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.
