2022ApJ...928..142S

As the only known intelligent civilization, human beings are always curious about the existence of other communicating extraterrestrial intelligent civilizations (CETIs). Based on the latest astrophysical information, we carry out Monte Carlo simulations to estimate the number of possible CETIs within our Galaxy and the communication probability among them. 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 a variety of combinations of f_c and F. Our results indicate that for optimistic situations (e.g., F = 25% and f_c = 0.1%), there could be 42,777_{-369}^{+267} CETIs and they need to survive for {3}_{-2}^{+17} yr ({2000}_{-1400}^{+2000} yr) to achieve one-way communication (two-way communication). In this case, human beings need to survive {0.3}_{-0.298}^{+0.6} Myr to receive one alien signal. For pessimistic situations (e.g., F = 75% and f_c = 0.001%), only {111}_{-17}^{+28} CETIs could be born and they need to survive for {0.8}_{-0.796}^{+1.2} Myr ({0.9}_{-0.88}^{+4.1} Myr) to achieve one-way communication (two-way communication). In this case, human beings need to survive {50}_{-49.6}^{+250} Myr to receive one signal from other CETIs. Our results may quantitatively explain why we have not detected any alien signals so far. The uncertainty of the results has been discussed in detail and would be alleviated with the further improvement of our astronomical observation ability in the future.