The Seven Numbers
factor ⇄ how we know · live
R★ · star birth: ~1.65 M☉ per year, ≈ 3 stars
Licquia & Newman 2015 · the best-measured factor
MEASURED
fp · planets per star: ≥ 1 · planets outnumber stars
Cassan 2012, Nature · microlensing + Kepler
MEASURED
ne · temperate rocky worlds: η⊕ ≈ 0.4 per system
Bryson 2021 · extrapolated from Kepler counts
MODELLED
fl · life, given a world: unknown
one sample, and it had to be positive · Spiegel & Turner
READING
fi · minds, given life: unknown
~4 Gyr and one technological species here
READING
fc · broadcasting, given minds: unknown
we chose radio · a quieter species scores zero
READING
L · years on air: unknown
our own count so far: ~110 and holding
READING
The silence: no confirmed signal in 66 years
Ozma 1960 → Breakthrough Listen · a real null result
MEASURED
View
GalaxyN civilisations, seated in the disk
Ledgerthe equation as a log waterfall
Preset
Green Bank 1961Drake's blackboard · N ≈ 1,000
The telescopesfirst three factors, measured
The optimistSagan-flavoured · a crowded galaxy
Rare EarthWard & Brownlee · maybe just us
DissolvedSandberg, Drexler & Ord 2018
Fermi's questionone front · 2% c · the whole disk
Reading · The 1961 blackboard guesses · N ≈ 1,000
Readout · live, from your sliders
N · on air now
…
Civ. births /yr
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Nearest expected
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Signal round-trip
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P(we're alone)
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Our bubble
110 ly · fixed
What is measured vs modelled vs read-in
Measured: the star formation rate, the fraction of stars with planets, and 66 years of silence: Project Ozma heard nothing in 1960 and Breakthrough Listen's wideband surveys have confirmed no signal since. Modelled: ne extrapolates Kepler's occurrence rates beyond what was directly counted, and the marker placement is a statistical seating; the annulus those markers sit in is Lineweaver, Fenner & Gibson 2004's published 7 to 9 kpc band. Read-in: fl, fi, fc and L, every factor right of the wall, argued across six orders of magnitude from a single sample: us.
Where the equation strains
It is not a law: nothing in it can be derived, and its factors hide correlations (a planet that evolves minds may need the same rare stability that makes it habitable). Critics call it a way to package ignorance ; Drake would have agreed, since organising the ignorance was the point. The 2018 uncertainty analysis shows the honest output is not a number but a distribution, wide enough to contain both a crowded galaxy and an empty one. The sliders are the argument; N is only where you set them.
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