Extraterrestrials

Where is everybody?

Legend has it that this is the question that Enrico Fermi asked when discussing the existence of alien civilizations. If, as we know now (but Fermi did not), there appear to be many planets forming many solar systems around stars in our Milky Way galaxy. And some fraction of those stars should be able to support life, and on some of those planets life actually emerges, why have none of those civilizations made contact AND remained in contact?

In 1975, Michael Hart made the case that the reason that "there are no intelligent beings from outer space on Earth now" is that there are none. He made some fairly good arguments, but his dismissal of the so-called "temporal explanation" seems the most dubious to me. The temporal explanations state that other civilizations have arisen so recently (or at least become capable of interstellar travel so recently) that they have not had time to reach Earth (assuming that that are colonizing the galaxy). Hart estimates that it would take a civilization (namely, humans) about 2 million years to colonize the galaxy. Since this is so much less than the age of the universe (~10 billion years), it seems unlikely that if other species existed they would not have visited us yet.

However, it is well known that life as we know it, based on carbon, needs elements other than hydrogen and helium to evolve. Since H and He were the only elements created in the big bang (this is an incredibly bad term, but I'll leave that to another post), stars are required to process H and He into heavier elements through nuclear fusion. Several stellar generations are needed, though, because the death of one start, via a supernova, for example, provides fuel for the start of another solar system. If stars are needed, then galaxies are needed, so that there are many stars around to feed on each other. Therefore, it is plausible that a certain minimum time is need for the primordial hydrogen and helium atoms to condense into galaxies, then stars, then have several generations needed to produce the stuff from which life is made. Exactly what this minimum time is is open to debate, but I think it's relatively well accepted that there is a minimum time. So that tells me that the temporal explanation might just be the right one.

But all this is to point out a recent paper in which the authors argue that it is still a good idea to look for extraterrestrials, and their idea is that we should look in the infra-red. If a civilization is advanced, they must use lots of energy, and will have lots of waste energy, and that will ultimately end up as radiation in the infrared region of the spectrum.

The G-hat infrared search for extraterrestrial civilizations with large energy supplies
Wright, et al.
arxiv:1408.1133

It's a brief review of the history of SETI. Well worth a read.