But a less advanced civilisation, say similar to our own would be inadvertently broadcasting radio into space just like we are, and transit time means we can see backwards in time, a civilisation like ours on the other side of the Milky Way making its first radio broadcasts would be reaching us today 105,000 years later.
In 1957, astronomers of the Ohio State University started to scan the cosmos by means of a radio-telescope searching signs of radio transmissions from hypothetical extraterrestrial civilizations. Acting with an anthropocentric mind, they addressed their instruments to the stars
Tau Ceti (spectral class G) and
Epsilon Eridani (spectral class K) for being the nearest among those like our Sun. In their search, they selected the wavelengths between 18 and 21 centimeters, corresponding to the spectral lines of the hydroxyl radical and the hydrogen, the band of electromagnetic spectrum named
Water hole. They were looking for the existence of water in these star systems.
But water is not the only possible solvent for life chemistry, it all depends on environmental conditions. In 99 per cent of the cosmos, temperatures are so low that water can be replaced by liquid ammonia, liquid methane, liquid hydrogen fluoride, liquid nitrogen, liquid oxygen, liquid hydrogen, liquid helium, or liquid argon. Even mercury and lava could be considered in environments with extreme temperatures and pressures. Carbon is also not essential, which can be replaced by phosphorus or silicon.
The search has continued to this day using increasingly sophisticated equipment capable of simultaneously monitoring a large number of radio frequencies, but so far they have found nothing.
It is naive to assume that intelligent creatures from a distant star system use such an unpractical communication as radio (limited to the speed of light in a universe full of interference) just because it was the best channel we had in 1957.
Interstellar distances are so huge that the transmissions of our most powerful equipment are 50,000,000 weaker when they reach the Centauri System four years later and 1,000,000,000 weaker when they reach, if ever, the center of the Galaxy within 26,000 years. It may be assumed that a more advanced civilization than ours has had time to develop something more efficient.
Science Fiction writers have 'solved' the issue by imagining a subspace transmitter using Faster Than Light (FTL) technology, something that violates Einstein's established paradigm. Everyone knows this is impossible, but my
neural network by defect tells me it will finally be built by someone who did not know it... or we will stay here until the Sun freezes!
The FTL hypothesis starts out from the assumption that technological progress has no upper limit. But we must consider the possibility that the human mind be incapable of solving the problem. Nor can we lift a ton of weight, fly, run at 100 mph, breath underwater, or see Uranus, the heat, or the bacteria... but we have found a way to build machines that do it for us.
Maybe
Artificial Intelligence will uncover the secrets of the FTL.
In my opinion, the search for ET is poorly raised because, in short, it all depends on what we are looking for, how we are looking for it and what we hope to find.
Alien life is very possible due to the abundant variety of amino acids found by radio astronomers in interstellar clouds.
Intelligent alien life is statistically probable (Frank Drake equation).
Intelligent alien life interested in interstellar travel is less probable. It would have to be an energy rich culture, with the curiosity of a young civilization (in ours, it only lasted 40 years) and with the conquest spirit of some mad individuals. Difficult that all these factors coincide in the same society.
Chronologically speaking it is not very probable that two civilizations evolve at the same time and are ready for contact. A few thousand years are not much for a galaxy and a too long period for the UN, NATO, or the Third Reich.
There is the theory of cyclical history as proposed by Oswald Spengler and developed in fiction by A.E. van Vogt, Robert A. Heinlein, and Isaac Asimov. According to that theory, a civilization can be ‘young’ and even barbaric, and yet high-tech, since civilizations comes and goes, though most of the technological progress done by the previous civilizations are not forgotten.
