“But where is everybody?” asked Enrico Fermi, wondering about why no alien civilizations have ever visited us!! Are there not enough Earth like planets in our galaxy where complex life forms have evolved?

If you think evolution of life is the rarest thing that can happen, you are wrong, because the evolution of such a Universe that can habitat life is even more rare!!

Evolution of Life is difficult

Is evolution of life, leading to that of intelligent life forms that could then develop advanced tech to do interstellar travel such a rare probability? If so, Earth is indeed a very rare planet that has supported habitable life long enough for intelligence life form to evolve.

We have many reasons to believe that evolution of advanced life forms is a very rare phenomenon. The number one reason being no one has really visisted us yet.

We need a lot of co-incidences to happen, like the planet should be at the right distance from its star so that it is neither too hot, nor too cold. It should be protected from large collisions with other bodies like comets and asteroids from great walls like Jupiter that absorbds a lot of impact, clearing the neighbor hood around Earth. It should develop a magnetic field to shield life forms from dangerous solar radiation and cosmic rays. The star itself should be a medium star that can survive long enough allowing for life to evolve. These are some of the very basic co-incidences required for meaningful life forms to evolve.

Interstellar Travel is even more difficult

The most important obstacle might be interstellar travel, rather than evolution of life itself. Most life forms, no matter how advanced they are, would confine themselves to their solar system, because interstellar travel is very difficult, requires lots of resources, and the ROI is almost zero.

The time required to travel from one star to another is so large, that generations of humans would be required to born and survive during the travel itself, far away from their home planet. Our nearest star Proxima Centauri is 4.2 light years away! Which means, the fastest thing in the universe requires 4.2 years to travel to our nearest star. With our current technology, any space mission sent to this star would require around 1,48,000 years to reach there!!

So even if interstellar travel were possible, the first aliens to visit us would be alien robots rather than aliens themselves, because sending robots on interstellar travel is always a safer option, than going ourselves.

Hence, the chances are that intelligent life exists in many places, but nobody in our vicinity have managed to master the tech to make interstellar travels.

A Stable Universe is the most difficult system we know

We know that our Universe was born in a big bang. And our universe would have collapsed back into itself in no time if not for all the right combination of physical constants, and the right things that happened at the right time, like the inflation. This right combination has made our Universe a Fine Tune Universe!

If not for the right values of the fundamental physical constants, there would have been no protons, electrons, and no matter as we know it. No stable Universe means, no evolution of galaxies, and hence no stars, and hence no star systems, no planets, no life!!

The most difficult thing to evolve, therefore was not life, but our Universe itself. The fundamental constants of our Universe have made life possible on Earth. Because if not for a stable star, stable galaxies, there would have been no Sun or Earth in the first place!

Our Universe is not the only one. Just like numerous stars are continously born and dead in our Universe, our universe is part of larger multiverse where an infinite number of big bangs have been taking place. Most of them, have the wrong combination of physical constants, and hence are not stable and collapse back before any meaningful stuff could evolve in it.

Our Universe lives in a multiverse where new universes are constantly born, and where most die very young. In the String Theory Landscape, the number of possible combination of Universes is almost infinite like, and to have a bigbang where all the fundamental constants are fine-tuned so precisely as ours, is like find a needle in a haystack that knows no bounds!!

The first meaningful stuff that evolves in a universe is matter like protons, electrons etc. From matter evolves structure, where nebula, stars, galaxies are formed. And then evolve stable star systems with planets around them. And then in very few of those planets, simple life evolves like microbes. And in very very few of those, life evolves into complex forms like reptiles and mammals, and then even rarely it gets transformed into intelligent life forms like humans who go on to build technology.

So the catch here is that, just like we have been very lucky to be in the right place at the right time, ie Earth — which had everything available to evolve us. We have also been even more lucky that our Universe evolved in the first place with all the right physical constants that lead to stability — matter, structure, star formation.

Our Stable Universe is even more a rare combination than our Earth