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UT AST 309L - Cultural Evolution, II

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Cultural Evolution, IIEvolution of Concept of Universe“Interest” part of fcRequires the following:1. Understand the size and nature ofUniverse2. Understand place in Universe (not thecenter)3. Optimistic Drake EquationAre ability and interest linked?Both very recentIs this a coincidence?Evolution of Worldview• Early astronomy had dual nature– Calendar Astronomy (observations)• Precise calendars in agricultural societies• Dates back to at least 3800 BCE• Mayans: length of year to 0.001% accuracy– Cosmic Myth (theory)• Tied to religion, origin storiesOyster World• Flat Earth• Dome of Heavens• Waters above• Waters below• Stars fixed to the dome• Sun moves acrossLate Babylonian World• Three heavens• Underworld• Babylon is center• Created by Marduk– City god of Babylon– Sliced up Tiamat– Separated watersabove and watersbelowEgyptian World• God of the Air (Shu)• Separated his parents• Sky and Earth• Mother was sky– Unusual choiceOrigin of Natural Philosophy• Around 500 BCE, Greeks on Ionian islands• Thales and others• Search for universal substance– Tried water, air, earth, fire• Key feature is search for natural explanation• Distinguished planets from stars• Began to think about larger UniverseOne group of Greek philosophers (the Atomists)believed in other worlds.Epicurus 4th Century BCEInfinite atoms implies infinite worlds, living creaturesIn contrast: Aristotle“The world must be unique”Lucretius (Roman poet and philosopher)“It is in the highest degree unlikely that thisEarth and sky is the only one to have been created…”But even the Atomists did not have a correct visionof the nature of the Universe, starsPlato and the dominance of the circle• Plato chose the circle – the most symmetric– “And he gave the universe the figure whichis proper and natural…”– “… he made it move with circular rotation”• Both from TimaeusAristotle and the two spheres• Aristotle distinguished the two spheres– Sub-lunary• Four elements, natural motion dictated by nature• Unnatural motion requires constant force– Celestial• Quintessence• Eternal, uniform, circular motion• Crystalline spheres• Moved by Prime Mover• All motion centered on EarthSaving the phenomena• Ptolemy’s Earth Centered model– Size: 19,865 Earth Radii• No voids (but cheated)– To match observed motions of planets,• Eccentrics, epicycles, equants• Extremely contrivedPtolemy’s modelFor moving models, checkhttp://faculty.fullerton.edu/cmcconnell/Planets.html - 7Model showing Ptolemy’s “tricks” Images and Animated Prepared by Tommy Huerta and Craig McConnellAstronomy & ReligionAugustine (420 CE)Neo-platonism incorporated into ChristianityIgnore ObservationWorld-view regressedAquinas (13th Century)Aristotle incorporated into ChristianityPtolemaic systemHereticse.g. Giordano, BrunoStars are Suns with Planets, LifeTwo Thousand Years of Error“There is perhaps no other example in the history of thought ofsuch dogged, obsessional persistence in error, as the circularfallacy which bedevilled astronomy for two millennia.”Arthur Koestler, in The Sleepwalkers, pg. 58)Example: the supernova of 1054 was recorded in China,American southwest, …, but NOT in Europe. It did not fit thetheory.Copernican Model (1540)• Sun at center• All planets orbit around Sun• Circular motion• Uniform speed• To explain planets:– Still need:– Epicycles• To avoid apparent motion ofstars– Much bigger universe– 7,850,000 Earth radii– voidsTycho Brahe (late 1500s)• Before the telescope• Very large circles forsighting positions ofplanets• Observed supernova• Careful records• Hired Kepler• Compromise world viewBrahe’s Compromise World ViewKepler• Worked with Brahe’s data• Found that he could fit the Mars data if– 1. Planets moved in elliptical orbits– 2. At different speeds at different places– 3. P2 = a3• P is period (how many years to complete orbit)• a is semi-major axis ~ radius or orbit• P in Earth years, a in earth orbit radii (AU)Elliptical OrbitsThis would be an extremely elliptical orbit.In fact, orbits in Solar system are nearly circles.Galileo• Used telescope (recently invented)– New “planets” (moons of Jupiter)– Sunspots– Craters on Moon– Many more stars• New physics– No force needed to keep a body in motionNewton completes the revolution• Newton (1687, Principia)– Unifies celestial and sublunary physics– Newton’s Laws of motion– Theory of Universal Gravitation– Together these explain both• Motion of planets• Motion on EarthNewton’s Laws of Motion• 1. A body in motion tends to remain in motionunless acted upon by external forces.– Momentum (p = m v) remains constant• 2. The rate of change of momentum with time isequal to the force.– F = (dp/dt) = m (dv/dt) = m a, if m constant• 3. An object exerting a force on a second objectexperiences an equal and opposite force.– F12 = – F21Newton’s Law of Gravity• Every object with mass exerts a force onevery other object with mass.– The force is proportional to the product ofthe two masses– The force is inversely proportional to thedistance between the two masses.– F = (GM1M2)/r2The Copernican Revolution• Copernicus (heliocentric but circular)• Tycho Brahe (meticulous observations)• Kepler (ellipses, not circles!)• Galileo (constant motion needs no force)– The Earth can move but we don’t feel it.– The inquisition was not persuaded.– “eppur, si muove”• Newton (unified physics)Copernican Principle• Removal from the center– geocentric before 1543 (Copernicus)– heliocentric 1543 to 1915 (Shapley)– galactocentric 1915 to 1923 (Hubble)– nowherecentric 1923 to present (Einstein,…)• Nothing special about us• No “fine-tuning” to allow usThe Universe as we see it now• The Observable Universe (Horizon)• Very large (about 13 billion light years)• Very clumpy on “small” scales– planets, stars, galaxies, clusters, superclusters• Very empty on average– about one atom every 40 cubic meters• Expanding (galaxies moving apart)– velocity proportional to distanceEvolution of World View• Need for a correct world view– If solar system is whole Universe– No possibility of other civilizations– Need to learn how big Milky Way is– (Note that we consider only the Milky Way)• Time to communicate with other galaxies is


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