Lecture7 Special Relativity

Lecture7 Special Relativity - Special Relativity Hawley &...

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3/9/09 Astronomy 309R - Spring 2009 1 Special Relativity Hawley & Holcomb, Chapters 6 and 7
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3/9/09 Astronomy 309R - Spring 2009 2 The Nature of Light Light and any other electromagnetic radiation, including radio waves, microwaves, X-rays, gamma-rays, etc. can propagate (i.e., travel) in empty space, which we call “vacuum.” When electromagnetic radiation travels in vacuum, an electric field (just like that after you have combed your hair) and a magnetic field (just like that of a bar magnet) perpetuate each other: each one oscillates, just like a wave on the surface of water. Because the electric field creates a magnetic field, and the magnetic field creates an electric field, the wave travels in space. Experiments carried out at the end of the 19 th century showed that the speed of light was the same under all conditions, that is, is independent of the velocity with which the source of light is moving relative to the observer, and is also independent of the motion of the earth around the sun.
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Michelson Interferometer 3/9/09 Astronomy 309R - Spring 2009 3
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3/9/09 Astronomy 309R - Spring 2009 4 The Speed of Light “Postulate” The speed of light is equal to all observers. In particular, the speed of light does not depend on the velocity of the light source relative to the observer. The constancy of the speed of light was first noticed in the experiment of Albert Michelson & Edward Morley in 1887. This postulate, which is at the heart of Albert Einstein’s special theory of relativity (1905), has since been tested an confirmed by numerous experiments. The constancy of the speed of light applies to the propagation of light in vacuum . In transparent media (air, water, glass, etc.) light propagates slightly slower than in vacuum, but this is due to microscopic interactions and does not contradict special theory of relativity.
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3/9/09 Astronomy 309R - Spring 2009 5 Implications for Simultaneity: The Paradox of Light Spheres light pulse light pulse Smiley face and green star emit a light pulse when they are right next to each other. They both claim to be at the center of the sphere defined by the light pulse. However, the two spheres are not the same sphere, because events simultaneous to one observer are not simultaneous to the other. See explanation of the paradox on the following slide. event B event A
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3/9/09 Astronomy 309R - Spring 2009 6 Paradox of Spheres: An Explanation Smiley face and green star emit a light pulse in all directions when they are right next to each other. Since the speed of light is the same to all observers, the emitted light is one and the same pulse to all observers. In the reference frame in which smiley face is at rest, in given a time interval t , the pulse has reached the same distance from the smiley face in all directions and is thus a sphere centered on smiley face. In this frame, the pulse reaches points A and B simultaneously. However, green star will have covered some distance, and will be displaced
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This note was uploaded on 09/14/2009 for the course AST 309 taught by Professor Johnlacy during the Spring '08 term at University of Texas at Austin.

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Lecture7 Special Relativity - Special Relativity Hawley &...

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