General Relativity explains the path a falling body makes (ex. An apple falling toward the center of the Earth) as a geodesic in curved spacetime. What explains the direction the apple falls? In other words: why doesn't the apple follow the same geodesic away from the Earth? What provides the kinetic energy in the "downward" direction?

  1. For a given timelike geodesic that would be the direction that points into the future light-cone in spacetime.

  2. The geodesic itself depends (besides on the geodesic equation) on conditions, such as e.g., initial conditions or boundary conditions.

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  • $\begingroup$ This answer seems incomplete, since when I toss an apple upward it also follows a timelike geodesic into the forward light-cone, but one that for awhile heads away from the Earth. Am I missing something? $\endgroup$ – G. Smith Jan 5 '19 at 20:19
  • $\begingroup$ I updated the answer. $\endgroup$ – Qmechanic Jan 5 '19 at 21:00
  • $\begingroup$ Could the direction of travel along the geodesic be determined by Entropy (i.e. The Second Law of thermodynamics)? Or do we need a theory of quantum gravity to provide gravitrons to explain why gravity is attractive and not repulsive? $\endgroup$ – Paul D Jan 13 '19 at 3:39

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