Lecture 38. The Fermi Paradox, Von Neumann Machines, Galactic Colonization. reading: Chapter 13

‘When he submitted his famous paper on beta decay to the prestigious journal Nature, the journal's editor turned it down because "it contained speculations which were too remote from reality". Thus, Fermi saw the theory published in Italian and in German before it was published in English. beta decayNature editor Italian German English He never forgot this experience of being ahead of his time, and used to tell his protégés: "Never be first; try to be second".’ Enrico Fermi Built the first nuclear reactor (under Stagg Field at the University fo Chicago), demonstrated the concept of the nuclear chain reaction in Novel Prize in Physics for work on induced radioactivity.

The Fermi Paradox First articulated by Enrico Fermi in If there is a high possibility for ETL then where is everybody??? Given: -planets (protoplanetary 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.

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.

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. Why would robots be better to use than humans for this task??

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.

Von Neumann Machines and SETI So, if there aren’t Von Neumann Machines all over the universe, we must be alone. Therefore, is SETI a waste of time/money???

How Likely Is It That We Are First? Intelligent civilization on Earth has only been around for ~4,000 years/3.8 Ga. Perhaps it takes 4.5 Ga to evolve intelligent civilizations. Suppose intelligence arises around 1/1 million stars. Given 100 billion stars in the galaxy, 100,000 should evolve civilizations. If the first of these arose 6 Ga (age of the solar system Ga) then an intelligent civilization should be arising somewhere in the galaxy every 60,000 years. (If you use 1/1 billion stars, then you expect 100 civilizations, or one arising every 60 million years).

How Likely Is It That We Are First?

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. If you travel at 0.01 c, and it takes 5,000 years between hops it would only take 100 million years to colonize the entire galaxy.

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

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??)

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.

Lecture 39. Does Life Elsewhere Seem Likely? The Psychological Impact of Finding Life Elsewhere. reading: Chapter 14