The Fermi Paradox

Why have we not yet been visited by extra-terrestials? This seemingly innocent question still beckons for answers...

Italian physicist Enrico Fermi was one of the greatest minds of the twentieth century. Among his many great contributions to science was a special statistic which now bears his name and is of fundamental importance in quantum physics. He once was confronted with a much more simple statistic: Given the number of stars in our galaxy and the number of galaxies in the universe there should be a large number of civilizations out there much more advanced than our own.

Fermi answered with a question: Why then have we not been visited by any of them yet? This clear and simple question has since become known as the Fermi Paradox.

Some people claim that we actually have been visited by aliens and point to certain petroglyphs in the South American desert for proof. In this case there would be no Fermi Paradox. But the vast majority of scientists reject this theory.

One possible answer to the Fermi Paradox would be that we are alone in the vastness of space. But an even larger majority of scientists thinks that the opposite is true. From all we learned so far in astrophysics and astrobiology particularly during the last two decades one should expect that there is plenty of life out there.

Simple lifeforms probably develop rapidly in many places but having a civilization intelligent enough to travel into space takes a lot more time and maybe special circumstances. Only during the past 50 years did our own human race join the club. Although men have only ventured as far as the Moon there are four human artifacts leaving the solar system all together: Pioneer 10 and 11 as well as Voyager 1 and 2.

From our own limited perspective it seems to take a few billion years of evolution in a benign environment (a stable energy source from the parent star, liquid water, no dangerous radiation from nearby supernova explosions etc.) to come up with an advanced civilization. So you would need an earth-like planet in a stable orbit around a sun-like star far from the center of the galaxy.

This might proof to be too conservative as we are still far from understanding under which extreme conditions life (and also intelligent life) might exist. But even a conservative estimate will still come up with a large number of andvanced civilizations simply because there are an awful lot of stars and galaxies out there.

So we are back to the question of why nobody has knocked at our door yet. Is it physically impossible to travel over astronomical distances fast enough? Our four spacecraft leaving the solar system need some tens or hundreds of thousand years to get anywhere near another star which far exceeds the lifetime of humans.

Maybe there are aliens who live for more than ten thousand years and could travel over long distances even in comparably slow spacecraft? Or have they mastered the art of tunnelling through spacetime which is a favorite of science fiction writers? And if travelling across galactic distances is difficult enough that is nothing compared to the problem of navigating precisely along the way.

But these are details I don't have to worry about. After all I'm just a mediocre member of a barely developed civilization. It's those aliens who are the smart guys and should have figured out how to travel from A to B across the galaxy.

So let's assume we already have the means to travel over galactic distances. Would we visit every planet in the universe? Certainly not. We would mostly be interested in places with other advanced civilizations. In order to know whether there is an advanced civilization elsewhere we would be looking for certain signs like radio signals or characteristic changes in the environment of the planet.

Those signals however travel at the speed of light. It's probably only during the past fifty years or so that signs of intelligent lifeforms on our own planet have become visible to the outside world. This limits the radius to where you would detect our civilization to about 50 light years. Add to this the fact that civilizations will not exist forever but have a limited lifetime of their own the probability of having an advanced civilization at just the right time of their existence within those 50 light years is rather small.

Given those odds I don't find it surprising that we have not been visited yet. But if there are civilizations which were probably out there for hundreds of thousands of years their signals should have reached us already. Why have we not heard from them yet? You might call this the Modified Fermi Paradox.

Programs to search for extra-terrestial intelligence in radio signals have been underway for some time now. But there are only so many channels which can be searched at any one moment and you cannot look in every direction at the same time.

Maybe some intelligent signals have already been received but we failed to identify them for what they were. For our interplanetary spacecraft special coding schemes were developed which allow a reliable communication with very weak signals over large and noisy distances. Advanced civilizations might have optimized those schemes to such a degree that we would not distinguish the signals from the background noise.

Of course there might be means of efficiently communicating over galactic distances other than radio waves. It would be dangerous to assume that we already know everything there is to know in physics! We have come a long way in science but there is still a long way to go.

It is the hallmark of a great scientist to be able to pose a simple and easy to understand question which is anything but trivial to answer. The Fermi Paradox is likely to keep us scratching our heads for some time to come...