[ExI] taxonomy for fermi paradox fans

BillK pharos at gmail.com
Tue Jan 27 19:15:46 UTC 2015

On 27 January 2015 at 18:44, Dan wrote:
> It's been a while since I read Webb's book, but, if memory serves, he
> examined fifty different solutions -- pro and con there being intelligent
> life not from Earth -- and ended by concluding N=1. It was more like an FAQ
> sandwiched between book covers than an extended essay on the subject --
> again, to my recollection.

Here is Stephen Webb's 50 list --------------



Where is Everybody?
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

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

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!


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

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

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 requite the
action of a gamma ray burster 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.


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