Have you ever asked yourself if we are really alone in the universe? With how old the universe is and an incomprehensible amount of planets that could possibly harbour life in the universe, statistics show it is nearly impossible that we are the only ones. So where are all the aliens?
To attempt to answer this question, we cannot focus on the entire observable universe because it is an estimated 96 billion light years in diameter and we will probably never be able to leave our galactic cluster–the local group which is a pocket of space that includes the Milky Way and other galaxies. We will probably never leave the local group because of the rate at which the universe is expanding, so let’s focus on the Milky Way.
The Milky Way has about 250 billion stars, but this is a very rough estimate, some astronomers say more, some say less. Only 10 percent of these stars are sun-like stars leaving us with 25 billion stars. If 1 in every 5 of these stars had a planet located in the habitable zone (this a conservative estimate according to Wait but Why), this would leave us with 5 billion possible habitable planets in our galaxy alone. This number does not include planets rotating cooler stars like white dwarfs. Just because a planet is in the habitable zone, it does not automatically mean it is habitable. A planet could have a weak or very strong atmosphere. This would change the temperature of the planets, affecting their ability to harbour life. It could also have noxious such as carbon monoxide gas in the air of the planet.
When considering possible life forms on other planets, we also have to consider that they might be at various stages of development. The Kardashev scale is used to group 3 stages of civilizations. A type 1 civilization can use all of the energy that comes from their star to their home planet, they also have full control of their planet, and also has no need for fossil fuels. A type 2 civilization could directly harness the energy of their star–we are not sure exactly how this would be done but has been speculated with concepts like the Dyson sphere or swarm that surrounds the sun and collects massive amounts of energy. A type 3 civilization would control the energy of their whole galaxy. 5 billion planets in a galaxy that is about 13 billion years old, should provide millions of chances for an intelligent species to develop into a type 3, multi-planetary species.
One attempt to explain why we have not found aliens is known as the Fermi Paradox, a theory named after physicist Enrico Fermi. It includes multiple theories and nobody knows which one is correct. One of the big ideas from the Fermi Paradox is that there are filters that every intelligent life form goes through removing them from possible contact, and that it is extremely unlikely for a species to make through it. The Great Filter is something that is really hard for life to overcome and get past, it shows that the reason we haven’t seen any type 2 or type 3 aliens is because there are none.
There are three possible outcomes from the Great Filter: we (life on Earth) are unusual and unique; we’re the first intelligent species; or we are doomed and the worst is ahead of us. The first scenario would mean we are the lucky ones and the filter is behind us and it is incredibly rare that life ever starts. The positive to this is we are ahead of the filter so we will not run into almost certain death. Although a negative is we are alone in the universe meaning we will probably never find other life and if we do, it will be extremely rare. In this case, it would be up to us to keep life in our galaxy going. It is possible that the universe has been a very dangerous place to live until just recently, making us one of the first intelligent species to be able to develop without being destroyed. This is probably the best possible scenario because it means we will be one of the first type three species, but will still get to see and meet aliens. In the final scenario, the filter is still ahead of us and we are closer to the end than the beginning. An example of a possible coming threat could be AI. Elon Musk is constantly warning us about AI and that at a certain point there is no going back with technology. It is possible that almost every species gets to a point where they create a new piece of technology that ends up destroying them. The chance that species make through these filters is super low which would explain why we haven’t seen anything. If this theory is correct, let’s hope the filter is not ahead of us.
The second theory is that there is highly intelligent life in our galaxy but we just don’t know it. There are a lot of different possibilities for this theory and more than one of them could be true. There could be a highly intelligent type 3 predator species hiding somewhere that exterminates any life once they get to a certain level of intelligence. This would explain why we haven’t seen any life. The few intelligent species that are alive would know better than to send out any signals to stay hidden, which would also explain why we haven’t received any signals. If you are wondering why we haven’t been found yet it’s because the size of our galaxy alone is ginormous, it would be similar to trying to talk to someone through a walkie talkie that has a billion different channels and you don’t know what channel they are on. Our form of communication could also be too primitive for them to pick up or, maybe they know we are here and we just aren’t close to being a threat yet. Another idea is that we just can’t detect all the intelligent species especially if they don’t wanna be found. Our technology may not be advanced enough to actually find anything even though life is there. It’s possible that we can’t perceive what they have done or are doing, even though we can see it. We could also be being observed by aliens. Like I mentioned earlier if intelligent aliens didn’t want to be found they won’t. They might wait until we reach a certain intelligence till they reveal themselves to us and until then they could observe and study us for whatever reason that may be.
There are also less likely but possible theories as well. I am sure you have heard of the idea that we all live in a simulation. If this was true then maybe aliens aren’t programmed into the simulation and that could solve our paradox. Or maybe highly intelligent beings believe that physical reality is stupid and a waste of time. Instead, they are all in their own virtual reality, escaping into a perfect paradise.
We still haven’t found any life in the universe, but we are certainly trying. METI (messaging to extra-terrestial intelligence) and active SETI (search for extra-terrestrial intelligence) are programs that sending and receiving signals in hopes of making contact with an extraterrestrial civilization. Also, the Voyager project sent out a golden phonograph record containing sounds of the earth along with other symbols of our intelligence. If an extraterrestrial found this and interpret it, they would know that we exist. There could be life out there, but the size of the universe makes it extremely hard to make contact. Unless we can figure out how to travel faster than the speed of light, we aren’t likely to actually venture out and find life in person.