In recent decades our understanding of life has advanced rapidly. Scientists now know that life can exist under a much wider range of conditions than were previously thought possible, and are discovering life in the most extreme parts of the earth: in rocks a kilometer under the ground, in the antarctic ice, in the boiling water of underwater geothermal vents, etc.
Meanwhile, astronomers have discovered many planets oribiting other stars, enough to realise that planets are commonplace in our galaxy. As the ability to detect planets outside our solar system improves, we are discovering more and more of the smaller earth-like planets.
As a result, most scientists now think that life is unlikely to be unique to planet Earth; and that the universe may be teeming with life. This raises the question: if there are so many aliens, how come we haven't seen any evidence of them? If life is widespread throughout the universe, how come those civilizations that arose before us haven't contacted us or visited us? This is known as the Fermi paradox.
The Drake equation tries to put this into numbers. It starts with the rate of star formation and assigns probabilities to:
- the likelihood of each star having planets that can support life,
- the likelihood of intelligent life developing on each of those planets, and
- the chance that those civilizations will develop a technology that allows them to become known to us
The numbers plugged into the Drake equation are necessarily speculative. Under the most pessimistic assumptions, humans may be the only intelligent life in the observable universe. Under other assumptions, the universe may support very large numbers of intelligent civilizations. So why haven't they visited us, or at least beamed us a signal? Many theories have been proposed, for example:
travel is difficult, and may be impossible. The other civilizations may simply live too far away.
- The aliens may not have discovered radio communication.
- They may have no urge to explore.
- They may be deliberately hiding, perhaps out of fear of other civilizations.
- They may be so different from us that we don't recognize their communications (for example, if their timescales are a thousand times faster or slower than our own).
- Perhaps other forms of life have prospered without needing to evolve intelligence or develop technology.
But there's one other factor in the Drake equation: after a civilization becomes intelligent and technologically-capable, how long does it last? If the average civilization only lasts a few hundred years before it self-destructs, the chance of these other civilizations being around at the same time as us becomes vanishingly small.
It's hard to guess what the lifespan of a civilization is likely to be. Maybe all civilizations soon blow themselves up, or inflict disease upon themselves, or irrevocably damage the ecosystem of their planet.
Alternatively, the civilizations might endure, but become undetectable to us. After the aliens become technologically capable, their energy use must skyrocket. One speculation is that they then build a spherical shell around their sun to capture all of its energy. Such a shell, called a Dyson sphere, would make them extremely hard to detect.
What I think is more likely is that, soon after becoming technologically-capable, the extra-terrestrials tend to eschew the physical world for a virtual reality. Brains in jars on a shelf, perhaps, hooked up to a totally immersive, totally addictive experience that they find more satisfying than their previous life with a physical body. We seem to be seeing early signs of that already on planet earth.
One suggestion is that the aliens are out there, but they have placed us in a limited environment—a sort of wildlife refuge. Or perhaps we are in captivity, like a goldfish held in a bowl from which it has no chance of seeing any of the other goldfish in the world.
Or maybe the explanation is much simpler than that. Maybe we really are alone.
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