In his 1964 Nikolai Kardashev (1932 – ) reviews and articulates the basic requirements for interstellar communication using radio waves. Further, it considers the noise sources in the radio and attempts to establish the ideal wavelength range for space communication. Not only this, calculating the energy requirements for such communication he classifies the three types of civilizations (now known as Kardashev civilizations) based on their energy harnessing capacities.
He starts off by stating that the 1 GHz – 100 GHz frequency space should be ideal for interstellar communication since the absorption due to the interstellar medium is minimum at these frequencies. The limiting factor for communication within the galaxy is the background from the galactic disk. However for intergalactic communication we also need to start worrying about radiation from the halo of the galaxy. Considering both these cases for communication, Kardashev again states that the 1 – 10 cm wavelength range (~ 1 GHz) would be the optimum band since the noise background reduces.
He then talks about the information content of the signal and the minimum power that would be required to establish a signal of appreciable statistical significance over the noise. To consider the maximum power of which a signal a civilization can transfer, he classifies levels of civilization and technological advancement.
As shown in the Figure below, a Type I civilization can garner all the energy incident on a planet from its host star.
Type II civilizations can utilize all of the energy of the star. As discussed in Dyson (1960), a potential way to do this by using a (now called) Dyson sphere or Dyson swarm, an artificial cloud of material to enshroud the star and absorb all of its energy output. There are also hypothetical scenarios proposed by Roger Penrose (1931 – ) where the the civilization can feed its star into a black hole and utilize the waste heat produced by the accretion disk.
Type III civilizations are those which can harness the entire energy output of their galaxy, potentially the supermassive black holes at the center and even quasars.
Humanity in its current era, is more of a 0.7 on this scale. Michio Kaku posits that it would take humanity another century or two to reach a type I stage.
It is interesting to note that one of the potential extraterrestrial sources he mentions in this paper (CTA-102) was later discovered to be a quasar.
Also the fact that he posits the development of radio telescopes 100,000 m^2 in area, the same year that the construction for Arecibo began. Arecibo has an area of about 70,000 m^2.
I think this paper is of historical significance as the first mention of the Types of civilizations which are now famous as Kardashev civilizations. Not only that, but also the fact that it takes the idea of the Dyson sphere and provides it with a much broader framework as to its significance for an advancing civilization.
I believe it is also important in the context of Science Fiction, the possibility of a realm where a single civilization can harness the energy of the entire galaxy. The concept seems to be too far fetched to humanity in the present, but it provides fodder for the imagination of authors and Sci-Fi creators to run wild with possible technologies of the future which can achieve this. Technologies and achievements which could potentially be realized in the future.