Week 10 Notes
Week 10 Notes GE 70B
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This 5 page Class Notes was uploaded by Sarah Doberneck on Saturday March 12, 2016. The Class Notes belongs to GE 70B at University of California - Los Angeles taught by Dr. Friscia in Winter 2016. Since its upload, it has received 35 views. For similar materials see Evolution of Life and the Cosmos in General at University of California - Los Angeles.
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Date Created: 03/12/16
Well what makes a planet habitable? • All life on Earth is based on Carbon chemistry with water as the principle solvent. • Hydrogen, Oxygen, Carbon, and Nitrogen are by far the most common elements everywhere making water the most common molecule after H2 and carbon the most common building block. • Good chance that the most common life forms are cellular organisms (to isolate their chemistry from the environment) with some form of carriers of heredity (like RNA or DNA) using C,H,O and N. • Other forms might be possible, but that won’t affect our fractions much. Which Stars make good Suns? • They must be old enough so that life could arise (Possibly a billion years) – this rules out the massive O & B main sequence stars • They must allow for stable planetary orbits – this may rule out many binary and multiple stars (about half) • They must have relatively large habitable zones – region where large terrestrial planets could have a surface temperature that allows water to exist as a liquid And even good suns don’t stay good. • In 7 billion years, our Sun will swell into a red giant roughly 200 times its present size. • Tidal effects on the Earth will have caused us to slightly spiral inward and we will be engulfed by the outer envelope of the swollen sun. fhp= • So even if 1/5th of stars like the sun have planets in a “habitable zone”, many other stars don’t for age and stability reasons. • And for 5/6ths of the age of our solar system, the Earth isn’t habitable. • So for a round number let’s take fhabitable=0.01 so 1% of stars have habitable planets now. flife? • So what is the fraction of habitable planets where life actually forms? • Detection of life or fossils on Mars or Europa would make this fraction basically 1. • No discovery makes this fraction very small. • flife could be 1 or could be 1/1,000,000 or smaller • Let’s choose flife=0.1. • So fhabitableflife=0.001 – 1/1000 stars have a planet with primitive life. On what fraction of living worlds do civilizations develop? fciv • Again, we have only 1 example (1 for 1, or 1 in a billion)? • Some are not optimistic. They point out that only one branch of bacteria has ever made it to multicellular status. – Perhaps all complex life evolved from only a single lucky bacteria. – Perhaps most living worlds are just covered in bacteria. • Europa, Titan, Enceladus, Mars, Ceres, Ganymede all have some suspicion. • Let’s assume fciv is only 0.01 (1 in a 100 living worlds develops a civilization during 2 Billion years) Civilizations out there now? fnow • Carl Sagan and others pointed out that technical civilizations may be short lived. • We might destroy ourselves – Nuclear War – Biological warfare – Run out of resources (over-population) – Run away greenhouse • Or we are “naturally” destroyed – Natural diseases – Nearby supernova – Major asteroid impact • If lifespan is 100 years, then fnow=0.00000005 • But could be fnow>0.005 (10 million year life out of 2 Billion year window of habitability) Why is Interstellar Travel Difficult? • The distances between the stars are HUGE! Nearest star is a little over a parsec away (1 pc =3.12 ly) • We will most likely be limited by the speed of light. • Our current interstellar spacecraft, Pioneers 10 & 11 and Voyagers 1 & 2, will take 10,000 yrs to travel 1 light-year. These are the fastest manmade objects ever produced (up to 106,000 miles/hour-~30 miles/sec) • Our spacecraft need to go 10,000 times faster in order to travel to the stars within human lifetimes • This will require new types of engines and new energy sources. • Accelerating the USS Enterprise to half the speed of light would require 2,000 times the total annual energy output of the entire world and still take 8 years to get to nearest star. Starship Propulsion • Chemical rockets are impractical for interstellar travel. o Only tiny fraction of mass is converted to energy, so speeds will always be limited to tiny fractions of c. o Just to get to the moon, the Apollo rockets weighed 62 times as much as the final lander. Mostly fuel. o A 100 stage rocket could reach 0.1% of c. o Would still take 4,000 years to reach alpha centauri. • Nuclear powered ships produce more energy per kg of fuel. o Two designs, employing fission & fusion, have been studied o Fission reactors are good for getting around the solar system (JIMO) o NASA has had multiple fission programs including one designed to carry men to mars. Developed in the 1960’s it involved more than 1800 scientists and engineers. o Fission would still take many centuries to reach Alpha Centauri. • Fusion power (star power) o Project ORION was first true starship design (Los Alamos Nuclear Lab – 1960s). o Mile long craft would drop small fusion bombs out the back. They explode and push the spaceship. o In theory, it could reach alpha centauri in a century. o Cancelled in 1965. Incredibly ambitious, but requires no radically new technology • Ships that do not carry their own feul o Solar sails can harness the photon pressure from sunlight, good for solar system but would require a powerful laser to reach high speeds • Starship Propulsion o Most fundamental problems might be radiation shielding needed for deep space, and problems of running into things along the way. – At 10% of the speed of light, even a dust grain can pack a punch. o A small marshmallow weighs 0.1 grams. If it hits a spaceship at a velocity of 0.1c, the energy is 1/2mv2=45x109 Joules or about the energy in 45,000 sticks of dynamite. o So might be better to go a bit slower and have some kind of shield. Galactic Colonization? • With modest improvements in technology it is plausible that… • Within a few millennia, out of curiosity or boredom, we could visit/colonize a few nearby stars. • An ORION starship could carry several thousand colonists to another world (1000 years) • Hybernate, breed along the way, only carry embryos, artificial life, many possibilities… • In <1000 years that colony would start sendin gout its own colonies, and we could send out others as well • At any given time, the edge of our colonial empire would be travelling at about 1 star system per 1000 years, or about 1/200th the speed of light (~5 light years between stars) • In 1,000,000 years, such a colonial system would expand to 5,000 light years in radius. • In 10 million years, we’d reach the opposite side of the Galaxy Where are the Aliens? • But there are stars like the sun that are 5 Billion years older! • IF there are other civilizations, the first could have arisen more than 5 billion years ago • There should be many civilizations which are millions or billions of years ahead of us • They have had plenty of time to completely colonize the Galaxy. • So again…where is everybody? Why haven’t they visited us? • This is known as Fermi’s paradox • Named after physicist Enrico Fermi, who first asked the question in 1950 • Fermi also produced first controlled nuclear reaction. • Stronger version – Time to evolve a new civilization is longer than the time to colonize the entire Galaxy. • So it should be winner takes all! Search for ExtraTerrestrial Intelligence • Maybe travel is much harder than we think. What about communication instead of spaceships. • That is the idea behind the SETI program. • Use radio telescopes to listen for encoded radio signals. • search strategies are used to decide which stars to observe • now they scan millions of frequencies at once • Due to 50+ years of no detections and large amount of time required, SETI is now privately funded. Project Phoenix (largest program to date) ended March 5, 2004. • Searched all 710 star systems within 150 light- years of Earth. They searched 70 million radio channels simultaneously. Nothing! • “Does imply there are not large numbers of civilizations transmitting at many frequencies” Frank Drake (2004) • Other programs: SERENDIP, META, BETA have tried other strategies: Nothing! Solar Motion • The sun and other stars are moving at about 220 km/sec around the Galaxy. • But we aren’t moving identically. Typical stars move 10 or 20 km/sec faster or slower than this average rate. • Sun moves 16.5 km/sec compared to local average Changing Neighbors • So each year the sun moves 1.7x10-5 pc compared to the average neighbor. • In a Billion years we cover 17,000 pc through our neighboring stars. • With 0.4 stars/pc3 locally, this leads to 200,000,000 stars which have passed within 100 pc of the Earth during the last Billion years (close enough for proposed life detecting telescopes). • >400,000,000 stars have past by since the atmosphere became oxygenic! – Each was that close for an average of 6 million years. • But no one appears to have stopped by. Could we be aliens? • Fossil record shows a direct link from extremely primitive life to us. • Some have suggested that the early earth was seeded with primitive life. • But why not just send intelligent colonists or robots? • To date, there are no artifacts on the moon, Mars or the Earth of any previous intelligence. – Most of mars and moon have now been mapped to roughly boulder sized objects. Little if any erosion on the moon during the past 4 Billion years. No monoliths. Possible Solutions to Fermi’s Paradox • We are alone: o – One or more of our fractions is way too optimistic. o – Earths are much rarer than we think. o • Moon needed to stabilize rotation. Otherwise our axis would often tip over creating climate extremes. o • Jupiter needed to sweep inner solar system of debris. Otherwise bombardments would cause 100 times more mass extinctions. o • Most solar systems are dynamically unstable so orbits change. – Life almost never becomes complex or rarely starts. Lucky bacteria theory o Civilizations are extremely rare and we are the first one to arise. o Consequences: We are unique, the first components of the Galaxy to attain self-awareness • No one colonizes or signals o Perhaps interstellar travel is much harder or costlier than we imagine o But EVERY civilization must have failed over all time periods. o Perhaps most civilizations have no desire to travel or colonize o But for this to work, ALL civilizations would need to avoid colonization. o Maybe they have better things to do… o Perhaps most civilizations have destroyed themselves before they could explore o But surely some intelligent life evolves from peaceful herbivores, or even photosynthetic life. o Consequences: We will never explore the stars, because it is impossible, we will do something else or we will destroy ourselves. • They deliberately conceal themselves o They are Billions of years more advanced than us. They have solved energy problems beyond our wildest dreams. They have solved every problem we’ve never even thought of. o We are the Galaxy’s rookies. o They have some kind of prime directive to not disturb the cute humans (anthropology or zoo) • Consequences: We are on the verge of a great adventure.
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