Fifty Solutions to the Fermi Paradox and the Problem of Extraterrestrial Life
Stephen Webb
Stephen Webb is a physicist working at the Open University in England.
Fermi's Paradox:
Enrico Fermi, in response to a lunch time discussion about extraterrestrials, asked the question "Where is Everybody?" (i.e. where are extraterrestrials). This became known as the Fermi Paradox (1950).
Given the vastness of the universe (around 1022 stars), the age of the universe (13 billion years), and the universal laws of chemistry and physics, many assume that life must be present elsewhere in the universe.
The Fermi Paradox. (1) According to the Copernican Principle, there is nothing special about Earth or humanity, so whatever is true here should be true elsewhere in the galaxy. (2) Following this reasoning, Frank Drake predicted the existence of tens of thousands of advanced extraterrestrial civilizations (ETC) and Carl Sagan likewise predicted perhaps a million ETC. Such estimates are typically based on the Drake Equation (1961). (3) At least some of these ETC's must be millions or even billions of years more advanced than us. (4) Given some very reasonable assumptions, even one such ETC could colonize the entire galaxy in as short as 2-5 million years or more conservatively 10-60 million years. (5) If ETC's can spread so rapidly, they should have already reached Earth (or at least we should have detected their existence). Given that there is no definitive evidence (direct or indirect) that we have been contacted by any of these civilizations, then something must be wrong with this chain of reasoning.
If there are a billion ETC's, then the nearest one would be around 300 light years away. If there are only a thousand ETC's, then the nearest one would be around 1,000 light years away.
They Are Here:
This class of solutions posits that advanced extraterrestrial civilizations exist and have visited or are visiting. This is the most popular interpretation among the general populous.
1) They Are Here and They Call Themselves Hungarians
A tongue-in-cheek explanation for famous Hungarian scientists (e.g. von Neumann)
2) They Are Here and Meddling in Human Affairs
Flying saucers, UFOs, etc. are extraterrestrial spacecraft
3) They Were Here and Left Evidence of Their Presence
"Face" on Mars, backside of the Moon, etc.
4) They Exist and They Are Us – We are the Aliens!
Panspermia
5) The Zoo Scenario
A no interference "Prime Directive" is in effect
6) The Interdict Scenario
The presence of many civilizations prevents spreading to Earth
7) The Planetarium Hypothesis
We are inside a giant computer simulation (e.g. like movies Matrix and Truman Show)
8) God Exists
They Exist But Have Not Communicated:
This class of solutions posits that advanced extraterrestrial civilizations exist but we have not made contact. This is by far the most popular interpretation among research scientists
9) The Stars Are Far Away
Long travel times between stars because of speed of light limits travel
Possible solutions: generation ships, hibernation, worm holes, etc.
10) They Have Not Had Time to Reach Us
Diffusion model of galactic colonization
11) A Percolation Theory
12) Bracewell-von Neumann Probes
Self-replicating robots explore the universe on behalf of the extraterrestrials
13) We Are Solar Chauvinists
They might favor stars very different from our own sun
14) They Stay at Home …
15) … and Surf the Net
They prefer virtual reality simulations rather than actual exploration
16) They Are Signaling But We Do Not Know How To listen
ET signals might not use light waves, e.g. gravity waves, neutrinos, tachyons, etc.
17) They Are Signaling But We Do Not Know at Which Frequency to Listen
The electro-magnetic spectrum is broad: radio, infrared, visible, x-ray
The period where a civilization is "radio-bright" (i.e. detectable by signal leakage) might be short
Different SETI strategies targeting different portions of the electro-magnetic spectrum
18) Our Search Strategy Is Wrong
19) The Signal Is Already There in the Data
20) We Have Not Listened Long Enough
If ETC's are be short lived, then detecting them would be difficult
21) Everyone is Listening, No One is Transmitting
22) Berserkers
Berserker probes would eradicate young civilizations to prevent later competition
23) They Have No Desire to Communicate
24) They Develop Different Mathematics
25) They Are Calling But We Do Not Recognize the Signal
26) They Are Somewhere But the Universe Is Stranger Than We Imagine
Aliens transcend physical matter or are exploring alternate universes
27) A Choice of Catastrophes
Perhaps advanced societies always destroy themselves: nuclear or biological warfare, overpopulation, nanotechnology run amok, environmental catastrophes, particle physics disasters, or nearby gamma ray bursts (GRB)
28) They Hit the Singularity
Given Moore's law (computing power doubles every 2 years), aliens might achieve transcendence prior to exploring the stars
29) Cloudy Skies Are Common
Delayed space exploration because of difficulties doing astronomy
30) Infinitely Many ETC's Exist But Only One Within Our Particle Horizon: Us
The particle horizon represents the farthest out it is possible for us to observe but the universe might be infinite in extent
They Do Not Exist:
This class of solutions holds that we are alone in the universe—no other ETC's.
31) The Universe Is Here for Us
If the number of "difficult" steps in the development of advanced life is too large, advance life might not appear before the parent sun becomes too unstable. For perspective, humanity appeared on early about halfway though through the sun's lifespan
Anthropic principle
32) Life Can Have Emerged Only Recently
Life cannot appear anywhere in the galaxy until certain elements build up to certain levels, so ETC's would not have started long before life started here.
33) Planetary Systems Are Rare
Some early models suggested the planetary formation required special circumstances and so would be rare but these models have been overturned
34) We Are the First
Life could not appear anywhere until sufficient quantities of certain elements had built up
35) Rocky Planets Are Rare
Some models suggested that rocky planet's like Earth may require the action of a gamma ray burst but there are more plausible models that do not require this event
36) Continuously Habitable Zones Are Narrow
The Habitable Zone refers to the distance a planet must be from the sun to maintain liquid water. The Continuously Habitable Zone is the region where liquid water is maintained for billions of years as the parent star changes in luminosity.
37) Jupiters Are Rare
Large gas giant planets may be common but they need to be in the right place and have a circular orbit to allow for habitable planets
38) Earth Has an Optimal "Pump for Evolution"
Extinction events (e.g. asteroidal collisions) make room for new life
39) The Galaxy Is a Dangerous Place
Black holes, supernova, and gamma ray bursts
40) A Planetary System Is a Dangerous Place
Snowball earth, super-volcanoes, and mass extinction events
41) Earth System of Plate Tectonics is Unique
Active plate tectonics is needed to recycle critical elements
42) The Moon is Unique
Our moon is necessary for maintaining stability, however, its formation requires very unusual circumstances
43) Life's Genesis Is Rare
44) The Prokaryote-Eukaryote Transition is Rare
Prokaryotes are the simplest organisms. Eukaryote cells are the basis for multi-cellar life
45) Toolmaking Species Are Rare
46) Technological Progress Is Not Inevitable
47) Intelligence at the Human Level Is Rare
48) Language Is Unique to Humans
49) Science Is Not Inevitable
Ancient science was developed by the Greeks (c. 500 BC) but modern science appeared late (17th-18th century)
50) Stephen Webb's Solution to the Fermi Paradox
It is likely that we are alone in the galaxy. Rather than a single solution (cause), it is likely some combination of the above listed solutions. Simple life (i.e. bacteria) may be common but we are likely the only advanced intelligent life.