## Energy and momentum

When Einstein began to develop the theory that became general relativity, there were a number of elements that were going to be essential to the result. The basic papers on special relativity were published in 1905 and general relativity was not made public until 1915. Special relativity had already dealt with coordinate transformations between inertial frames of reference; general relativity was to incorporate accelerated frames of reference. Einstein would also have had in mind the equivalence principle; namely, that a gravitational field was equivalent to an accelerated frame of...

Read More## Geodesics and general relativity

“What is a geodesic?”, you ask. The answer is simple, it is the straightest possible line that you, or anything else for that matter, can travel on through space. If space were flat (Euclidean), then the geodesic is just a straight line. On the surface of a sphere, a geodesic is a Great Circle. If you’ve ever looked at one of those maps in the back of the airline flight magazine while flying from point A to point B, you will have seen that the Great Circle routes that airlines like to fly, especially on a Mercator projection map of the Earth. The arcs curve to the north in...

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