1. Fermi Paradox Where are they?
Obviously, this planet can’t support intelligent life!
ASTR 1420
Lecture 22
Section 11.3 & 13.3
I see only a little snow at the poles.

2. High probability of ETIs!
Fermi Paradox
High probability of ETIs!
No evidence yet?
High probability of advanced civilizations + large age of our Galaxy (10Gyr)
there should be wide-spread ET civilizations!
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.

3. Ancient fundamental question
There are infinite worlds, both like and unlike this world of ours.
-Epicurus ( BC)
There cannot be more worlds than one.
-Aristotle ( BC)
Epicurus taught that pleasure and pain are the measures of what is good and bad, that death is the end of the body and the soul and should therefore not be feared, that the gods do not reward or punish humans, that the universe is infinite and eternal, and that events in the world are ultimately based on the motions and interactions of atoms moving in empty space.

4. 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 one of the youngest civilizations in the Galaxy!
1st civilization might have a 10Gyr head start (if they survived)
with 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?

5. Ancient Polynesian
Space travel for us now is similar to ancient polynesian’s bold journey 1600 yrs ago!
1,600 years ago, they explored through the fierce sea and reached as far as 10,000 km on a small canoe!

6. Robotic Emissaries
 if manned mission is difficult, then a robotic mission!
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?
von Neumann : Mathematician ( )
* Frank Tipler (1981), Mathematics and Physics, “Theory of resurrection of the dead”  no von Neumann machines  no alien  SETI is a waste of time!
!But, such machineries will quickly exhaust Galactic resources from their exponential growth! Therefore, smart aliens capable of building such machine would not make them!

7. Voyager Golden Record Greetings in 55 languages
Various sounds : thunder, volcano, animals.
27 international music pieces.

8. Isn’t Galaxy too large? Galactic colonization : coral model

9. Galactic colonization
Doesn’t take that
much time!
Great size of the Galaxy and limited traveling speed cannot be barriers!

10. Coral Model of Galactic colonization
Colonization Speed
Example 1:
speed 0.1c
settling time 150 yrs
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!
Crossing the entire Milky way at the speed of light takes about 0.2Myr!
Coral Model of Galactic colonization *

11. Possible solutions to Fermi Paradox
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.

12. Earth may be the fortunate outcome of several lucky accidents!
1. Rare Earth Hypothesis
Earth may be the fortunate outcome of several lucky accidents!
Special place in the Galaxy
Galactic Habitable Zone (only 10% of space)
enough metal
too many SNs
Heavy bombardments and Jupiter
no Jupiter  many more sterilizing impacts!
Climate Stability
Plate tectonics
Large Moon

13. 2. They are here! UFOs : no scientific background (yet!)
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?
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
Sentinel Hypothesis :
they are waiting for us to grow
up and discover them.
2001 Space Odyssey
check L4 & L5 points!

14. They exist (or existed)
Have not had enough time due to technological difficulties
 unlikely! consider the age of Galaxy and our rate of technological advance!
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?
They do not want to talk!
could be that all aliens are sociologically reticent
and human is an odd ball!
 against Copernican Principle.
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.

15. Chapter/sections covered in this lecture : 11.3 & 13.3
In summary…
Important Concepts
Important Terms
Three possible solutions to the Fermi paradox
Example cases of the above
Fermi paradox
zoo hypothesis
sentinel hypothesis
rare Earth hypothesis
Chapter/sections covered in this lecture : 11.3 & 13.3
UFOs: next class!!

16. Theological Aspects of SETI signal
When presented with a hypothetical scenario about the detection of an information-rich signal from extraterrestrials, the more religious American undergraduates in the study—as compared to their less religious counter- parts—were more likely to assume that the senders of the message had hostile intentions
Also the religious group was more resistant to sending a reply message.
belief in extra- terrestrial beings is consistent with both science and Christian theology