious way.  We would perhaps have a subconscious intuition ultimately for quantum mechanical construction on nanotechnological devices, so you could do the sorts of things that all of human science requires great focus and concentration to do just like that. 

This might be another reason why many of our expectations about who we might see or hear from in the universe, what they might tell us, might really be rather fanciful: that maybe consciousness, just like these lectures and all sorts of other good things, really must come to an end.

© John D Barrow, Gresham College, 2007