The Quiet Galaxy

The equation · piece 6 of 7 · fc

The share of intelligent species that make themselves detectable

Given an intelligent species, how likely it is to do something we could notice from here: leak or send radio, light up its planet, change its atmosphere, build things in space. Drake had deliberate radio messages in mind; the modern reading is broader, since our own radar, television and city lights have made Earth detectable to a neighbour with a good enough telescope without our choosing it. The factor also covers the choice not to be found, which is where several of the explanations for the silence live. Nothing here is measured; every figure is an assumption.

The range this site uses 0.01 to 1 reviewed
set on 16 October 2026

When the number on the home page is computed, a value for this piece is picked at random from this range each time, and every tenfold step between the two ends is equally likely.

Why that range

The 2018 paper's span of 0.01 to 1, kept as it is: it covers Drake's one in ten to one in five, Sagan's one in ten and Westby and Conselice's 1, and nothing has been measured that would move either end. The floor of 0.01 allows for species that never build radio, or build it and stay quiet, which several explanations (the dark forest, the zoo hypothesis) say is the norm. Evenly spread across its powers of ten, as in the 2018 paper.

Rests on: Sandberg and others, 2018.

The published estimates

4 estimates on the record, oldest first. Each says who made it, when, how, and where to find it. A range is set from these by hand, with the reason above, never computed from them: the reason is the arguable part.

WhenWhoEstimateHow it was reached
1961 Frank Drake and the Green Bank meetingGreen Bank meeting of November 1961, as recorded in Drake and Sobel, Is Anyone Out There? (Delacorte, 1992), pages 55 to 62 (drake-1961) 0.1 to 0.2 Educated guess: one in five to one in ten intelligent species would build radio and choose to signal.
June 2018 Anders Sandberg, Eric Drexler and Toby OrdSandberg, Drexler and Ord, 'Dissolving the Fermi Paradox', arXiv:1806.02404 (2018), Table 1 (sandberg-2018) 0.01 to 1 A span evenly spread across its powers of ten, from the published estimates of 0.01 to 1, with the paper's remark that a broader range is clearly intended by some authors.
June 2020 Tom Westby and Christopher ConseliceWestby and Conselice, 'The Astrobiological Copernican Weak and Strong Limits for Intelligent Life', Astrophysical Journal 896, 58 (2020) (westby-2020) 1 Assumed: every civilisation that develops as ours did is taken to be communicating, for an average of 100 years.
June 2020 Alex De VisscherDe Visscher, 'Artificial versus biological intelligence in the Cosmos: clues from a stochastic analysis of the Drake equation', International Journal of Astrobiology 19(5), 353-359 (2020) (de-visscher-2020) · in the log 0.01 to 1 For intelligent biological species able to communicate across space, took the 2018 paper's span of 0.01 to 1 as it stands, so it is not an independent estimate. The paper also assumes 0.0001 to 1 for the further chance that such a species builds a machine intelligence able to communicate, as the first chance squared; that is a figure for a different thing and is not used here. Added on 2026-10-17 (outside review, item 16).

What would move this

The discovery that would narrow the range

A published argument with a number on how often technological species stay quiet or go dark and that isolates this factor; survey limits on how many transmitters or technological signatures could be nearby (the Breakthrough Listen limits) are logged here too, but they bound the product of all seven factors and move this range only together with such an argument.

When a paper or a survey of that kind is published, it is logged in the story so far with a note saying whether it moved this range. A move is written as a dated revision below, with the old number, the new one and the reason.

The last reading

Once a week the records are read against the news, each against the test above. Open a line to see what that week held. Every reading ever made of this record is on the reading page.

  • 2026-10-16 Logged, no change By hand

    No detection. Four new bounds and follow-ups: Mason, Garrett and Siemion (2025-11-25) limit steady beacons of at least five times ten to the sixteen watts to fewer than about one in a hundred thousand stellar systems within 2.5 kiloparsecs, by counting every star that fell in the beam of a Breakthrough Listen survey; Huang, Tao and Zhang (2026-01) find no galaxy-scale waste heat in nearby galaxies, with at most about one in 6,500 able to host it; Korn and others (2026-07-28) find two of the seven Dyson-sphere candidates of 2024 are background galaxies and none confirmed; Garrett (2026-08-31) proposes a search for cold waste heat at submillimetre wavelengths. The bounds are on strong, steady transmitters, which the range's low end of 0.01 already allows to be rare, so the range stays.

    Mason, 2025Huang, 2026Korn, 2026Garrett, 2026 source 1source 2source 3source 4

How the range has moved

DateWhatFromToWhy
16 October 2026 estimate (Carl Sagan, 1980) value 0.1, 'a round guess, one tenth' removed Checked against Cosmos (1980), chapter 12, on 2026-10-16: Sagan writes 'let us choose fi x fc = 1/100' and gives no value for the detectable factor alone, so one tenth is not a published estimate of it. The range and its first-pass reasoning are untouched; the reasoning's mention of Sagan's one in ten should be revisited at the outside review.
17 October 2026 what_would_move_this A survey result that bounds how many transmitters or technological signatures could be nearby (the Breakthrough Listen limits), or a published argument with a number on how often technological species stay quiet or go dark. A published argument with a number on how often technological species stay quiet or go dark and that isolates this factor; survey limits ... are logged here too, but they bound the product of all seven factors and move this range only together with such an argument. Ruled by John 2026-10-17 on the outside review (item 17, accepted in part): the reviewer rightly said a radio search bounds the product of all seven factors and not this one alone, but the line also drives the weekly reading's searches and survey limits are worth logging here, so the survey half stays with that warning.
17 October 2026 estimate three published estimates four: De Visscher (2020), 0.01 to 1 for the biological route, added; Shostak's chapter (2015) logged as a publication without an estimate Ruled by John 2026-10-17 on the outside review (item 16). The figures the reviewer attributed to the two papers could not be confirmed as estimates of this factor: De Visscher's 0.0001 is for the machine route, and Shostak's chapter could not be read. The range is unchanged.Because of Artificial versus biological intelligence in the Cosmos: clues from a stochastic analysis of the Drake equation.
17 October 2026 range.status first-pass reviewed The outside review of the seven ranges ran on 2026-10-17 and John ruled on every item the same day (docs/reviews/2026-10-17-rulings.md).

The explanations that lean on this piece

An explanation for the silence is, in the end, a claim about one or more of the seven pieces. 13 explanations say this piece is smaller than it looks; 7 leave it alone and say the silence is about us.

The papers that bear on it

Books and films that take it up

Notes

The founding pessimist papers, Hart (1975, hart-1975) and Tipler (1980, tipler-1980), do not estimate this factor; they argue from the absence of visitors and of self-copying probes that no detectable civilisation exists in the galaxy at all, and Brin's 1983 survey of the silence (brin-1983) collects the answers. Those are explanations and are written up as such. Tarter (2001, page 544) notes that searches at today's sensitivity could detect the equivalent of strong television transmitters at 1 light-year, military radars at 300 and the Arecibo planetary radar at 3,000, so by the broad reading this factor is at least the share of species that reach our level of technology and do not hide. Corrected 2026-10-16 against the sources: Carl Sagan's one tenth was removed (his Cosmos, chapter 12, gives only the product of the intelligence and detectable factors, one in a hundred, never this factor alone; see the revision below); Forgan and Elvis (2011) is a paper on asteroid mining as evidence of aliens and says nothing about Earth being detectable, so it is dropped; and the range's reason below still names 'Sagan's one in ten' and should be revisited at the outside review (the range is unchanged). Written 2026-10-16. Outside review, 2026-10-17, ruled by John the same day: lowering the floor from 0.01 to 0.0001 (item 15) was accepted only if De Visscher (2020, de-visscher-2020) gives a range reaching 0.0001 for this factor, and the condition was not met, so the floor stays at 0.01. The paper does print 0.0001 to 1, but for a different quantity: the chance that an intelligent biological species goes on to build a machine intelligence able to communicate across space, which the author sets by assuming the extra step is as selective as the first, so that it is the first one squared. For the biological route, this factor's own subject, the paper takes the 2018 paper's 0.01 to 1 as it stands, which is the range used here. The figure is an assumption for a different thing, and it cannot lower the floor of a share that is already counted over either route (at 0.0001 the scratch headline was 65% alone in the galaxy against 59%). If John reads the paper otherwise, the change is one line and a revision. The reviewer's second paper (item 16), Shostak's chapter 12 in the 2015 Cambridge volume, is in the log (shostak-2015) with its figure left out because the chapter could not be read. The test line was reworded to say survey limits are logged but move the range only with an argument that isolates this factor (item 17).

Think the range should move? Argue it, with a source. Spotted an error on this page? Say so. Or read how disagreement gets folded in. The record itself is data/factors/detectable.toml in the public repository.