A short survey about the possibility of extraterrestrial life. This survey deals with potential intelligent life. That exludes primitive microbial life forms which may well be discovered within our own solar system.

The Probability of Civilizations in the Universe

From the Atlas of the Universe we can see that our visible universe - which spans over 14 billion light years - contains an estimated 30 000 000 000 000 000 000 000 stars, or 30 billion trillion stars. Within that astronomical number of stars it seems mathematically inconceivable that there wouldn't be many other civilizations somewhere.
( For details see   An Atlas to the Universe )

However, it is also obvious that we will never be able to communicate over such an enormous length of time, and any intelligent signals over such an immense space would diminish practically to zero before reaching us - for it would be swallowed up by millions of intervening galaxies.

Our own galaxy - the Milky Way - contains 200 billion stars, but it is still too large for effective communications, because its size - 90 000 light years - would require 180 000 years for a two way communication.


The Probability of finding other Civilizations

For practical reasons we have to confine our search for other civilizations to our immediate neighborhood. If we reduce our search to an area of 12,5 light years the number of stars shrinks dramatically to about 33 stars, out of which most are red dwarfs. That leaves about 6-7 normal stars. It would be highly unlikely to find intelligent life within such a small number.

A 50 light years large neighborhood contains about 2000 stars, with 80% red dwarfs - still a relatively small number.

The mathematical probability of finding alien life improves if we extend our search over 250 light years radius, within which we find an estimated 260 000 stars. Should we ever receive any intelligent signal it would most likely come from this small part of the Orion arm which constitutes part of the Milky Way.

However, we have to keep in mind that at this distance communications would be mostly a one way street. Because if we receive a signal from 250 light years away it would require another 250 years to answer and a return answer would not come back before 500 years!

Further obstacles to find other civilizations
A further complication can be demonstrated by the following consideration - let's call it

The Eggshell Theory postulated by the author - Andreas Nothiger

The thickness of an expanding eggshell in this theory would represent the space-timespan within which a scientific civilization would be able to broadcast its signals to other worlds before its own civilization expired.

Let's assume some distant planet had developed an advanced scientific civilization. It would be extremely unlikely that such a civilzation would have developed exactly at the same time as our own here on earth. It could have occured - let's say - two million years ago. ( If we take into account that the formation of our own solar sytem occured around 5,000 million years ago a 2 million year difference would be miniscule ).

Let's further assume that the lifespan of a scientific civilization would be 100 000 years - a very generous assumption. We only have to look at the fragile state of human affairs here on earth. Is it very likely that mankind will survive for more than a few thousand years after the onset of the nuclear age? ( If we consider the risks of nuclear war, the long term effects of global warming and a myriad of other problems it seems problematic that our scientific civilization will survive for more than 100 000 years).

That would not neccessarily be different on an another planet. If such an alien civilization had broadcast its existence for all of 100 000 years, but starting two million years ago, the timespan - or the thickness of the eggshell - would have radiated out from the originating planet at the speed of light. It would mean this electronic wave ( the eggshell ) would have hit our earth two million years ago but it would have lasted for only 100 000 years. So we would have missed it since at that time only primitive hominids roamed the African continent, barely having invented primitive stone tools.

Similarly, our own electronic projection - starting less than one hundred years ago - would hit another planet millions of years after its civilization had expired, or alternatively, long before it evolved. In that case no communication would be possible because we would attempt to communicate with each other millions of years apart. The probability that an expanding eggshell from one civilization would intersect another civilzation exactly at the right time when the receiving civilization would be able to detect such signals would certainly be 'astronomically' small, that is, almost non-existent.

On the other hand, if we assume a much more optimistic estimate - let's say a potential civilization would have evolved - 'progressed' - over several million years, then another possibility arises. The late Dr. Carl Sagan pointed out that such an extremely advanced civilization might have no interest whatsoever in our primitive human species. The gap would be too immense and such aliens would conceivably look at us, as we look at lower forms of life, such as insects for example. Or they might just ignore us as so underdeveloped that communication with us would be a waste of their time.

A more optimistic scenario

It has to be conceded that if the "Search for Extraterrestrial Intelligence" (SETI) should find any intelligent signal in the future the above theories would have to be revised!
For more optimistic persons see Setiathome. Setiathome is a scientific experiment that uses Internet-connected computers in the Search for Extraterrestrial Intelligence. Anyone can participate by running a free program that downloads and analyzes radio telescope data.

Should we detect any intelligent signals from outer space that certainly would be very exiting because it would expand our understanding of alien life forms enormously.
However, the fact remains that communication would - for all practical reasons - still be a one way street because our answer would not receive a response from a potential alien world for several hundred years !