|MadSci Network: Astronomy|
Hi! This is the way gravitational interactions are described in general relativity - as a warping of space (and time), rather than as action at a distance (like the Newtonial theory of gravity) or via the exchange of particles (as in the quantum field theories of the weak, electromagnetic, and strong forces). We can measure the amount that space is warped by the presence of a mass by using light beams - we define a straight line to be the path taken by a light beam, and then see how that path changes when a mass is present (this has been done by looking at the light from stars as they graze the limb of the sun during a total solar eclipse). The plots you see in books are an attempt to draw the result; they are incomplete and can be misleading, as spacetime is four-dimensional already (three space dimensions, one time), and warping is usually indicated by distorting the drawing into another space dimension. That's why the pictures just show two dimensions. You are correct, though, in that light going directly towards (or away from) a mass will not be deflected (except in the case of a black hole, which is infinitely deep, where it won't get out at all). Light passing by will be deflected, however. Here are some Web pages which give a basic introduction to general relativity: Eric's pages on general relativity: http://ourworld.compuserve.com/homepages/eric_baird/gr_home.htm General relativity, with references (this one is more historical): http://www-groups.dcs.st-and.ac.uk/~history/HistTopics/General_relativity.html A brief introduction, with a description of the "rubber sheet" diagram: http://webhome.idirect.com/~yanspace/einstein/grelativ.htm The relativity FAQ: http://math.ucr.edu/home/baez/physics/relativity.html
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