"Where is everybody?" Enrico Fermi.
Many people are aware of the FERMI PARADOX. When we observe our Milky Way galaxy, we find no evidence for intelligent life. If life is common throughout the universe, as most scientists believe likely, then surely there ought to be other intelligent species out there. But despite decades of searching, no indication of another civilization has been found. We now know planets are common. Several thousand have been detected so far.
Recent estimates, based upon frequencies of already detected extrasolar planets place the number of potentially Earthlike worlds, orbiting in habitable zones (where liquid water can exist) in our galaxy as high as 40 BILLION. 11 BILLION of these worlds orbit sunlike stars. (1) Given that the universe is 13.7 billion years old, and that sunlike stars and Earthlike planets existed before our own world condensed out of its primal nebulae, intelligent life would be expected to abound. Yet the heavens are silent.
I suspect that this is partly explained by the Rare Earth hypothesis. Basically this hypothesis agrees that life is common throughout the universe. However, multicellular life is rare. Animal life still rarer. Intelligent life even rarer. Basically it all comes down to location, location, location. If our planet lacked a large moon to stabilize its precession about its axis (seasons), it would wobble erratically. This would create random, often extreme seasons. Complex multicellular life would never be able to develop. If our sun's orbit about the plane of the galaxy were different we would regularly plow in to vast dust clouds in the various spiral arms generating ionizing radiation which would at a minimum, kill multicellular life. And on and on. Readers unfamiliar with all of the constraints on our existence should click on the above hyperlink for this topic and review it before continuing.
Further, the emergence of an intelligent species which develops civilization, need not always lead to the development of a technological civilization. Technological civilizations may destroy themselves swiftly by nuclear war, as we nearly did during the Cold War. A species just slightly more aggressive than ours almost surely would have done so. "Klingons" are improbable for this reason.
Even for a galaxy containing billions of Earthlike worlds, across a timeframe of billions of years, the rise of technological civilizations would be an infrequent occurrence. So infrequent, that we may well be the only technological civilization currently existing in our galaxy.
STILL, one civilization is all that is needed to populate the entire galaxy in no more than a few million years assuming travel at only 1% of light speed. The 100 Year Starship Project has demonstrated pretty convincingly, that it is theoretically possible to build interstellar craft which could attain about one tenth of light speed. All we need is about another century of continuous technological development"
And yet, across billions of years, our galaxy remains un-colonized. WHY? I believe that the answer is all around us right now.
The laws of physics are known to be universal. Intelligent life, insofar as understood presently, is based upon carbon. This is because it has eight valence electrons, and so is free to form a vast array of organic compounds. Nothing else in the Periodic Table is like it.
If we assume a planetary origin for a carbon based intelligent species, then it follows that in the deep past of their planet large amounts of plant life would have been sequestered in the ground. Coal, oil, natural gas would exist in significant quantities.
A species which learned to exploit these concentrated sources of energy, would by definition, become technological by doing so. Steam engines, electricity... all would follow logically once these energy sources were exploited. An industrial revolution would occur. Society would come to be based upon the convenient, compact, relatively cheap energy provided by hydrocarbon fuels.
Eventually the carbon content of the atmosphere would begin to rise remorselessly. However, the civilization's political economy would come to be based upon hydrocarbon energy. When alien scientists begin to sound the alarm about planetary global warming, they would be resisted by the local powers that be whose position in society is based upon maintenance of business as usual. Actions, if any, to break the carbon habit would come too late and be too little.
Either the species would so destabilize its planetary biosphere as to drive itself into extinction, or its civilization would collapse amid planetary atmospheric chaos. Either way, that species would never go to the stars. If it went extinct, the reason why is obvious. Even if it survived, it would have exhausted all of its easy to obtain hydrocarbon resources. Without advanced technology it would be impossible then to rebuild, as these easy sources of energy, which, literally, fuel the rise of a technological civilization, would be permanently depleted. Advanced technological civilization can exist only once in a planet's history. The planet would be a cosmic Easter Island, its population stranded forever in the sea of space.
Nature is not cruel. However, it is utterly unforgiving. Make a mistake, die as a result. There are no "do overs." We, humanity, are in the final stages of making this fatal and eternally unrecoverable mistake. If it's any consolation, all other intelligent species appear to have also fallen for the "carbon trap." And perished, or forever isolated themselves.
Possibly, it is still not too late for us to break out of the carbon trap. However, doing so now requires a planetary revolution. The existing order, EVERYWHERE needs to be overthrown. WE need to change our thinking. We are part of vast systems--not isolated, selfish individuals.