Different cosmic bodies are at different distances from us and therefore we “see” them in “different past”. Thus, when observing the Universe, the time period available for study increases, taking into account the capabilities of modern astronomical technology, at least up to 7-8 billion years.
Of course, interstellar travel has been the most effective method for searching for alien civilizations. However, the problem is extraordinarily complex. At the present time, it is even difficult to say whether the interstellar ships of the future will be photonic rockets and whether they will use the reactive principle at all. In any case, in the light of what is known to science and technology today, the practical implementation of a photon starship appears to be very problematic, if not completely impossible. But, on the other hand, the possibility is not excluded that in the course of time some unknown principles will be discovered that will allow us to create thrust in outer space and develop speeds comparable to the speed of light.
Thus, the implementation of interstellar flights, in any case, is a matter of a relatively distant future. Therefore, the idea naturally arises to use radio methods to detect intelligent beings of other cosmic worlds and establish contacts with them.
Probably, our descendants will read the history of the first attempts to make space connections as a fascinating adventure novel. Indeed, there is everything that can usually be found in works of this kind: amazing riddles, mysterious phenomena, unsolved traces, unjustified hypotheses, unexpected turns.
The first attempt to detect artificial radio signals from other civilizations, known as the OZMA project, was undertaken by the American astronomer Drake in 1960. The idea of this experiment was to register cosmic radio signals coming from some stars close to us and try to isolate from them an artificial component.
First of all, the question arose as to which stars should be chosen as objects of observation. Of course, the level of our knowledge does not yet make it possible to make such a choice for sure. However, stars of some types can undoubtedly be excluded in advance and thereby narrow the range of searches. First of all, exclude hot stars. Hot stars are, as a rule, young objects, and, as we have already said, in order for life to arise on the planet and reach the highest stages of its development, sufficiently long periods of time are needed. Cold dwarfs are not suitable either, as they emit too little energy. Planets revolving around multiple stars are hardly suitable for life, since frequent fluctuations of physical conditions should occur on these planets. Finally,Drake and his collaborators opted for the two stars Tau Ceti and Epsilon Eridani - the closest solitary stars like the Sun.
Observations were made during May, June, and July 1960, but were unsuccessful.
But, on the other hand, it was difficult to hope that with a random selection of objects of observation, artificial radio signals could be detected at the very first attempt. After all, this requires the coincidence of a number of circumstances. Not only is this planetary system habitable and sending radio signals, it is also necessary that our observations coincide in time with radio transmissions. And if these transmissions are conducted by a directed beam, then it is also necessary that Lemlya at this time was just on his way. It is clear that the likelihood of such coincidences is practically small.
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Therefore, the idea of searching for artificial radio signals proposed by the Soviet radio astronomer NS Kardashev is much more promising. In 1964, he made extremely interesting calculations, the results of which are very significant.
Since intelligent life does not arise on different planets at the same time, there should be societies among space civilizations that have reached different levels of development. They can be roughly divided into three types. Civilizations of the first type are at about the same level as modern humanity. The second type includes civilizations that have mastered energy on the scale of their star, and the third - on the scale of their galaxy.
On the other hand, it can be assumed that highly developed civilizations strive to establish radio contacts, exchange useful information. In this case, the most effective should be precisely all-directional radio transmissions. Indeed, with such transmissions, the greatest probability of connecting the stump of new subscribers is ensured, that is, of receiving radio signals by all new civilizations.
Therefore, it seems very likely that intelligent civilizations of the second and especially the third type allocate a significant part of the energy resources at their disposal for the implementation of all-directed transmissions.
However, the question arises: is it really possible for a society of intelligent beings to master such powerful sources of energy and achieve such a high level of power availability?
To get an answer to this question, it is best to try to assess the energy capabilities of earthly humanity. As you know, on Earth, the history of intelligent life is only a few thousand years old, and the history of science and the modern understanding of the word, in fact, is only a few centuries. But mankind has already reached the level when it has powerful enough energy sources and technical devices to "enter into space radio communication." And it can be calculated that in about 3200 years, if a further increase in power availability continues at the same pace as at present, people will acquire energy comparable to the energy of the Sun, and in 5800 years - with the energy of the Galaxy. The astronomical timeframes are very short. Moreover, considering that science and technology are developing with acceleration,then these periods may in reality turn out to be much shorter.
Of course, the practical mastery of such enormous energy resources will require much more time, perhaps several million years, because for this, mankind, apparently, will have to master a colossal area of space.
It is interesting that the need to expand human activity into space is dictated not only by considerations of a scientific nature. The fact is that the development of energy, an increase in energy production, even if it proceeds at the current pace, in a few hundred years will inevitably lead to a significant change in the thermal regime of the Earth.
Some foreign scientists believe that in order to eliminate the danger of overheating, it will be necessary at some point to prohibit the further development of energy and to stabilize it at a certain acceptable level.
However, such a measure is hardly feasible. Then mankind will have the only way out of the situation: to take the power plants into outer space. Incidentally, this will prove necessary for another reason. Most likely, in the near future, nuclear fuel will become the main source of energy, and the deployment of a large number of nuclear installations on Earth is associated with a radiation hazard for humanity.
Thus, the experience of mankind testifies to the fact that as it develops, an intelligent civilization should expand the scope of its activities, covering more and more areas of outer space. It is interesting, in particular, to note that thanks to the implementation of space flights, people have already increased the scope of their activities many thousands of times.
If you look into the more distant future, then having fully mastered its planetary system, humanity will begin to develop neighboring planetary systems or the vicinity of nearby stars by creating artificial biospheres around them, that is, such “structures” on which people could live. It can be assumed that such an operation should take several thousand years.
The “branch” of the terrestrial civilization created in this way, in turn, can take the next step, to other stars, and so on, until the entire Galaxy has been mastered in several tens of millions of years.
But what is true for humanity must also be true for other civilizations. And it is very likely that artificial radio signals that contain the richest scientific information are continuously coming to us on Earth from outer space. And while you are in your room and read this book, artificial signals from other cosmic civilizations continue to arrive on Earth. They penetrate through the roof of the building, through the ceiling, fill the space around you. These signals, perhaps, contain information about many unsolved problems of science, answers to many questions of concern to people. Unfortunately, we have not yet learned how to pick up these signals and decode them.
But can we catch them at the current level of development of science and technology? The answer to this question is the main thing in Kardashev's calculations.
It turns out that the minimum power of omnidirectional transmissions is such that they can be recorded by modern radio astronomy equipment even if at least one civilization of the second type exists within our Local System of Galaxies, or at least one civilization of the third type exists within the entire observable region of the Universe. In addition, there is a real opportunity in the coming years to create receiving devices that could provide not only the capture of signals, but also the reception of the information they contain. This means that it makes quite real sense to organize searches for artificial radio signals not in the direction of individual stars, as was done in the American OZMA project, but in the direction of large star clusters or entire galaxies, for example, the Andromeda galaxy …
The primary search for civilizations of the third type makes sense also because their signals should be more powerful and contain more useful information.
Of course, this radiation must also be modulated, that is, certain information must be put into it. However, almost no additional energy consumption is required for coding. Thus, if civilization has sufficiently powerful sources of energy, the whole problem, in essence, boils down to the creation of the necessary transmitting and coding equipment.
It is also possible that alien civilizations can use some natural sources to transmit information by means of omnidirectional transmissions, artificially modulating their radiation in one way or another. You can, for example, surround a star with a solid sphere and somehow change its transparency for radio waves. In this case, all the difference between such an artificial signal and the natural one will be only in the nature of the modulation.
It can be assumed that the main purpose of omnidirectional transmissions, if they exist, is to transfer information from more advanced civilizations to less developed ones. As for the exchange of information between supercivilizations, it is, most likely, carried out through highly directional communication channels.
However, the possibility is not excluded that only a small number of supercivilizations conduct omnidirectional transmissions. In his famous science fiction novel The Andromeda Nebula, Soviet writer I. Efremov described the “great ring” of civilizations - a constantly operating system of communication between the societies of intelligent beings inhabiting various cosmic worlds, and designed for the regular exchange of information.
It is quite possible that systems of this kind really exist and function in the real Universe. But then it is logical to assume that between the participants of such a "ring" there should be a certain division of functions and the transmission of information to the Universe is carried out by some one civilization, and the rest give only signals such as callsigns or in general participate only in mutual exchange, which is conducted through narrow directional channels … There may be other options as well. But if such reasoning is correct, then the number of artificial radio signals in the Universe available to terrestrial observation should be much less than we would expect based on statistical calculations.
Of course, searching for omnidirectional radio broadcasts is not the only option. Civilizations of the first type, which do not have unlimited reserves of energy, most likely send their radio signals in narrow, directional beams.
Such civilizations should be sought in the relatively close vicinity of the Sun. Academician V. Kotelnikov made such a proposal at the Byurakan meeting. The scientist calculated that if we restrict ourselves to a sphere with a radius of 1000 light years for the first case, then it will be possible to examine 64,000 stars located within it.
However, when searching for artificial radio signals, another problem inevitably arises: on what wavelength should we look for transmissions from the inhabitants of other cosmic worlds?
When you turn on your radio and want to hear the transmission of a particular station, you will definitely tune it to a specific frequency. What frequency should the radio telescope be tuned to when searching for alien civilizations?
On this occasion, a number of witty considerations were expressed. In particular, it was proposed to conduct searches on a radio wave of 21 cm - a wave of interstellar hydrogen, since it can be assumed that at the disposal of alien civilizations there is equipment operating on this wave. But, on the other hand, it is on this wave that cosmic interference is very strong, arising due to the disordered radiation of hydrogen atoms that exist in outer space. Therefore, some scientists believe that half the wavelengths are more suitable for inter-space transmissions, since they are less sensitive to various interference.
However, the validity of all these assumptions, unfortunately, can only be assessed in the future. Therefore, apparently, the most effective means for searching for signals from other civilizations could be a multichannel signal receiver, that is, a radio telescope that would simultaneously cover a sufficiently large frequency range.
In turn (at least from a theoretical point of view) mankind already has sufficient means to send special radio signals into space to establish contacts with other civilizations and thus declare its existence. With the current state of radio physics, such signals can cover a distance of the order of several hundred light years. This means that there are already about half a million stars in their range. Of course, we still do not have at our disposal such energy capabilities that would allow us to carry out omnidirectional transmissions, or at least transmissions with a sufficiently wide cone of radio waves. For now, we will have to restrict ourselves to sharply directed beams, precisely "addressed" to certain stars.
It is possible, for example, to transmit radio signals such as callsigns. Such callsigns could serve as a kind of "ready signal". They would inform other civilizations that the Earth is ready for inter-cosmic radio exchange.
Sending readiness signals by the Earth to nearby stars can greatly facilitate the establishment of interspace connections. Indeed, if these civilizations have reached a level of development close to that of the earth, they still have no opportunity to carry out omnidirectional transmissions, and the transmission of information in narrow beams without an exact address is devoid of practical meaning. After receiving a signal from the Earth and making sure that there is a civilization in the solar system capable of entering into intercosmic communication, the intelligent inhabitants of another planet will begin to transmit information in the direction of the Earth. Of course, with this method, the beginning of the arrival of information to the Earth is postponed for a period that depends on the distance to the radio partner. This period may be several millennia. And yet, perhaps the result in this case will be achieved faster than if we wait,when the alien civilization reaches such a high level of development that it itself can start omnidirectional broadcasting.