Fermi Paradox
Where are they?
FYOS
Lecture 11
I see only a
little snow at
the poles.
Obviously, this
planet can’t
support
intelligent life!
Fermi Paradox
High probability of
ETIs!
No contact yet??
• High probability of advanced civilizations + large age of our Galaxy (10Gyr)
 wide spread ET civilization!
• There should be a widespread evidence of them! However, we don’t see
them. Where is everybody?
The Fermi Paradox is
the apparent contradiction between the high probability of extraterrestrial
civilizations' existence and the lack of contact with such civilizations.
Ancient fundamental question
There are infinite worlds, both like and
unlike this world of ours.
-Epicurus (341-270 BC)
There cannot be more worlds than one.
-Aristotle (384-322 BC)
Many advanced Galactic civilizations
• Earth is only 4.5 Gyr old and we are only 10,000 yrs old.
• Our Galaxy is 10 Gyr old
 we are the youngest civilization in the Galaxy!
 1st civilization(s) had a 10Gyr head start (if they survived)
with a modest technology and imperative need for more resources, they
would have started a Galactic colonization.
So, the real question is
Where are super-advanced aliens?
Robotic Emissaries
• Aliens even with a modest
level of technology could
send a self-replicating
robotic messengers
throughout the Galaxy!
von Neumann probes
self-replicating machine using
the local resource to duplicate
itself and embark on a new
journey to next stars!
Similar to our voyager and
pioneer spacecrafts!
But, we don’t see them!
Solution?
Ancient Polynesian
Space travel to us is similar to ancient polynesian’s bold journey!
Colonization Speed
Great size and limited traveling speed
cannot be barriers!
Example 1:
• speed 0.1c
• settling time 150 yrs
Coral Model of Galactic colonization
 expansion speed of 0.01c.
 It takes only 10 Myrs!
Example 2:
• speed 0.01c
• settling time 5000 yrs
expansion speed of 0.001c
 takes only 100 Myrs!
Galactic colonization
Galactic colonization
• Doesn’t take that
much time!
Possible solutions to Fermi Paradox
1.
2.
3.
They do not exist  “Rare Earth Hypothesis”
They are here, but we have not realized it yet.
They exist, but we couldn’t make a contact yet.
Rare Earth Hypothesis
Earth may be the fortunate outcome of several lucky accidents!
1.
Special place in the Galaxy
o
Galactic Habitable Zone

enough metal

too many SNs
2.
Heavy bombardments and Jupiter
3.
Climate Stability
o
o
Plate tectonics
Large Moon
They are here!
1. UFOs : no scientific background (yet!)
2. We are one of them: We are the descendents of ancient alien civilizations.
 Where are the original aliens? Where are all the other alien civilizations?
3. Zoo hypothesis: We are in a cosmic “wild life refuge”!  aliens have a
cosmic treaty against making contacts with infant civilizations
 “Prime Directive” of StarTrek
4. Sentinel Hypothesis :
they are waiting for us to grow
up and discover them.
2001 Space Odyssey
check L4 & L5 points!
They exist (or existed)
1.
Have not had enough time due to technological difficulties
 unlikely! consider the age of Galaxy and our rate of technological advance!
2.
3.
Different communication technology : Electromagnetic waves, gravity
waves, exotic particles (e.g., tachion). But, if there are many of them,
some will use EM.  need more surveys done via SETI
Galaxy filled with killer robots!
 where are killer robots coming after us?
4.
They do not want to talk!
could be that all aliens are sociologically reticent
and human is an odd ball!
 against Copernican Principle.
5.
They were all self-destructed!
self destruction technologies obtainable nearly at the same time as radio
communication and inter-stellar travel technologies!
Ex) nuclear warfare, over population, nanobots (gray goo problem), dangerous
particle experiment, etc.
In summary…
Important Concepts
Important Terms
• Three possible solutions to the
Fermi paradox
• Fermi paradox
• Example cases of the above
• zoo hypothesis
• sentinel hypothesis
• rare Earth hypothesis
Chapter/sections covered in this lecture : 11.3 & 13.3
UFOs: next class!!