It is interesting to think about the potential for contact with alien life. I read a very interesting article by a grad student in some flavor of astronomy. He was comparing various timescales. According to him, the time it would take a civilization to colonize/contact our galaxy is small compared to other relevant astronomical timescales. For example, if a civilization could make a ship to achieve 1% of light speed, they could colonize our galaxy in a few million years. Compared to the billions years it took for humans to emerge on earth, that is a tiny amount of time. Moreover, we’ve only been really developing science for the past 1,000 years or so and we already have a satellite outside our solar system. Even if civilization collapsed and we had another dark age, what is a 1,000 year setback compared to the million year time scales of colonization?
The question he then asked is: why have we not encountered alien life yet? Earth has been here for a very long time, but we have no evidence of ever being visited by alien life. He proposed that there must be one or more “great filters” which prevent all life from embarking on a quest to colonize the universe. He proposed that it might be the origin of life itself, that the specific chemistry required for life might be so astronomically improbable that there are no other planets with life on them in our galaxy. Another candidate was the evolution of multi-cellular life. However, he postulated that it is also possible that the “great filter” lies in our future, rather than the past. In other words, we may still have to face some inevitable calamity or challenge that has prevented all other life from ever traveling to other planets. We may invent some super weapon, or make some other discovery that subsequently destroys our entire civilization.
He concluded that looking for life on mars and other planets is exciting and worthwhile, but we shouldn’t be too happy if we find it, because if we find life, it weakens the proposition that the great filter is in our past.