Spacetime Contactability in Galactic Civilization Networks: Contact Counts and Percolation

[astro-ph.IM] The number of Galactic civilizations does not determine whether they are in contact. Allowing civilizations to appear at different times separates listening, audibility, mutual-listening, and relay networks, with a distinct reply-closing condition requiring a return signal within the sender’s lifetime.

For each network we derive the contact counts and estimate the connectivity threshold in a uniform population, then apply the results to a structured Galactic disc, separating the detection range dmax from the light horizon cL through χ=d_max/cL.

The contactability is η=πnd²_max, where n is the active civilization surface density. Averaged over civilization appearances, the lifetime listening count is 2η for all χ and, at fixed detection range, for any lifetime distribution. Audibility and mutual-listening counts sum to 4η, but connectivity depends on threshold behavior rather than counts alone. As χ→0, the listening threshold decreases from the coeval value 4.51 to 1.32±0.01. Assuming an ideal complete survey, our own non-detection gives the reference bound η(R₀)≤2.75 at the Sun’s Galactocentric radius.

This benchmark lies above the listening, audibility and relay thresholds throughout the range studied. Unlike the coeval model, the spacetime model can satisfy both this benchmark and a local connectivity criterion, moving the local-contact frontier outward by 3.2 kpc. Under the same benchmark, civilizations hear at most 5.5 others on average, while the mean reply-closing count falls below unity for χ≳1. Contact may therefore be common even when completed replies remain rare.

Kohji Tsumura

Comments: 24 pages, 4 figures, 5 tables, submitted to Acta Astronautica
Subjects: Instrumentation and Methods for Astrophysics (astro-ph.IM); Astrophysics of Galaxies (astro-ph.GA); Popular Physics (physics.pop-ph)
Cite as: arXiv:2611.12923 [astro-ph.IM] (or arXiv:2611.12923v1 [astro-ph.IM] for this version)
https://doi.org/10.48550/arXiv.2611.12923
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Submission history
From: Kohji Tsumura
[v1] Sun, 1 Nov 2026 07:39:44 UTC (675 KB)
https://arxiv.org/abs/2611.12923

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