Lect. 12 Summary, Clas. Phys. - Michelson-Morley Exp. Summary of Classical Physics Announcements Start the Revolutions of Modern Physics • Today: Summary of Classical Physics Classical Physics in 1880’s • The beginning of a new scientific revolution • Does the earth move? The Michelson-Morley Experiment Is Conceptual Physics finished? Motion through • Lightman Ch 3, March Ch 8 the Ether? Only details left? • Next Time: Einstein and the Birth of Relativity mirror • Lightman Ch 3, March Ch 9 What does a scientist do? half-silvered Seek to: mirror Ask the right question. • Give out Homework 6 – due Wed., Oct. 22 mirror Find the crucial experiment light • Homework 5 due Wednesday, Oct. 15 that provides a definitive test of the theory Michelson-Morley Experiment Summary of Classical Physics – I Timeline Middle “Modern” Asia, Egypt Greece, Rome Renaissance Ages Physics • Physics as it stood near the end of the 19th Century Mesopotamia Al-Khawarizmi Copernicus • Fundamental quantities (Primitives): • Time flows the same everywhere for all observers 0 -1000 1000 2000 • Space is described by 3 dimensions (Euclidean Geometry) Plato Ptolomy • Mass is never created nor destroyed (conserved) Fibanacci Galileo Aristotle Erastosthenes • Charge (plus and minus) total is conserved – defined by force Kepler Euclid Aristarchus Franklin Maxwell • Units for primitives (standards) Ampere Newton Coulomb Faraday • Time - second – defined by standard clock in Paris – other clocks Volta are brought to Paris; if they agree with the standard, they become secondary standards; it is assumed that each measures “time” valid for everyone 1600 1700 1800 1900 • Space - meter – defined by standard meter in Paris • “Classical Physics” was complete around 1880 • Mass - kilogram – defined by standard kilogram in Paris • See Timeline description of lives of various • Charge - Coulomb – defined by standard kilogram in Paris scientists on WWW pages. Summary of Classical Physics – Ia Summary of Classical Physics – II • Physics near the end of 19th Century (Continued) • Physics as it stood near the end of the 19th Century • Fundamental Objects: • Derived quantities: • Particles have mass and move according to Newton’s laws • baseballs, rockets, ….. • Velocity – directly defined by space and time • Waves are moving patterns in a medium • Acceleration – directly defined by space and time • Sound, Light, ….. • Force originates in interactions between particles of matter • Energy changes form but is conserved • Momentum is conserved • Waves exhibit interference • …… • Particles do not exhibit interference 1
Lect. 12 Summary, Clas. Phys. - Michelson-Morley Exp. Summary of Classical Physics - III Summary of Classical Physics - IV • Physics near the end of 19th Century (Continued) • Physics near the end of 19th Century (Continued) • Laws that describe particles and waves were • Question 1: How does one know these laws and formulated by concepts are “true” ? • Newton – three laws + law of gravity describe motion of particles • What does “true” mean in science? • Maxwell – four laws describe all electromagnetic effects The law applies to nature. including light • How does one “know” ? • First and second laws of thermodynamics describe heat and irreversible behavior By careful, reproducible experiments • Fundamental physics appeared finished – The laws • The laws give a framework - a paradigm - that allows appear to be so comprehensive that all that questions to be asked and answered by experiment remained was more precise measurements and new -- Is the set of laws internally consistent? discoveries of force laws -- Does the law apply within the accuracy of the • The laws are deterministic - If one could measure experiment? the positions of all objects at one time the future would be completely determined Summary of Classical Physics – V What does a scientist do? • Ask clear well-defined questions that: • Physics near the end of 19th Century (Continued) Address the fundamental issues - the foundations • Question 2: To what extent have the laws of Can be tested by decisive experiments classical physics passed the tests? • Already deep issues can be seen. Recall: • By the 1880’s they appeared to pass every test Galileo’s principle of relativity (also called superposition) – all attempted, but there are limitations: velocities are relative – no experiment can detect absolute • Many things were not (yet?) explained …… motion – no experiment can determine whether or not the earth • Accuracy of measurements could only test each of the is moving at a constant velocity conservation laws (mass, energy, momentum) to some • Can this be tested using sensitive experiments and level what has been learned about light? • New experiments were becoming possible • By careful formulation of questions and consideration of to small details, complete revolutions in science emerged - with all their consequences for humanity Toward the question: What is light? Can we detect absolute motion • According to Galileo, Newton all motion is relative • Proposed to be an wave in the “ether” • Also our common sense – examples …… • Ether: Substance that permeates all space • But waves present another possibility All waves known by physicists in 1800’s traveled Matter (planets, ….) can move through the either through some medium at a fixed speed in that with no resistance medium • Sound waves in air (around 340 m/s) Yet the ether must be very tight tom transmit light at • Waves on a string (depends on string) extremely high speeds Waves on water • An experiment can detect motion relative to this • If it exists, this is the universal medium that can medium define “absolute rest” – motion can be measured relative to the ether 2
Lect. 12 Summary, Clas. Phys. - Michelson-Morley Exp. Example – boat in water Fundamental Question • According to Galileo, Newton all motion is relative • Can we detect the earth moving through the ether? • Also our common sense – examples …… V boat (relative to the water) Earth V wave (relative to the water) V wave (relative to the water) V earth (relative the the ether) • Measured from the boat: • front wave moves at V wave - V boat • back wave moves at V wave + V boat • Can this be the definition of absolute motion; • Difference = (V wave + V boat ) - (V wave - V boat ) = 2V boat absolute rest? • An experiment can detect motion relative to the medium Measurement of Speed of light Beginning of the New Revolutions • Michelson (1870’s) established his scientific • The Michelson-Morley experiment (1887) reputation first by greatly improving on the previous • At the time, Classical Physics had not really been measurement of the speed of light by Foucault tested at speeds comparable to the speed of light Mirror still Light beam Mirror rotating Mirrored cube • How to find the key experiment? This is the creative challenge of experimental physics! Important but not the key experiment! Michelson-Morley Experiment Key Part of Classical Physics Tested by Michelson-Morley Experiment • If light is a wave in some medium (ether) then if the earth moves in the ether it should be detectable • Addition of velocities - Applied to light • If Newton’s laws are correct the earth “moves” • Superposition principle of Galileo - Also contained around the sun: in Newton’s law of inertia Sun • Leads to the formulas for addition of velocities: Earth Example swimmer in river moving at speed v With respect to • In 1887 Michelson & Morley do experiment to try to v v B (smimmer wrt bank) = measure the velocity of the earth with respect to v s v B (wrt) the “ether” v (river wrt bank) + v s (swimmer wrt river) • Testing key ideas: Does the ether exist, as appears to be required by electromagnetic waves? Can we Important point: s = speed of swimmer = magitude of v s (swimmer wrt river) depends only on the swimmer. measure the “absolute motion of the earth”? s does not change if swimmer is going up, down or across the stream 3
Recommend
More recommend
