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I'm a retired engineer trying to get my head around GR, its effects in our everyday non-relativistic world, and its reduction to Newtonian gravity. I hope this is not too much of a digression from the current string. As I understand it, the presence of mass/energy causes curvature in space-time and that curvature causes mass/energy to travel along a force-free geodesic. If I toss a ball five meters high and five meters horizontally it takes about 2 seconds to land. On a worldline the ct axis increment is (2 sec)*(3 e8 m/s) or about 6 e8 m. The xy plane space increment on the worldline is 5 m in the x and y directions. Is the ball traveling force free along the worldline? Is there any physical significance to the ct increment of 6 trillion meters or is this just a mathematical byproduct to be ignored? When one considers the Earth's movement in one day about the sun, is the earth traveling in space along a geodesic arc of about (360 deg/365.24 days)? Is the ct component of this worldline (3 e8 m/sec)*(86,400 sec/day) and what does that mean? If a test particle is released at the Earth's position withe zero tangential velocity it will travel toward the sun. Is this also a geodesic path and does the geodesic path depend on the velocity of the test particle relative to the sun? Thanks for any understanding you can provide. Bob R.

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# I Geodesics in curved spacetime

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