Conceptual computer artwork of multiple universes: Some physicists believe that there are an infinite number of parallel universes, created for each possible quantum mechanical outcome. The collective name for these universes is the multiverse.

Credit: Mehau Kulyk/SPL

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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.

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