A probability-weighted conclusion engine with 30 solutions, error bars, and calibration ranges. Built from hundreds of expert sources + peer-reviewed biological and astrophysical science, in Claude Code.
Finished with deep gap-filling research by Gemini.
Starting from 400 billion stars in the Milky Way, how many survive each filter?
| Filter | Pass Rate | Survivors |
|---|---|---|
| Stable star (FGK, not M-dwarf) | 20% | 80 billion |
| Rocky planet in habitable zone | 20% | 16 billion |
| Surviving atmosphere + magnetosphere | 10% | 1.6 billion |
| Earth-like water (oceans + dry land) | 5% | 80 million |
| Abiogenesis (single-celled life) | 30-80% | 24-64 million |
| Eukaryogenesis (complex cells) | 10-6 to 10-8 | 0.02 - 64 |
| Multicellularity (given eukaryotes + O2) | 99% | same |
| Human-level intelligence | 10-40% | 0.002 - 25 |
| Rare Fire (land + O2 + hands + combustion) | 5-20% | 0.0001 - 5 |
| Fossil fuel bootstrap (Carboniferous accident) | 1-10% | ~0 - 0.5 |
| Language + technological accumulation | 1-10% | ~0 |
The galaxy is probably teeming with bacteria (tens of millions of planets).
Complex life? Maybe a handful. Tech civilizations? We might be it.
Everything above is scoped to the Milky Way — one galaxy among roughly two trillion. The observable universe contains approximately 1024 stars. Even if the probability of a technological civilization per star is 10-12, that still yields a trillion civilizations somewhere out there. At the scale of the full universe, the question flips from "where is everybody?" to "they're almost certainly out there — but so what?"
But first — how far has humanity's voice actually traveled? We've been broadcasting electromagnetic radiation for roughly 100 years. Our radio bubble extends about 100 light-years in every direction, encompassing approximately 15,000 stars. The Milky Way is 100,000 light-years across. Our signal has covered one-thousandth of the diameter of our own galaxy. And those signals are so weak that even our most powerful radar pulses are indistinguishable from background noise beyond a few light-years.
The problem is distance. The nearest major galaxy, Andromeda, is 2.5 million light-years away. A signal sent from Andromeda when it was transmitted would have been launched when Homo habilis was chipping the first stone tools in East Africa — before our species even existed. The Virgo Cluster is 54 million light-years out. The Laniakea Supercluster — our cosmic neighborhood — spans 500 million light-years. And between the superclusters lie cosmic voids: stretches of nearly empty space 100 to 300 million light-years across where there is essentially nothing at all.
Even at lightspeed, communication across these distances is a geological process. A round-trip conversation with a civilization in the Virgo Cluster would take 108 million years per exchange. By the time their reply arrives, the species that sent the original message may have evolved into something unrecognizable — or gone extinct entirely. The expansion of the universe makes it worse: galaxies beyond a certain distance are receding faster than light itself, permanently unreachable regardless of any technology. Roughly 94% of the galaxies in the observable universe are already beyond our cosmic event horizon and will never be contactable — not in a million years, not in a billion, not ever.
So the math of all galaxies is simultaneously hopeful and crushing. Life is almost certainly out there — probably in staggering abundance. But the universe has built walls between its neighborhoods that no engineering can breach. The Fermi Paradox is a local question with a local answer: in this one galaxy, the filter cascade may have produced exactly one technological civilization. And the rest of the universe, teeming or not, might as well be a 4th-generation photocopy of a 100-year-old photograph of a cat taken from 50 yards away.