When you think of the size of the universe, there must be life elsewhere. There are 100 – 400 billion stars just in our galaxy, and just as many galaxies in the universe. If we estimate that 5% of all stars are sun-like, which is on the low side. There should be 500 quintillion sun-like stars. From a PNAS study, 22% of those stars might be orbited by an earth-like planet, which means there should be 100 quintillion earth-like planets. That is equal to 100 earth-like planet for every grain of sand in the world. That means there are 1 billion earth-like planets in our galaxy alone. If 1% of those planets advances intelligent life like Earth, there should be 100,000 intelligent civilization in our galaxy.
So. Where is everyone?
Our sun is young compared to the universe. There are countless older stars with countless older earth-like planets.
Let’s look at the Kardashev Scale. It groups civilizations into three categories.
Type I: A civilization has the ability to use all the energy on their planet.
Type II: A civilization has the ability to harness all of the energy of their host star.
Type III: A civilization has the ability to harness the enemy comparable to the Milky Way galaxy.
We, humans, are around a 0.7 according to Card Sagan.
If 1% of of intelligent civilizations survive to Type III, there should be around 1,000 Type III civilizations in our galaxy.
One explanation is that is there no signs of higher (Type II and III) civilization because there are none. If there is none out of all the stars and all the earth-like planets, there must be something preventing that. That something is the Great Filter. The Great Filter theory states that at some point while trying to get to Type III intelligence, there is a wall or filter that all attempts at life hits. That there is some stage of evolution that is impossible or near impossible.
Where is the Great Filter for us?
There is 3 possibility: We are lucky, we are first, we have not hit it…. yet.
1) We are lucky. The Great Filter is behind us. We, humans, have passed the filter, which means life is rare to make it to the same level of intelligence as us. This would explain why there are no Type II or Type III civilizations as many civilizations gets caught by the Great Filter before even reaching Type I.
2) We are first. The Great Filter will not occur, but since we are the first and most advance civilization and we are on our way to becoming a Type II civilization, we will not contact life outside of Earth unless we do it.
3) We have not hit it……..yet. If we are not lucky and passed the Great Filter or we are not the first intelligence civilization to reach this level of intelligence, then we must have not hit the Great Filter yet. The Great Filter must be in our future and something will prevent us from going further and reaching high intelligence to contact other life.