The Milky Way contains somewhere between 100 and 400 billion stars. The observable universe contains an estimated two trillion galaxies. The fraction of stars with orbiting planets has been revised steadily upward over the last thirty years as detection methods improved; it now appears to be close to one. The fraction of those planets in conditions potentially suitable for life is smaller but not negligible. Apply even conservative assumptions about the probability of life emerging given suitable conditions and you produce numbers large enough that "probably not alone" is the closest thing to a scientific consensus available.
And yet: silence. No signals. No visitors. No responses to any transmissions. No artefacts in our solar system. Nothing. This is the Fermi Paradox, and it is considerably more alarming than it first appears.
The Numbers Don't Add Up
The galaxy is about 13.6 billion years old. Even at speeds far below what physics permits, a civilisation that arose a billion years ago could have physically colonised every star system in the Milky Way several times over by now. The question Enrico Fermi asked his colleagues in 1950, "where is everybody?", is deceptively simple. The numbers strongly imply that if intelligent life arose anywhere with significant frequency, we should by now have evidence of it. We don't. The absence needs explaining.
The explanations on offer are not reassuring. The most popular are: life is far rarer than the naive calculations suggest (the Rare Earth hypothesis); civilisations reliably self-destruct before achieving significant expansion (the Great Filter hypothesis); and we're simply not looking in the right way or at the right time. Each answer has problems.
The Great Filter and Its Implications
The Great Filter idea, developed by economist Robin Hanson, holds that somewhere between the formation of stars and the colonisation of the galaxy, there is a step that almost nothing gets through. The question is where that step falls. If it's behind us, if the hard transition was, say, the development of eukaryotic cells, or sexual reproduction, or multi-cellular life, then we might be genuinely unusual, possibly unique, and the silence is explained. This is the optimistic interpretation.
If the Great Filter is ahead of us, if every intelligent civilisation tends to destroy itself before achieving significant expansion, then the silence is evidence for our own likely extinction. Every absence of a signal from an older civilisation is one more data point for the hypothesis that advanced civilisations don't last. The discovery of microbial life on Mars or Europa, far from being cause for celebration, would narrow the window for the filter to our future, not our past.
We should perhaps be relieved that we haven't found life elsewhere. Not because life elsewhere would be bad, but because its absence is the version of the story that leaves the filter behind us rather than ahead.
Not Alone, But Not In Contact
The probabilistic case for other life somewhere is strong. The observational case for detectable life anywhere near us is weak. These positions are compatible. The universe is large enough that "not alone" and "not in contact" can both be true, especially given that the window of coincidence required for two civilisations to overlap in time and space and be detectable to each other is narrow by cosmic standards.
But the Fermi Paradox is not primarily a question about whether life exists. It is a question about whether something systematically prevents it from lasting. That question does not have a comfortable answer available.
We are almost certainly not alone. Whether that's good news depends entirely on where the filter is.
Disagree? Say so.
Genuine pushback is welcome. Personal abuse is not.
