The universe is big. Really, really big. In our Galaxy alone, there are about 400 billion stars – 20 billion of which are similar to our own. Of those sun-like stars, about one-fifth have earth sized planets orbiting a habitable zone – not too far as to freeze, not too close as to burn. If only 0.1% of those planets developed life, there would be 1 million planets with life in our galaxy. And that’s just our galaxy.
This thought process was formalized and made famous by Frank Drake who proposed an equation to estimate the number of active, communicative extraterrestrial civilizations in the galaxy. Drake originally estimated the number of these advanced extraterrestrial civilization between 20 and 50 million.
So where is everyone? This was the question that Enrico Fermi posed in his famous paradox. If the universe is so large and old, and if it only took us 250 thousand years to develop radio communication and space flight, why then is there no evidence of intelligence elsewhere in the universe?
What we are looking for
In 1964, Nikolai Kardashev, a Russian astrophysicist, defined three levels of civilization based on their ability to harness energy.
A Type I civilization would be able to harness all of the available energy on its planet. They would be able to control the weather, earthquakes, and everything else on their planet. We are currently around a Type 0.73. We use dead plants for fuel but we also have the internet.
A Type II civilization would be able to harness all of the available energy in its home star. This would make them 10 billion times more powerful than a Type I civilization. Think of the Federation of Planets in Star Trek. A Type II civilization would be able to spread across the stars, making it immune from natural disasters such as meteor impacts or a star’s death.
A Type III civilization would be able to harness all of the available energy in its galaxy. Think the Empire in Star Wars. A Type III civilization would be able to colonize billions of solar systems and would appear godlike to us.
If Type III civilizations exist in our solar system, it is reasonable to think we would have come in contact with them in some shape or form. For example, we would probably notice their space pillaging drones as they converted all useful matter on our planet (including us) into raw resources. The question still remains, where are they?
Barriers to advancement
One proposed solution to the Fermi paradox is the idea of great filters that stop life from advancing past a certain point. Maybe life is really common in the universe, but it gets destroyed when it reaches a certain level of technological advancement. Think nuclear war or the invention of a warp drive that instantly destroys a solar system. Or maybe there is a Type III civilization monitoring all life in the galaxy. When they reach a certain level of technological advancement they are destroyed.
Or maybe the great filter is behind us. Maybe the development of life is a lot rarer than the Drake equation would suggest. For example, we were lucky enough to be blessed with a large moon. If we didn’t have that, the Earth’s axis would probably have become unstable making life impossible. We were also blessed with a magnetic field that shields us from cosmic rays. Add in ice ages, meteor impacts, super volcanic eruptions, gamma ray bursts, and there were plenty of chances for life on earth to be filtered out.
The issue of timing
Say there were thousands of space faring civilizations in our galaxy, nothing suggests that they would be around for the small 200 year window in which we could detect their radio signals. Maybe our galaxy is littered with the ruins of death star like technology.
If they do coexist in our time period, maybe they are using a much higher level of communication than radio signals. Or maybe they have better things to do with their time then spend the tens of thousands of years it takes to traverse the galaxy at the speed of light. For example, maybe they advanced to have an even better version of World of Warcraft.
The rest of the universe
There are over 100 billion more galaxies in the universe, but we have only focused on our own small corner. This was for good reason, the universe is expanding. Even with speed of light travel, we would be trying to reach a destination that is quickly moving away from us. It would take billions of years to reach the closest destination outside of our local group of galaxies.
This brings up another point. When traveling times and evolution are combined, one species quickly branches into many. What’s to say they will be any more friendly with each other than we are with apes. Civil war might be the downfall of many advanced civilizations.
The Fermi paradox asks why, if there is so much space and time for alien civilizations to advance in, have we not met any? This question ultimately boils down to the limits of technology. In 200 years, we went from solely using human power to harnessing the power of the atom. At this rate, it is easy to project a future where technology advances to a point that we can do anything.
But it is equally likely that we are already at the limits of what technological advancement can bring. In this scenario, humanity is at its twilight, not its dawn. Either way, we really don’t know. Maybe the major religions have it right. Maybe the minor religions have it right. Maybe we are alone in the universe. As Douglas Adams said:
It is known that there are an infinite number of worlds, simply because there is an infinite amount of space for them to be in. However, not every one of them is inhabited. Therefore, there must be a finite number of inhabited worlds. Any finite number divided by infinity is as near to nothing as makes no odds, so the average population of all the planets in the Universe can be said to be zero. From this it follows that the population of the whole Universe is also zero, and that any people you may meet from time to time are merely the products of a deranged imagination.