1. THE FERMI PARADOX First articulated by Enrico Fermi in 1950. If there is a high possibility for ETL (extra terrestrial life) then where is everybody??? Given: -planets (proto=planetary disks) are common -life originated early on Earth, is easy to evolve -only a matter of time before intelligence arises -the vastness of the universe 100 billion stars in the Milky Way galaxy 100 billion galaxies in the universe -the tremendous age of the universe Then: -expect to have a large number of civilizations. It is only a matter of time before they develop the ability for intergalactic travel.

2. THE FERMI PARADOX, CONT. However there is no evidence of galactic colonization. Herein lies the paradox. Maybe we are the first? We have already built robots to travel to other planets in the solar system. It is reasonable that we will keep building better robots for exploration of our solar system. It didn’t take a great deal of technology to land on the Moon. We (beings) don’t have to colonize - machines we construct could do the job.

3. Von Neumann Machines Self-replicating machines. Travel to and explore other worlds. Dig up resources. Use those resources to build more robots. Disseminate and colonize additional worlds. Would spread from star system to star system. Requires technology only slightly more advanced than our own.

4. Extension of the Fermi Paradox In 1981, Frank Tipler used the idea of colonization by self-replicating Von Neumann machines to argue that machines would spread throughout the galaxy as soon as any civilization reaches a level to build these machines. Because it doesn’t take much more technological capability than what we already have. And if civilizations are common. The universe should be overrun by self-replicating machines.

5. HOW LIKELY IS IT THAT WE ARE FIRST? Suppose intelligence arises around 1 in one million stars. Age of galaxy = 12 billion years but you have to wait a few billion years For a sufficient number of supernova to deliver heavy elements To the galaxy. Indeed, this is borne out by observations of planetary Systems around nearby stars: Given 100 billion stars in the galaxy, 100,000 should evolve civilizations. If the first of these arose 6 Ga then an intelligent civilization should be arising somewhere in the galaxy every 60,000 years.

6. How Long Would It Take To Colonize The Galaxy? If: -you could travel at 10% the speed of light, 0.1 c (3 x 10 7 m/sec) And: The average distance between stars is 5 light years (50 years) And: After 150 years you can spread to the next system, sending new craft to one or two other systems. Then: You could colonize the entire galaxy in 10 million years if you start at the edge of the galaxy.

7. One day, some civilization decides to use its resources to launch 1000 inhabited space ships in random directions. Each spaceship is capable of a propulsion velocity of 0.1 c. 99% of all spaceships are destroyed before reaching another planet Upon arriving at another planet, on average 10,000 years later, the surviving members of the civilization spend 10,000 more years on that planet converting its resources into 1000 more spaceships to repeat this exercise.

8. GALACTIC COLONIZATION, CONT. Would other civilizations want to colonize? Right now we have no motivation - the costs outweigh the need. However, the human desire for colonization is strong - filling every niche. Reasons for colonization: - escape war - escape persecution - lack of resources - too much competition for resources - protect your lineage/civilization from extinction Stupid Civilizations can’t colonize because they use up all their resources first

9. POSSIBLE SOLUTIONS TO THE FERMI PARADOX 1.We are alone / we are the first / there is no galactic civilization. 2.Civilizations are common, but no one has colonized the galaxy. Why? - technological difficulties - interstellar travel is difficult or vastly more expensive or dangerous than we think - maybe our desire to explore is unusual and other societies would not choose to leave their stars - maybe civilizations tend to destroy themselves 3.There is a galactic civilization and it is deliberately avoiding us / hiding their existence from us / or we just haven’t found them yet (haven’t looked at enough stars??)

10. The Prime Directive / The Zoo Hypothesis Other civilizations are aware of our presence, but they have deliberately kept us in the dark about their existence. Or they have kept our solar system “off limits” to leave us alone. The Sentinel Hypothesis: The monolith in Arthur C. Clarke’s “2001: A Space Odyssey” ETs leave a device that sends a signal when our civilization becomes sophisticated enough.