人类作为唯一已知的智慧文明，始终好奇宇宙中是否存在其他可交流的地外智慧文明(Communicating Extra-Terrestrial Intelligence civilizations, CETIs)。随着19 世纪电磁辐射的发现及20 世纪射电望远镜的发明，使得地外文明搜寻计划(Search for Extra-Terrestrial Intelligence, SETI) 逐渐具备科学性，成为天文学中的一个重要领域。基于最新的天文观测数据及天体物理学信息，我们采用蒙特卡罗模拟的方法来模拟恒星形成历史及CETIs 的诞生，以此来估计银河系内可能的CETI 的数量以及它们之间的交流概率。由于目前已知的智慧文明只有人类，我们提出有两个了解甚少的参数可能会对结果造成很大的影响，其中一个参数是生命出现在类地行星上并最终演化成CETI 的概率(fc)，另一个参数则决定了CETIs 将在其宿主恒星演化的哪一阶段诞生(F)。为了保证模拟的完整性，我们考虑了fc和F 分别取三个值的九种模型，其中fc 取不同数量级的情况(0.1%, 0.01%, 0.001%)? F 测试了CETIs 诞生在恒星演化早期，中期及晚期的情况(25%, 45.7%, 75%)。为了简化计算，我们假设每一个CETI 的交流寿命(ΔT, 即文明发射和接收可传播信号的时间长度) 是相同的，以此来计算实现交流的临界寿命(ΔTc) 及不同ΔT 对交流概率的影响。我们的结果表明，对于乐观的情况下(F = 25% 及fc = 0.1%)，可能有4.3+0.03-0.04× 104个CETIs 且他们需要生存3+17-2 年(2+2-1.4 × 103 年) 才能实现一次单向交流(双向交流)。在这种情况下，人类则需要存活0.3+0.6-0.298 Myr (106 年) 才能接收到一次来自其他CETIs 的信号。在悲观的情况下(F = 75% 及fc = 0.001%)，可能只有1.1+0.3-0.2 × 102 个CETIs 诞生并且他们需要存活0.8+1.2-0.796 Myr (0.9+4.1-0.88 Myr) 才能实现一次单向交流(双向交流)，这种情况下人类则需要50+250-49.6 Myr 才能接收一次信号。我们的结果也许可以定量解释为什么到目前为止我们没有探测到任何其他CETIs 发出的信号。此外，我们对结果的不确定性也进行了详细的讨论，这些不确定性将随着未来天文观测能力的进一步提高而减小。

As the only known intelligent civilization, human beings are always curious about the existence of other Communicating Extra-Terrestrial Intelligent civilizations (CETIs). With the discovery of electromagnetic radiation in the 19th century and the invention of the radio telescope in the 20th century, SETI (Search for Extraterrestrial Intelligence) has gradually become scientific and become an important field in astronomy. Based on the latest astronomical observation data and astrophysical information, we carry out Monte Carlo simulations to simulate the star formation history and the birth of CETIs, so as to estimate the number of possible CETIs within our Galaxy and the communication probability among them. Because the only known intelligent civilization is human, we propose two poorly known parameters have a great impact on the results. One is the probability of life appearing on terrestrial planets and eventually evolving into a CETI (f_c), and the other determines at what stage of their host star's evolution CETIs would be born (F). In order to ensure the completeness of the simulation, we consider nine models where fc and F take three values respectively, in which f_c takes different orders of magnitude cases (0.1%, 0.01%, 0.001%)? F tests the birth of CETIs in the early, middle and late stage of stellar evolution (25%, 45.7%, 75%). To simplify the calculation, we assume that the communication lifetime (?T, the length of time during which civilizations could release and receive communicable signals) of each CETI is the same, and then calculate the critical life of communication (?T_c) and the influence of
different ?T on communication probability. Our results indicate that for optimistic situations (e.g. F = 25% and f_c = 0.1%), there could be 4.3^+0.03_-0.04 ×10⁴ CETIs and they need to survive for 3⁺¹⁷_-2 yr (2⁺²_-1.4 × 10³ yr) to achieve one-way communication (two-way communication). In this case, human beings need to survive 0.3^+0.6_-0.298 Myr (10⁶ yr) to receive one alien signal. For pessimistic situations (e.g. F = 75% and f_c = 0.001%), only 1.1^+0.3_-0.2 × 10² CETIs could be born and they need to survive for 0.8^+1.2_-0.796 Myr (0.9^+4.1_-0.88 Myr) to achieve one-way communication (two-way communication). In this case, human beings need to survive 50⁺²⁵⁰_-49.6 Myr to receive one signal from other CETIs. Our results may quantitatively explain why we have not detected any alien signal so far.