So let’s say there’s intelligent life in the galaxy outside of Earth. We don’t have the technology to visit them and our ability to search for them is limited. Centuries or millennia might pass before we develop the technology to find, communicate, and visit other civilizations in the galaxy.
But what about them? If extraterrestrials are part of a significantly older civilization and have had much longer to evolve and develop advanced technology, why have we not heard from them? According to estimates obtained from the Drake Equation, there should be many communicative civilizations in the Milky Way, and exponentially more in the universe. Of these civilizations, some should be relatively old and have much more advanced technology. Considering that these civilizations have this technology, why have we not heard from them already and why are they not everywhere in the galaxy? This is the basis of the Fermi Paradox.
There are several explanations for why other civilizations have not contacted us:
1. They don’t exist. There is the possibility that Earth is unique. The conditions that allow Earth to be hospitable for life could be very specific, and life could be a very rare occurrence. In addition, if both life and intelligent life are extremely difficult to develop, then it would make sense for us not to find any civilizations. This explanation is based on the assumption that life on Earth is the exception and that humans are very special.
2. Destruction: having the technology to communicate with other civilizations probably means having the technology to destroy civilizations. Self-destruction is a more likely end to a civilization, but destroying other civilizations as they were encountered would not be surprising. Self-destruction would not have to be complete annihilation; if there was a conflict within a civilization, the result could just be that the civilization is reduced to a primitive state and is not communicative. Essentially, there might be a narrow “window” where a civilization has the technology to communicate before it resets itself. Abiotic factors can also destroy civilizations, such as the catastrophic meteorite impact that wiped out the dinosaurs.
3. The third possibility is that other civilizations do exist, but finding them is harder than we think. Interstellar space travel is impossible at the present time, but we have ideas that may become reality in the future. However, even with improved technology, it probably will not be easy to travel across the galaxy. It might require too many resources. For example, we have the technology to send space probes into space, but we are not constantly doing it because it costs too much. An advanced civilization capable of space travel might only be able to afford a handful of spacecrafts. Progress and colonization of the galaxy would be slow. Also, if civilizations were separated by thousands of light-years, then communication between them would take thousands of years, and a civilization might go extinct before any messages can be exchanged.
Assuming that they have the ability to communicate, they probably have the ability to hide themselves as well. If they do not want to communicate, then they can be aware of our existence but we will not find them. Possible reasons might be self-preservation or the zoo hypothesis.
Which one of these is most likely? Who knows. Weighing the probabilities, I believe that it seems more likely for intelligent life to develop and either be destroyed or be extremely hard to find than for there to be no other civilizations in the galaxy. Destruction seems very likely given the short history of humans: the Cold War brought humans very close to nuclear warfare. If self-destruction of a civilization does not occur, then destroying other civilizations would be likely. The European conquests of the Americas during the 1500s are a good example of how civilizations of different technological capacities interact. If alien civilizations have a similar history, they might pursue an isolationist policy towards other civilizations.
All of these explanations are purely hypothetical. The quickest way to explain the Fermi paradox is to discover a communicative civilization and end the speculation.
