Kardashev, N. S.: Soviet Astronomy, vol. 8, n° 2, septembre-octobre 1964, pp. 217-221 Astronomicheskii Zhurnal, vol. 41, n° 2, pp. 282-287, mars-avril 1964.
Article d'origine soumis le 12 décembre 1963.
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La durée prolongée de propagation de signal est un facteur déterminant dans la transmission unidirectionnelle d'information à travers l'espace. Une réception fiable, ou toute réception quelle qu'elle soit, de signaux par des abonnés inconnus requiert nécessairement une émission isotropique. Le optimum signal spectrum for transmitting the maximum amount of information in the presence of quantum noise and the background of cosmic radio-frequency emission has been calculated. It is shown that a civilization located at any distance in the universe and in possession of power on the order of LO ≈ 4 × 1033 erg/sec or more, which it is capable of transmitting in a coded isotropic radio-frequency signal, may be detected by conventional radio astronomical techniques. The expected distinguishing properties of artificial sources of cosmic radio-frequency emission are enumerated. It is speculated that even some sources known to us today (notably CTA-21 and CTA-102) may be artificial radio sources.
1. The principal factors which exert a determining effect on the range of space radio communications are the transparency of the insterstellar medium to radio signals, the level of the equipment noise and space noise, and the power of the transmitters. The greatest possible range for establishing space communications could be set most likely in the range from 109 to 1011 cps [1]. The absorption coefficient of the interstellar medium is negligibly small at those frequencies. The equivalent noise temperature may be represented in the form TN = Tn + Tt + Tq, where Tn and Tt are respectively the temperature due to synchrotron radiation and due to background thermal cosmic radio emission, and Tq = hf/k is the equivalent noise temperature due to quantum fluctuations in the minimum detectable signal (h and k are the Planck constant and the Boltzmann constant). The expression for TN gives an estimate of the limiting sensitivity which might be achieved in the case of an ideal noise-free receiver and observations outside the earth's atmosphere. In Fig. 1, we find plots of TN as a function of the frequency in accord with up-to-date radio astronomy date [2]. The paramount role in the establishing of long-range communications within the confines of our galaxy will evidently be played by thermal and nonthermal radio-frequency emission from the galactic disk (over a range of ±50° in longitude on either side of the center of the galaxy). In that case, we have
TN = 2·1027·f-2,9 + 1019 f-2 + 4,8·10-11 f. (1)
In dealing with the problem of possible success in setting up communications between the galaxies, we must take into consideration the brightness temperature may be represented in the form TN = Tn + Tt + Tq, where Tn and Tt are respectively the temperature due to synchrotron radiation and due to background thermal cosmic radio emission, and Tq = hf/k is the equivalent noise temperature due to quantum fluctuations in the minimum detectable signal (h and k are the Planck constant and the Boltzmann constant). The expression for TN gives an estimate of the limiting sensitivity which might be achieved in the case of an ideal noise-free receiver and observations outside the earth's atmosphere. In Fig. 1, we find plots of TN as a function of the frequency in accord with up-to-date radio astronomy date [2]. The paramount role in the establishing of long-range communications within the confines of our galaxy will evidently be played by thermal and nonthermal radio-frequency emission from the galactic disk (over a range of ±50° in longitude on either side of the center of the galaxy). In that case, we have
TN = 2·1027·f-2,9 + 1019 f-2 + 4,8·10-11 f. (1)
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In dealing with the problem of possible success in setting up communications between the galaxies, we must take into consideration the brightness tempeature of the background at high galactic latitudes, which is due to synchrotron radiation from the halo and from the metagalaxy. In this case, we have
TN = 1026·f-2,9 + 4,8·10-11 f. (2)
In both cases, the noise temperature will display a deep-sloping minimum in the decimeter and centimeter wavelength ranges, which renders this range more suitable for space communications over exceptionally vast distances.
2. Let us evaluate the information content of communications channels for application to this problem.
The upper bound of the rate of information transmission at a specified average transmitter power and specified noise distribution is determined by the corresponding Shannon theorem [3]:
R =
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In line with the estimates arrived at, it will prove convenient to classify technologically developed civilizations in three types:
4. Estimates of the possibility of detecting a type I civilization [7] and related experiments in the "OZMA" project in the USA have revealed the extremely low probability of any such event. Consider the possibility of detection and reception of information sent by type II and type III civilizations. First of all, we assume here that one of the principal tasks of such communication efforts would be the transmission of information from a more highly developed civilization to a less highly developed one. Starting from the present level of development of radio physics as point of departure, we see that in principle it is possible to build antennas, within the next two decades, with an effective area of 105m2 and with receiving apparatus featuring a noise temperature TN ≈ 1°K. If a transmitter is designed for this system to receive and record information, Eq. (3) will show that the radio emission flux at the receiving point will be not less than 1.4 × 10-26 W/m2 · cps, an amount which is well within the recording capabilities of presently existing radio telescopes. [Of course, far simpler equipment will be required to detect the signals since, in contrast to the reception of information, here we may utilize the averaging techniques common in radio astronomy, and the radiometric gain ...
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