If it seems to you that you have not yet communicated with aliens … then, it seems to us, it only seems to you. When SETI "listens" to the sky, a new study says, it does so without thinking about how they shout into the sky.
The work of David Messerschmitt from the University of California at Berkeley (USA) is quite voluminous, and, without pretending to cover all of its more than two hundred pages, we will briefly dwell on some points of the work.
It has already been noted more than once: the assumption that radio is the best and final form of communication is not obvious, since we have been using it for a century, and because of this, it is rather naive to consider our own technologies to be the crown of communication evolution. But let's suppose that the technology of the 19th century is really the last word for all the civilizations of the Universe that ever existed. So to speak, "what if."
Even with an arbitrarily advanced forward error correction, it will still be impossible to avoid the fundamental limit associated with the increase in noise with increasing signal power. (Here and below graphs by D. Messerschmitt.)
How exactly will interstellar radio communication be organized in this case? Obviously, notes Mr. Messerschmitt, it is worthwhile to assume that this will be done rationally. For example, imagine a radio transmission of data at a rate of one bit per second for a range of at least 1,000 light years. If you conduct it in those frequency ranges that SETI is now listening to, then for the emission of such a signal from the indicated distance, power will be required twice as high as that of the Large Hadron Collider.
And if you limit yourself at least to the bandwidth of banal WiFi, then a terrestrial civilization will begin to have serious economic problems. Again, this is the case if we are transmitting only in one direction. If, for some reason, you do not have the exact address of intelligent aliens, then the broadcasts will have to be conducted in all directions, and against this background, the energy consumption of the LHC will very soon cease to be perceived as a unit of measurement - the scale of interstellar communication problems will be significantly greater.
Such an overload can be avoided, the scientist believes. The first is to minimize the transmission rate. Since it will still require an abyss of time (the speed of light is limited), it makes no sense to drive it at a high speed, and the probability of the initial detection of a signal grows in direct proportion to its duration. In addition, although initially the increase in the average power of the transmitters leads to an increase in the ability of information to reach the receiver, after a certain limit this no longer allows one to overcome the noise created by the signal propagation medium when interacting with it. Thus, one of the best optimization strategies would look like "not too loud": the transmit power must be strictly limited.
Of course, other optimization strategies suggest themselves. For example, for a more economical signal transmission, you can try using the polarization of electromagnetic waves and various types of multiplexing. Yes, in theory it will save energy, but then another problem appears: applying more and more advanced methods of increasing the density of information transmission, we will constantly focus on contact only with those civilizations that have already mastered such technologies. And everyone who listens to space at the level of those same earthlings of the 1960s (when SETI first appeared), remain outside the intercivilizational dialogue. That is, you cannot over-optimize too much in this direction.
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But there are other optimization elements that should remain relatively universal for all intelligent beings in the universe. At the same time, they never clearly pedalized by those who were looking for "extraterrestrials", listening to space.
On Earth, the range in which it is possible to send signals and receive information was limited from the very beginning: too many programs were broadcast on the air almost from the earliest years of radio. Therefore, the narrowness of the channel used for communication was a priority, and the transmission power - due to the small distances by interstellar standards - did not really matter. Therefore, when the SETI project began listening to space, its approach remained quite earthy: narrow frequencies were listened to.
At the same time, Mr. Messerschmitt notes, by using the widest possible range of the entire microwave window available in space, the average transmission power should be much more economical than the fixed-frequency broadcasting approach. So, we should start looking for higher bandwidth signals of lower power and data rate - something that, alas, SETI has not yet done. Among other possibilities is the search for signals with a very long period, much longer than that used in ground communications.
Well, well, but what if a civilization lives in some astronomical units from a black hole and can use it as an energy source to transmit information in all directions, or even as a gravitational lens in order to amplify these same signals?
It's okay, the scientist is sure: it will not be able to bypass the fundamental limit of communication - the more high-power signals it sends into the interstellar medium, the faster these signals, interacting with interstellar gas, will start to dangerously raise the level of interference and complicate further intercivilizational communications. That is, even supercivilizations, using radio communication for contacts with less developed beings, cannot bypass the fundamental limit in any way, and the SETI strategy should be reoriented even in relation to them.
Among the main practical conclusions that, in the opinion of David Messerschmitt, are worth making SETI, the most important are the following. The organization is looking for a long-term signal with frequent repetitions. In such a scheme, in order to distinguish a "false alarm" from a real signal, a rather simple method of long-term "listening" to a specific sector of the sky is assumed.
The author does not hide his attitude to this element of the search strategy: those who came up with this "truth check" did not even try to estimate how much energy would be spent on such a long repetitive signal. In other words, this tactic is unwise. All this again brings us to the signal "Wow!" 1977 year. Recall that when a signal that looked "artificial and extraterrestrial" arrived at the Big Ear radio telescope, the method of repeated listening to the same sector of the sky was used to verify its origin. However, such auditions were not carried out on an ongoing basis - not then, not now.
The use of multilevel keys in interstellar communication at each time interval will allow any pulse to take values M from 0 to 8 and, therefore, represent log2 = 3 bits of information.
If we assume that the civilization that sent the signal was simply saving energy, then there was no need to repeat it more often than, say, once every few years. By the way, earlier the same strategy of signal transmission was proposed by another researcher, who noted that from the point of view of minimizing the risk of collision with an aggressive CC, it is best to send a signal after significant periods of time, which increases the likelihood of its registration, but reduces the chance of receiving it by an aggressive CC.
So, David Messerschmitt believes, the SETI project has probably already caught a signal from one of the energy efficiently seeking contact of civilizations in the space around us. But due to the limitations of their own search strategy, the Earthlings classified it as a "false alarm." What can you do to avoid such failures in the future?
The author proposes to "refrain" from using the concept of "false alarm" at all, systematically and for a long time to examine each sector of the sky, and not one by one "listen" to different surroundings, as is done now, and also maintain a single database of all signals that may be of artificial origin. Trivial, how to wash your hands? Alas, in the sense of looking for a VTS, we seem to have not even reached the understanding of the need to start eating with clean hands.
A preprint of this study can be viewed on the arXiv website.