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Lecture12_10 - Physics19 GreatIdeasofPhysics Lecture12...

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Click to edit Master subtitle style Physics 19 Great Ideas of Physics Lecture 12: Intro to Special Relativity
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Review The wave model of light was apparently confirmed  by observation of interference Maxwell’s equations (and subsequent experimental  verification) showed that light is an electromagnetic  wave, moving at a speed  c = 3 × 108 meters/second. The speed of light is fixed to a specific value by the  strength of the electric and magnetic forces Two questions: What allows electromagnetic waves to propagate at such  great speed through apparently empty space?
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Click to edit Master subtitle style Galilean Relativity and Light
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“Galilean” Relativity Galileo argued that the laws of physics  are the same in a laboratory moving at a  constant speed, and a laboratory at rest Remember the discussion of an object  dropped from the mast of a moving ship... Think of other examples Newton argued that time is an absolute,  and flows uniformly without reference to  anything else To translate measurements made in two  frames, moving with relative speed E �: t’  = t x’ = x -  t y’ = y, z’ = z This means that measured velocities  simply add: v’ =  v - c x x’ y y’
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Galilean Relativity and Light According to these principles: If the speed of a light pulse  emitted by a lighthouse at rest is  c, its speed should be measured  as c - �  in a spaceship flying by  at speed ˚ If so, then Maxwell’s equations,  then (speed of light = c,  constant) must be talking about  the speed that light travels  through the “ether” Maxwell’s equations would only  be valid for a laboratory that is  stationary in this ether The speed of light would be  Lighthouse on planet (at rest) Pulse of light (speed = c) Spaceship (speed =  )
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The Michelson-Morley  In 1887, the Americans  A.A. Michelson and E.W.  Morley tried to use the  Earth’s motion to detect  the ether According to the ether  theory, they should have  seen a clear signal Their experiment saw no  sign of motion with  L L Earth’s Direction
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The Speed of Light is Constant Michelson and Morley’s experiment (and  later ones) showed two surprising things: There is  no luminiferous ether  –  Electromagnetic waves can propagate through a  vacuum and don’t need a medium Maxwell’s equations (laws of electromagnetism)  are valid in all laboratories: the speed of light  (electromagnetic waves) is always the same –  independent of the motion of the source or the 
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The Puzzle There are three principles: Newton’s concept of Absolute Time Time flows uniformly and unaffected by anything else The Principle of Relativity:
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