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Unformatted text preview: The Principle of Relativity Lecture I General Relativity (PHY 6938), Fall 2007 Copyright 2007 by Christopher Beetle Why Spacetime? Space and time are tied together in the idea of motion. Space and time are commonly regarded as the forms of existence in the real world, matter as its substance . A deFnite portion of matter occupies a deFne part of space at a deFnite moment of time. It is in the composite idea of motion that these three fundamental conceptions enter into intimate relationship. Hermann Weyl (1921) There are no preferred points of space or moments of time, but there are preferred (inertial) motions through spacetime. Every body continues in its state of rest, or of uniform motion in a right line, unless it is compelled to change that state by forces impressed upon it. Isaac Newton (1687) Newton is right to emphasize that there exist preferred, forcefree motions, but he also makes an assumption about what they are : straight lines through Euclidean space. x y t x y t x y t Inertial motions are properties of spacetime. Test Particles and Inertial Motions How can we observe inertial motions experimentally ? A test particle is: isolated (no contact forces), small, nonspinning (no tidal forces), electrically neutral (no electromagnetic forces), and not massive (no gravitational radiation). Test particles do not couple to known, longrange forces, except for gravity , and so follow natural paths through spacetime. But, we must measure what these inertial motions are, not postulate them. The Principle of Relativity Through any point of space, at any moment of time, there is exactly one inertial motion for each initial velocity a test particle might have at that point. The fundamental laws of physics do not distinguish these motions. Relativity and Electrodynamics Lorentz/Fitzgerald approach (Bell 1976) Electric and magnetic felds o moving charges modiy the dynamics o (classical) atoms. They get atter and the orbital period gets shorter....
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 Spring '08
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 General Relativity

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