Ask the average scientist about the possibility of an encounter with an extra-terrestrial lifeform, and it's likely you'll find you've prompted 'the giggle factor'. After rolling their eyes, they remind you that the distances between stars are so vast, it's virtually impossible for any aliens to visit us.

But a potential flaw is assuming an extraterrestrial civilisation would be only a few hundred years ahead of us in technology. How about civilisations that may be a million years ahead of ours?

The late scientist and author Carl Sagan once asked: "What does it mean for a civilisation to be a million years old? We have had radio telescopes and spaceships for a few decades; our technical civilisation is a few hundred years old ... an advanced civilisation millions of years old is as much beyond us as we are beyond a bushbaby or a macaque."

This question is no longer just a matter of idle speculation. Soon, humanity may face an existential shock as we discover Earth-sized twins of our planet orbiting nearby solar systems. This may usher in a new era in our relationship with the universe, so that we will never see the night sky in the same way. Realising that scientists may eventually compile an encyclopaedia identifying the precise coordinates of perhaps hundreds of Earth-like planets, gazing at the night sky, we will forever after wonder if someone is gazing back at us.

Every few weeks, yet another planet about the size of Jupiter is discovered outside our solar system, adding to the list of several hundred extrasolar planets that have been discovered in the short period we've been searching. The problem is, most are too large to sustain the kind of complex life that, from our one example here on Earth, we know.

But over the next few years, new spaceborne telescopes will finally become powerful enough to identify twins of Earth. The Kepler telescope, to be launched in 2008, will probably be able to identify terrestrial planets – rocky worlds rather than gas giants like Jupiter and Saturn. Until 2012 it will scan as many as 100,000 Sun-like stars up to 2,000 light years away, and perhaps identify hundreds of Earth-like worlds by detecting the slight loss of light they cause as they pass in front of their mother star. Kepler will hopefully identify 185 such planets with less than 1.3 times the radius of Earth, and as many as 640 terrestrial planets less than 2.2 times.

Kepler will pave the way for the Terrestrial Planet Finder, expected to be launched in about 2014, which should identify an even greater number of Earth-like planets. It will scan the brightest 1,000 stars within 50 light years of our tiny home world, and focus on the 50 to 100 brightest planetary systems. Also, it will analyse the faint light reflected from these planets to determine if they can support the organic chemicals that make life possible.

All this, in turn, will stimulate an active effort to discover if any of them harbour life, perhaps some with civilisations more advanced than ours. Although it's impossible to predict exact features of such civilisations, their broad outlines can be analysed using the laws of physics. No matter how many aeons separate us from them, they still must obey the laws of physics – which we have determined to such an extent that we can explain the behaviour of the cosmos from the subatomic world to the large-scale structure of the universe, through a staggering 43 orders of magnitude (a factor of 10 million billion billion billion billion).

Civilisations may be ranked by their energy consumption, using the following principles:

• The four laws of thermodynamics describe transport of heat and work. Even an advanced civilisation is bound by the laws of thermodynamics, especially the First and Second, and can hence be ranked by the energy at its disposal. The first law states that "energy can be changed from one form to another, but cannot be created or destroyed". While, "in all energy exchanges, if no energy enters or leaves the system, the total amount of disorder always increases" is the second law.

• The laws of stable matter. Matter in the universe clumps into three large groupings: planets, stars and galaxies. This is a well-defined structural product of stellar and galactic evolution, thermonuclear fusion, and so on. Thus, the energy of a hyper-advanced civilisation will also be based on three distinct types, and this places upper limits on their rate of energy consumption.

• The laws of planetary evolution. Any advanced civilisation must grow in energy consumption faster than the frequency of life-threatening catastrophes, such as meteor impacts, ice ages, supernova explosions, and so on. If their growth rate stays any slower, they are doomed to extinction. Thus, this places mathematical lower limits on the growth rates of these civilisations.

In a seminal paper published in 1964 in the Journal of Soviet Astronomy, Russian astrophysicist Nicolai Kardashev theorised that advanced civilisations must thus be grouped according to three Types: I, II and III, signifying mastery of, respectively, planetary, stellar and galactic forms of energy usage. He calculated that the energy consumption of these three types of civilisations would be separated by a factor of about 10 billion.

Human civilisation has only recently begun to master planetary energies: fossil fuels, passive solar, wind, geothermal and nuclear fission, and may one day soon crack nuclear fusion. But how long will it take to reach Type II and III status? Less time than most realise.

Our entire planetary energy production is now about 10 billion billion ergs per second (an erg is a unit of measurement, equal to 10-7 joules). That sounds like a lot, but it's actually a small fraction of the energy we receive from the Sun. The Earth is bathed with about one billionth of its mother star's energy – we utilise about one millionth of that.

But our energy growth is rising exponentially, and we can calculate how long it will take to rise to Type II or III status. "Look how far we have come in energy uses once we figured out how to manipulate energy, how to get fossil fuels really going, and how to create electrical power from hydropower, and so forth," says Donald Goldsmith, a University of California at Berkeley astronomer and author. "We've come up in energy uses by a remarkable amount in just a couple of centuries compared to billions of years our planet has been here ... and this same sort of thing may apply to other civilisations."

Freeman Dyson, a physicist at the Institute for Advanced Study in Princeton, New Jersey, estimates that, within a century or two, we should attain Type I status. In fact, growing at a modest rate of 1 per cent per year, Kardashev estimated that it would take only 3,200 years to reach Type II status, and 5,800 years to reach Type III status.

A Type I civilisation is a truly planetary one, which has mastered most forms of planetary energy. Their energy output could be between thousands and millions of times our own current output. Mark Twain once said, "Everyone complains about the weather, but no one does anything about it". This could change with a Type I civilisation, which has enough energy to modify the weather. It also has enough to build cities beneath oceans and alter the occurrence of earthquakes and volcanoes.

Currently, our energy output qualifies us for Type 0 status; Carl Sagan estimated that we qualify as a Type 0.7 civilisation. We derive our energy not from harnessing global forces, but by burning fossil fuels (oil and coal). But already, we can see the seeds of a Type I civilisation: when you read the newspaper, you see evidence everywhere that we are an emerging Type I civilisation. For example:

• The Internet will be our planetary 'telephone system'. Already, the Internet is the universal communication for science, commerce, the arts, politicians, and individuals. If the leader of a nation were attempt to ban the Internet, most people would simply laugh. It has become unstoppable.

• The planetary language of our future Type I civilisation will be English. This is already the number-one second language on Earth, spoken by most scientists, engineers, politicians, artists, and business people. For people in Asia, with so many local languages, the most convenient way to communicate is via everyone's second language. There will be one umbrella language for the entire planet – English – but beneath that umbrella, there will be hundreds of local languages.

• The planetary economy will be global. Already, we can see the emergence of the European Union, whose member nations have, for centuries, slaughtered each other's citizens. The E.U., in turn, was formed as a consequence of competition from an economically united North America under NAFTA (the North American Free Trade Agreement, covering Canada, the USA and Mexico).

• The planetary culture will be mass culture and youth culture. Already, movies, songs, books, art,
and ideas are distributed on a planet-wide scale. Like
planetary language, this culture will be everywhere on Earth, but will co-exist with local ones.

By definition, an advanced civilisation must grow faster than the frequency of life-threatening catastrophes. Since large meteor and comet impacts take place once every few thousand to million years, a Type I civilisation must master space travel to deflect space debris within that time, which should not be much of a problem. Ice ages may take place on a time scale of tens of thousands of years, and so a Type I civilisation must learn to modify the weather within that period.

Artificial and internal catastrophes must also be negotiated. Global pollution is a mortal threat for a Type 0 civilisation, but not a Type I civilisation, which has lived for several millennia as a global force and necessarily achieved ecological balance with its home planet. Internal problems such as wars do present a serious recurring threat, but emerging civilisations have thousands of years in which to solve their racial, national, and sectarian conflicts. Since it would take centuries or even millennia for a Type I civilisation to terraform nearby planets, its peoples will have plenty of time to work out their internal differences on the same planet before they finally leave the mother planet in any significant numbers.

Eventually, after several thousand years, a Type I civilisation will exhaust the power of its home planet, and derive its energy by consuming the entire output of energy available from its sun –
roughly a billion trillion trillion ergs per second.

With an energy output comparable to that of a small star, it should be visible from space. Freeman Dyson has proposed that a Type II civilisation may even build a gigantic sphere around its star to utilise more efficiently its total energy output. Even if it tries to conceal its existence, it must – to comply with the Second Law of Thermodynamics – emit waste heat. From outer space, its planet may be seen to glow like a Christmas-tree ornament. Dyson has even proposed looking specifically for infrared emissions (rather than radio and TV) to identify these Type II civilisations. So far, no evidence of such an infrared planet has been found (although hardly anyone has searched).

Perhaps the only serious threat to a Type II civilisation would be a nearby supernova explosion, whose sudden eruption could scorch their planet in a withering blast of X-rays, destroying all life. Thus, perhaps the most interesting civilisation is a Type III civilisation, for it is truly immortal. It has exhausted the power of a single star, and has reached out to other star systems. No natural catastrophe known to science has the capacity to destroy a Type III civilisation.

Faced with a neighbouring supernova, it would have several alternatives, for example altering the evolution of a dying red giant star which is about to explode, or leaving this particular star system and terraforming a nearby planetary system.

However, there are roadblocks to an emerging Type III civilisation. Eventually, it bumps into Einstein's theory of relativity. Nothing can travel faster than light, which is about 300,000km a second (for a possible loophole, see the end of this article). Since the universe is so vast and space is so empty, this absolute speed limit tends to hold back a civilisation's successful expansion. Dyson estimates that this roadblock may delay the transition from a Type II to a Type III civilisation by perhaps a million years or more.

But even with the light-speed barrier, alternatives exist for expanding at near-light velocities. For example, the ultimate measure of a rocket's capability is something called 'specific impulse', defined as the product of the thrust and the duration, and is measured in units of seconds. Chemical rockets can attain specific impulses that last from several hundreds to several thousands of seconds. Ion engines can attain specific impulses of tens of thousands of seconds. But to attain near-light velocity, one has to achieve specific impulse of about 30 million seconds, which is far beyond our current capability, but not that of a Type III civilisation. Various propulsion systems would be available for sub-light speed probes, such as ramjet fusion engines, photonic engines, anti-matter engines, and others we cannot even imagine.

Because the distances between stars are so vast, and the number of unsuitable, lifeless solar systems so large, a Type III civilisation would be faced with the next question: what is the most efficient way of exploring the hundreds of billions of stars in the galaxy?

In TV science fiction, the search for inhabitable worlds is shown as the exclusive dominion of heroic captains boldly commanding a single starship, or as the murderous Borg society of Star Trek – a Type III civilisation which absorbs lower Type II civilisations (such as Star Trek's United Federation of Planets). However, the most mathematically efficient method to explore space is far less glamorous: to send fleets of 'von Neumann probes' throughout the galaxy. These are named after John von Neumann, the Hungarian-born mathematician who defined the mathematical laws of self-replicating systems.

A von Neumann probe is a robot designed to reach distant star systems and create factories that will reproduce copies of themselves by the thousands. For von Neumann probes, a planet is a less ideal destination than a dead moon; these have no atmosphere and no erosion, which means the probes can easily land and take off and can 'live off the land', using naturally occurring deposits of iron, nickel and other minerals to build replicants for dispersal in search for other star systems.

Similar to a virus colonising a body many times its size, eventually ever-increasing numbers of von Neumann probes would expand in all directions, at a fraction of the speed of light. So in this fashion, even a galaxy 100,000 light years across may be fully explored within, say, a half million years.

If a von Neumann probe only finds evidence of primitive life – such as an unstable, warlike and savage Type 0 civilisation – it might simply lie dormant on the moon, silently waiting for the Type 0 civilisation to evolve into a stable Type I civilisation. After waiting quietly for several millennia, it may be activated when the emerging Type I civilisation is advanced enough to set up a lunar colony. Physicist Paul Davies of the Australian Centre for Astrobiology in Sydney, has even raised the possibility that a von Neumann probe could be resting on our own Moon, left over from a previous visitation in our system aeons ago.

If that sounds familiar, that's because it was the basis of the film, 2001: A Space Odyssey. Originally, Stanley Kubrick began the film with a series of scientists explaining how probes like these would be the most efficient method of exploring space. Unfortunately, at the last minute, Kubrick cut the opening segment from his film, and the famous monoliths – von Neumann probes – became almost mystical entities.

This raises the question asked by Italian physicist Enrico Fermi: if von Neumann probes exist, where are they? If such advanced civilisations exist, they would have already visited us years ago. Yet we see no evidence for them. But by analogy, think of walking down a country road and encountering an anthill. Do we bend down to the ants and say to them, "I bring you trinkets and beads. I bring you nuclear energy and space travel. Take me to your leader"? Or, are you perhaps tempted to step on a few of them? Or ignore them altogether?

It's humbling to realise that the developmental gulf between a miniscule ant colony and our modern human civilisation is only a tiny fraction of the distance between a Type 0 and a Type III civilisation – a factor of 100 billion billion, in fact. Yet we have such a highly regarded view of ourselves, we believe a Type III civilisation would find us irresistible and would rush to make contact with us. The truth is, however, they may be as interested in communicating with humans as we are keen to communicate with ants.

Of course, such an encounter with a Type III civilisation has its perils. The danger is not that its citizens would want to eat us or steal our resources – themes that have propelled many a plot in science fiction films. Their DNA, if they have any, would be incompatible with ours, and hence our proteins would be indigestible to them; furthermore, there are plenty of uninhabited planets in the galaxy with more natural resources than Earth, so why would they need to plunder inhabited real estate?

The main danger we face is the same as that faced by our ant colony: the danger of being paved over. Many lifeforms are being imperilled on Earth not because humans want to conquer and plunder them, but because they get in the way of what humans want to do. No ant would understand the construction of a highway or the flooding of a dam, but they would be its victims.

Furthermore, citizens of a Type III civilisation probably wouldn't resemble anything we'd be able to recognise immediately. Humanity's pathway to intelligence required only three basic elements: eyes, hands, and language. Beyond those requirements, almost anything goes. There is nothing sacrosanct about the human shape or form. One can, for example, imagine that it's possible to breed a race of intelligent octopods if we had a few million years to play with. Lastly, if we ever encounter a vessel from outer space, chances are it will be a von Neumann robotic probe of some sort, rather than a transport ship occupied by the aliens themselves.

But by the time a civilisation reaches Type I stage of development stage, it will probably have discovered biotechnology and computers. Hence, there's a likelihood they would gradually abandon the inherited shape and form from evolution and adopt more radical designs. It is well within the laws of biology and computing to imagine a civilisation that creates bodies which are immortal, possess great strength, or have other key attributes.

Since Kardashev's original ranking of civilisations, scientific developments have refined and extended our perceptive analysis, including recent advances in the fields of nanotechnology, biotechnology and quantum physics.

For example, nanotechnology may facilitate the development of von Neumann probes. As American physicist Richard Feynman observed in his seminal essay, 'There's Plenty of Room at the Bottom', there is nothing in the laws of physics to proscribe building armies of molecular-sized machines. At present, scientists have already built atomic-sized, entertaining curiosities, ranging from an atomic abacus with buckyballs to an atomic guitar with strings, measuring about 100 atoms across.

Paul Davies speculates on the idea that a spacefaring civilisation could make good use of nanotechnology to construct miniature probes to explore the galaxy, perhaps no bigger than the palm of your hand. "The tiny probes I'm talking about will be so inconspicuous that it's no surprise that we haven't come across one," he says. "It's not the sort of thing that you're going to trip over in your backyard. So if that is the way technology develops – namely, smaller, faster, cheaper – and if other civilisations have gone this route, then we could be surrounded by surveillance devices."

The development of biotechnology has also opened the possibility that such probes may act as lifeforms, reproducing their genetic information, mutating and evolving at each stage of reproduction to enhance their capabilities, and may have artificial intelligence to accelerate their search.

Information theory has modified the original Kardashev analysis. Current SETI (search for extra-terrestrial intelligence) projects only scan a few frequencies of radio and TV emissions sent by Type 0 civilisations, but perhaps not an advanced civilisation. Because of the enormous static found in deep space, broadcasting on a single frequency presents a serious source of error. Instead of putting all your eggs in one basket, a more efficient system is to break up the message and smear it out over all frequencies and then reassemble the signal only at the other end. In this way, even if certain frequencies are disrupted by static, enough of the message will survive to reassemble the message accurately via error-correction routines. However, any Type 0 civilisation listening to the message on a single-frequency band would only hear nonsense. In other words, our galaxy could now be teeming with messages from a number of Type II and III civilisations, but through our Type 0 radiotelescopes, we would only hear gibberish.

Lastly, there is also the possibility that a Type II or Type III civilisation might be able to reach the fabled Planck energy with their machines – that is, a staggering 1,019 billion electron volts, which is a quadrillion times larger than our most powerful atom smasher.

The Planck energy only occurs at the centre of black holes and the instant of the Big Bang. But with recent advances in quantum gravity and superstring theory, there is renewed interest among physicists about energies so vast that quantum effects rip apart the fabric of space and time. Although it is uncertain that quantum physics allows for stable wormholes, this raises the remote possibility that sufficiently advanced civilisations may be able to move across inconceiveably vast distances of space in the blink of an eye, by using a wormhole as a 'back door' around the speed of light – almost like Alice's looking glass.

And if these civilisations can navigate through stable wormholes, attaining a specific impulse of a million seconds is no longer a problem. They merely take a short-cut through the galaxy. This would greatly cut down the transition time needed to progress from a Type II to a Type III civilisation.

Such an ability to tear holes in space and time may come in handy. Astronomers, analysing light from distant supernovae, have concluded recently that the universe may have been accelerating, rather than slowing down, since the Big Bang – and this is still occurring. If this is true, there may be an antigravity force – perhaps even the Cosmological Constant proposed by Albert Einstein and later retracted – which is counteracting the gravitational attraction of distant galaxies.

But this also means that the universe may expand forever, the stars burn out of fuel and the universe will eventually darken – ending in a Big Freeze, with temperatures approaching near absolute zero. Several papers have recently laid out what such a dismal universe may look like. It'll be a pitiful sight: any civilisation that survives will be desperately huddled next to the dying embers of fading neutron stars and black holes. All intelligent life must die when the universe dies.

Contemplating the death of the Sun, the British philosopher and mathematician Bertrand Russell wrote: "All the labours of the ages, all the devotion, all the inspiration, all the noonday brightness of human genius, are destined to extinction in the vast death of the solar system, and the whole temple of Man's achievement must inevitably be buried beneath the debris of a universe in ruins".

Today, we realise that sufficiently powerful propulsion systems may one day spare us from the death of our Sun in some five billion years when the Earth's oceans boils and the mountains melt. But how do we escape the death of the universe itself?

There could be a way out, according to John Barrows, a theoretical physicist at the University of Cambridge: "Suppose that we extend the classification [of advanced civilisations] upwards. Members of these hypothetical civilisations of Type IV, V, VI … and so on, would be able to manipulate the structures in the universe on larger and larger scales, encompassing groups of galaxies, clusters, and superclusters of galaxies." Civilisations beyond Type III may have enough energy to escape our dying universe via holes in space.

Lastly, physicist Alan Guth of the Massachusetts Institute of Technology, who helped create the inflationary universe theory, has even computed the energy necessary to create a new 'baby universe' in the laboratory: it requires a temperature of 1,000 trillion degrees Celsius, which is within the range of these hypothetical civilisations.

Of course, until someone actually makes contact with an advanced civilisation, all of this amounts to speculation – albeit tempered with the laws of physics – that is no more than a useful guide in our search for extraterrestrial intelligence.

But one day, many of us could gaze at the encyclopaedia that contains the coordinates of perhaps hundreds of Earth-like planets in our sector of the galaxy. Then we will ponder with wonder, as Sagan did, what an intelligent civilisation a millions years ahead of ours will look like.

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