The History and Philosophyof Astronomy (Lecture 14: Newton)Instructor: Volker BrommTA: Jarrett JohnsonThe University of Texas at AustinAstronomy 350L (Fall 2006)Isaac Newton: Founding Father of Physics • 1642 (Woolsthorpe) – 1727 (London)• Principia MathematicaPhilosophiae Naturalis(“Mathematical Principles ofNatural Philosophy”, 1687)- universal gravity (inverse-square law)- three laws of motion• invented calculus(differentiation and integration)Newton: Timeline and Context • building upon Galileo, Kepler, and Descartes• completes Copernican Revolution!DescartesNewton: Geography of his Life N D1642: Birth in Woolsthorpe• born in rural Lincolnshire• father died before his birth (‘posthumous child’)1642 –49: The English Civil War • bitter struggle between King (Charles I Stuart)and Parliament (“Cavaliers” vs “Roundheads”)• King desires to rule without Parliament1649: Execution of the King • King Charles I (Stuart) beheaded1642 –49: The English Civil War • Victory for Parliament• Republic (“Commonwealth”)• Oliver Cromwell (1599-1658)- Lord Protector• Anarchy after his death• Army recalls son of former (executed) kingfrom exile1660: The Restoration • Return of the Stuarts: Charles II (son of behead king)London Coffee-House Culture • New venue for meetings of intellectualsThe Royal Society of London • founded 1660: institution to foster exchange ofscientific knowledgeGreshamCollegePhilosophical Transactions • published by Royal Society• first scientific journal• a public registry of newscientific ideas• professionalization of science1661: Newton enters Cambridge University • one of oldest universities in the world1661: Newton admitted to Trinity College • admitted as “subsizar” (has to perform menial duties for older or richer students)Student in Cambridge (1661-65) • Study Descartes’mechanical philosophy!• Principia Philosophiae(1644)• No vacuum, no atoms!• Force by direct contact(pressure and tension)Student in Cambridge (1661-65) • Study all the mathematicsthat there is to know!• John Wallis ArithmeticaInfinitorum(1656): - predecessor of integralcalculus- introduces symbol forinfinity ( )John Wallis, 1616-170381665: The Great Plague ~100,000 dead in London1666: The Great Fire of LondonChristopher Wren: England’s Greatest Architect • 1632 – 1723• Rebuilt London after Great Fire of 1666• > 50 new churches• St Paul’s Cathedral• Savilian professor ofastronomy at OxfordWren: Rebuilding London • St Paul’s CathedralNewton during Plague Year: AnnusMirabilis • Return to Woolsthorpe• 3 Great Discoveries:- Calculus- Nature of Light- Universal Gravity “The Miraculous Year”(1665-66)AnnusMirabilis I: Calculus DifferentiationIntegrationyx• independently discovered by Leibniz in Germany(giving rise to ugly priority dispute later on…)AnnusMirabilis II: Optics • white light is composed of different colors!AnnusMirabilis III: Universal Gravity • Newton asks: What if the same force (gravity) causesfall of apple and keeps Moon in orbit around Earth???AnnusMirabilis III: Universal Gravity • Moon is constantly falling toward Earth (as is apple)! F~v2/rCentripetal forcevrAnnusMirabilis III: Universal Gravity • Earth’s gravitational pull is ~ 1/3600 weakerat location of Moon compared to surface (apple)!AnnusMirabilis III: Universal Gravity • Intuitive nature of inverse-square law!(compare to dilution of light over growing surface)Newton’s Return to Cambridge• 1669: Lucasian Professor for MathematicsIsaac Barrow: 1stLucasian ProfessorStephen Hawking: 17thLucasian ProfessorNewton’s Return to Cambridge • 1671: Design for new (reflecting) telescope • earns him membership (as fellow) in Royal SocietyEn Route to the Principia • Newton was very reluctant to publish! • Edmond Halley (1656-1742)• member of Royal Society• Halley’s Comet• first astronomer to observeSouthern Sky (from St Helena)• Convinced Newton to publish PrincipiaNewton’s Principia (1687) • Philosophiae NaturalisPrincipia Mathematica(Mathematical Principles of Natural Philosophy)• Challenges Descartes’Principia Philosophiae (1644)- Descartes: qualitative- Newton: quantitative, predictive• The foundational text formodern physics and astronomy!Newton’s Principia: Overall Structure Book 1: Basic Laws Book 2: Demolition of Cartesian System Book 3: System of the World (applications)Principia Book 1: Basic Laws • Scholium: Absolute Space and Time“Absolute space…without relation to anythingexternal, remains always similar and immovable”“Absolute, true, and mathematical time, of itself,…flows equably without relation to anything external ”• Passive stage for all motionPrincipia Book 1: Basic Laws • Newton’s bucketexperimentPrincipia Book 1: Basic Laws • Newton’s 1stLaw of Motion:• Law of inertia (Galileo, Descartes)• State of rest and uniform motion are equivalent(relative to observer)Principia Book 1: Basic Laws • Newton’s 2ndLaw of Motion:• Same force (F) exerted on a larger mass (m)produces a correspondingly smaller acceleration (a)Force = mass x accelerationPrincipia Book 1: Basic Laws • Newton’s 3rdLaw of Motion:Force = Counter-forcePrincipia Book 1: Basic Laws • Newton’s Law of Gravity:• Gravity is universal and follows inverse-square law!Principia Book 2: Demolish Descartes • Newton proves mathematically:- Descartes’ vortex motion cannot be sustained!- it would soon stop because of friction(objects have to move through `plenum’)Principia Book 3: New System of the World • Newton’s laws celestial motions (ellipse, parabola,hyperbola,