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<blockquote data-quote="fuindordm" data-source="post: 6689286" data-attributes="member: 5435">Just to add a gloss to Umbran's very nice answer:Special Relativity can be derived from the assumption that light travels at the same speed in every reference frame.General Relativity can be derived from the same assumption, plus a second assumption that inertial mass and gravitational mass are identical (or at least proportional).&nbsp; That second assumption is called the Equivalence Principle, and it can be expressed in several other ways having to do with the nature of reference frames, but that's the simplest and my personal favorite. The tensor calculus required by general relativity stitches together lots of local, small-scale, and essentially flat coordinate systems into global coordinate system with curvature that satisfies both assumptions. It is rather analogous to modeling a curved surface as a patchwork of flat tiles, like what game consoles do to render 3D graphics.&nbsp;The equivalence principle is one of my favorite mysteries to introduce to first-year physics students. They all learn F=ma and F=mg in short order, but there is nothing in Newtonian physics to explain why the same physical property plays two such different roles.&nbsp; The first one resists a change in motion due to any external force; the second actually creates a force on other objects.&nbsp; Lots of very clever experiments were done before and after Einstein to find out whether the two masses really had the same value in all cases.Maybe string theory has something to say about that as well? I know it does some very clever things with gravity.Ben</blockquote>

[QUOTE="fuindordm, post: 6689286, member: 5435"] Just to add a gloss to Umbran's very nice answer: Special Relativity can be derived from the assumption that light travels at the same speed in every reference frame. General Relativity can be derived from the same assumption, plus a second assumption that inertial mass and gravitational mass are identical (or at least proportional). That second assumption is called the Equivalence Principle, and it can be expressed in several other ways having to do with the nature of reference frames, but that's the simplest and my personal favorite. The tensor calculus required by general relativity stitches together lots of local, small-scale, and essentially flat coordinate systems into global coordinate system with curvature that satisfies both assumptions. It is rather analogous to modeling a curved surface as a patchwork of flat tiles, like what game consoles do to render 3D graphics. The equivalence principle is one of my favorite mysteries to introduce to first-year physics students. They all learn F=ma and F=mg in short order, but there is nothing in Newtonian physics to explain why the same physical property plays two such different roles. The first one resists a change in motion due to any external force; the second actually creates a force on other objects. Lots of very clever experiments were done before and after Einstein to find out whether the two masses really had the same value in all cases. Maybe string theory has something to say about that as well? I know it does some very clever things with gravity. Ben [/QUOTE]

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