The Quiet Galaxy

Explanations · They are there and we cannot see them

Shouting is costly: a beacon that could be heard across the galaxy is expensive to run, so anyone sending one sends short pulses, rarely, and our searches almost never catch them

Proposed by Gregory Benford, James Benford and Dominic Benford, 2010 fc smaller

The searches were designed to find a steady, powerful signal aimed at us or at everyone. But a transmitter that could be heard across thousands of light-years would cost a great deal to run, and any civilisation that counted the cost would do what lighthouses do: flash briefly, sweep the beam around, and repeat on a long cycle. A search that points at a star for a few minutes would almost never be looking at the moment the beam swept past. On this view the galaxy may contain beacons by the hundred, each announcing itself as cheaply as it can, and our searches have been looking for a light that is always on.

Three members of the Benford family, two physicists and a NASA astronomer, published a pair of papers in 2010 asking a question the search community had mostly skipped: if you were building a beacon to announce yourself to the galaxy, and you had a budget, how would you build it? The answer, worked out from the physics of radio transmission and the economics of large projects on Earth, was nothing like the signal the searches look for. A continuous, all-directions signal strong enough to be heard across the galaxy would be enormously powerful and expensive. The cheap alternative is a narrow beam, far more powerful for the same energy, swept across the sky so that each target gets a short pulse every so often. The pulse could be seconds long and the return interval months or years. The frequency would be higher than the searches had favoured, because higher frequencies carry further for the same cost. And the beacon would most likely be pointed along the plane of the galaxy, where the stars are, which is also where the searches have the most trouble with background noise.

The consequence for the searches, worked out in the second paper, is that almost all of them would have missed such a beacon. A search that looks at a star for a few minutes and then moves on would catch the pulse only by luck. Even a long stare at one star would miss a beacon on a year-long cycle. The Benfords argued that the right strategy was to stare at the galactic plane for long stretches and look for brief, repeating pulses, and that the absence of detections so far said little about how many beacons existed.

The bearing on the Drake Equation is on the share of intelligent civilisations that are detectable, which this explanation lowers in a specific sense: the civilisations are transmitting, but switched on toward any one listener for so small a share of the time that detection requires a different kind of search. It belongs with the explanations that say the silence is about how we listen.

The objections: a civilisation that can afford interstellar beacons can probably afford a continuous one, so the cost argument may apply only to civilisations near our own level; beacons of the kind described would still be found eventually by the wide-field, long-duration searches now being built; and the argument assumes a civilisation wants to be found at all, which several other explanations deny. The Benfords accepted the first point and noted that it was a reason to expect young civilisations' beacons to be the first found, if any are. The argument's lasting effect has been on search design rather than on belief: surveys that watch large parts of the sky continuously for brief signals are now standard, and they are the test of this explanation.

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:

The test: what would strengthen or weaken it

Would strengthen it: Detection of a brief, repeating, artificial radio pulse by a long-stare survey of the plane of the galaxy, where most of its stars lie; a published count showing that past searches had almost no sensitivity to signals that are switched on only a small share of the time.

Would weaken it: Years of continuous wide-field monitoring of the plane of the galaxy with no brief artificial signal; a detection of a continuous beacon, which would show that at least some civilisations do not economise.

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 Nothing new By hand

    No brief repeating artificial pulse from the plane of the galaxy, and no published count of past searches' sensitivity to intermittent signals beyond the 2023 Galactic Centre fast-folding search. The LOFAR weekly technosignature survey has a conference abstract and no results.

How seriously it is taken

Respected and influential on how searches are planned; as an explanation of the silence it is regarded as partial, applying to deliberate beacons from civilisations that count the cost, and testable by the long-stare surveys now running.

The papers that bear on it

Books, films and other work that take it up

Read next

Notes

Checked 2026-10-16. Both papers are in Astrobiology 10 (5), June 2010: Messaging, pages 475-490 (https://doi.org/10.1089/ast.2009.0393) and Searching, pages 491-498 (https://doi.org/10.1089/ast.2009.0394). The preprints (https://arxiv.org/abs/0810.3964 and https://arxiv.org/abs/0810.3966) support the conclusions given: short dwell times and infrequent revisits, higher microwave frequencies near 10 gigahertz, and a strategy of watching the plane of the galaxy for years for brief repeating pulses. A galactic-scale beacon needs more than 10^17 watts of effective radiated power. Corrected 2026-10-16: the body had said a continuous all-directions signal would need power "on the scale of a nation"; that phrase is not in the papers, which call such a beacon enormously powerful and expensive (the second paper) and put the cost of beaming quasi-continuously into the whole sky at ten times that of the narrow-beam design (the first), so the body now uses their wording. Pulse lengths of about a second and revisit times of a day to a year are in the papers. The authors are Gregory Benford and James Benford (identical twins, both physicists) and Dominic Benford (James's eldest child, NASA's chief technologist for astrophysics): https://en.wikipedia.org/wiki/James_Benford and https://en.wikipedia.org/wiki/Gregory_Benford.

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/shouting-is-costly.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.