I've been a sci-fi fan since before Star Trek's Captain Kirk split his first infinitive ("To boldly go ..") - but I have to doubt if we'll ever encounter any of his intelligent alien friends or even any unintelligent alien microbes.
Some will assert that UFOs are a fact - but this only seems to mean that objects have been seen flying that were not identified - hence, by definition, they are 'unidentified flying objects' - but that does not make them alien spacecraft - just unexplained observations. Unfortunately, UFO data is not very convincing of an alien connection - and if aliens are amongst us, they are keeping a very low profile.
One hears statements such as: "Given the billions of stars in the universe it is beyond reasonable doubt that some sort of life exists in outer space"...and..."There's no question but that we live in an inhabited universe, that has life all over it"
Unfortunately, the above type of statements tell us nothing - and until such time as genuinely solid evidence arises regarding UFOs, aliens, close encounters and/or alien abductions, we can really only weigh up the likelihood of extra terrestrial life based on the data that is available to us.
It's certainly possible to make a plausible-sounding case for extra terrestrial life. For a start, we know that life exists on earth and we infer that it once did not exist. So at some point in the 4 billion year history of the earth, life arose. If it arose via a natural mechanism, then we may infer that the same mechanism might occur on any earth-like planet anywhere in the universe. Thus, alien life may seem plausible, even probable.
The 'Drake Equation' (Frank Drake 1961) goes beyond this - it seeks to makes an estimate of actual number of potential planets in the universe with life. The calculation is based on various assumptions, the key one surely being that life is highly likely to exist on any given earth-like planet - and the equation then implies that that there could be millions of planets throughout the universe with life - many life forms would be intelligent - and many would be more technologically advanced than ourselves.
However, a problem with the key Drake assumption arises as a direct consequence of Darwin's Theory of Evolution. This theory asserts that the earth's organic complexity was produced gradually, over a period of several billion years, by the action of natural selection, causing gradual changes over many successive generations of self-replicating organisms.
However, consider this - the number of generations along any sequence of evolution must be finite - one cannot fit an infinite number of successive generations of self-replicators into the finite history of the earth, no matter how short each one may be. Hence, if traced backwards, every evolutionary sequence must lead to a very first member - a self-replicating molecular entity that arose spontaneously (without the aid of natural selection, since there were no generations prior to its existence) some 3000+ million years ago. Indeed, every evolutionary sequence would lead back to the very same first self-replicating molecular entity, the common ancestor to all subsequent life forms - unless more than one self-replicator arose independently.
So, for life to exist on any planet, anywhere in the universe, a self-replicating molecular entity would have to arise spontaneously, for Darwin-like evolution to then follow. There is no "mechanism" involved, that we may envisage having once operated on the earth, and being equally likely to operate on any earth-like planet elsewhere in the universe - since the occurrence of a fist self-replicator is an entirely random event.
My own estimates indicate that, even if we assume every single star in the universe has an earth-like planet in orbit, and even if the first self-replicator was far simpler than the simplest known cells, the probability of its spontaneous origin on any earth-like planet is mind-bogglingly small.
Given our lack of knowledge about a potential first self-replicator, my assumptions about it for the purpose of my calculation are, of necessity, arbitrary - but I suggest that no reasonably realistic alternatives would justify the Drake-like assumption that life is highly likely to exist on any earth-like planet.
Indeed, there may well be millions of planets throughout the universe possessing the essentials for life (water & even organic molecules) and yet which are totally dead - because the self-replicating molecular entity, essential for subsequent evolution, did not spontaneously arise on any of them. The number of planets supporting life may well be just one - this one.
However, on the plus side (!) - all those millions of planets will be unoccupied, unpolluted Edens, ideal for 'seeding' and for our future colonisation, allowing us to spread throughout the universe unopposed by aliens. Future interplanetary wars envisaged in alien movies such as "War of the Worlds", may well be between those who remain on the earth and the descendants of those who've made their home on other planets, subsequently seeking their independence from the 'home world' - not a bad story line for a sci-fi film!
(C) Copyright 2009 Andy Chains (andychains.com)
Andy Chains - Born 19 Mar 1952, Rotherham, South Yorkshire, England. BSc Physics (1st class) 1976. PGCE. MSc Scientific Computing. Longest continuous job - FE Teacher - Physics/Maths for 12 years.
My homepage ( [http://www.andychains.com] ) has lots of quality ebooks to read for free.
