鈥淲here is everybody?鈥� wondered physicist Enrico Fermi when ruminating on the question of life in the cosmos. He reasoned that, if life鈥檚 genesis is not too difficult, the universe could be teeming with little green creatures on the many trillions of planets out there.
It鈥檚 been nearly 14 billion years since the big bang, so if some of these alien societies had become technologically advanced and spacefaring, evidence of their existence should be obvious. So why do we see no sign of them?
This apparent absence of evidence is known as the Fermi paradox. It has led to considerable head-scratching for more than half a century. Now, , rephrasing Fermi鈥檚 question to: 鈥淲here was everybody?鈥� In particular, one answer could be our own solar system. He wonders if 鈥減rior indigenous technological species鈥� arose here, and what trace might they have left behind?
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The Fermi paradox has many proposed solutions: that we are truly alone in the cosmos, for example, or that Earth is kept isolated from the interstellar community until it becomes a responsible galactic citizen. The scariest possibility is the idea of a Great Filter, some inevitable sticking point that means all civilisations have a relatively short shelf life, perhaps because they develop and fall victim to self-destructive technology. In which case, on cosmic timescales, the chances of two coexisting in close proximity would be vanishingly small, and they would always appear to be alone.
Before humans roamed on Earth
So the universe could be littered with the debris and detritus of such dead civilisations. Rather than contemplating extinct life 鈥渙ut there鈥�, which would be very hard to detect, Wright is contemplating the possibility of technologically advanced life that rose and fell in the early solar system, and succumbed to the Great Filter many hundreds of millions, if not billions, of years before humans walked on Earth.
If they existed here, or on the other planets and moons, what signs should we look for and where? In the crushing environment of Venus, and the churning plate tectonics of Earth, buildings and monuments would be eroded and destroyed on such long timescales. But on slow-changing Mars, our moon, and possibly the frozen satellites of outer solar system planets, the tunnels and cities of ancient lost civilisations could survive buried under the soil and ice.
Other signatures would be more durable still, with the slow decay of nuclear power sources apparent for billions of years, with distinct mixtures of elements and radioactivity.
This is pure speculation. But remember our space probes have barely scratched the surface of any planet, and when we finally start digging into the dirt of other worlds, we might uncover definitive signs that someone else has been there before.
That someone may not have crossed the depths of space, as SETI usually assumes, but may have been there all along.
Geraint Lewis is a professor of astrophysics at the University of Sydney, Australia
More on this topic: Alien Life
