The Quiet Galaxy

Explanations · 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

Proposed by Jacob Haqq-Misra and Seth Baum, 2009 L smallerfc smaller

A civilisation becomes visible from space by growing fast: burning fuel, building, transmitting, changing its atmosphere. That kind of growth cannot continue on one planet for long. Either the civilisation collapses when its planet can no longer support it, or it learns to live within its limits, in which case it stops growing and stops being loud. On this view there is no galaxy-spanning expansion because expansion is exactly the habit that kills, and the civilisations that last are the ones that gave it up. The sky is quiet because the loud ones are gone and the quiet ones are, by definition, quiet.

Jacob Haqq-Misra and Seth Baum put the argument in a short 2009 paper. The reasoning behind the Fermi question, from Hart in 1975 onward, assumes that a civilisation capable of spreading between stars would do so exponentially, filling the galaxy in a few million years. But exponential growth is not something any living system sustains. On Earth, every population that grows that way runs into the limits of its habitat and either crashes or levels off. A civilisation's growth draws on energy and materials from a finite planet, and the faster it grows, the sooner it meets the wall. The authors concluded that civilisations able to sustain exponential expansion may not exist, and that those which persist do so at a scale and pace that does not make them visible across the galaxy.

Adam Frank and his colleagues gave the idea a mathematical form in 2018, modelling a civilisation and its planet as a pair that affect each other, as humans and Earth's climate do now. Their models produced three broad kinds of outcome: a smooth settling into a steady state the authors call sustainable; a boom followed by a die-off, in which the population falls to a fraction of its peak; and a rapid collapse. In the authors' own description of the work, collapse comes in two forms: one where the civilisation never changes the resources it draws on, and one where it switches to low-impact resources but too late. Which outcome occurred depended on how the civilisation drew on resources and how strongly the planet pushed back. The authors drew the lesson that a civilisation's growth phase is a dangerous and probably short passage, and that humanity is in it now.

The bearing on the Drake Equation is on the last two factors. The detectable lifetime is short, because the loud phase is the dangerous one and does not last. The share of intelligent species that remain detectable is lowered as well, since the survivors are the ones that stopped doing the things that can be seen. The argument sits between self-destruction and the "unseen" family: the loud ones destroy themselves, and the quiet ones are still there.

Objections: a civilisation that learned to live within limits could still choose to expand slowly, settle other worlds and eventually be visible on a longer timescale, so the argument needs the quiet ones to stay quiet forever; the limits of one planet are not the limits of a solar system, and a civilisation that gets even a little way off its planet has vastly more to draw on; and the models are built on Earth's situation and may not generalise. The reply is that getting off the planet in a serious way is itself a product of the fast-growth phase, and that the window in which a civilisation is both able to expand and not yet in trouble may be narrow.

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 2 pieces:

The test: what would strengthen or weaken it

Would strengthen it: Evidence of an atmosphere around another star bearing the traces of industry but no present signal; models of coupled civilisation-and-planet systems that find collapse to be the usual outcome under a wide range of assumptions; humanity's own trajectory continuing toward its planet's limits without correction.

Would weaken it: A detected civilisation that is both long-lived and expanding; evidence that a civilisation can move its industry off its planet before running into limits; a model showing soft landings are the common outcome.

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.

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

    Blanco, Haqq-Misra and Profitiliotis (2026-04-15) simulate an Earth-born civilisation collapsing and recovering over a thousand years and find the rate of resource depletion the most influential lever, with a civilisation technologically active for as little as 38% of its span: a model of the coupled civilisation-and-planet kind the test names, though it finds intermittent activity rather than final collapse to be the usual outcome. No atmosphere bearing the traces of industry has been found; the instruments that could see one are still being planned. Logged.

    Blanco, 2026 source

How seriously it is taken

A recent and well-regarded line of work, especially since the 2018 paper, and congenial to researchers who study climate and sustainability; it is taken as one serious candidate for a Filter ahead rather than as a settled answer.

The papers that bear on it

Books, films and other work that take it up

Read next

Notes

Checked 2026-10-16. Borne out: Haqq-Misra and Baum, 'The Sustainability Solution to the Fermi Paradox', Journal of the British Interplanetary Society 62, 47-51 (2009), whose abstract questions the assumption that civilisations grow exponentially or very fast (https://arxiv.org/abs/0906.0568); Frank, Carroll-Nellenback, Alberti and Kleidon, Astrobiology 18(5), 503-518 (2018), https://doi.org/10.1089/ast.2017.1671. Corrected: the paper's abstract names three classes of outcome (sustainable steady states, die-off, collapse), not four; the four-way list (die-off, sustainability, collapse without a change of resources, collapse after a change of resources that came too late) is the University of Rochester's description of the paper (https://www.sciencedaily.com/releases/2018/06/180604172652.htm), so the body now gives the three named classes and says the two forms of collapse come from the authors' own description. The earlier sentence that the outcome depended on how quickly the civilisation recognised the problem was not found in either source and was replaced.

Think this page gets the explanation wrong, or misses an objection? Say so, or read how disagreement gets folded in. The record itself is data/explanations/civilisations-burn-out.toml in the public repository. This page's address will never change; if the explanation is ever retired, the address will say what replaced it.