[Paper Review] Can Collimated Extraterrestrial Signals be Intercepted?

Duncan H. Forgan|arXiv (Cornell University)|Oct 27, 2014

Space Science and Extraterrestrial Life11 references3 citations

TL;DR

This study uses Monte Carlo simulations to assess whether Earth can intercept unintentional collimated optical signals from extraterrestrial civilizations. It finds that interception is rare unless beams are weakly collimated (opening angle ~0.1 radians), suggesting optical SETI is more likely to detect deliberately targeted signals than accidental ones.

ABSTRACT

The Optical Search for Extraterrestrial Intelligence (OSETI) attempts to detect collimated, narrowband pulses of electromagnetic radiation. These pulses may either consist of signals intentionally directed at the Earth, or signals between two star systems with a vector that unintentionally intersects the Solar System, allowing Earth to intercept the communication. But should we expect to be able to intercept these unintentional signals? And what constraints can we place upon the frequency of intelligent civilisations if we do? We carry out Monte Carlo Realisation simulations of interstellar communications between civilisations in the Galactic Habitable Zone (GHZ) using collimated beams. We measure the frequency with which beams between two stars are intercepted by a third. The interception rate increases linearly with the fraction of communicating civilisations, and as the cube of the beam opening angle, which is somewhat stronger than theoretical expectations, which we argue is due to the geometry of the GHZ. We find that for an annular GHZ containing 10,000 civilisations, intersections are unlikely unless the beams are relatively uncollimated. These results indicate that optical SETI is more likely to find signals deliberately directed at the Earth than accidentally intercepting collimated communications. Equally, civilisations wishing to establish a network of communicating species may use weakly collimated beams to build up the network through interception, if they are willing to pay a cost penalty that is lower than that meted by fully isotropic beacons. Future SETI searches should consider the possibility that communicating civilisations will attempt to strike a balance between optimising costs and encouraging contact between civilisations, and look for weakly collimated pulses as well as narrow-beam pulses directed deliberately at the Earth.

Motivation & Objective

• To evaluate the likelihood of Earth intercepting collimated optical signals not intentionally directed at us.
• To determine how beam collimation and the number of communicating civilizations affect interception rates.
• To explore whether weakly collimated beams could serve as a cost-effective strategy for interstellar network formation.
• To assess the feasibility of detecting unintentional signals in optical SETI searches.

Proposed method

• Conducted Monte Carlo Realisation simulations of interstellar communication between civilizations in the Galactic Habitable Zone (GHZ).
• Modelled a circular annular GHZ with 10,000 stars hosting intelligent civilizations.
• Simulated collimated signals with variable beam opening angles and measured interception frequency by third parties.
• Used a time-dependent simulation to track beam intersections over time, assuming constant communication rates.
• Calculated interception rate as a function of beam divergence and fraction of communicating civilizations.
• Applied geometric and statistical models to estimate signal intersection probability based on spatial distribution and beam width.

Experimental results

Research questions

• RQ1 How frequently are collimated interstellar signals intercepted by third parties not intended as targets?
• RQ2 How does the beam opening angle affect the probability of unintentional interception?
• RQ3 What is the relationship between the number of communicating civilizations and the rate of signal interception?
• RQ4 Can weakly collimated beams be used as a cost-effective method to build a network of communicating civilizations?
• RQ5 To what extent do geometric constraints of the GHZ influence interception rates compared to theoretical expectations?

Key findings

• The interception rate increases linearly with the fraction of communicating civilizations and cubically with the beam opening angle.
• Interceptions are unlikely unless the beam opening angle is approximately 0.1 radians or larger.
• The observed cubic dependence on beam angle is stronger than theoretical predictions, attributed to the geometric structure of the GHZ.
• For a 10,000-civilisation GHZ, unintentional interception of highly collimated beams is improbable without very wide beams.
• Civilizations may intentionally use weakly collimated beams to encourage interception and build a network at lower cost than isotropic beacons.
• Optical SETI is more likely to detect signals deliberately targeted at Earth than accidental intercepts of interstellar communications.