The whole question of intelligent life elsewhere in the universe comes down to two questions:
1. How difficult is it for life to come into existence out of non-life.
2. How unique is the Earth's environment and its environmental history in the universe and how necessary was that environmental history to the development of intelligent life (man).
The first question addresses whether, given the right environment, would it be likely that life would start up anywhere? We suspect that the answer to that question is: Very Likely. The reason is that the more we look around the universe the more precursors to life we find. The more we look around the Earth the more we find evidence that life on earth happened very soon after the Earth was created (geologically speaking). The question of life elsewhere in our solar system is probably the most important question that all of our space exploration is aimed at answering. We have not found any slam-dunk evidence for life elsewhere but we continue to find places where it could have developed and we continue to look.
The second question addresses the nurturing of primitive life into complex organisms. On Earth (our only example of life we currently have) it took over 3.5 billion years to come up with man; three and a half billion years where the environment on the Earth was relatively stable. For about 3 billion of those years life was mostly single celled. During this immense time period the environment on Earth was favorable to the continued development of life even in the face of a drastically changing solar environment. Is it likely that such a planet would exist elsewhere in the universe? The numbers of stars are staggering so it might seem a virtual certainty that there are “Earth-like Planets” elsewhere in the universe. Consider this: the Earth has developed life and as far as we currently know Mars and Venus have not. They are Earth-like planets - very similar planets to the Earth. The answer must be more complicated than just “Earth-like planets.”
Another thing to consider: our sun, Sol, has over the last 4 billion years increased its output by about 25% due to normal stellar evolution. That is an enormous change in output and without some compensating factor the Earth, over that time period, would have gone from an ice ball to a barren Venus like planet. It didn't- but we don't know what the factor or factors were that maintained such a constant environment.
Now consider the likelihood of the Earth receiving visits from aliens. The chances of alien life forms visiting us come down to three additional questions. One addresses the spatial problem and one the temporal problem. The third addresses actual travel possibilities.
1.If a technologically sophisticated life form did develop elsewhere in the universe would it be likely that they would be contemporaneous with man when he is also technologically sophisticated.
2.Is it likely that a technologically sophisticated life form could develop near enough to us so that they could reasonably visit the Earth?
3.Does there really exist a “new physics” that would allow travel over immense distances in reasonable times?
When alien extraterrestrial life is considered what we usually referred to is: technologically sophisticated life. We aren’t usually talking about bacteria or worms or even Homo Erectus visiting the Earth. Humans have been technologically sophisticated for only about 100 years with radio communications and rockets. Our first satellite to orbit another body was only 42 years ago. That is 100 years out of a total history for the Solar System of 4,500,000,000 years and of 13,500,000,000 years for the universe. The chances of a second life form becoming technologically sophisticated and also overlapping with man- also being technologically sophisticated seem vanishing small. Alien civilizations may have come into being, developed and become extinct hundreds of millions of years before or after man.
The problem of travel is also daunting. With all of our present sophistication we do not have the ability to travel to even the nearest star. The development of super-sophisticated intelligent extraterrestrial life capable of traveling to nearby stars would first require a nurturing environment existing for billions of years and then the development of science and technology far beyond ours. Put all together that in itself is very, very, very unlikely. For intelligent life to happen a second time NEAR enough to our star, Sol, so travel would be possible is vanishingly unlikely. In talking about alien visitors to Earth there is an implied uncovering of a ‘new physics’ that would allow what our present physics does not allow- travel over large distances in reasonable time frames. It may be that that physics does not exist. If you indulge in new physics then you are opening yourself up to creating a fantasy world that may not exist in reality
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