Are We Alone In The Universe? The Fermi Paradox

  

  Are we the only living species in the entire Universe? The observable Universe is about 90 billion light years in diameter. There are at least 100 billion galaxies in the observable Universe each with 100 to 1000 billion stars. Recently, we have found that planets are also very common, which orbit these stars and there are probably trillions of habitable planets. Which means there is a lot of opportunity for life to develop and exist. But the question is, where is it? And why we have not seen it yet? 

  Even if there are alien civilizations in the other galaxies there is no way we will ever know about them. Basically, everything outside of our local group is out of our reach forever because of the expansion of the Universe. Even, if we had really fast spaceships, it would take billions of years to reach our nearest galaxy. So, finding life there would be impossible for us. So, lets focus on our home galaxy Milky Way.

Possibility of Life in Milky Way

  Our home galaxy, the Milky Way consists of up to 400 billion stars. That’s a huge number of stars. There are 20 billion sun like stars in Milky Way and fifth of them have Earth like planet, which lies in its habitable zone. If 0.1 percent of those planets contained life, there would be 1 million planets with life in the Milky Way. The Milky Way is about 13 billion years old. 1 to 2 billion years after the formation of Milky Way first habitable planets were formed. The Earth is only 4 billion years old. So, there have been a lot of chances of life to exist and evolve even before earth was formed. If only a single of them had developed into a space travelling civilization, we would have noticed it now. How would such a civilization look like? Advanced civilizations are classified by the Kardashev scale. We will discuss it briefly.

Kardashev Scale

  This classification of advanced civilizations was first proposed by Russian astronomer Nikolai Kardashev in 1964. He was not satisfied looking for alien civilizations without any idea of what he might be searching for. Scientists like to quantify the unknown, so he introduced a scale that ranked civilizations on the basis of energy consumption. His ranking is given below.

Type I Civilization

  A Type I utilizes all of the energy of its home planet including the sunlight falling on that planet. They use all of the resources of their planet to get energy. We are currently 0.73 on the scale and we will reach on level of Type I in the near future. Transition from Type 0 to Type I civilization is most difficult because it is the most uncivilized, both technologically and socially. 

Type II Civilization 

  Eventually, a Type I civilization may exhaust the power available on its home planet and look to exploit the enormous energy found in the sun itself. A Type II civilization is capable of harnessing the whole energy its sun produces. This would require some amazing technology. It could be done by the concept of Dyson Sphere (A large mega structure built around the sun, which is able to harness the energy of sun).

Type III Civilization

  Eventually, a Type II civilization may exhaust the power of not just its home star but all the nearby stars and gradually start the journey to become a Type III civilization, which is galactic. A Type III civilization is able to utilize the energy of the entire galaxy. Not only can a Type III civilization harvest the energy from billions of stars, it can also harness the energy of black holes, like the supermassive one located at the center of the Milky Way galaxy, which weighs as much as two million suns.

If there are any of these civilizations, why have not we encountered them yet? If life evolved on other planets before Earth, they would have more time than us to become advance enough to develop space travel. So, why Earth have not been visited yet by aliens?

Fermi Paradox: Where Are They?

  If any advance civilization exists, then surely they would leave a mark, maybe even visit us, yet we see no real evidence of alien visitation. Here are some possible answers to this question. Maybe the probability of life spontaneously appearing is so low that Earth is the only planet in the galaxy or in the observable universe on which it happened. Another possibility is that there was a reasonable probability of forming self-reproducing systems, like cells, but that most of these forms of life did not evolve intelligence. We are used to thinking of intelligent life as an inevitable consequence of evolution, but what if it isn’t? It would be consistent with the hypothesis that the probability for life to develop intelligence is low. In this case, we might expect to find many other life forms in the galaxy, but we are unlikely to find intelligent life. 

  Another way in which life could fail to develop to an intelligent stage would be if an asteroid or comet were to collide with the planet. In 1994, we observed the collision of a comet, Shoemaker–Levy, with Jupiter. It produced a series of enormous fireballs. It is thought the collision of a rather smaller body with the Earth, about sixty-six million years ago, was responsible for the extinction of the dinosaurs. A few small early mammals survived, but anything as large as a human would have almost certainly been wiped out. It is difficult to say how often such collisions occur, but a reasonable guess might be every twenty million years, on average. If this figure is correct, it would mean that intelligent life on Earth has developed only because of the lucky chance that there have been no major collisions in the last sixty-six million years. Other planets in the galaxy, on which life has developed, may not have had a long enough collision-free period to evolve intelligent beings.

  There are filters which life have to overcome in order to evolve. In this case, a filter is like a barrier which is harder for a life to overcome. Some of great filters are like nuclear war, climate change and asteroid impact etc. One possibility is that we have passed some great filters. May be, it is way harder for life to develop and this process is not completely figured out yet. May be it is a really complicated process and life on Earth is unique. Another possibility is that their are great filters and they are ahead of us. May be life exists everywhere in the universe, but it gets destroyed by some great filters. May be there were some civilizations in the Universe but they did not overcome these barriers and got wiped out.

  Currently, we have no direct evidence of any alien life in the Universe. The Universe appears to be lonely and empty. But, who knows may be in the near future we will be able to find some evidences of life elsewhere in the Universe.