In terms of leaving traces, aliens come in two varieties.
In the mass understanding, the search for aliens is inextricably linked with ufologists, who carefully collect information about UFO sightings, about the contacts of people with aliens who allegedly visit Earth, or even try to make contact with them.
It is much less known that the problem of extraterrestrial intelligence is one of the largest scientific problems, which was first posed in the 17th century. The most outstanding scientists worked on it. Few people also know that the search for alien civilizations has been going on in the USSR since 1964.
A closer look at the problem of the search for extraterrestrial intelligence shows that many prominent scientists attached great importance to this matter. This gives us reason to believe that the topic of aliens is not only the lot of freaks.
In the further presentation we will use instead of the term "aliens", tracing paper from the English word aliens, which, from our point of view, much more accurately reflects the essence of the topic under consideration. We do not yet know anything concretely about aliens, and no one guarantees, for example, that their habitat, in fact, are the planets.
The search for strangers while unsuccessful
For the first time, the search for extraterrestrial intelligence began in the United States with the project of the radio astronomer Frank Drake "Ozma", who in 1960 searched for signals of artificial origin from the stars Tau Ceti and Epsilon Eridani, which are 11 light-years distant from Earth.
In the USSR, searches began after the book by I. S. Shklovsky "Universe, Life, Mind". The All-Union Meeting on Extraterrestrial Civilizations was formed, on the recommendation of which in 1964 a section "Searches for signals from extraterrestrial civilizations" was created in the Academy of Sciences of the USSR, which worked until 1987.
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Despite the efforts made and numerous methods tried, no convincing traces of aliens have been found. Optimists from among the participants in the SETI program say that any more or less development of an alien civilization must come to the creation of systems of radio television or radar signals that can be detected at a distance of at least 100 light years. But only 0.1% of promising stars have been checked and no one can vouch that among 99.9% of other stars there are no signals of an alien civilization.
By the way, mankind itself shows its presence by powerful radio emission, originating both from the operation of communications and radio broadcasting, and from the operation of radio telescopes, which is comparable to the radio emission of the Sun, and in the past even exceeded it. From the point of view of observers from the depths of space, about 70 years ago, some astronomical event took place in the solar system, as a result of which the radio emission from the star increased sharply. If aliens are also looking for other civilizations in space, then they can find the Earth precisely by this powerful radio emission.
Skeptics argue that the negative result of the search for aliens is natural, since in a vast space, infinite in time and space, different civilizations simply do not intersect.
Space must be habitable
All our current knowledge of space suggests that intelligent life in the Universe should be a fairly common and commonplace phenomenon. The fact is that the Sun is a very common “yellow dwarf” star G2 in the Galaxy. There are probably more than a dozen billion such stars in our galaxy alone. If we count with other galaxies, then the number of such stars should be huge.
The similarity of a number of stars to the Sun has led to the assumption that these stars must have planetary systems that can be inhabited. These hypothetical planets have been called exoplanets. This was hypothesized in 1916 by Eduard Bernard, but the first exoplanet was discovered in 1991 by the astronomer Alexander Wolshchan near the neutron star PSR 1257 + 12. In 1995, a large planet was discovered near the star 51 Pegasus. According to the wobble of the star, the exoplanet should be as large as Jupiter, only located close to the star. By November 2007, 260 exoplanets were discovered in 220 planetary systems. Now the discovery of new planets occurs about once a week, and sometimes more often.
The discovery of exoplanets was the first step towards discovering alien intelligence. The second step was the discovery of planets, according to conditions close to earthly ones. Because it is the terrestrial conditions, according to the ideas of modern science, are considered the most suitable for the emergence of life. In 2004, in the system of the star Mu Altar, the first planet was discovered, close in parameters to the Earth. Now photographs of exoplanets have already been obtained (for example, the gas giant near the star Epsilon Eridani) and it is assumed that soon the scientists will have at their disposal equipment that will allow them to study the chemical composition of the exoplanet atmosphere and detect exoplanets with a nitrogen-oxygen atmosphere.
The assumption that the Earth is “non-unique”, which follows from modern ideas about the isotropy of the Universe, finds more and more confirmation. Humanity has only three steps left to discover alien intelligence: first, to find exoplanets with an oxygen atmosphere and water, second, to discover exoplanetary life, and third, to directly discover alien minds.
Aliens do not exist, and there may be reasons for this
But while the cosmos is silent, and there is no proven evidence of the existence of an alien mind. In an attempt to resolve this contradiction, several hypotheses were created about what caused the "non-existence" of aliens.
The first, and most likely version is that the appearance of intelligence, of course, occurs regularly, but the Universe is so vast in space and time that we simply do not intersect with strangers. This hypothesis suggests that all that we can find in space are some incomprehensible artifacts, millions of years old, which are completely useless for us.
But here the question arises, why, in this case, we do not observe civilizations that have existed for these very millions of years. This question is given a preliminary answer by the data of biological evolution, according to which the lifespan of successful biological species capable of producing intelligent life is limited to only a few million years. While a number of species, such as sharks and crocodiles, have existed for hundreds of millions of years or more, they are, alas, unreasonable. Most likely, the lifespan of alien civilizations is also short, and we, at best, will find some of its remnants.
From the emptiness of the Universe around us, we can also draw conclusions that humanity in its current form is very unstable.
Traces of strangers
As already mentioned, traces of strangers have not yet been found. But this does not mean that nothing can be said about the places where they are supposed to be. In terms of leaving traces, aliens come in two varieties.
The first type is aliens, to some extent coinciding with human ideas about them. That is, these are traces left by a certain intelligent race, but which still has something similar to technology in our understanding. The existence of such aliens from the point of view of modern ideas is considered just the least probable, but it cannot be discounted.
Where should you find them? First of all, it is definitely not necessary to search on other planets, and even more so on Earth. The atmosphere, and even more so the biosphere, will destroy practically any conceivable material over millions of years. Accordingly, traces of the presence of such aliens can only remain in space. More specifically, the most likely location for the artifacts is the Lagrange points, the places where the gravity of the sun and the planet balance. By the way, three terrestrial objects are located at the Lagrange points.
The second variety is aliens, sharply exceeding the earthly level, and having nothing remotely similar to technology. Naturally, when it comes to a technology that is many orders of magnitude superior to that of the earth, it makes no sense, based on our technical level, to assume what someone else's technology can and cannot. But the search technique still exists. This is a search for anomalies. After, by examining the solar system, it becomes clear what natural phenomena are common for it, it makes sense to focus on those phenomena that stand out from this series. Thus, if such outsiders left their artifact in the system, there is a chance to find it.
Philistine ideas about strangers
Naturally, when covering the topic of strangers, one should nevertheless mention how the arguments of ufologists have to do with real strangers. Let's say right away - no. Modern ufology is a good field for the study of human prejudice and in this regard is very interesting.
A large amount of factual material accumulated by ufologists, where people who are completely unfamiliar with each other, having no benefit from this, but having problems, with the same words (often the descriptions coincide even in details), talk about contacts with aliens, at first glance, make an impression. Do such cases take place? Have. Are contacts related to reality? Have. But conclusions from the facts should be made quite different from those made by ufologists. Ufologists from this conclude that aliens exist and visit Earth.
At the same time, it is striking that the stream of UFO reports is of relatively recent age and the year when they began is known. The first UFO sightings and the dynamics of the spread of sightings coincide with the beginning of the Cold War. The first UFO reports were in 1947, the year of Fullton and Churchill's speech. It would be strange to seriously assume that it was this time that the strangers chose this very time to start penetrating. In the Middle Ages, people saw angels, in modern times - ships, in our time - UFOs. Such a clear correlation between the appearance of technology and the mass perception of outsiders shows that ufologists turn everything upside down. UFO reports are a reflection of global social and psychological processes in societies of the European type, especially in the United States and Europe. We do not yet know how this is explained.
The logically correct conclusion sounds like this: contacts with strangers are exactly as ufologists show them, because such is the idea of them in mass psychology. Therefore, before they flew on sailing ships in the sky, then on super airships, nowadays on "flying saucers". This explains the paradox that information has been accumulated about tens of thousands of cases of observations and contacts, but not a single material evidence, not a single artifact, even the smallest, that would have a proven extraterrestrial origin was found.
Will there be a war with strangers?
The favorite topic of science fiction writers and partly of ufologists, after the great novel by H. G. Wells, The War of the Worlds, is the war between earthlings and strangers. It is even difficult to list how many times and for what reasons this topic was addressed.
Only the current state of ideas about space and the state of problems of an alien civilization exclude this possibility. If a terrestrial or alien civilization reaches such a level of development that it will be able to overcome distances of tens of light years (hundreds of trillion kilometers) in a relatively short time, then for it other intelligent life forms will only be of scientific interest, since by this time all economic and energy problems will be resolved long ago.
It is not a fact that if the terrestrial civilization manages to reach exoplanets with alien life, it will discover precisely the mind and civilization, and not the ocean with the simplest organisms, not the biosphere inhabited by unreasonable beings. It is not a fact that earthlings on an exoplanet will not encounter something huge and incomprehensible, like Solaris.
In other words, for a situation of conflict and war, it is necessary that the terrestrial and alien civilizations have an approximately comparable level of development, which, in the framework of current ideas, looks extremely unlikely.
It would not be superfluous to also mention that now, even theoretically, it is possible to imagine the achievement of only the nearest stars. But space is huge, and the size of our Galaxy, according to modern concepts, is about 100 thousand light years in diameter and about 1000 light years in thickness. How to overcome such distances, now, even in theory, is very difficult to imagine. Most likely, huge distances are an obstacle for someone else's mind.
Most likely, if we establish contact with someone else's mind, then only remote contact, which will exclude in general any possibility of not only a conflict, but a conflict situation in general. A possible conflict in this case is resolved by simply turning off the transceiver equipment.
Why do we need outsiders?
In the philistine view, strangers have a high level of technology that earthlings can adopt, which is why ufologists diligently exaggerate the topic that governments of different countries, especially the United States, have long been engaged in contacts with strangers in a secret regime for military purposes.
However, it is most likely that the aliens who can reach the Earth have such a high level of technology that the earthlings will not only be unable to adopt, but even understand the principles of these devices. Or we will receive artifacts long out of order, the principle of operation and the purpose of which will also be incomprehensible to us.
With all this, strangers have some practical value for earthlings. We are now living in many amazing times. The idea of the unity of mankind, which for centuries and millennia was the property of only its best representatives, is now becoming a reality. Of course, the world of globalization is not a world of utopia; it also has a lot of shortcomings. But, perhaps, it is the extraterrestrial aliens that mankind lacks so that the ideas about the enemies inherent in our culture find an external embodiment. Perhaps it is strangers that we lack so that the concept of "we" encompasses all earthlings, without distinction of nationality and race.
This moment has been reflected in a number of science fiction films. For example, in James Cameron's film "Aliens", Ellen Ripley initially threw a tantrum about the fact that an android robot was in the crew of the rescue expedition. But after fighting with strangers, her hatred for androids disappeared. Also, apparently, it will be on Earth. To any, the most inveterate white racist who sharply hates people of other races, after a personal acquaintance with a stranger, the Zulu from South Africa will seem like a brother. Aliens will put a decisive and irrevocable point in the existence of any doctrines of racism and nationalism that divide humanity.