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As the only known intelligent civilization, human beings are always curiousabout the existence of other communicating extraterrestrial intelligentcivilizations (CETIs). Based on the latest astrophysical information, we carryout Monte Carlo simulations to estimate the number of possible CETIs within ourGalaxy and the communication probability among them. Two poorly knownparameters have a great impact on the results. One is the probability of lifeappearing on terrestrial planets and eventually evolving a into CETI ($f_c$),and the other determines at what stage of their host star's evolution CETIswould be born ($F$). In order to ensure the completeness of the simulation, weconsider a variety of combinations of $f_c$ and $F$. Our results indicate thatfor optimistic situations (e.g. $F=25\%$ and $f_c=0.1\%$), there could be$42777_{-369}^{+267}$ CETIs and they need to survive for $3_{-2}^{+17}$ yr($2000_{-1400}^{+2000}$ yr) to achieve one-way communication (two-waycommunication). In this case, human beings need to survive$0.3_{-0.298}^{+0.6}$ Myr to receive one alien signal. For pessimisticsituations (e.g. $F=75\%$ and $f_c=0.001\%$), only $111_{-17}^{+28}$ CETIscould 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-waycommunication). In this case, human beings need to survive $50_{-49.6}^{+250}$Myr to receive one signal from other CETIs. Our results may quantitativelyexplain why we have not detected any alien signals so far. The uncertainty ofthe results has been discussed in detail and would be alleviated with thefurther improvement of our astronomical observation ability in the future.

The Number of Possible CETIs within Our Galaxy and the Communication Probability among These CETIs