Lecture13_10 - Physics19 GreatIdeasofPhysics...

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Click to edit Master subtitle style Physics 19 Great Ideas of Physics Lecture 13: Special Relativity
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Review: Simultaneity and Dilation The Postulates of Relativity: The laws of Physics are the same in all inertial frames The speed of light in vacuum is always the same Simultaneity is relative Simultaneous events for one observer will not be  simultaneous for a second observer moving with some  velocity relative to him Clocks which are moving with respect to each other  cannot be kept synchronized Clocks run slower in a frame moving with respect  to the observer:
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Simultaneous or Not? With our synchronized clocks,  we can tell if two measurements  or “events” are simultaneous in a particular frame of reference Events which are  simultaneous in one inertial  frame may not be  simultaneous in another  inertial frame which is moving  with respect to it! Simultaneity is relative – it  depends on the observer’s  state of motion! A B M A B M A B M A B M
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Time Dilation Our inability to keep moving  clocks synchronized with each  other arises because  they tick  at different rates ! The stationary observer sees  the moving clock take more time  between ticks than his own  identical clock Each tick of the stationary  observer’s clock occurs at the  same place, so it is the  minimum (“proper”) interval The interval between ticks of the  moving clock is longer, because  each tick occurs at different  L L vt At = 2L/ c
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The Dilation Factor ( ) The moving clock is dilated  (slowed) by a factor C  = (1- 2)-1/2, where  =v/ c For relative speeds small  compared to the speed of  light ( ~0), the dilation factor   is close to 1 (no dilation) The effect only becomes  significant when the relative 
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Click to edit Master subtitle style Making Sense of Time Dilation
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We have an apparent paradox. According to an  observer in the station, clocks on the moving train  are running slow But an observer on the train considers himself to  be at rest in an inertial frame, with a perfectly  functioning clock, while the observer in the station  is moving backward with respect to him Remember, all inertial frames of reference are equally  valid! According to the observer on the train, clocks in 
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Lecture13_10 - Physics19 GreatIdeasofPhysics...

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