[论文解读] Can Collimated Extraterrestrial Signals be Intercepted?

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

Space Science and Extraterrestrial Life参考文献 11被引用 3

一句话总结

本研究利用蒙特卡洛模拟评估地球能否拦截来自地外文明的非故意准直光学信号。结果表明，除非光束的准直度较弱（发散角约0.1弧度），否则拦截极为罕见，暗示光学SETI更可能探测到故意定向的信号，而非偶然的信号。

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.

研究动机与目标

• 评估地球拦截非有意图指向自身的准直光学信号的可能性。
• 确定光束准直度及通信文明数量对拦截率的影响。
• 探讨弱准直光束是否可作为构建星际通信网络的低成本策略。
• 评估在光学SETI搜索中探测非故意信号的可行性。

提出的方法

• 在银河系宜居带（GHZ）内对文明之间的星际通信进行蒙特卡洛实现实验。
• 将圆形环形GHZ建模为包含10,000颗恒星的系统，每颗恒星均可能孕育智慧文明。
• 模拟具有可变光束发散角的准直信号，并测量第三方的拦截频率。
• 采用时间依赖的模拟方法，追踪随时间推移的光束交叉情况，假设通信速率恒定。
• 计算拦截率随光束发散角及通信文明比例的变化关系。
• 应用几何与统计模型，基于空间分布与光束宽度估算信号交叉概率。

实验结果

研究问题

• RQ1 非目标第三方拦截准直星际信号的频率如何？
• RQ2 光束发散角如何影响非故意拦截的概率？
• RQ3 通信文明数量与信号拦截率之间存在何种关系？
• RQ4 弱准直光束是否可作为构建通信文明网络的低成本方法？
• RQ5 与理论预期相比，GHZ的几何约束对拦截率的影响程度如何？

主要发现

• 拦截率随通信文明比例线性增加，随光束发散角呈立方关系增长。
• 除非光束发散角约为0.1弧度或更大，否则拦截几乎不可能发生。
• 光束角的立方依赖关系强于理论预测，归因于GHZ的几何结构。
• 在10,000个文明的GHZ中，高度准直的光束若无极宽发散角，其非故意拦截几乎不可能发生。
• 文明可能有意使用弱准直光束以促进拦截，从而以低于全向信标的方式低成本建立通信网络。
• 光学SETI更可能探测到专为地球定向的信号，而非星际通信中的偶然拦截信号。