Humanity is at a crossroads, says Michio Kaku. We can decide to unite in harnessing the power of the stars and thrive or we can succumb to primitive urges and self-destruct

About 5,000 generations have preceded us since modern humans first left the forests of Africa about 100,000 years ago and began the great diaspora, populating the various continents of the Earth. Of these myriad generations, ours is the most important because we control the destiny of the planet. For the first time in history, we possess the power to destroy life on Earth completely or to create a paradise. To gain some insight into the evolution of civilisations, physicists have scanned the heavens for signs of intelligent life that have perhaps already negotiated the numerous obstacles we may face in the future. Indeed, the Russian astrophysicist Nikolai Kardashev introduced the ranking of Type I, II and III civilisations to guide this search.

A Type I civilisation is a planetary civilisation that can harness all planetary energy. It can control or modify the weather at will, tame volcanoes and calm earthquakes. The amount of energy it can control is equal to the total amount of energy it receives from its sun. A Type II civilisation has outgrown the power of its planet and can directly harness the total energy output of its sun, exploiting solar flares to energise its engines. Its total energy output is equal to that of a star. A Type III civilisation can harness galactic power by exploiting the energy of billions of star systems.

Each civilisation is 10 billion times more powerful than the previous one.

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On this cosmic scale of energy consumption, where is our civilisation? Sadly, we rank as Type 0. We obtain our energy from dead plants, and we harvest only the tiniest fraction of energy from our sun.

Kardashev also introduced a way of calculating when we might reach these higher civilisations. Assuming that a civilisation grows at a modest 1 to 2 per cent a year in terms of energy consumption, it will be about 100 years until we reach Type I status, several thousand years until we attain Type II status, and perhaps 100,000 years or more before we achieve Type III.

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Each civilisation type faces unique challenges. For us, the transition from Type 0 to Type I may be only a century away, but we can see evidence of advancement everywhere: n A truly planetary communication system is emerging from the germ of the internet

* English is emerging as a planetary Type I language. It is already the world's most used second language

* A Type I economy can be seen emerging in huge trade blocs such as the European Union and that formed by the North American Free Trade Agreement

* Cultural convergence can be seen in the emergence of satellite TV, the internet and the wiring of the planet. A universal youth culture is also developing

* A planetary response to global crises such as the greenhouse effect can be seen

* Although nation-states and wars will remain with us, their relative power (compared with that of, say, the Romans) will be diminished. Any nation-state that tried, for example, to ban the internet would be laughed at.

However, anything so momentous in human history is inevitably going to produce a backlash. Fundamentalists, for example, instinctively realise the threat that this transition poses to their parochial beliefs.

Unfortunately, we are still plagued by the same savage instincts that haunted us when we first emerged from the forest, which today can be seen in the ugly faces of sectarianism and racism. The difference this time, however, is that these dark forces may have access to weapons of modern technology, including biotechnology, nuclear weapons and chemical weapons.

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Designer germs, suitcase atomic bombs and their ilk are the nightmares of every scientist.

So we are at the most crucial crossroads in human history. Decisions made by the current generation will determine the ultimate destiny of the human race. The climb from Type 0 to Type I is the most dangerous of civilisational transitions, and it can be negotiated only with the utmost maturity and wisdom.

If we negotiate the transition to Type I, we would still face continuing threats to our existence. But a Type II civilisation is nearly immortal as it can avoid most natural threats, modifying ice ages and deflect comets and meteors. The only possible threat might be the death of its sun, but it might be able to move its planet or even reignite the fading sun.

A Type III civilisation might face yet another threat: the death of the universe itself. The evidence is overwhelming that the universe is accelerating, careering out of control in a runaway expansion, and that it will die in a Big Freeze, when the stars exhaust their nuclear fuel, the sky becomes dark, the oceans freeze over and the universe consists of the dying embers of neutron stars and fading black holes.

In this distant future, a Type III civilisation may have no recourse but to leave the universe. The latest theories in physics, such as inflation and string theory, postulate the existence of parallel universes, some perhaps hovering just above ours in a higher dimension. A Type III civilisation may use the laws of physics to embark on a crash programme to build gigantic banks of laser cannons and atom smashers the size of star systems that can focus the intense temperatures, energies and densities necessary to open a hole in space. Facing the death of the universe, it may have no choice but to find a warmer, more hospitable home.

But for us, there is one final warning from our search for civilisations in space. We have scanned star systems with our radio detectors out to several hundred light years from Earth, and yet we find no evidence of any civilisations. Drake's equation, which tries to estimate the number of intelligent life forms in our Milky Way galaxy of 200 billion stars, indicates that there should be 10,000 intelligent life forms. But where are they?

Some believe that perhaps we are too primitive for them to communicate with us; these other life forms might treat us in the way that we treat ants.

But there is another possibility. Perhaps 10,000 planets did spawn intelligent life, but only a handful made the transition from a Type 0 to a Type I civilisation.

This is sobering because we are now making that same transition. If we ever soar into outer space and visit nearby stars, perhaps we will visit planets whose atmosphere is radioactive as a result of nuclear war, or whose atmospheres are blisteringly hot because of the greenhouse effect.

So the choice is ours. Our generation will decide whether we successfully negotiate this transition to the new Golden Age or face the abyss.

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Michio Kaku is professor of theoretical physics at the City University of New York. This article is adapted from his book Parallel Worlds , published by Penguin Books on February 3, ?18.99. His website is