Fermi ParadoxWhere are they?

ASTR 1420

Lecture 22

Section 11.3 & 13.3

I 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) 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.

High probability of ETIs! No evidence 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 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?

Ancient PolynesianSpace travel for us now is similar to ancient polynesian’s bold journey 1600 yrs ago!

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?

Voyager Golden Record

• Greetings in 55 languages• Various sounds : thunder, volcano, animals.• 27 international music pieces.

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

Galactic colonization

• Doesn’t take that much time!

Great size of the Galaxy and limited

traveling speed cannot be barriers!

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 yrsexpansion speed of 0.001c takes only 100 Myrs!

Coral Model of Galactic colonization

Crossing the entire Milky way at the speed of light takes about 0.2Myr!

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.

1. Rare Earth Hypothesis

Earth may be the fortunate outcome of several lucky accidents!

1. Special place in the Galaxy o Galactic Habitable Zone (only 10% of space)

enough metal too many SNs

2. Heavy bombardments and Jupitero no Jupiter many more sterilizing impacts!

3. Climate Stabilityo Plate tectonicso Large Moon

2. 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!!