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von Neumann probe

Drake equation: is there extraterrestial civilizations out there

It has been calculated there are about 7×1022 stars in the observable Universe. That is more than the total number of grains of sand in all the Earth's beaches and deserts. But that is only the stars in the visible Universe within range of our telescopes. The actual number could be still much, much bigger. Given the amount of stars in the universe, it would be almost unbelievable coincidence that there is only one planet in world that is hospitable to complex life and later to technological civilization. Sure it is possible but just sounds unlikely.

But when we are considering possible interactions between different civilizations, more relevant question to ask is: is there other technological civilizations in our own Milky way galaxy. Most of the stars in universe are so far away that communication even with radio would be extremely difficult, as it would take millions or even billions of years for message to arrive. Stars inside galaxy are much closer: the nearest star from Sun is only 5 light years away. That means it would take 5 years for a radio message to reach it, and about 50 years for a spaceship with cruise speed of 10 percent of speed of light to travel that distance.

The famous Drake's equation tries to count how many technological civilizations there are in our Milky way galaxy at any moment. Frank Drake's own estimate puts the number of communicating civilizations in the galaxy at 10,000 NASA. This is, of course, just a crude assumption, but it is still better than nothing.

The proponents of extraterrestial life often believe in principle of mediocrity: an assumption the Earth is a pretty much normal planet in a pretty average Solar system. However, there are people who prefer rare Earth hypothesis: an assumption that life on Earth as we see it is a result of many lucky coincidents. It should be noted even rare Earth hypothesis doesn't doesn't deny the possibility of extraterrestial life, it just claims the probability is lower than expected.

It could be argued that lack of contact to other intelligent civilizations is a proof that they don't exist. I strongly reject that idea. There are many possible reasons why we are not communicating with extraterrestials even if they exists. Some of those ideas are considered later on article. For other suggested explanations for lack of contact, see Fermi's paradox.

So, conclusion about extraterrestial civilizations is there seems to be good changes we are not alone in Milky way galaxy.

What is von Neumann probe

Von Neumann probe is a hypothetical robotic space probe. It is programmed to search space for raw materials and energy in order to build copies of itself. It can be considered as a mechanical bacterium or virus. Besides self-replicating, von Neumann probes will execute tasks they are programmed to do. You can read more about von Neumann probe on Wikipedia.