Passive SETI Detection Probability

If N civilisations have been broadcasting for Ī“ years, what is the probability that at least one signal crosses Earth? The Civiletti (2025) geometrical framework ā€” the principled way to read a null result.

šŸ”¬ Established (geometry) āš  Theoretical (priors)
Parameters
10
Number active in the Milky Way today
100 yr
How long the civilisation has been broadcasting
50,000 ly
Milky Way disk radius ā‰ˆ 50,000 ly
P(ā‰„1 detection)
ā€”
ā€”
N required for detection confidence level (current Ī“, R)
50% confidenceā€”
90% confidenceā€”
99% confidenceā€”
99.9% confidenceā€”
P(ā‰„1 detection) vs. N ā€” four signal durations
Vertical line = your selected N Ā· Horizontal dashed = 50% / 90% / 99% thresholds
šŸ“” Observational context ā€” COSMIC / VLA survey (Sheikh et al. 2025)

The COSMIC system searched 950,000+ objects during the VLA Sky Survey (covering 75% of sky), placing EIRP upper limits of 10Ā¹Ā¹ ā€“ 10Ā¹ā¶ W on narrowband radio transmitters. No technosignatures were confirmed. Under the Civiletti framework, this null result constrains the product N Ɨ Ī“/R: the 950,000-object sample implies that if a transmitter at OC distance (~17,900 ly) were broadcasting with sufficient power, we would have detected it. The absence updates our posterior downward on NƗĪ“ but cannot rule out civilisations broadcasting below the sensitivity threshold or in other channels.

ā†— Drake Monte Carlo (prior on N) ā†— Radio SETI (power limits) ā†— Great Filter Localizer ā†— Constraint Stacker (OC window)

The Civiletti 2025 framework

The standard Fermi paradox formulation asks why we see no evidence of extraterrestrial intelligence when our best estimates of N suggest there should be many. But that framing conflates two questions: "does ETI exist?" and "would we detect it if it did?" This tool answers the second question geometrically.

Civiletti (2025) constructs a model in which N civilisations in the Milky Way each broadcast an EM signal for duration Ī“ (years), with the signal expanding as a spherical shell at light speed. The probability that at least one such signal intersects Earth's spacetime position is:

pā‚ ā‰ˆ 0.6 Ɨ Ī“ / R   (per civilisation, R = galaxy radius in light-years)

For N independent civilisations: P(ā‰„1) = 1 āˆ’ (1 āˆ’ pā‚)į“ŗ

The factor 0.6 comes from the geometry of a uniform spherical distribution of sources and an Earth-based receiver ā€” roughly 60% of the available "EM shell area" of a galaxy-crossing signal passes through the galactic disc where we reside.

What the tool does not model

This is a geometric null-detection framework. It does not account for: directionality of transmissions (beamed vs. isotropic), frequency selection effects, signal power (sensitivity is handled by Radio SETI ā†—), or the anthropic constraint (we are only here because our civilisation arose, which correlates with N > 0). For the prior on N, use the Drake Monte Carlo tool ā†—.

Key reference

Civiletti, M. (2025), "Quantifying the Fermi paradox via passive SETI: a general framework," Open Journal of Astrophysics. arXiv:2505.00062. Submitted April 2025; revised December 2025.

COSMIC survey: Sheikh et al. (2025), AJ. arXiv:2501.17997.

Tool v1.0 Ā· 2026-06-01 Ā· Code: MIT Ā· Prose: CC BY 4.0 Ā· omegacentauri.me/tools/passive-seti.html