Thursday, 29 December 2011

The Reason Why

The Reason Why: The Miracle of Life on Earth by John Gribbin

Literary Review, November 2011

In the summer of 1950 the New Yorker magazine published a cartoon suggesting that aliens were behind the mysterious disappearance of rubbish bins from the streets of New York. Not long after the cartoon appeared, a group of scientists at the Los Alamos Laboratory in New Mexico were joking about it with their distinguished visitor, the Italian physicist Enrico Fermi. During lunch Fermi suddenly asked, ‘Where is everybody?’ It took a moment for his colleagues to realise that he was referring to extraterrestrials.

With hundreds of billions of galaxies, the universe could easily be teeming with extraterrestrial life. However, the enormous intergalactic distances involved rules out the possibility of a visit. Yet Fermi was not thinking about the entire universe, only our tiny part of it – the Milky Way. After a quick calculation, then and there, he concluded that space-faring aliens, should they exist, would have colonised our galaxy long ago and therefore have visited Earth. As Fermi was regarded as one of the great physicists of the twentieth century, his back-of-the-envelope reasoning was taken seriously and led to what was called the Fermi Paradox: ‘If they are there, why aren’t they here?’ The reason why, argues science writer John Gribbin, is simple: ‘We are alone, and we had better get used to the idea.’

Gribbin begins his entertaining polemic by referring to an equation devised in 1961 by the American astronomer Frank Drake that attempted to quantify the chances of detecting intelligent life elsewhere in our galaxy. Starting with the total number of stars, Drake calculated how many of them were Sun-like, and then asked what fraction of these had planets, how many of them were capable of supporting life, and so forth. With one guesstimate piled upon another it’s a futile approach that, at best, succeeds only in demonstrating our ignorance, for the answers generated range from zero to the hundreds of billions.

Gribbin, though, is prepared to play the numbers game while treating Drake’s equation for what it is, ‘a kind of mnemonic to remind us of the sort of things we have to take into account when considering the possibility of finding intelligent life elsewhere’. For Gribbin, the reasonable thing to do is to look at the history and geography of our galaxy and try to understand why, and how, intelligent life emerged on Earth. For he seeks to understand not whether we are alone but why we are alone. It’s Fermi’s paradox repackaged as: ‘If they are not there, why are we here?’

Gribbin sets out the arguments that the Earth occupies a special location in both space and time that has allowed the development of the only technological civilisation in the Milky Way. The Sun, for example, is fortuitously situated in what is called the Galactic Habitable Zone, a kind of Goldilocks region that’s just right for life. Critically, five billion years ago when the solar system was being formed, the zone had an abundance of metallic elements that allowed the formation of a planet like Earth. Any closer to the galactic centre would have been hazardous for life because of radiation from supernovae explosions, while the outer regions were poor in metals.

Gribbin only entertains the possibility of ‘life as we know it’ and therefore the presence of water is essential. Fortunately for us, Jupiter is of the right size and in the right place to have sent water-rich asteroids and icy comets crashing to Earth early in its history. In yet another piece of luck, the Earth acquired a relatively large moon. Others have covered before much of what Gribbin describes about the Moon’s essential role in the development of life on Earth, but he does so in an easily accessible style befitting the author of more than 100 books. He explains how the fledgling Earth was formed close to Theia, another proto-planet the size of Mars, and that when the two collided the surface of the Earth turned into magma as it swallowed up Theia’s core.

The vast quantities of debris ejected into space as the result of these worlds colliding eventually formed the Moon. The collision tilted the Earth on its axis and set it spinning much faster than it does today. The Moon has acted as a stabilising influence ever since and continues to shield the Earth from the full effect of Jupiter’s gravity. And our neighbour has certainly prevented cosmic debris from striking the planet, says Gribbin. He argues convincingly that the Moon is the single most important factor in making life on Earth possible.

The collision that led to the creation of the Moon resulted in the Earth having a thin crust and, through plate tectonics, the dynamic surface required to sustain the temperature range required for liquid water – and therefore, eventually, us. Otherwise it would probably have become a hot desert like Venus or a frozen world as cold as the Moon.

Life on Earth, argues Gribbin, is the product of such an improbable sequence of chance events that the possibility of finding any other technological civilisation in the galaxy is effectively nil. Yet the fact remains that we are here to ponder the reason for our existence because things are the way they are, because they were the way they were. Elsewhere they could have been different, leading to life, but not as we know it.