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phi1123
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So I recently watched the new movie Interstellar, and I've been inspired to do some more general relativity. At one point in the movie they mention that 1 hour on a planet orbiting a black hole is 7 years back on Earth, and so I decided my first project would be to figure out exactly how close they would have to be. So, taking the geodesic equations off of Wikipedia (because I'm too lazy to figure that out)
##0=\frac{d^2\phi}{dq^2}+\frac{2}{r}\frac{d\theta}{dq}\frac{dr}{dq}##
##0=\frac{d^2t}{dq^2}+w^{-1}\frac{r_s}{r^2}\frac{dr}{dq}\frac{dt}{dq}##
##0=\frac{d^2r}{dq^2}+\frac{w}{2}\frac{d(w^{-1})}{dr}(\frac{dr}{dq})^2-rw(\frac{d\phi}{dq})^2+\frac{c^2}{2}w\frac{dw}{dr}(\frac{dt}{dq})^2##
Where ##w=(1-\frac{r_s}{r})## and ##r_s## is the Schwarzschild radius. Using these three equations, setting ##q=\tau##, and assuming circular orbits (##\frac{dr}{dq}=0##), I ended up with 4 equations and 4 unknowns. Solving for ##\frac{dt}{d\tau}##, I end up with
##\frac{dt}{d\tau}=\sqrt{\frac{1}{1-\frac{3}{2}\frac{r_s}{r}}}##
SO my questions: Is this process valid, or am I missing something? I was expecting the time dilation to go to infinity at the event horizon, ##r_s##, however it seems to become undefined at ##\frac{3}{2}r_s## instead. What is special about this distance?
##0=\frac{d^2\phi}{dq^2}+\frac{2}{r}\frac{d\theta}{dq}\frac{dr}{dq}##
##0=\frac{d^2t}{dq^2}+w^{-1}\frac{r_s}{r^2}\frac{dr}{dq}\frac{dt}{dq}##
##0=\frac{d^2r}{dq^2}+\frac{w}{2}\frac{d(w^{-1})}{dr}(\frac{dr}{dq})^2-rw(\frac{d\phi}{dq})^2+\frac{c^2}{2}w\frac{dw}{dr}(\frac{dt}{dq})^2##
Where ##w=(1-\frac{r_s}{r})## and ##r_s## is the Schwarzschild radius. Using these three equations, setting ##q=\tau##, and assuming circular orbits (##\frac{dr}{dq}=0##), I ended up with 4 equations and 4 unknowns. Solving for ##\frac{dt}{d\tau}##, I end up with
##\frac{dt}{d\tau}=\sqrt{\frac{1}{1-\frac{3}{2}\frac{r_s}{r}}}##
SO my questions: Is this process valid, or am I missing something? I was expecting the time dilation to go to infinity at the event horizon, ##r_s##, however it seems to become undefined at ##\frac{3}{2}r_s## instead. What is special about this distance?
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