宇宙中，也许历来不贫乏地外文明的存在。然而，要晓得人类拥有发出可用于在星际间交换的信号的才能的时间，不外是短短60年。与剖解学意义上的现代人呈现在地球上的时间（10万年）比拟，前者不外是后者的0.06%。也就是说，即便在已知“地球上存在聪慧生命”的前提下，一个外星文明向地球发出信号，地球上的人类能承受并做出回应的概率，也不外是0.06%。若是人类可以持续拥有那一手艺到达1000年，那一概率会上升到1%，但人类能否对峙那么久的时间也是很可疑的。此外，从4亿年前大型陆生动物呈现在地球上，到太阳光度的上升使得6亿年后地球不再能撑持那些复杂生命的保存，申明地球会在10亿年的时间里契合“宜居星球”的尺度。那一尺度也恰是我们在宇宙中寻找“宜居星球”的尺度。但当我们把60年和10亿年比拟的时候，得到的成果还不到百万分之一。地外文明可能向地球发出了大量的信息，但是没有一个刚好在人类可以领受到它的时间段里到来。同样，即便我们向一个极为类似地球的宜居行星发出信息，对方能收到那一信息的概率也不外是百万分之一。
因而，发现一个地外文明的希望长短常渺茫的。我们以至能够想象，公元前3000年时有外星飞船拜访了地球。其时处于新石器时代的人类将它们视做神明，那一接触也促成了人类早期宗教的产生。然而那一外星文明在公元前2800年时已经扑灭了（此处的扑灭，其实不必然指完全消亡，可能只是失去了与宇宙中其他文明沟通的才能），可能是因为内部抵触、资本耗尽、天然灾祸，以至是文化守旧主义下自愿放弃手艺。若是今天我们向他们发送一个迟到的回复，那颗已经死寂的行星上不会再有回音了。时间标准的差距否认了星际交换的一切可能性。
There may have been always extraterrastrial civilisations in the universe. However, it was just 60 years since humanity gained the ability to give out signals that could be used to interact between planets. Compare it with the time that anatomically modern human live on the earth (100,000 years), The former time is just 0.06% of the latter. That means, if an extraterrestrial civilisation sends a signal to earth, and even if the preposition that “There are intelligent animals on earth” is true, the probability that human on earth could interpret an answer the message, is a mere 0.06%. If human could keep this ability for 1,000 years, this probability will raise to around 1%, but whether human could sustain such a long time is doubtful. Moreover, large terrastrial animals appeared on earth since 400 million years ago, and the raise of sun luminosity means earth could sustain these animals until 600 million years from now, that means, Earth is a planet that is hospitable enough to sustain complicated life for one billion years. This is how we find in the universe for planets that could sustain life, and Earth will be such a planet for one billion years. If we compare 60 years with one billion, that is literally a one-in-a-million chance. Extraterrestrial civilisations may send lots of messages to the earth, but none of them happen to come within the timeframe that human could receive and interpret them. That means, even if we could find a planet just as hospitable as earth, and send a signal there, the chance that there exists a civilisation that could reply us, is merely one in a million.
As a result, the hope to ever find an extraterrestrial civilisation is really dim. We could even imagine that extraterrestrial spaceships visited earth in 3,000 BC. Neolithic human at that time just viewed the visit as a “god”, and the visit from the space gave them inspirations towards an early religion. However, this civilisation has perished by 2,800 BC (By saying perish, it does not mean that they are completely destroyed, but rather means they lost their ability of interstellar connection) either due to internal strife, depletion of necessary resources, natural disaster, or cultural conservatism that made them give up their technologies voluntarily. If we give them a late reply today, we could not expect any message from that long-dead civilisation. Difference in the scale of time makes any effort of interstellar connection deem to be in vain.