# Would time dilation cause a planet to rotate?

For the sake of the argument lets use a black hole so the time dilation effects are greater and the planet in question is orbiting close enough to have significant TD effects but far enough to not fall in. So, in this scenario one part of the planet would be closer to the BH and therefore would experience a slower passage of time relative to the part of the planet on the other side. Would this cause the planet to "roll on time" due to the time dilation effects?

Assume it is not tidally locked.

• Small point, but in reality, I think the tidal effects would be many thousand times stronger, if not millions of times stronger, so it would be hard to detect. That and the Roche limit for your average planet around a black hole would be well outside any significant time dilation. In theory, I don't see why it wouldn't happen and even be measurable. Commented Mar 8, 2015 at 18:08
• This would be interesting math. How do you figure it could be calculated?
– Joe
Commented Mar 10, 2015 at 23:33
• And does that mean the mass that is creating the time dilation is losing energy?
– Joe
Commented Mar 11, 2015 at 0:09
• It would depend on a few factors. The size of the planet and how close it was orbiting. I can give running the numbers a try a bit later. As to the 2nd question, Er, hmm. I don't think that's how it works. The mass doesn't spend any energy creating the time dilation - that wouldn't make sense. Similar to how the magnet doesn't spend any energy creating the magnetic field. What changes is the potential and/or kinetic energy of the object in orbit. But, I'm not a professional, so, you might want to verify that with someone who is. Commented Mar 11, 2015 at 12:35
• Any luck? I tried running the numbers and quickly got stuck.
– Joe
Commented Mar 17, 2015 at 23:36

So we are to assume that the planet is not gravitationally locked for some period of time. This could possibly happen for a neutron star because it's gravitational bulging would be minimal owing to its structural integrity. We also have to assume that $t_{rot}\ll t_{orbit}$, because of $g^{tt}$ varies appreciably from one side of the planet to the other, then so does $g^{rr}$ and I'm not sure what kind of orbit that would be (presumably different than the particle orbit one finds in a GR exercise).