Are We Alone In The Universe?

Mainstream SETI projects are seeking out radio signals at frequencies thought best suited for interstellar communications; others are seeking signals at optical wavelenths which might be used for laser communications beamed in our direction; and yet others are seeking evidence of biological forms in exo-planetary atmospheres. So the search for life in the Universe continues but still nothing has been found. Where are the aliens?

Maybe they are only looking but not transmitting. (A bit like like us, then!).

Maybe they are at different stages of civilisation - either they are not yet capable of inter-stellar communications or have degenerated beyond that point in their history.

Maybe all civilisations have a short "use by" date, not long enough to make themselve known. 

Maybe there are not many alien civilisations at all.  

Maybe primitive life is common but advanced lifeforms with an ability to develop a technological society are not. Perhaps there are exo-planets teeming with fish, insects or rodents without any intelligence as we know it arising.

Maybe we are the only ones. When you consider how life here on Earth evolved - with life beginning relatively easily at an early point in our history but taking nearly four billion years for humans to appear - that is a distinct possibility.

• The right kind of galaxy.
• The right orbital location in that galaxy.
• Orbiting at the right distance from the right type of star.
• A solar system with the right orbital arrangement of planets.
• A continuously stable orbit within the habitable zone.
• A terrestrial planet of the right size.
• A planet with plate tectonics.
• A large stabilizing moon.
• An evolutionary trigger for complex life.
• The right time in evolution.

Then there other necessities like a large proportion of water to land mass, a suitable rotation period, sufficient axial tilt to create seasons, radiation belts to protect us from harmful radiation, a climate which produces suitable weather conditions and an atmosphere which retains just the right amount of heat without trapping too much, the right amount of gravity.

How many essential accidents of galactic and stellar evolution occurred since the Big Bang before primitive life evolved on Earth? How many generations of stars, how many supernovae, how many inter-stellar cloud condensations, how many freak interactions?

After that, how many freak events occurred in the protodisc of our newly forming Solar System, to form a planetary system and produce an Earth-Moon system at an ideal distance from the Sun and protected by outer gas giant planets? 

Evolving proto-disc of the star HL Tau. Photo: ALMA.

Given a suitable planet in a suitable place at a suitable time, even the appearance of our own species was a result of an unknown number of flukes, accidents and disasters right here on an evolving Earth. The tree of life, shown here, is a very simplified version of the real tree, which would contain millions of "twigs" at the top of the branches - of which one very short one would represent little old us. How many mass extinctions and other accidents of evolution were necessary for that one little twig to evolve and begin using technology?

Then of course there is the big question of how long a technically clever civilisation can sustain itself before it collapses under its own run away population growth, suffocates itself in its own pollution, exhausts all of its natural resources or simply self-destructs under deadly conflict. I give it 1000 years at most, probably a lot less. If other civilisations have arisen, then the shorter this period, the less likely are they to exist at the same time.

Is it so crazy to entertain the depressing thought that intelligent, technological civilisations may be so sparse - despite the projected huge proliferation of exo-planets- we may effectively be alone?