It is unclear how frequently life and intelligence arise on planets. I consider a Bayesian prior for the probability P(ETI) that intelligence evolves at a suitable site, with weight distributed evenly over ln(1 - ln P(ETI)). This log log prior can handle a very wide range of P(ETI) values, from 1 to 10^(-10^122), while remaining responsive to evidence about extraterrestrial societies. It is motivated by our uncertainty in the number of conditions that must be fulfilled for intelligence to arise, and it is related to considerations of information, entropy, and state space dimensionality. After setting a lower limit to P(ETI) from the number of possible genome sequences, I calculate a Bayesian confidence of 18% that aliens exist within the observable Universe. With different assumptions about the minimum P(ETI) and the number of times intelligence can appear on a planet, this value falls between 1.4% and 47%. Overall, the prior leans towards our being isolated from extraterrestrial intelligences, but indicates that we should not be confident of this conclusion. I discuss the implications of the prior for the Search for Extraterrestrial Intelligence, concluding that searches for interstellar probes from nearby societies seem relatively effective. I also discuss the possibility of very small probabilities allowed by the prior for the origin of life and the Fermi Paradox, and note that similar priors might be constructed for interesting complex phenomena in general.
The SERENDIP III 70 cm Search for Extraterrestrial Intelligence
我们使用了SERENDIP III系统和阿雷西博射电望远镜来寻找可能的人造外星信号。在四年的搜索中，我们覆盖了93%的阿雷西博可观测的天空至少一次，44%的天空五次或更多，灵敏度为~3E-25 W/m2。数据被发送到一个400万通道频谱分析仪。从超过1 E +14个独立数据点获得信息，然后通过一套模式检测算法分析结果，以识别不容易识别为人类活动产物的窄带光谱功率峰。我们分别选择了与我们的天空调查中偶然遇到的有趣的附近G矮星相一致的数据，以获得多余功率峰值的建议。这两个数据集中的峰值功率分布与随机噪声一致。我们报告上限可能的信号从恒星调查，并提供最有趣的候选人在天空调查中确定的例子。本文件原计划于2000年出版，现将2000年提交给ApJS的版本原封不动地在此介绍。
The Breakthrough Listen Search for Intelligent Life: A 3.95-8.00 GHz Search for Radio Technosignatures in the Restricted Earth Transit Zone
我们报告了在受限的地球过境区内搜索20颗恒星的人造窄带信号，这是为期十年的突破性聆听（BL）搜索外星智慧的一部分。限制地球凌日区是一个天空区域，从这个区域观察者可以看到地球以小于0.5的碰撞参量凌日。这片天空在几何学上是独一无二的，为外星智慧生物发现太阳系提供了一条潜在的途径。目标就在附近（7-143 pc），搜索覆盖了3.95-8.00 GHz的电磁频率范围。我们使用了罗伯特C。伯德绿岸望远镜执行这些观测与标准的BL数据记录器。我们在这些数据中搜索多普勒漂移率为$\pm 20$ Hz s $^{-1}$的人工窄带（$\sim$ Hz）信号。我们在目标HIP 109656上发现了一组潜在的候选信号，然后发现其与人为射频干扰的已知特性一致。在我们的观测中，我们没有发现来自外星智慧生物的无线电技术特征的证据。观测活动实现了最小可探测通量，这将允许探测发射，其强度分别为最近和最远恒星的阿雷西博雷达发射机信号能力的$10^{-3}$至$0.88$倍。我们的结论是，至少$8\%$的系统在150 pc内的限制地球过境区不具备在本次调查中搜索的发射机类型。据我们所知，这是第一次有针对性的搜索外星智慧的限制地球过境区。本文中使用的所有数据都可通过Breakthrough Listen Public Data Archive（http：//seti.berkeley.edu/bldr2）公开获取。
