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Spacetime - the relativistic view of time and space -
June 10, 2012
از تاریخ معاصر
2 other people
"From the preceding reflections on time dilation, we learn that Albert Einstein has overthrown commonsense assumptions about space and time that were valid for centuries. Relative to the observer, distances appear to contract while clocks tick more slowly when moving at velocities close to the speed of light. These are the practical consequences of Special Relativity, the work for which Einstein became famous. Einstein did not stop at this point. In 1916, he published his General Relativity, which further challenged conventional wisdom. The paper proposed that matter causes spacetime to curve. Gravitation is understood as the warping of spacetime, not a force acting at a distance, as Newton had suggested. A massive object causes spacetime to curve, which is often illustrated with the picture of a bowling ball lying on a stretched rubber sheet: Contrary to appearance, the diagram does not depict the three-dimensional space of everyday experience. Instead it shows how a 2-D slice through familiar 3-D space is curved downwards when embedded in flattened hyperspace. We cannot fully envision this hyperspace. Flattening it to 3-D allows us to represent the curvature and helps us visualise the implications of Einstein's General Theory of Relativity. Gravitation bends light rays. Since light has no mass, it is not subject to Newton's law of gravity, and hence, in Newtonian physics gravity has no effect on light. If space is curved, however, it follows that a ray of light seemingly moving in a straight line really travels in a curved line following the curvature of space. This is comparable, in some way, to the itinerary of a plane. Because the earth is a sphere, the shortest path between two points on earth is described by a geodesic, a curved line. While moving along the geodesic it would appear to the passengers of the plane that they are moving in a straight line, although they are not. Similarly, the light of distant stars travels through the curved geometry of space...
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