Science Magazine describes a provoking paper by Piran (HUJI Jerusalem) and Jimenez (U. Barcelona, Harvard) accepted for publication to prestigious Physical Review Letters two weeks ago:
Complex life may be possible in only 10% of all galaxies (Science)The basic claim is rather simple.
On the role of GRBs on life extinction in the Universe (arXiv, PRL)
This extraterrestrial citizen works with Alza.cz and Alza.sk and spreads tablets and other things all over our homelands. The negative feedback suggests that the Czechoslovak folks may suffer from green xenophobia, after all.
The extraterrestrial ethnic minorities face another problem: (long) gamma-ray bursts, likely to be close to centers of small enough galaxies, take place too often and kill (the ozone layer and) complex life too early. Only 10% of galaxies – typically those that are large enough and sufficiently metallic, similarly to the Milky Way – has a chance to pass this cataclysmic test, and life seems to have a chance far away from the center only.
If true, it has far-reaching consequences. Not only complex life is rarer in the Universe than many people think. But SETI is in double trouble because they are selectively focusing on communication with places that seem to be sterile due to this GRB-induced Holocaust.
As faithful readers of this blog know, I am among those who believe that the null results of our observations of the extraterrestrials should be taken somewhat seriously and life is probably rare. Moreover, life on Earth might have needed some "panspermia" – preevolution on comets or cosmic dust of some kind, just like the working hypothesis behind the Rosetta mission assumed.
The number of similar conditions – what has to happen in the environment for life to collect all the prerequisites to start; and what mustn't happen if the life is supposed to survive for a long enough time – may be rather high and it may reduce the estimate of the density of complex living civilizations by orders of magnitude and perhaps dozens of orders of magnitude. The number of promising galaxies may be much smaller than the 10% announced in the articles above, and the regions of such promising galaxies that are sufficiently hospitable and safe may be very rare, too.
The anthropic principle in the form that prevails in the literature is completely irrational.
But "similar thoughts", while speculative in character and lacking the precision and rigor we expect in physical sciences, could be interesting and could have some true answers. While the anthropic people believe that their principle is trying to maximize the overall number of observers – and that's another reason why they tend to believe, ignoring the observations, that life is literally everywhere (the first reason is that these people are almost always leftists and the idea of life as omnipresent trash seems to strengthen their political prejudices) – I think it is more likely that a similar but corrected principle actually implies that the number of living civilizations is "of order one" in the visible Universe so that our life has barely emerged.
At any rate, if someone is expecting a clear detection of intelligent extraterrestrials within 10 years, I am happy to make a 200-to-1 bet against him or her.