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| | Encyclopedia: Special relativity |
 | | The differential of distance(ds) in cartesian 3D space is defined as: In mathematics, the word differential has various meanings: In calculus, a differential is an infinitesimal change in the value of a function. |
 | | In the geometry of special relativity, a fourth dimension, time, is added, with units of c, so that the equation for the differential of distance becomes: Cherenkov effect in a swimming pool nuclear reactor. |
 | | We see that the null geodesics lie along a dual-cone: in physics, and specifically general relativity, geodesics are the world lines of a particle free from all external force. |
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