"Shadow Biospheres" - the parallel-life ALIENS AMONG US
But even a superpowered radio lighthouse would need to stay operational for incalculable thousands or even billions of years to give us a decent chance of detecting it - Davies also raises the point that Sun-like stars, perhaps with attendant Earthlike worlds and civilisations, existed in the Milky Way well before our solar system even formed. There would be a major risk that any such beacon, even if built, was subsequently destroyed along with its parent civilisation and star - or was allowed to run down after its builders turned themselves into floating disembodied intelligences or whatever and forgot about it. By the time Mr Marconi got to work on newly-formed Earth, the fading pulses of long lost early-Galactic lighthouses would be lost far out in the vastness of intergalactic space.
So, what should we be doing instead of/alongside SETI?
According to Davies there are several rather more promising lines of enquiry which might help us get a handle on whether we are simply a bizarre statistical freak in a beautiful but cold and empty vastness, or in fact but one of countless civilisations scattered through time and space.
First up, there's the idea of looking into the likelihood of life developing given Earthlike conditions, which would help to put some actual numbers into the famous "Drake equation" postulated by the founder of SETI. If it is in fact reasonably likely for life to appear in liquid-water environments, Davies says, it might actually have happened several times right here on old Earth. Along with the microscopic life which eventually assembled itself into plants, animals and so forth, there might also be one or more "shadow biospheres" of microbes built along different chemical lines which have never started to organise themselves into bigger lifeforms.
Most terrestrial life is microbial, and biologists have only scratched the surface of the microbial realm. Many bizarre micro-organisms have been discovered, including the so-called extremophiles that thrive in conditions lethal to most known forms of life, but so far all of these organisms have turned out to belong to the same tree of life as you and me. However, this means little. Biologists customize their techniques to target standard life, so any microbes with a radically different form of biochemistry would tend to be overlooked.
In theory at least, the shadow-life might also be present here on Earth in larger forms - simply ones we haven't yet encountered. Various earthly environments haven't yet been much explored, for instance the deep oceans.
Then, there's the matter of intelligence. Even if the galaxy and universe beyond is hopping with biospheres containing life at various levels of organisation, it may still be that intelligence is so wildly unlikely as not to have arisen elsewhere - or indeed, it might be a counter-survival trait which tends to wipe its possessor out. Perhaps, every time intelligence arises, that species swiftly thereafter destroys itself by the use of planet-busting weaponry, or becomes enervated by spending all its time in immersive artificial game/entertainment environments and dies out because it can't be bothered to breed.
Is it just me, of has the prof just rifled through the average SF fan's bookshelf and pinched a load of the material found therein? Granted much of SF is inspired by 'proper' scientific research, but either way this guy is saying nothing (at all!) I haven't read in fiction over the years (e.g. the shadow biosphere in 'Behemoth', or the ephemeral nature of civilisations in, oooh, a shit-load of places).
I have yet to see any evidence of intelligence having arisen on Earth yet, let alone elsewhere
It is simple
They do not visit us because we play cricket!