Are there really aliens out there? Then where are the aliens?

The simple answer is 'Yes'. Statistically, it is inconceivable that there is no other life in the universe is just too improbable. In our galaxy there are between 200-400 billion stars in our galaxy. There are over 200 billion galaxies in the universe.

The two big real questions are, how frequently does life occur, and how frequently does intelligent life appear?

To calculate the frequency of intelligent life we should be able to detect, we have the drake equation. To calculate how many aliens we should discover, a number of constants must be plugged into the drake equation and the challenge lies in determining the value of these constants. However the constants are calculated, the fact is there are clearly less intelligent aliens out there then we would expect. This surprising lack of evidence of intelligent alien life is referred to as the Fermi paradox.

There is solid evidence that the building blocks for life exist in space. There is ambiguous evidence that remains of actual life have been found on meteorites, but this is not conclusive. But when it comes to intelligent civilisations, we should be able to at least detect their radio signals from thousands of light years away yet a very intensive search, SETI, has found nothing.

So we are left with one of two conclusions. Either life itself is remarkably rare, or the evolution of life sufficiently intelligent to transmit radio signals in the manner we do from Earth is extremely rare.

Why do we have a rare intelligent civilisation here on Earth?
The question arises, that if intelligent civilisations are so rare, how did we manage to get one on Earth.
Here are some of the theories:
1) The 'rare earth' theory. This theory proposes that Earth enjoys a set of conditions necessary for life that are extremely rare in the universe.
2) The 'low probability of intelligence' theory, which proposes that even on Earth the probability of intelligent civilisation are very low.
3) The 'self destruction theory' which proposes that intelligent civilisations may normally self destruct so quickly that they are a fleeting existence.

Any others?

I will discuss these alternatives soon.