Ships in the night: civilisations are common but short-lived against the age of the galaxy, so almost none are alive at the same time
The galaxy is about thirteen billion years old. Suppose it has produced many thousands of civilisations in that time, and suppose each one is detectable for ten thousand years, which is longer than human civilisation has existed. Then at any given moment, only a handful are alive, scattered across a hundred thousand light-years, with their signals arriving at each other long after the senders are gone. On this view the question is not why nobody is out there but why anyone expected the timing to work. Civilisations could be as common as the optimists hope and still almost never overlap.
This explanation is built into the Drake Equation itself. The equation counts civilisations detectable right now, and the way it gets there is to multiply the rate at which they appear by how long each one lasts. Drake noticed at once that the whole result hangs on the last factor: if civilisations appear once a century somewhere in the galaxy and each lasts a thousand years, there are about ten alive at a time; if each lasts a million years, there are ten thousand. The number of civilisations the galaxy has ever produced is not what matters. What matters is how many of their lifetimes overlap with ours.
Shklovskii and Sagan drew out the consequence in their 1966 book, which did more than any other to bring the question to a wide audience. If the typical civilisation is detectable for a few thousand years, then the galaxy's whole history is a sequence of brief flashes, each gone before its light has crossed much of the galaxy, and two flashes close enough in both space and time to notice one another would be rare. The picture has been tried out in computer models since, among them a random-trial model of the whole galaxy built by Duncan Forgan in 2009, in which signal lifetime is one of the inputs and the number of civilisations on the air at any moment is something the model counts rather than assumes.
The point is sharper when distance is included. A signal from a civilisation ten thousand light-years away shows us that civilisation as it was ten thousand years ago. For a signal to be received, the sender has to have been transmitting when the light left, and the receiver has to be listening when it arrives, and the two windows have to be lined up across the gap. Short lifetimes make that alignment unlikely.
The bearing on the Drake Equation is on the last factor only: the explanation is a statement about what a short detectable lifetime implies, not a claim about why it would be short. The pages on self-destruction, burn-out and natural catastrophes offer the reasons; this page is about the arithmetic. If the lifetime is long, in the millions of years, the explanation fails and the silence needs another answer.
The main objection is Hart's argument of 1975: a single civilisation with a long lifetime would spread across the galaxy and leave a mark everywhere, so the ships-in-the-night picture requires that no civilisation ever lasts long or expands, which brings the question back to why. It also assumes that detectability ends when a civilisation ends, whereas a civilisation that built things in space might leave traces for far longer than it lived.
What it says about the equation
Every explanation is a claim about one or more of the seven pieces that are multiplied to give the number of detectable civilisations. This one bears on 1 piece:
- L · How long a civilisation stays detectable, in years piece 7 of 7 says this piece is smaller than it looks
The test: what would strengthen or weaken it
Would strengthen it: Evidence that detectable lifetimes are short, such as detected signals or traces that have already stopped; a well-founded argument that civilisations stop being detectable within thousands of years even when they survive.
Would weaken it: Any detected civilisation that is old; evidence of long-lived structures or activity around other stars; a persuasive case that civilisations which survive their first few thousand years go on for millions.
These two lines are what the record is read against when a new paper or discovery is logged. A result of the kind named here, published with its evidence, is what would settle it; a press claim on its own settles nothing.
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.
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2026-10-16 Logged, no change By hand
Three arguments toward the explanation: Rahvar and Rouhani (2026-02-24) bound the lifetime at about 5,000 years if civilisations are common; Garrett (2025-09-28) argues the detectable phase may be mere decades; Blanco and others (2026-04-15) find civilisations detectable for only part of their span. No signal or trace that has already stopped has been found. Logged.
Rahvar, 2026Garrett, 2025Blanco, 2026 source 1source 2source 3
How seriously it is taken
Universally accepted as a correct piece of arithmetic and the reason the lifetime factor is the one everyone argues about; as an explanation it stands or falls with the claim that lifetimes are short, which is the subject of its sister pages.
The papers that bear on it
- Projections of Earth's Technosphere: Civilization Collapse-Recovery Dynamics and Detectability 15 April 2026 Celia Blanco, Jacob Haqq-Misra, George Profitiliotis · logged; moved no range
- Constraining the Lifespan of Intelligent Technological Civilization in the Galaxy 24 February 2026 Sohrab Rahvar, Shahin Rouhani · logged; moved no range
- Blink and you'll miss it: How Technological Acceleration Shrinks SETI's Narrow Detection Window 28 September 2025 Michael Garrett · logged; moved no range
Books, films and other work that take it up
- Rendezvous with Rama 1973 Book · Arthur C. Clarke
- Roadside Picnic 1972 Book · Arkady and Boris Strugatsky
Read next
- Self-destruction: civilisations destroy themselves soon after they learn to build the weapons that make them detectable They are gone
- Civilisations burn out: the fast growth that makes a civilisation detectable also uses up its planet, and the ones that survive are the ones that slow down and go quiet They are gone
- Natural catastrophes: asteroid strikes, giant eruptions and nearby exploding stars end civilisations before they can last long enough to be noticed They are gone
- Interstellar travel is impractical: the distances are so great and the costs so high that nobody ever goes anywhere, so nobody has come here They are there and we cannot see them
- We have not looked long enough: all the searching ever done amounts to a glass of water dipped from the ocean They are there and we cannot see them
- Expansion stalls: civilisations do spread between stars, but settlement spreads like a stain, not a flood, and leaves large regions, perhaps ours, never reached They are there and we cannot see them
Notes
Checked 2026-10-16. Forgan's paper is correctly cited: International Journal of Astrobiology 8 (2), 121-131, published online 23 January 2009 (https://doi.org/10.1017/S1473550408004321; preprint https://arxiv.org/abs/0810.2222). Corrected 2026-10-16: the body said the paper reports that, for short lifetimes, the chance of any two civilisations making contact is small. The paper is a method paper: it sets out a random-trial model of the galaxy and reports counts of inhabited planets, civilisations that destroy themselves and the distribution of signal lifetimes under three hypotheses. I found no contact-probability result in it (the full text was searched, not read line by line). The body now describes what it does. The sentence about Shklovskii and Sagan's discussion stays in the verify list.
Claims still to be checked
These were written from memory and have not yet been checked against a source. A later pass clears each one.
- That Shklovskii and Sagan's 1966 book discusses the overlap-in-time problem in those terms; check the chapter on the lifetime of civilisations. Tried 2026-10-16: the book's text is not freely available (the Internet Archive copy is lending-only), review pages and the book's contents were not reachable, and a 1980 summary by Peter Sturrock (https://ntrs.nasa.gov/citations/19800014518) confirms only that Sagan weighed a long and a short lifetime and typically put the number of communicating civilisations near a million; whether the book frames the overlap problem as described could not be confirmed.
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