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We want to be able to calculate the trajectory of a freely falling object (massive or mass less). In special relativity, and in a local intertial frame, a freely falling object moves in a straight line / has a constant velocity. From there, we can go to general coodinates by using the ``trick'' of changing from an ordinary derivative to a covariant derivative. This gives the following equation of motion (also known as the geodesics equation):


tex2html_wrap_inline1253 , the affine parameter, is a parameter along the curve, which is unique up to a transformation of the form tex2html_wrap_inline1255 (Misner, Thorne, & Wheeler 1973).

A light ray, which I want to study for this project, moves with the speed of light, which is the same as it has a null separation, i.e. tex2html_wrap_inline894 . If we have a metric like the Kerr one and solve for tex2html_wrap_inline896 we get a 2nd order equation:


where we have used the symmetry of the metric tensor, i.e. tex2html_wrap_inline898 . This equation can be readily solved. Notationwise, we could just as well have used tex2html_wrap_inline1258 instead of tex2html_wrap_inline1260 .

Web Exhibition: Null Geodesics Around a Kerr Black Hole

Bo Milvang-Jensen (
Mon Jun 17 11:54:08 MDT 1996