Fermi’s Paradox was actually a question originally: “Why aren't they here?”. The paradoxical form is “aliens are everywhere, except for here”. This document looks at various resolutions of the paradox.
Since writing this page, I’ve found a very good book on possible resolutions of Fermi’s Paradox. It has the incredibly long title of If the Universe Is Teeming with Aliens… Where Is Everybody? Fifty Solutions to Fermi’s Paradox and the Problem of Extraterrestrial Life, and is well worth reading:
- If the Universe Is Teeming with Aliens… at Amazon.com
- If the Universe Is Teeming with Aliens&hellip at Amazon.co.uk (for British readers)
Basic resolutions
Resolutions of the paradox fit into five broad categories, outlined below.
- Life is rare
- Life rarely evolves intelligence
- Intelligent species are destroyed before exploring the galaxy
- Intelligent species decide not to explore the galaxy
- They have explored the galaxy, but we don’t see them
1 Life is rare
1.1 Suitable planets are rare
Planets must form in the ‘habitable zone’, the region where liquid water can exist, to be able to support life. If a planet is too light, its atmosphere will escape into space and it will not be able to support life. If a planet is too heavy, it will attract hydrogen and helium and become a gas giant, and will not be able to support life.
Although most planets will fail one of these criteria, scientists believe there are still millions of planets fulfilling these criteria in our galaxy alone.
1.2 ‘Suitable’ planets are not suitable enough
Even if a planet is in the habitable zone, it must have sufficient quantities of appropriate chemicals for life to start.
Recent studies have shown that large quantities of certain amino acids are present in interstellar gas clouds, and so it is likely more suitable planets will also have sufficient quantities.
2 Life rarely evolves intelligence
2.1 Too many mass extinctions
Mass extinctions tend to wipe out the advanced species on a planet and leave the more primitive ones. For example, the dinosaurs died out to leave the early mammals, which were very primitive in comparison.
On Earth mass extinctions happen roughly every 500 million years, which should leave plenty of time for intelligence to evolve (humans evolved from the early mammals in only 60 million years).
2.2 Too few mass extinctions
The dinosaurs showed no sign of evolving intelligence, and if they hadn’t been wiped out by that asteroid the early mammals would not have had their chance.
This is more plausible than 2.1, but it seems unlikely — new creatures can always evolve, even in well-established ecosystems.
2.3 Wrong kinds of creature
For half a billion years (before the dinosaurs) Earth was ruled by Ediacarans, creatures with no brain and no internal organs (similar to today’s jellyfish). These became more and more advanced, but showed no sign of become intelligent.
This is based on the assumption that intelligence needn’t necessary be the end result of evolutionary processes. However, we have seen on Earth that the same advantageous features evolve in creatures with unrelated evolutionary histories. For example, bats and birds both have wings, but their last common ancestor 500 million years ago was a four-legged creature halfway between a reptile and a mammal.
3 Intelligent species are destroyed before exploring the galaxy
3.1 Natural disaster
A natural disaster such as an asteroid impact tends to have a greater effect on the more advanced species.
The arguments against this are the same as those against 2.1.
3.2 Scientific accident
Scientists may accidentally create a black hole/huge nuclear explosion/deadly plague etc.
This is quite plausible. Scientists frequently conduct potentially dangerous experiments without knowing the possible consequences (for example in particle accelerators). Even when careful controls are in place, and accident at a biological lab could release a deadly virus.
3.3 Scientific ‘on purpose’
The self-righteous president of a superpower could start a nuclear war, or a mad scientist (probably a member of some religious cult) could release a deadly virus etc.
A nuclear war seems unlikely, as this would be mutually assured destruction. The mad scientist theory is technically possible.
3.4 Pollution
Much as we like to pretend we’re independent of nature, if the ecosystem collapses, we couldn’t survive, and this applies to aliens too.
It’s hard to assess the likelihood of this point - it seems we are going this way at the moment.
4 Intelligent species don’t explore the galaxy
4.1 Not capable
Dolphins, for example, don't have the ability to use tools (no hands etc., you see...) so could never build a spaceship.
We already know of one case of a tool-using intelligent species, and that is us. It would seem likely that as tool-using intelligent species have evolved on one planet, they would evolve on many.
4.2 Scientific barriers
It is too difficult to explore galaxy, because of the limits imposed by the speed of light, for example.
This seems unlikely — it will soon be possible for humans to create an enclosed ecosystem that can sustain itself for thousands of years (especially with human management). It is hard to believe that once this technology is developed, there won’t be a group of people willing to start colonising the galaxy, even if it takes many generations to journey between stars.
4.3 Fundamentalist religion/philosophy
Science stops, and people don’t want to know what’s out there.
This seems unlikely. On Earth scientists kept working even when their work was condemned by religious or political leaders.
4.4 Virtual reality
Society turns in on itself, and people are not even aware of actual reality.
Though possible, for this state to be persistent it would require millions of robots to deal with maintenance (including self-repair) and also external threats.
5 They have explored the galaxy, but we don’t see them
5.1 The Prime Directive
Aliens study life on Earth (from a distance) but don’t interfere and make sure we are not aware of their presence.
This is a very pleasant idea. As it is unlikely only a single race has explored the galaxy, it would require a number of races to agree on rules of interaction (an interstellar United Nations) and be able to enforce these rules.
5.2 Advanced technology and oversight
The aliens haven’t noticed Earth, and because their technology is so advanced, we don’t recognise signs that they have been elsewhere in the galaxy.
It would not require much effort for a race to send out probes that systematically survey every solar system in the galaxy.
5.3 Transcendence
Aliens leave our universe into something different.
Much like arguments for the existence of a god, there is no way to disprove this one however unlikely it seems.