First of all, it is important to define what is meant by general "extraterrestrial life". Normally, they are including little green men in spaceships with front-drive Wharp. But that probably would be the "highest level" of extraterrestrial life. It had completed a stage of development the people ahead of us. Because with the currently known laws of nature and technology, but are denied human interstellar travel, but the distances are too great.
Extraterrestrial life is not necessarily intelligent. Life - that could be bacteria, plants, fungi, animals and us be unknown creatures. It is quite possible that there were bacteria on Mars again. And the Jupiter moon Europa might harbor simple life forms. Even the simple discovery of bacteria on another celestial body would be a scientific sensation - but it would make our uniqueness one more time in question.
For a long time, scientists thought that life somewhere in the universe is very unlikely. Prerequisite for this is, namely, that there are planets or moons that orbit at a suitable distance to a star and have a hospitable climate. But the existence of so-called extrasolar planets (planets of other star) has long been regarded as very unlikely. Only in recent years succeeded in repeatedly to demonstrate extrasolar planets in our immediate environment indirectly. This was another major breakthrough in astronomy.
It can therefore be assumed that some planets "normal" in our Milky Way. It houses about 100 billion stars. If only every 1000th of a planetary system and if there were life on every 1000th of them, there would be alone in our galaxy 100000 animated planet! And when you consider that it in turn upon billions of galaxies in the universe, it could be trillions of life-bearing planet. If only every 1000th intelligent life has brought forth these are still trillions of civilizations. And if there were any 1000th technologies owned by them, which could overcome the familiar confines of space and time, there would, in principle, one billion species of intergalactic space tourists who might visit our little earth. Of course, the assumptions in this calculation are completely arbitrary - it could be every 100th or 100,000 each.
A simple consideration shows that it is purely statistical two likely possibilities for a second man in space: If there are in our Milky Way and about a hundred billion stars in the universe contains billions of galaxies, so these are 10 to 22 stars, which are 10 sextillion stars. Let us assume that the probability of intelligent life in a star system would be relatively high, eg One in a billion. Then there was the universe 10 quadrillion (10 ^ 13) intelligent civilizations. If they are relatively low, so for example 1 to 10 to 30, then humanity would probably unique in the universe. The probability that the huge number of stars, the probability of intelligent life, just keep the balance, so that only a few planets in the universe are civilizations, is extremely low. This means that statistically we are either unique or there are many civilizations in the universe.
Even if the latter is the case, then the question remains whether we get our "extraterrestrial brothers and sisters" (or there may have very different gender?) Ever seen, because we are separated by space and time.The cosmic distances within our galaxy amount to thousands of light years in nearby stars is still around 10 light years. Even these "small" distances by means of our physics can not be overcome. And even if extraterrestrial beings have the knowledge of natural laws that go beyond the limits of our physics, only a small time window exists for a reciprocal visit. The civilized man has existed since a few thousand years, and we do not know if they will see the year 3000. A few thousand years is not much when you consider that the universe is billions of years old. When the expedition reached the Andromeda Galaxy some day our earth, it might be too late.
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