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Outline

Review

Rare

Intelligent Life Homework due Saturday morning 9 am Exam Tuesday 11:30 am Quantities Some knowledge Unknown Intelligent Life SETI I Drake equation Travel Fermi paradox I SETI Posed Solutions I Energetics of

I Fermi Paradox Outline

Review

Rare Earth

Intelligent Life

Drake Equation Quantities Some knowledge Review: Life (not intelligent) Unknown SETI

Travel

Fermi paradox Posed Solutions Outline Question concepts Review

Question #1: Life formed on Earth quite early, shortly after Intelligent Life bombardment ceased 3.5-3.8 billion years ago. Drake Equation Quantities Question #2: Microbial life is thought to be common in the Some knowledge Galaxy because it arose early on Earth, and Earth-like Unknown SETI are thought to be common. Intelligent life, in Travel contrast, is thought to be rare because we have detected no Fermi paradox evidence other despite looking. Posed Solutions Question #3: Of the basic necessities for the formation of life - energy, carbon compounds, and liquid water - liquid water is the least common. In what ways might the Earth be rare?

Possible unique things about Outline Earth/ Review Rare Earth

I position in galaxy Intelligent Life I Jupiter reducing asteroid Drake Equation Quantities impacts Some knowledge Unknown I Stable climate SETI

I Large Moon Travel

I Fermi paradox Posed But each of these might also Solutions be fairly general or only – regions modestly unlikely features with enough heavy element enrichment but not too much Also some might only supernova radiation adversely affect complex (multi-cellular) life Outline

Review

Rare Earth

Intelligent Life Intelligent Life Drake Equation Quantities I Drake equation Some knowledge SETI Unknown I SETI I Energetics of interstellar travel Travel

I Fermi Paradox Fermi paradox Posed Solutions Quantifying ignorance

Outline

Review Drake equation – how to think about possibility of other Rare Earth

intelligent life (short version) Intelligent Life

Number of civilizations we could detect Drake Equation Quantities Some knowledge = ((NHP × flife) × fciv) × fnow Unknown SETI

Travel I N – Number of habitable planets (in galaxy) HP Fermi paradox Posed I flife – fraction of which develop life Solutions I fciv – fraction of which develop civilizations

I fnow – fraction of which are currently ”transmitting” Each of these will be less than 1, decreasing total number for more strict conditions Factors with some knowledge

Outline

Review flife – fraction of habitable Rare Earth that develop life Intelligent Life Drake Equation I Assumes is in Quantities Some knowledge habitable zone Unknown

I Earth developed life very SETI early - but sample of 1 Travel Fermi paradox NHP – Number of habitable planets doesn’t really prove Posed in galaxy anything Solutions

I Many with a habitable I Basic necessities for zone simple life are common I Rocky planets appear to be common (proven in 2012!) I Maybe near 1 - might know soon from mars I About half of stars are in binaries (their planets?) I Easily billions in Galaxy Factors with little knowledge

fciv – fraction of planets with life that develop Outline Review I Took billions of years for complex Earth life Rare Earth Brain/body ratio: Intelligent Life I But perhaps complex life is inevitable, just need time Drake Equation Quantities Some knowledge I Actual civilization didn’t Unknown happen until very recently SETI I Could be near 1 or almost Travel zero Fermi paradox Posed Solutions fnow – fraction of civilizations currently transmitting I We have not been Is a large brain sufficient? transmitting very long (about probably not 50 years) I Civilizations might ”fail” and stop transmitting I Again entirely unknown Outline

Review

Rare Earth

Intelligent Life Question concepts Drake Equation Quantities Some knowledge Question #4: The Drake equation is useful for discussing Unknown how common intelligent life might be but it has limited SETI usefulness for calculating this because several critical terms Travel in it are wildly uncertain. Fermi paradox Posed Solutions Search for Extraterrestrial (SETI)

Outline

Search for radio emissions Review

from other systems Rare Earth

Intelligent Life Assumes ET usage of radio Drake Equation for long-range communication Quantities Some knowledge (like us) Unknown SETI I Omnidirectional Travel

I Directed at us Fermi paradox Posed Solutions We don’t emit as much clear Very laborious to search many radio signals as we used to – systems with good sensitivity now more used for low-power communication – looks like Unknown how long a noise from far away civilization might generate these signals Outline

Review

Rare Earth

Intelligent Life Question concepts Drake Equation Quantities Some knowledge Question #5: One of the critical assumptions of SETI as it Unknown has been conducted so far is that civilizations will commonly SETI use radio signals to communicate. Travel Fermi paradox Posed Solutions Interstellar Travel and Energy

Outline Need to accelerate fuel for Review Basic relativity: infinite energy further acceleration – major Rare Earth required to reach speed of limitation on speed Intelligent Life Drake Equation light Quantities Proposals have been Some knowledge developed to do this with Unknown SETI existing (atomic) technology – Travel can reach few % of lightspeed Fermi paradox Posed Unclear if such a large Solutions resource expenditiure is viable. However, Propulsion by repeated given that workable design nuclear explosions exists, it seems likly this will be accomblished if economically/socially viable. Outline

Review

Rare Earth

Intelligent Life Question concepts Drake Equation Quantities Some knowledge Question #6: Because we have starship concepts that could Unknown be built, though at great cost, interstellar travel is thought SETI to be difficult but possible. Travel Fermi paradox Posed Solutions Where is everyone else?

Outline Assumption: Once interstellar colonization begins... Review only a few million years or so to fill galaxy Rare Earth – even with fairly slow travel (few % of light) Intelligent Life Drake Equation Quantities Implies that first civilization Some knowledge Unknown to develop interstellar travel SETI

will fill galaxy ”quickly” – i.e. Travel

in much less time than Fermi paradox Posed development time Solutions Obvious question is: where are they?? they should be everywhere This is called the Fermi Paradox Possible solutions to the Paradox

Not all happy... Outline Can reconcile with the Review absence of galactic civilization Rare Earth in several possible ways: Intelligent Life Drake Equation Quantities I We are the first – and Some knowledge only Unknown SETI I Colonization is hard Travel

I interstellar travel is Fermi paradox actually hard Posed Solutions I interstellar travel not persued I self-destruction is common

I There is a Galactic civilization – we just don’t know it yet Outline

Review

Rare Earth Question concepts Intelligent Life Drake Equation Quantities Question #7: Microbial life is thought to perhaps be Some knowledge common in the Galaxy because it arose early on Earth, and Unknown SETI Earth-like planets are thought to be common. Intelligent life, Travel in contrast, is thought to be rare because we have detected Fermi paradox no evidence other civilizations despite looking. Posed Solutions