- 7 -

- 11 -

Earth’s full, go home

How the fact there are aliens on Earth is telling us either we are the first intelligence to arise or some unknown factor prevents the

evolution of space-faring civilisations

Sometimes I think we are alone, sometimes I think we are not. Either way, the thought is staggering.

Buckminster Fuller

I'm sure the Universe is full of intelligent life. It's just been too intelligent to come here.

Arthur C. Clarke

One striking feature of the world is so obvious that, like the darkness of the sky at night, it is almost never remarked upon. It does not matter what country you live in, what continent you are on, where at all you are on the planet. There are no aliens. They are not loitering on street corners, coasting angelically through the clouds above your head or materialising and de-materialising like crew members of the "Star Trek" Enterprise.

The fact there are no aliens on Earth is widely believed to be telling us something profound about intelligent life in the Universe. Unlike the case with the other everyday observations in his book, however, no one is quite sure what that profound thing is.

Over the years, many people have realised that the lack of aliens on Earth is a deep puzzle. However, the person who articulated it in the most memorable way was the Italian physicist Enrico Fermi. One of the last physicists to combine the roles of front-rank theorist and an experimentalist, not only did Fermi come up with a theory of radioactive beta decay, which predicted the existence of the ghost-like "neutrino, but he constructed the first nuclear reactor - on an abandoned squash court under the west stand of the University of Chicago's Stagg Field. Fermi's "nuclear pile", which went "critical" on 2 December 1942, made the "plutonium" for one of the two atomic bombs dropped by America are on Japan [1]. Those bombs were tested in the desert of New Mexico. And it was, while visiting the bomb lab at Los Alamos in the summer of 1950, that Fermi made his memorable observation about extraterrestrials.

He was having lunch in the canteen with Herbert York, Emil Konopinski and Edward Teller, the "father of the H-bomb". The physicists had been discussing ETs because of a recent spate of newspaper reports of "flying saucers". Although the discussion had turned to more mundane subjects, Fermi had gone quiet, deep in thought. Suddenly, in the middle of the ensuing conversation, he blurted out: "Where is everybody?" The others around the table immediately knew what he was referring to - ETs. They also recognised that Fermi, a man with a reputation as a deep thinker, had articulated something important and profound.

Fermi was a renowned for his back-of-the envelope calculations. For instance, at the explosion of the first atomic bomb at Alamogordo in the New Mexico desert on 15 July 1945, he had dropped a scrap of paper from shoulder height and watched how it was deflected by the shock wave from the Bomb. Knowing that Ground Zero was 9 miles away, he estimated the energy of the blast – the equivalent of more than 10,000 tonnes of TNT [2].

Implicit in Fermi's "Where is everybody?" question was a similar back-of-the-envelope calculation. How long it would take a civilisation that developed a star-faring capability to spread to every star system in our Milky Way galaxy?

Fermi never revealed the details of his reasoning. However, more likely than not he realised that the most efficient way to explore the Galaxy would be by means of self-reproducing space probes [3]. Such a probe, on arrival at a destination planetary system, would set about constructing two copies of itself from the raw materials found there. The two daughter probes would then fly off and, at the next planetary system, build two more copies. In this way, the probes would infect the Galaxy relatively rapidly like bacteria spreading throughout a host.

Using plausible estimates for the speed of such probes and the time required to make copies, it was possible to estimate how long it would take to visit every star in the Milky Way. And the answer was surprisingly modest – between a few million and a few tens of millions of years. Since this was a mere fraction of the 10 billion-year lifespan of our Galaxy, one conclusion was unavoidable. If a star-faring race had arisen at any time in the history of our Galaxy, its space probes should be here on Earth today [4]. So, in Fermi's immortal words, "Where is everybody?"

Really, Fermi's question is saying two things. Why don't we see any sign of ETs on Earth or in our Solar System? And, why do we see no sign of ETs when we look out at the Universe? For instance, why have we not seen some kind of technological artefact with our telescopes or picked up an ET signal?

Some people, of course, would maintain that there is evidence that ETs are here - Unidentified Flying Objects. The majority of UFOs turn out to be rare atmospheric phenomena, high-flying weather balloons, the planet Venus seen under unusual conditions, and so on. However, a minority of sightings remain unexplained. This does not mean there is no natural explanation. Simply that one has not been found. There is a serious objection to UFOs being extraterrestrial spacecraft, however, and that is the lack of material evidence. Despite claims of sightings of such craft - and in some instances even claims of direct contact with their occupants - nobody has come forward with a single alien artefact.

The American astronomer Carl Sagan voiced another objection to the idea that UFOs are alien spacecraft visiting Earth. He pointed out that such craft were suspiciously similar to modern high-tech aircraft, which often have the appearance of flying wings. To Sagan, it was like a Victorian looking up at the sky and seeing steam-powered flying machines. He called this tendency to see vehicles similar to present-day vehicles “temporal chauvinism”. This flew in the face of an observation made by science fiction writer Arthur C. Clarke. Any sufficiently advanced civilisation - certainly one capable of crossing the vast distances between the stars – ought to be indistinguishable from magic. To Sagan, UFOs - though not necessarily hoaxes – were definitely in the eye of the beholder.

But what about alien signals? Surprisingly, radio telescopes over the years have picked up numerous broadcasts with the characteristic expected for an ET transmission - a signal spanning a narrow "band" of frequencies much like a terrestrial radio station. The most famous of these is the "Wow!" signal, registered on 15 August 1977 by the 79-metre "Big Ear" telescope of Ohio State Radio Observatory. When astronomer Jerry Ehman saw the off-the-scale signal, he immediately scrawled "Wow!" in the margin of the paper print-out. It is a name that has stuck. But, despite re-observations of the sky in the direction of the 37-second-long transmission - the constellation of Sagittarius - the signal has never repeated.

This is characteristic of all the unusual signals that have been intercepted by radio telescopes, including 11 signals picked up in the late 1980s by the Planetary Society's Mega Channel ET Assay (META). They never repeat perhaps because they are from some transient source - maybe secret spy satellites in Earth orbit - or because they are the result of some freak electrical malfunction in the detecting equipment.

There does, however, remain the faint possibility that some of the unexplained signals are real. Joseph Lazio of the National Research Council in Washington DC and his colleagues have pointed out that there is electrically charged, or "ionised", hydrogen gas drifting between the stars. It is distributed in an uneven, clumpy manner, and such irregularities are known to cause radio signals from distant astronomical objects such as "pulsars" to fluctuate in brightness. Similar irregularities in the Earth's atmosphere cause stars to "twinkle". According to Lazio and his colleagues, such "interstellar scintillation", can depress or boost a radio signal by as much as 20 times its normal strength. The scientists therefore suggest that the unusual signals could be ET broadcasts on the occasions when they are boosted.

Unfortunately, a signal subject to interstellar scintillation spends nearly all of its time depressed and is very rarely boosted. According to Lazio and his colleagues, it would require thousands or even tens of thousands of re-observations to see such a signal repeat. So far, however, no signals have been looked for in more than 100 follow-up surveys. The scientists conclude that, though the data is compatible with there being tens of thousands of ET civilisations in our Galaxy, it is equally compatible with there being none. ET may already have phoned Earth - only it is impossible to tell.

So, if there is no evidence of ETs physically on Earth or in the Solar System and there is no evidence we are picking up intelligent signals from space, what should we conclude? Well, one possibility is that we have not yet looked hard enough for long enough. Certainly, this is the view of those actively involved in the Search for Extraterrestrial Intelligence, or SETI. They point out that so far only a tiny fraction of the 200 thousand million or so stars in our Galaxy have been targeted for radio signals.

Nevertheless, according to Seth Shostak of the SETI Institute in Mountain View, California, we are now observing target stars at a furious rate. He believes that, to stumble on the first alien civilisation, we will need to observe a few million stars, a number that will be reached by 2015. He will be surprised, he says, if SETI does not meet with success by that date. Shostak’s tacit assumption, generally accepted in the SETI community, is that between 10,000 and a million alien civilisations are currently broadcasting radio signals in our Galaxy. Sceptics consider the estimate as at best optimistic and at worst wishful thinking. SETI scientists lean towards the view of the 16^th-century Italian philosopher who in his book On the Infinite Universe and Worlds, declared: “There are innumerable suns, and infinite number of earths revolve around those suns, just as the seven we can observe revolve around this sun which is close to us.”

The idea that we have not looked hard enough for long enough for ET signals could equally well explain why we have seen no sign of alien visitation in our Solar System. Recall that, in Stanley Kubrick and Arthur C. Clarke’s 2001: A space odyssey, an alien artefact is found on the Moon. Left there three million years before as a kind of "baby alarm" to alert its makers if intelligence ever arose on the Earth, its presence is not obvious because it is buried the Tycho crater, deep beneath the lunar dust. And, plausibly, an alien artefact might be left somewhere even less obvious. As physicist Stephen Webb points out, there are 50 billion billion billion cubic miles of space within a sphere that encloses the orbit of Pluto; and the Solar System extends to the Oort Cloud of comets, far beyond Pluto. "The chances of finding a small alien artefact by accident are essentially zero," he says.

Of course, we may have seen no signs of aliens and picked up no alien signals not because there are still so many places left to look but because we are using the wrong search strategy. There is a tendency for SETI scientists to target Sun-like stars for the obvious and sensible reason that we know of one such star where life has definitely arisen: the sun. But perhaps ET civilisations, because of their rapidly mushrooming energy needs, move from relatively cool stars like the sun, of "spectral type" G and K, to super-hot O and B stars. Such stars are rarely targeted by SETI searches because they burn up quickly and would incinerate any planets that happened to have formed.

Then again, perhaps most life is not to be found around stars at all. In an attempt to widen the debate about life in the Galaxy, planetary scientist David Stevenson of the California Institute of Technology in Pasadena, has suggested that life might most commonly be found on "interstellar planets", sunless orphans wandering through the dark, deep-freeze between the stars. As evidence for his idea, he points to computer simulations of the formation of our Solar System, which invariably show about 10 Earth-mass planets forming. Close-encounters with embryonic giant planets like Jupiter quickly cause the majority to be ejected into interstellar space.

Naively, it might seem that a planet without a sun to warm it and provide energy would be a dead loss for life. However, Stevenson points out that such planets will be swathed in a thick embryonic atmospheres of molecular hydrogen gas, the raw material from which stars are born, and this atmosphere could be anything from 100 to 10,000 times denser than the Earth's atmosphere. Crucially, under such high-density conditions, all gases become "greenhouse" gases, trapping heat with the effectiveness of a planet-wide duvet. According to Stevenson, with such good insulation, the heat coming from radioactive rocks - the same heat that to this day keeps the Earth's interior molten - could keep the surface of such a planet warm enough for liquid water to exist for at least 10 billion years, twice as long as the current age of the Earth. Remarkably, interstellar planets might be the most common places in the Galaxy to find life, most likely primitive microorganisms, but - who knows - maybe intelligence as well.

But even if we are not looking in the wrong place for ETs, another possible flaw in our search strategy might be that we are listening at the wrong radio frequencies. Searches for intelligent radio signals, for instance, tend to focus on a small number of special radio frequencies. These are the frequencies of radio waves emitted naturally by atoms and molecules commonly found in drifting on the currents of space. ETs will know about these frequencies, goes the argument, and they will know that other technological civilisations will know them too. The "Wow!" signal, for instance, was picked up at 1420 megahertz (MHz), the frequency at which the hydrogen atoms floating in interstellar space broadcast to all and sundry like miniature radio stations.

It is entirely possible that ETs are broadcasting at radio frequencies which we have not anticipated. Although searches for extraterrestrial intelligence have also been conducted at optical frequencies, the same logic applies. ETs might be using optical frequencies we have not yet tried.

But maybe our search strategy is even more flawed than this. Aliens, rather than using different radio waves or visible light to signal, may be using an entirely different communication medium altogether. For instance, they might be signalling with ghostly neutrinos or with gravitational waves - ripples in the fabric of space-time - or using some other communication mechanism we cannot begin to imagine. By looking for radio or optical signals, and expecting ETs to communicate in pretty much the way 21st-century humans do, we may be guilty of Sagan's temporal chauvinism.

In his book The Cosmic Connection, Sagan describes tribes that live in deep valleys in New Guinea and which use drums to communicate with people in adjacent valleys. When asked how an advanced tribe might communicate, tribesmen say by using a bigger drum. The irony is that, all the while, the babble of global radio traffic fills the air. In the same way, we might be completely oblivious to the babble of Galactic communication surging through the vacuum all around the Earth. Arthur C. Clarke had a similar thought. In Odyssey, an authorised biography, Neil McAleer quotes Clarke saying: “The fact that we have not yet found the slightest evidence for life - much less intelligence - beyond this Earth does not surprise or disappoint me in the least. Our technology must still be laughably primitive, we may be like jungle savages listening for the throbbing of tom-toms while the ether around them carries more words per second than they could utter in a lifetime."

The truth, of course, is that we have little choice in our search strategy. Either we abandon the search for extraterrestrial intelligence altogether or we listen with the means currently at our disposal and keep our fingers crossed.

But it is possible that it is not only ET communications that are unrecognisable. Alien artefacts - in the Solar System or elsewhere – may also be unrecognisable. After all, ETs could be millions or even billions of years ahead of us technologically. Does an ant recognise that is crawling over a paving stone in a city connected to other cities by land and sea and air? Does a bacterium? Once again, Arthur C. Clarke's remark is apt - a sufficiently advanced civilisation will be indistinguishable from magic.

But maybe we should take the fact that we have not seen any evidence of ET at face value. The question then is: Why have they not have come here or left any discernible sign of their presence in the heavens?

Could it be that it is physically impossible to cross interstellar space - that the distances are simply too vast? This seems unlikely. Even today we can envision mans of travelling to the stars at say 1 per cent of the speed of light. Although it is beyond our present-day capabilities, it seems unlikely it will always be.

If there is no physical barrier to travelling between the stars, then perhaps ETs find something better to do with their time than explore the Galaxy or beam radio messages to all and sundry. Perhaps they become preoccupied with art or introspection and decide to stay at home and ignore the wider universe. Or perhaps they find computer-generated artificial realities more seductive than the messy real world. Perhaps they self-destruct - snuffed out by nuclear war or global warming or any number of other possible environmental catastrophes. Or maybe ETs adhere to something like the "Star Trek" prime-directive on not interfering with the emerging civilisations. According to this "Zoo Hypothesis", our Solar System is cordoned off. We can expect to be to be left alone on our "nursery world" until, one day, we develop a star-faring capability and sail out into interstellar space to be welcomed as new members of the Galactic Club.

One of the problems with these kinds of explanations would appear to be that they require us to speculate on the motivations of ET civilisations far in advance of our own. Since we cannot know their motivations, we are on shaky ground to say the least. Fortunately, however, such explanations of why we see no sign of ETs can be dismissed regardless of ET motivations. As physicist Michael Hart of the National Center for Atmospheric Research in Boulder, Colorado, pointed out in a classic paper 1974, we can expect ETs to have a variety of motivations. Sure, some may blow themselves up. Sure, there may be stay-at-home ETs. Maybe even the majority will stay at home. But, just as a small minority of the world's population has gone out to explore the four corners of the globe while the majority has stayed at home, there will always be exceptions that buck the couch-potato ET trend. And, while most ETs might respect a zoo-like quarantine of the Solar System, would all of them? A policy of non-interference might be difficult to police over millions or even billions of years - and there is no evidence the Earth has been interfered with during its 4.55 billion-year lifetime.

According to Hart, all the explanations of the absence of ETs based on their motivation are undermined by the same thing. Inevitably, there will be exceptions who have different motivations to the rest or who ignore bans on visiting the Solar System - rogue ETs who nevertheless come to Earth.

So, might it be possible to explain the non-appearance of ETs on Earth in some way that does not appeal to ET motivations? Might there be some compelling reason not to go from star to star, exploring the Galaxy? One possibility is that it is too dangerous. “Any creatures out there may be malevolent or hungry,” pointed out radio astronomer Sir Martin Ryle at the University of Cambridge on hearing, on 16 November 1974, that astronomers at the giant radio dish in Arecibo, Puerto Rico, had announced our presence to the cosmos by broadcasting the first interstellar radio message in the direction of the globular star cluster M13 about 25,000 light years away. Perhaps, as Ryle worried, there is a xenophobic race out there that wipes out other civilisations soon after they make their presence known. Maybe self-reproducing space probes accrue mutations with every new generation just as living things do until, eventually, a mutation leads to a murderous species which sees as its mission the wiping out of all competing life-forms. Science fiction writer Fred Saberhagen envisioned just such a race of doomsday machines, which he called "Berserkers". If Berserkers are out there, it might be best to sit quiet and listen rather than broadcast out existence to the four corners of the Galaxy. The trouble is it may already be too late since our TV broadcasts, travelling at the speed of light, have already reached stars more than 70 light years away.

A non-Berserker reason why ETs might not want to explore the Galaxy is that there is some place to visit far more attractive, far more seductive. Astronomers have found that there is six times as much invisible, or "dark", matter in the Universe as normal matter - the atomic stuff that composes you and me, the stars and galaxies. We know of its existence only by the gravitational tug it exerts on the visible material. Even the dark matter is outweighed by the mysterious dark energy, with the two invisible cosmic components together accounting for 96 per cent of the mass-energy of the Universe. Could it be that the Universe is chock-a-block full of dark matter stars, dark matter planets and dark matter life? Might it be that all the activity, the cosmic commerce, is going on in the dark matter - that the dark not the normal matter is where it's at? As New Scientist reader Simon Williams observed on the magazine’s letters’ page in 2003: “We are told that only 4 per cent of the Universe is made of atomic matter that we can observe, and that the remaining 96 per cent is missing. Perhaps someone or something else is sitting out there trying to figure out where the missing 4 per cent is.”

Then, in addition to dark matter, there is a strong suspicion among physicists that there may be our other dimensions. According to "string theory", which views the ultimate building blocks of matter as tiny vibrating strings of mass-energy, there are 6 space dimensions in addition to 3 of space and 1 of time with which we are familiar. The extra dimensions are thought to be "rolled up" far smaller than an atom, but there are ways theorists believe they could big and still have escaped notice. Could ETs find more interest, more activity, in other dimensions? Or maybe they escape into other, more interesting universes, down short-cuts in space time called “wormholes”, which are permitted to exist by Einstein's general theory of relativity? Maybe even – God forbid – we are simply not interesting enough and aliens treat us with monumental indifference. “How would it be if we discovered that aliens only stopped by Earth to let their kids take a leak?” wondered American chat show host, Jay Leno.

But, of course, it is not just that ETs are not here on Earth, that they do not appear to have explored the Galaxy. We see no sign of them either. Even if they do not signal, we might expect to see signs of their presence. As pointed out, our TV broadcast have already leaked out into neighbouring space. On planets around stars 70 light years away, it is possible to pick up 1930s' Nazi broadcasts. And it is not just the "electromagnetic radiation" of TV. There are military radars and power lines and the whole electrical paraphernalia of our technological civilisation. Surely, if ours stuff is leaking out into space, we should also expect to see the same kind of electromagnetic leakage from advanced technological civilisations? It does not matter whether ETs, for whatever reason, decide to stay at home gazing at their alien navels, they should inadvertently reveal their presence. But they have not.

Of course, absence of evidence is not necessarily evidence of absence. As pointed out already, we may not yet have looked hard enough or long enough. However, it looks like there is no one out there. Some scientists say we should not be swayed by emotion, by our heart telling us that, surely, we cannot be totally alone in the Universe. Instead, we should take a cold, hard look at the facts and draw the logical conclusion without flinching. The facts are we see not the slightest piece of evidence for ETs. And the logical conclusion to draw is that this is because they do not exist. We are alone, the first intelligence to arise in our Galaxy.

Such a conclusion, though logical, not only flies in the face of our desire not to be alone in the cosmos but it collides with the idea that there is nothing special about our place in the Universe. In the 16th-century, the Polish astronomer Nicolaus Copernicus told us that the Earth orbited the sun along with all the other planets and so was in no special position in the Solar System. In the 19th century, Charles Darwin taught us that humans are just another animal species, the product of evolution by natural selection. Then, in the 20th century, we learnt that the Milky Way was but one among another 100 billion galaxies and in no special position in the Universe.

To accept that we are special - the only intelligent life in the Galaxy (possibly even the whole Universe) - flies in the face of the "Copernican Principle", also known as the principle of mediocrity. It smacks almost of religion in which the human race is assumed to be a special project of a Creator despite the fact that the Earth is but one planet among possibly trillions upon trillions of others. But, difficult as it is to accept that we are alone, it does appear to be the logical conclusion to draw from what American science fiction writer David Brin calls "The Great Silence".

But how could it be that we are the first intelligence to arise, at least in the Milky Way? Well, if we are, it must mean that the emergence of intelligent life is a spectacularly unlikely occurrence. Perhaps it requires a whole chain of accidents, each of low probability.

One low-probability event appears to be the formation of a giant satellite. The Earth's moon has been essential for life on Earth. For one thing its has stabilised the climate over billions of years. This is because the Earth, as its spins, has a tendency to wobble and fall over much like a top. Such wild variations in the spin axis of the Earth would result in wild fluctuations in its climate. In fact, this is exactly what happens on Mars, which has only two minuscule moons. As Elton John sang in Rocket Man: “Mars ain’t the kind of place to raise your kids.” However, whenever the Earth tips over too far, the gravity of the Moon pulls it back. The Moon can do this only because it is a quarter of the diameter of the Earth and so has an appreciable gravitational pull. In fact, the Earth-Moon system is effectively a double planet.

But look around the rest of the Solar System. There are hundreds of moons but only one other - Pluto's Charon - is comparable in size to its parent planet [5]. The rarity of such moons is believed to be because of the unusual circumstances required for their formation. Shortly after its birth, the Earth is believed to have been struck by a Mars-mass body. According to this "Big Splash" theory, the impact turned the exterior of the Earth molten, splashing some of it into space to congeal as the Moon. This may have been an unlikely event. So, if big moons are essential for life-supporting planets, then this greatly reduces the number of possible sites for life in the Galaxy.

In addition to the accident of the Moon, there is the fact that the Earth has both water and land. Although whales and dolphins are known to be as bright as the higher apes, and on the very threshold of human intelligence, it was only on the land that the step to human intelligence was taken. Perhaps it was only there that evolutionary pressures were sufficient to create a hand capable of dexterous manipulation of materials to make tools and weapons. If this is so, then the step to human-level intelligence would not be taken on planets entirely covered with water. And, since the Earth is itself almost entirely covered with water, it is conceivable that this might be the case for most life-supporting worlds. A galaxy of dolphin-like creatures might be a silent galaxy.

But, in addition to a big moon and some dry land, biologists can identify at least a dozen hurdles that life had to overcome on the road to producing human beings. A key one, however, appears to be the step from single-celled to multicellular organisms, which happened only about 700 million years ago, nigh on 3 billion years after the emergence of life on Earth. Since single-celled organisms such as bacteria can reproduce in a matter of hours, we are talking about many trillions of generations before cells hit on the idea of clubbing together to form higher organisms such as ants and human beings. Could this be the hurdle that has prevented life evolving to the level of human intelligence or beyond everywhere else in the Galaxy?

The physicist Brandon Carter, one-time office mate of Stephen Hawking, has provided an ingenious mathematical argument that there were 5 low-probability, “hard”, steps on the evolutionary road to human technological civilisation. Step 1 was the advent of the first bacteria, or “prokaryotes”; step 2 the complex cells with nuclei, or “eukaryotes”; step 3 multicellular life; step 4 intelligence; and step 5 human civilisation. Each step took roughly 800 million years. There is also the possibility that life started on Mars, which is smaller than the Earth and so cooled from its initial molten state more quickly, before being transferred to Earth on board a meteorite. If so, we may be talking about 6 hard steps. The upshot is that, if Carter is right about the 5 – possibly 6 – low-probability steps on the road to humans, technological civilisation is likely to be extremely rare in out Galaxy.

The obvious question is then: Why did it happen here on Earth first? But, of course, it had to happen somewhere first. Why not here? Certainly, the Great Silence is utterly compatible with the human race being the first technological civilisation in our Galaxy.

At this point, it worth pointing out that, if the step from single cells to multicellular organisms took an unimaginably long time, the step from non-life to life - from inert chemicals to the first bacterial cells - was remarkably rapid on the Earth. There is evidence of primitive life on Earth 3.8 billion years ago, within 800 million years of the planet's birth, which means it had to be around even earlier. In fact, it must have arisen almost at the moment the newborn Earth had cooled enough from its molten state for life to be possible. Despite this, however, it has proved impossible – even after more than half a century of effort - to turn non-life into life in the laboratory [6].

Some say the only way to resolve the paradox of life's rapid emergence on Earth and its difficulty in getting started is to say that the Earth was "seeded" from space – Mars being an uncontroversial possibility - with the most primitive microorganisms. If so, we are aliens - or at least we arose from aliens. This idea of "panspermia", though dating back to the late 19th century, was championed in the 20th century by the late Fred Hoyle and Chandra Wickramasinghe. According to the two astronomers, life is extremely difficult to get started but, once it starts - perhaps in only one place in the Galaxy - there is an efficient mechanism to spread it from star to star, planet to a planet. The proposed mechanism involves interstellar clouds and comets and a great cosmic cycle that continually ferries bacteria between the depths of space and planetary surfaces and back again [7].

Nobody whether panspermia is true. But it remains a viable alternative to the view that life arose on Earth in splendid isolation. Certainly, if it is true, then primitive life is widespread - a cosmic rather than a planetary phenomenon. Of course, if there is a big hurdle on the evolutionary road to intelligence - perhaps at the step from single celled to multicellular organisms - panspermia is still no consolation to us. The Universe could very well be teeming with bacteria but bereft of any complex or intelligent life. Still, we will have no one to talk to.

So, is this the depressing answer to Fermi's question - "Where is everybody?" - nowhere, because we are the first. Not necessarily. According to one man, intelligent life is inevitable. The trouble is there is a snag. Intelligent life is inevitable - but we will never, ever find it (at least not by looking out in the Galaxy).

As evidence, the British physicist Stephen Wolfram points to our communication signals. In order to squeeze more and more information into them - be they mobile phone conversations or computer data - we remove all redundancy or pattern [8]. If anything in a signal repeats, then clearly it can be excised. But this process of removing any pattern from a signal make it look more and more random - in fact, pretty much like the random radio "noise" that rains down on Earth coming from stars and interstellar gas clouds. According to Wolfram, if someone beamed our own 21st-century communication signals at us from space we would have a hard job determining whether they were artificial or natural. So what chance do we have of distinguishing an ET communication from the general background radio static of the cosmos?

And, as it is for ET signals, so it is for ET artefacts. According to Wolfram, they too will be unrecognisable, though this is more difficult to see. He uses the example of a train station seen from so high above that the details of the trains are invisible. What would tell you that the train station is the product of technology is the regularity with which trains arrive and leave. However, Wolfram maintains that such a pattern will be totally absent from transport systems of the future. He believes they will use lots of small cabs summoned on demand. Such a system, coordinated by computers, would look entirely random viewed from high. It would look far more like a natural artefact.

At present, says Wolfram, it is easy to distinguish a technological artefact such as a car from a natural object such as a tree. The tree is far more complicated. But, says Wolfram, this is simply because our technological artefacts are primitive. As they become more complex - with computer processors enabling them to make a moment-by-moment decisions - they will begin to look just as complex as trees and people and stars. So what chance do we have of distinguishing an ET artefact from a natural celestial object? Essentially none, says Wolfram.

If Wolfram is right and ETs are out there but we will not be able to recognise them - either in their communications or their artefacts - then of course they could be here in the Solar System and we would not have noticed, neatly solving the Fermi paradox. That tree on the corner of your street could be an ET. Wolfram, thinks this is unlikely, however. In fact, he thinks he has a cast-iron reason why ETs will not want to travel to Earth - or anywhere else for that matter. And it is all to do with computers.

This next bit is difficult to swallow. Wolfram thinks he has found nature's big secret - how it generates the complexity of the world, everything from a rhododendron to a tree to a barred spiral galaxy. It does it, he believes, by applying simple rules over and over again. Running a simple computer programs, if you like. He came to this remarkable conclusion in the early 1980s when he discovered that the simplest kind of computer program - known as a cellular automaton - can generate infinite complexity if its output is repeatedly fed back in as its input.

Crucially, Wolfram has found evidence that the kind of computer program that produces endless complexity can be implemented not just systems of biological molecules but in all sorts of physical systems - chaotic gas clouds, systems of subatomic particles and so on. He concludes that all over the Universe life - though definitely not life as we know it - will spring up spontaneously. It is a fundamental feature of matter.

This would appear to be good news. However, not only will ET life be unrecognisable, for the reasons already expounded, it will not want to come to Earth, according to Wolfram. The reason is subtle.

In Wolfram's view, everything in the Universe is the product of a computer program. In fact, he imagines an abstract cyber-universe of all conceivable computer programs, all the way from the simplest up to the most complex. This "computational universe" contains everything from the Apple Macintosh operating system to a programme for creating a faster-than-light starship. The existence of this computational universe is the crucial thing. For, if any ET civilisation contacted us, says Wolfram, what could it trade with us? In his view, all it could say is "Here are some useful programs we have found in the computational universe - what have you found?" But the reality is it would be it easier and more efficient to stay at home and use a computer to search the computational universe for useful programs rather than try to get the same information by hunting for ETs to talk to among the 200 billion or so stars in the Galaxy. "It's a simple numbers game," says Wolfram.

Perhaps you think such a stay-at-home ET would miss out on the pleasure of meeting us and learning about out civilisation first-hand - or anyone else's for that matter. But think again. "Remember, everything is generated by computer program - and that includes you and me," says Wolfram. "Someone halfway across the Galaxy could have found the computer program for you and conversing with you at this very moment."

If you do not like Wolfram's rather unorthodox explanation of the Fermi paradox, then you are left essentially with two plausible options: either there is some murderous race out there in the Galaxy - in which case perhaps we should be cautious about overtly announcing our presence - or we are the first intelligence to have arisen and therefore and utterly alone in the Milky Way. Then again, there could still be some explanation nobody has yet thought of.

Maybe someone has stuck a sign at the edge of the Solar System saying “Earth’s full, go home”. In any case, it seems right to end this book on everyday observations which tell us profound things about the Universe with an everyday observation - the lack of ETs in our midst - for which we do not yet know what the profound thing is. Speculate away. Your guess is likely to be as good as mine. Fermi’s reaction to a lecture he once attended encapsulates the situation nicely, so it seems fitting to leave the last word to him. “Before I came here I was confused about this subject. Having listened to your lecture I am still confused. But on a higher level.”

[1] Plutonium-239 is one of two heavy nuclei that splits, or "fissions", when struck by a neutron, liberating a large amount of energy. Since further free neutrons are created, more nuclei can be fissioned, causing a runaway nuclear "chain reaction", and the unleashing of a dam burst of energy. This is a an “atomic bomb". The other nucleus that can that support a runaway chain reaction is uranium-235.

[2] A more trivial “Fermi problem”, typical of the kind Fermi used to challenge his students was: “How many piano tuners are there in Chicago?” A rough estimate can be obtained by the following reasoning. Chicago has a population of about 10 million. Pianos tend to be owned by families not individuals (assuming pianos owned by schools, concert halls, and so on, account for only a small minority). If an average family has five members, then that makes 2 million families in Chicago. If 1 in 20 families owns a piano, then there must be 100,000 pianos. Say, each piano requires tuning just once a year. That makes 100,000 tunings a year. And, say, a piano tuner can tune 2 pianos a day and works about 200 days per year, so they can tune 400 pianos a year. Since there are 100,000 pianos in Chicago, the city must have about 250 piano tuners.

[3] The concept of self-reproducing machines would later be explored in detail by the Hungarian-American physicist John von Neumann, famous for inventing the modern computer program. Such probes are therefore often called Von Neumann probes.

[4] The first person to spell out this argument involving Von Neumann self-reproducing space probes was the American physicist Frank Tipler in 1981.

[5] Pluto is nowadays not considered a planet but just one among maybe 100,000 icy “Kuiper Belt” objects in the outer Solar System. So, essentially, Earth is the only planet with a moon comparable in size to itself.

[6] In the 1952, Stanley Miller and Harold Urey of the University of Chicago famously took a mixture of gases thought to have existed on the primordial Earth and subjected it to electrical discharges and ultraviolet light. Their experiment yielded aldehydes, carboxylic acids and amino acids, precursors of life. But things stalled there.

[7] See my book The Universe Next Door (Headline, 2002).

[8] See my book, The Never-Ending Days of Being Dead (Faber & Faber, 2008).