Fermi ParadoxWhere are they?

FYOS

Lecture 11I see only a

little snow at the poles.

Obviously, this planet can’t

support intelligent life!

Fermi Paradox

• 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.

High probability of ETIs! No contact yet??

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)

Ancient fundamental question

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!

Possible solutions to Fermi Paradox

1. They do not exist “Rare Earth Hypothesis”2. They are here, but we have not realized it yet.3. 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 Stabilityo Plate tectonicso 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 growup and discover them.2001 Space Odysseycheck 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. 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

3. 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• Three possible solutions to the

Fermi paradox

• Example cases of the above

Important Terms• Fermi paradox

• zoo hypothesis• sentinel hypothesis• rare Earth hypothesis

Chapter/sections covered in this lecture : 11.3 & 13.3UFOs: next class!!