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Donovan wrote:<BR>
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"In science the Theory of Gravity is still<BR>
just a theory." <BR>
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Gravity in Relativity no longer exists as it is described in Newtonian Physics. It has been replaced by mass altering the geometry of space/time. Objects near a mass (our Sun, for example) follow the geometry of space/time (Earth's orbit follows this geometry) that the mass creates, but are not acted upon by a force (gravity) at a distance, which is the intuitive notion that we often think of to imagine gravity keeping the Earth in orbit around our Sun.<BR>
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Nonetheless, the Newtonian equations dealing with gravity, speed, mass and force are perfectly workable to launch satellites into orbit above Earth. I doubt anyone at NASA uses Relativity to do the math to successfully launch a satellite into a required orbit, though I have not verified this. Newtonian physics does the job just fine.<BR>
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Though I disagree with you that the theory of gravity is "just a theory," given that its predictions have been empirically validated over and over given certain limitations, it is a good example of a scientific theory which has been replaced by a more accurate theory (Relativity) to predict the behavior of matter in those cases where it is required.<BR>
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<A HREF="http://en.wikipedia.org/wiki/Gravity">http://en.wikipedia.org/wiki/Gravity</A><BR>
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This below is from the link above:<BR>
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How spacetime curvature simulates gravitational force<BR>
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The <A HREF="http://en.wikipedia.org/w/index.php?title=Curvature_of_spacetime&action=edit">curvature of spacetime</A> considered as a whole implies a rather complex picture that is usually treated with the tools of <A HREF="http://en.wikipedia.org/wiki/Differential_geometry">differential geometry</A> and that requires the use of <A HREF="http://en.wikipedia.org/wiki/Tensor_calculus">tensor calculus</A>. It is possible though to understand - at least approximately - the mechanism of gravitation without tensors when the total curvature of spacetime is split into two components:<BR>
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<A HREF="http://en.wikipedia.org/wiki/Curvature_of_space">curvature of space</A> <BR>
<A HREF="http://en.wikipedia.org/wiki/Time_dilation">time dilation</A>. <BR>
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The above components of the curvature of spacetime, and only these, are responsible for the gravitation according to Einstein's theory.The effect of the first component, the curvature of space, is negligible in all cases when the velocities of objects are much smaller than the speed of light and when the ratios of masses divided by the distances separating their <A HREF="http://en.wikipedia.org/wiki/Center_of_mass">centers of mass</A> are much smaller than a specific constant, namely the ratio of speed of light squared to Newtonian gravitational constant: . So for the majority of cases in the universe, and certainly for almost all cases in our solar system except precession of perihelion of <A HREF="http://en.wikipedia.org/wiki/Mercury_%28planet%29">Mercury</A> and deflection of light rays in the vicinity of the <A HREF="http://en.wikipedia.org/wiki/Sun">Sun</A>, we may treat the space as flat, as ordinary Euclidean space. It leaves us only with the gravitational time dilation as a possible reason for the illusion of "gravitational force" acting at the distance. Assuming that the ratio of masses to distances between them are smaller than the constant above, the time dilation is tiny, but it is enough to cause "Newtonian gravitational attractive force" as we know it.The reason for this illusion is this: any mass in the universe modifies the rate of time in its vicinity this way that time runs slower closer to the mass and the change of time rate is controlled by an equation having exactly <I>the same form</I> as the equation that Newton discovered as his "Law of Universal Gravitation". The difference between them is in essence not in form since the Newtonian potential is replaced by the Einsteinian time rate <I>d</I>t / <I>dt</I>, where t is the time at a point at vicinity of the mass (the <I>proper time</I> of objects at this point in space, the time that is measured by the clocks in this point) and <I>t</I> is the time at observer at infinity, with the right side of the equation staying the same as in Newtonian equation (with accuracy to irrelevant constants). Because of the same form of both equations, the path of the object that takes an <A HREF="http://en.wikipedia.org/wiki/Extremum">extremum</A> of proper time while traveling, and by this taking a geodesic in spacetime, is the same (with accuracy to the negligible in this case curvature of space) as the Newtonian orbit of this object around the mass. So it looks as if the path of the object were bent by some "force of attraction" between the object and the mass. Since bending of the object's path is clearly visible and the time dilation extremely difficult to notice, a (fictitious) "gravitational force" has been assumed rather than a (real, presently measured with precise enough and formerly unavailable clocks) time dilation as the reason for bending the paths of objects moving in vicinity of masses.So without any force involved into keeping the traveling object in line the object follows the Newtonian orbit in space just by following a geodesic in spacetime. This is Einstein's explanation why without any "gravitational forces" all the objects follow Newtonian orbits and at the same time why the Newtonian gravitation is the approximation of the Einsteinian gravitation.In this way the Newton's "Law of Universal Gravitation" that looked to people who tried to interpret it as an equation describing a hypothetical "force of gravitational attraction" acting at a distance (except to Newton himself who didn't believe that "action at a distance" is possible) turned out to be really an equation describing spacetime geodesics in Euclidean space. We may say that Newton discovered the geodesic motion in spacetime and Einstein, by applying Riemannian geometry to it, extended it to the curved spacetime, disclosed the hidden Newtonian physics, and made its math accurate.<BR>
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V2020 post by Ted Moffett<BR>
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