How many places in the Universe have the appropriate conditions to house life? Is there extraterrestrial life? Or not?
Let's see. If around half of the stars of our galaxy similar to the Sun orbit a planet, in the right place to have a temperature favorable to the appearance of life, then in the Milky Way there would be ten billion planets similar to the Land.
Now, to know in how many of them there can be intelligent life and with technological ability, with which we could communicate by radio, we would have to know how likely it is to arise when conditions are right; how feasible it is to evolve to generate intelligent beings and, finally, how possible it is that they form a society of technological orientation.
The consideration of all these factors is beyond the domain of astronomy and is the responsibility of sciences such as biochemistry, biology or sociology. However, according to estimates by several scientists, a technically advanced civilization may arise on one in every 100 planets. Therefore, in the Milky Way there would be a hundred million planets on which, at some point in its development, a technological civilization emerged.
Not all civilizations necessarily evolve into technological societies. At Universe there may be many integrated by poets (who possibly survive better), very respectable by the way. But, unfortunately, we can never communicate with them using radio waves. Therefore, our focus is on technological civilizations not because we consider them "the most advanced" or the best in the Cosmos, but because only with them can we get in touch.
More urgent than knowing how many civilizations we expect are there, somewhere in the Milky Way, while waiting to communicate with us, it is important to solve a crucial problem: knowing what is the longevity of a technically advanced civilization. How long does a civilization of this nature live before destroying itself or succumbing to problems caused by itself and that it is unable to solve?
The only technologically advanced civilization we know is ours, and has lived as such (that is, with the ability to communicate via radio waves with other points in space) about 70 years. That is, a very small span compared to the life of the galaxy.
If advanced civilizations lacked sufficient wisdom to overcome the problems that technological advancement entails, and only lived (for example) one hundred years, the one hundred million civilizations of our galaxy would already be extinguished.
To know how many are alive today, just find out what percentage represents a hundred years in relation to the age of the galaxy, a life of the order of ten billion years. The ratio is one to one hundred million. That means that today only one of the one hundred million that existed in the Milky Way would be alive: ours.
But let's not be so pessimistic. Suppose that a technically advanced civilization lived a long time, about one hundred million years, for example, and that it solved all the problems that arise. In that case there would be a million civilizations throughout the galaxy that would be alive today and with which we could, in principle, establish contact by radio waves.
This number (one million civilizations) may seem very large, but the communication possibilities are smaller if you remember that the typical distance between two stars is about four light years. Even if we could know exactly which star contains the planet where the civilization is closest to ours, the possible conversation with its members would not be easy.
Suppose that intelligent life is discovered near Alfa Centauri, the closest among all stars of the universe. If at this time we said Hi!, our call would take more than four years to reach them; if they responded immediately another 4 long years would pass before their response to our greeting came back. Therefore, it is quite a very exciting possibility to talk on the phone back and forth, live and direct, with our closest neighbors.
Communications should be in only one direction. We could send a lot of information in specially coded messages for them to understand, and hope that someday, someone who listens to them, knows about our existence in the cosmos and learns something from us.
In the same way, we should listen to the radio bands with appropriate antennas to know if someone, from some point in the galaxy, has already broadcast a message announcing their presence and telling what civilization it belongs to is like. And be able to decipher the message, if it exists. It would be like practicing the activity of radio amateurs, but on a cosmic scale and without knowing the language. Is it really that easy?
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