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From my naive understanding of Einstein theory of relativity: gravity is not a force but only a consequence of inertia in a world where mass deforms the space-time. Without time, no gravity is possible.

Imagine, a mad scientist managed to reverse time! Time is now running backward at exactly the same rate but opposite rate (if that means anything) as before. People are still free to behave in ways that differ from what that've done in the past though.

How is gravity affected by this change in time direction? Is gravity negative (in the sense that objects are getting repulsing each others) or is gravity unchanged?

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    $\begingroup$ Interesting, but I think you will find better answers in Physics Stack Exchange. $\endgroup$ – Ender Look Apr 8 '18 at 17:00
  • $\begingroup$ All of the velocity vectors would suddenly be reversed and things would move back in the direction they came from, but that has nothing to do with gravity. $\endgroup$ – Spencer Apr 8 '18 at 17:27
  • $\begingroup$ You must think through what reversing time means; at first sight, this means reversing the 2nd law of thermodynamics, because at macroscopic level this is what generates the time arrow. Reversing the 2nd law of thermodynamics would have wondrous effects, for example the spontaneous appearance of gasoline in a car's tank after it comes thundering backwards into the garrage... $\endgroup$ – AlexP Apr 8 '18 at 17:47
  • $\begingroup$ I'm having a lot of trouble understanding what you are asking ("mad scientists" are not a particularly well-defined concept and "mad scientists reversing time" even less so). One thing that appears to be on topic for your question is that General Relativity is a time-symmetric theory, so reversing time would not have any observable consequences on the shape of spacetime and hence on gravity. $\endgroup$ – Mark Olson Apr 8 '18 at 18:57
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    $\begingroup$ Physics student here. Physicists take the axiom of unitarity. Pulling time in reverse results in G constant, being negative, negative eletric permitivity and negative magnetic permeability. Conveging lenses diverge light and visa versa. Probably the most interesting thing is entropy now is being destroyed. Chemical processes happen in reverse. But is there will-power in this reversed time world? Is time reversing or creating a new universe with negative time and the "future-past" is still undetermined? $\endgroup$ – Tsangares Apr 17 '18 at 3:44
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From my understanding, gravity is the action of mass upon the spatial and temporal fabric of the universe. As far as I can tell reversing the direction time flows in would not change the mass of an object, therefore the effect that mass has upon time and space would not change.

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