The existence of alien life is an interesting question.

There are many books about alien life. Some of those books are fictional and some books are more scientific. But the thing is that before we can say whether aliens exist or not, we should determine existence. What do we mean about existence? If alien civilization exists, it might not exist to us. 

That is one of the most interesting models of Scrödinger's cat, the thing that exists but doesn't exist. The alien civilization might not just answer to us. It means it can turn its back on us. So, if we compare this situation with parties where nobody talks to us, and everybody walks away or keeps their back turned to us we can say that those persons discuss. 

We can see that they exist. But what they say is a mystery. That means those people exist but same way they don't exist. We see how they dress and maybe we see some mobile telephones and other kinds of stuff. But those people don't share anything with us. This is the thing that makes existence interesting. 

Or otherwise, if an alien civilization is, let's say, at a distance of 3000 light-years it's the same to us, if that civilization exists or not. For us, it does not exist because we cannot see it, or communicate with it. If that civilization invented the radio 1000 years ago the signal would arrive on Earth in about year 4024. But until that signal arrives on Earth we are alone. 

The thing is that we can say that maybe the alien civilization can travel faster than light. But even if an alien civilization can make that thing it must turn its craft to our solar system. And those hypothetical aliens need some reason to come here. So can we someday meet aliens? Maybe that thing will happen far in the future. 

Or maybe it will happen tomorrow. Who knows what the thing called alien thinks? Do we know those species? There is no single confirmed attempt where aliens tried to make contact with us. There are some radio signals whose origin is unknown. Those signals happened only once. And that makes them interesting. But because they happened only once it's quite sure that they didn't mean to us. 

Cosmic black swans. 

That is one thing that we must realize. There is always a possibility that the cosmic Black Swan event creates unique radio signals. The cosmic black swan can be the case that a high-energy reaction like annihilation happens just when a radio impulse travels through that point. 

"The black swan theory or theory of black swan events is a metaphor that describes an event that comes as a surprise, has a major effect, and is often inappropriately rationalized after the fact with the benefit of hindsight. The term is based on a Latin expression that presumed that black swans did not exist. The expression was used until around 1697 when Dutch mariners saw them in Australia. After this, the term was reinterpreted to mean an unforeseen and consequential event. " (Wikipedia, Black Swan theory) 

That thing means that we are cosmic black swans. We are lucky because we live in a solar system between the galaxy's spiral structures. That means there are fewer particles like in the main spiral structures. So the possibility of collision is smaller than in the main spiral structure. The other thing is that the Milky Way, our galaxy seems to be in a cosmic bubble. 

That means energy travels out from our galaxy faster than in other galaxies. That means our galaxy is not as ordinary as we might think. And the location of the sun is not a typical star's location. So we are far from the typical solar system if we think of our location in the universe. 

Another thing that we must realize is that our knowledge of the universe is limited. We don't have any experience with things like large-scale high-energy nuclear reactions that form quark-gluon soup. The  CERN and Brookhaven produced quark-gluon plasma (QGP) for a very short time. In those tests, only a couple of particles can turn into QGP. 

But it's possible. The QGP can have long-term existence in the universe in places like black hole acceleration disks or in their relativistic jets. We don't have very much information about things like quark-gluon plasma and radiowave interactions. 

We don't know what happens if a small black hole's relativistic jet hits some planet or asteroid. Of course, we know that the asteroid turns into a molecular cloud. But it's possible. The high-energy jet that comes from stellar or substellar primordial black holes can smash those atoms together and form fusion in that object. 

We don't know how those extreme energy levels act on the material. And we don't have experienced what happens if the most high-energy thing in the universe grills objects for hours or even years. We have very little experience in the extreme nuclear reactions. And we don't know a thing about the large-scale extreme nuclear reactions. 

When we think about the possibility that the black hole's relativistic jet impacts with some cold planet there is very strong and thick ice that thing can form the antimatter on that planet. That antimatter destroys the planet immediately. And it's possible that things like radio waves travel through that thing in the same moment as an explosion happens. 

Of course, we don't know that Black Swans or extremely rare, but possible cases affect things like radio impulses. The antimatter particle may hit some asteroid. At the same moment when a radio wave travels through that. The possibility that this happens is minimal. But it can explain the reason why some radio signals are unique. 

https://en.wikipedia.org/wiki/Black_swan_theory

The models of alien civilizations could be the civilization of humans in the distant future.

The models of alien civilizations could be the civilization of humans in the distant future. 

During the interstellar flight, evolution has time to work

When we are creating models of an alien civilization, we are also creating models, what our civilization might look like tomorrow, and if we are thinking about the possibility to create stable colonies in our solar system and farther in the universe. The thing is that sooner or later every each of the stars spends its nuclear fuel and if the star is like our sun, it would end it's life as a nova, which means that the end is as the white dwarf. 

The bigger star would end it's life as a supernova. And if we are thinking this kind of thing at the point of view of evolution, that means that the citizens must flee from its planet or it must be terminated. In many models, the civilization can advance only in the planets, what is orbiting the yellow stars, what can live long enough. So the model could be wrong because there is a possibility that even the short-living blue star might have the planet, where is intelligent life-form. 

And if the intelligent life form is curious and it observes it's a star, it might notice that the end is near, and start to create the giant space arks, what it can use for evacuating. During this process, the civilization would terminate the non-wanted genetic material, and that means that the controlled evolution would make them different than they were at the beginning of the process. If that kind of civilization would put all its knowledge, and intelligence to that operation, it can survive. And when the entire civilization moves to the artificial planets, what the giant spacecraft are, there are not similar problems than in conventional space journeys. The scale of those projects is so huge, that we cannot even think that we would do that in modern days. 

But maybe in the distant future, we or our descendants are facing the situation, that they must leave this planet. And at least, when our star the sun would be ending its life as a star when it explodes as a nova, the civilization must leave the solar system to finding new energy sources. In some theories, the civilization would move to farther planets, when the star is growing, and in those scenarios, we have forgotten, that there is a possibility that solar system might have only one planet. 

But this kind of thing is pure speculation. There are many other models about civilization and the thing is that we could be that "alien civilization" in the day tomorrow. If we are thinking the hypothetical space civilization, that civilization might use 100 % of the energy of some star by simply closing the red dwarf in the giant ball, what is covered by using the silicon panels, and that giant structure can send energy to the alien crafts by using very powerful masers or electron cannons. 

There are many forms of the theoretical alien civilizations

Could the hypothetical alien civilization consist of two groups of aliens? The planetary and space race. The other group lives in planets, and other group lives in spacecraft, and they would ever land to the planets. There is a possibility, that the aliens could hybridize with their spacecraft. If the aliens would have a collective mind or consequence that means that it would eliminate the rebellion or violence. The thing is that the collective mind would benefit the community. 

But individual intelligence could danger the mission. This is the paradox of the hypothetical extremely long term missions, which could take even millenniums. One of the things, that we must realize when we are thinking about the hypothetical alien civilization, what might be traveled between the stars, is that those creatures might not make that thing for fun. They must have a reason for those journeys, which will take even millenniums. That thing means that the crew of those crafts might face the evolution, what we cannot even imagine. 

When we are thinking about things like the physiology or brain capacity of aliens, we must realize, that those creatures might be intelligent as a group. This means that the creatures might have microchips in their brains, and those microchips make them the ability to communicate with the computer of their craft, and in this case, they could have collective knowledge in their craft. The central computer would have the role, that it connects the nervous systems of those hypothetical humanoids, but the individuals would be like handicapped. 

The hybridization with spacecraft

Or there is a possibility that the evolution in the bodies of those creatures would make their nervous system communicate spontaneously with the wireless data network of their craft. That means that those creatures would be hybridized with their craft. At this moment many of you will say, that the evolution would need a very long time to modify their bodies. But if the journey will take tens of thousands of years that means, that evolution would have the time to make those modifications. 

And there is created a model, that the evolution of that crew would be controlled. In this scenario, the aliens would first travel a long time near their home star, and in that period the controlled evolution would hide the origin, where those creatures would have left for protecting their species. That's why they had to modify their bodies for hiding their origin. But this is only a hypothesis, nothing more or less. Just hypothesis, but the hypothesis is the road to theory, and theory is the road to a practical solution. That means in the future, we might be that alien civilization, what travels in the stars. 

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