Cliff Smith wrote:

Here are some of my thoughts on the existence of alien life.

In assessing the probability of other advanced civilisations existing
elsewhere in the galaxy, it's important to maintain both a sense of
perspective and a firm grasp of the laws of physics and chemistry, at
least as far as they are currently understood.
So far, 115 planetary bodies with a mass less than 18x that of Jupiter
have been discovered orbiting Sun-like stars relatively (in astronomical
terms) close to our own. The search is ongoing, and detection techniques
are being constantly refined.
It is consistent with current theories regarding planetary formation to
suggest that smaller, denser planets might accompany these gas giants so
far detected. Giant planets can be detected because of the perturbations
they cause in the movement of their respective stars. Smaller Earth-like
planets don't cause enough movement to be detectable with our current
instruments, but still may be present.
As is well understood, the chemical elements that make up our environment
were produced from lighter elements by the fusion processes within stars,
processes that are apparently consistent in most stars, therefore it is
likely that any solid planets orbiting other stars would be made up of
much the same elements that make up the planets in our own solar system.

There are very few of those chemical elements with the range of complex
bonding characteristics necessary to support the types of molecules
required for life as we know it to exist. It is possible that
silicon-based biochemical molecules could exist at higher temperatures,
and even that ammonia could form the basis of an alternative biochemistry,
but carbon is more stable at a wider range of temperatures, and therefore
much more likely. Therefore it is not unreasonable to assume that if life
exists elsewhere, the majority of it will be carbon-based. Speculations
about ethereal beings, machine intelligences and super-intelligent shades
of the colour blue are unhelpful and are best left in the realms of
science fiction. If we want to make contact with aliens, we need to be
able to recognise and communicate with them, so it will be helpful if we
at least have a basic biochemistry in common.

Another assumption is that our solar system is in no way out of the
ordinary. We occupy no special place either in space or in time. There are
stars far older than our own, as well as stars younger than our own. There
are over 100,000,000,000 (one hundred thousand million) stars in our
galaxy, so there are naturally many millions of stars close to the age of
our sun. If planetary formation is as common as it now appears to be,
there are very likely millions of planetary systems at approximately the
same level of stellar evolution as our own, and potentially capable of
supporting life.

The only real unknown is the likelihood of life arising in the first
place. Since we only have one experimental subject, our survey is far from
complete. We have as yet not completely ruled out the existence of some
form of life on Venus, Mars or the moons of Jupiter and Saturn in our own
solar system, so any speculation on the existence of life in other solar
systems is based on a very limited set of data. However, it is at least
possible that there could be a great many life-bearing planets in the
galaxy, and therefore it is also possible that many of them could be at
least as advanced as our own if not more so. One could argue that in an
infinite universe, anything that is not impossible is inevitable.

However, having established that life on other worlds is possible at least
in principle, we must turn to the likelihood of our ever being able to
communicate with, let alone visit, these hypothetical aliens. If life on
other worlds must obey the same rules of chemistry as us in order to
exist, then it must also obey the same laws of physics. For nearly a
century, it has been an established axion of relativity that information
cannot be propagated faster than the speed of light, which is as far as we
know a constant throughout the universe. We cannot travel faster than
light, and neither can the aliens. While we might be statistically certain
that there are other civilisations in distant solar systems, we will
almost certainly never be able to visit them and they will never be able
to visit us.

So there; anyone got anything to add?

where's your proof.
-- 
I would have gotten away with if it werent for those meddling kids!