Why didn't we met the aliens already?

There are two arguments about the probability of apparition of life in the Universe that strikes me, going in opposite directions: First it seems that life on earth appeared very early: The age of the universe is roughly 13 billion years. The age of the earth is around 4 billion, and life appeared on it 3 billion years ago. In fact, it seems that life appeared as soon as it was possible: before this date, the surface was still melted and constantly bombarded by meteorites. This seems to give some credit to the probability of life.

However, if life is probable, why didn’t we found any trace of life not based on ADN, or one of its derivate (ARN)? Is ADN the only structure able to carry and encode enough information to enable life? Did non ADN-based life form appeared sometime in the past, and competed with ADN-based life forms? If life was probable, it is surprising that no non-ADN based life forms were discovered on earth.

For me, the main reason why we didn’t discover extraterrestrial life is that… The Universe is big. It is big, both is space and time. It has, in fact, a very different scale than the “scale” in which life exists and develops. As an example, both the Egyptian and the Mayan civilizations existed in the past on earth. But they never met, for two reasons: first, they were on different continents, and none of them developed the oversea travel. Secondly, they lived at different times. In fact, we probably have a must greater chance to find fossile Alien life, that is, only evidences of a past alien civilization. This trace could be a radio signal, or a fossilized bacteria in a meteorite…

As a life form ourselves, we didn’t really swam far from the beach yet. We’ve been only on our own satellite, can you imagine? Space exploration really begun a short time ago, compared to the age of the universe. Take the SETI program, for example. If a similar program was developed by aliens on another planet, would they have detected us already? It’s unlikely. Strong electromagnetic emissions from earth didn’t really begun a long time ago, probably with the development of the radio, and later, TV. However, those signals become undistinguishable from background noise after travelling some light-years. This leaves a very small numbers of candidate planets for Aliens to detect us.

The universe is big, and it is expanding. This means that the farther two points are, the faster they move apart. As it is expanding, if you take two points of the universe sufficiently distant from each other, they move appart from each other at an ever increasing speed, making them in practice unreachable from each other. These points are completely, and definitely, isolated. If an alien civilization develops there, they are definitely out of reach and we will never be able to interact with them.

An alien civilization could have existed, developed and expanded, and then crumbled and disappeared, just like the Mayan civilization. Life in the universe may be like spots, that appear, grow and then shrink and disappear. With some luck, two spots may be close enough to overlap, both in time and in space. Is that our case? Anyway the time scales involved are so huge that, as persons, we probably won’t know during our life time.

To some extent, we can also apply the anthropic principle [2] to this question. The fact that we are here to ask the question “why no alien visited us yet?”, might bring an answer to the question in itself. Meeting an alien life, given our own state of spatial development and out-reach, seems to imply that our Alien counterpart be much more developed than us. This contact might not turn in our favor, and we would not be here anymore to ask the question. So the initial question might be self-answering.

[1] http://zidbits.com/2011/07/how-far-have-radio-signals-traveled-from-earth/
[2] http://en.wikipedia.org/wiki/Anthropic_principle

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