On Mon, Jun 18, 2012 at 12:43 PM, Terry Blanton <hohlr...@gmail.com> wrote:
Take that, Fermi: > > http://www.sciencedirect.com/science/article/pii/S0094576511003304 > > T > Assume that the odds that there are sentient beings on other planets is nonzero, and that, further, this proposition understates things for a universe where we have every reason to believe that the number of planetary systems is large. A problem to be explained, then, is that we have not detected any sign of such beings. If they survive and advance technologically, one can presume that they will eventually discover electromagnetic radiation and its ability to carry information. If they had something analogous to television and radio stations for an extended period of time, wouldn't we pick up the signal at some point? If they made use of anything comparable to what we presently have, it should be straightforward to infer from such a signal that it is not background noise. So perhaps we have one of the following: (1) there are no sentient beings on other planets (this contradicts our hypothesis); or (2) all other sentient groups did not advance to the point of controlling EM radiation (unlikely, given the numbers we're talking about); or (3) such groups quickly move to a mode of communication that is difficult for us to pick up on with our present technology. Perhaps they're using quantum cryptography with an enormously large key strength. With sufficiently advanced encryption, we could mistake the signal for white noise. Possibility (3) could be for reasons unrelated to us; i.e., the other civilizations simply like the domestic privacy such technology affords. Or it could be for evolutionary reasons, as the article suggests; but even then, the alien civilizations could have made the evolutionarily dubious choice of sending out a probe over an extended period before they came to their senses, as we earthlings already have. Or there could be something along the lines of Star Trek's prime directive -- don't interfere with the development of alien societies, with a possible corollary of not permitting them to detect you until they're sufficiently advanced. So to detect alien civilizations in our third scenario, we will either need to crack the ciphers once we get quantum computers rolling or we will need to happen upon the brief period of time (200 years, say) before which they figured out how to super-encrypt things. Even if we assume a large number of alien civilizations, their number will be vanishingly small compared to the surrounding cosmos, so that window of 200 years or so will be a very fleeting thing to attempt to pick up on, and presumably, given the relative weakness of the signal, you will need to point the radio telescope in exactly the right direction. (Here I am going beyond my knowledge of radio telescopes.) One moral of this story is that we need not conclude anything about the existence of alien civilizations on the basis of the SETI results. Eric
