2 ms·

I think this question is a bad one. You can ask it only for an in-falling observer. To extend that to someone who remains outside the event horizon, it become

I think this question is a bad one. You can ask it only for an in-falling observer. To extend that to someone who remains outside the event horizon, it becomes meaningless. There are many hints that there is something special going on at the event horizon itself.

raattgift 3y ago
> I think this question is a bad one. With respect, I disagree. A pair of comments I made elsewhere in this thread bear that out. > > Does frame dragging originate at the event horizon or at the singularity/ringularity? > You can ask it only for an in-falling observer Both the geodetic effect and the Lense-Thirring (LT) effect, forms of "frame dragging", have been demonstrated in Earth orbit, for example by Gravity Probe B <https://en.wikipedia.org/wiki/Gravity_Probe_B https://en.wikipedia.org/wiki/Gravity_Probe_B>. I think we can agree that there is neither an event horizon nor a singularity associated with Earth, and that since GP-B continues to orbit Earth after almost twenty years, it was not doing any sort of in-falling during its mission. The tl;dr answer to the question you think is a bad one is that Kerr exterior spacetimes -- even those that are not black holes -- induce LT, which is roughly an altitude-dependent mismatch between a vector pointing to something far away that is parallel-transported around the spinning central mass and around the same mass if it were not spinning. On extended objects occupying a range of altitudes above a spinning mass, an LT torque is induced. No black hole needed. The paper press-released in TFA the spinning object is a black hole, which lets one get much closer to the centre of mass (because a BH is compact compared to a planet) and because can spin much faster than a planet (which would tend to fly apart), which lets the LT effect be stronger. It also contemplates a large accretion disc in which LT torques can create pronounced observable effects. The singularity is not relevant to the analysis (it is not even mentioned in the paper). The horizon is barely relevant: towards the horizon they reduce the resolution of the adaptive mesh they calculate with and justify doing so in a couple of ways -- most of the interesting physics are at least 2R and mostly above 4R, where R := r_g which is the Kerr equivalent of the Schwarzschild radius r_s.

raattgift 3y ago

> I think this question is a bad one. With respect, I disagree. A pair of comments I made elsewhere in this thread bear that out. > > Does frame dragging originate at the event horizon or at the singularity/ringularity? > You can ask it only for an in-falling observer Both the geodetic effect and the Lense-Thirring (LT) effect, forms of "frame dragging", have been demonstrated in Earth orbit, for example by Gravity Probe B <https://en.wikipedia.org/wiki/Gravity_Probe_B https://en.wikipedia.org/wiki/Gravity_Probe_B>. I think we can agree that there is neither an event horizon nor a singularity associated with Earth, and that since GP-B continues to orbit Earth after almost twenty years, it was not doing any sort of in-falling during its mission. The tl;dr answer to the question you think is a bad one is that Kerr exterior spacetimes -- even those that are not black holes -- induce LT, which is roughly an altitude-dependent mismatch between a vector pointing to something far away that is parallel-transported around the spinning central mass and around the same mass if it were not spinning. On extended objects occupying a range of altitudes above a spinning mass, an LT torque is induced. No black hole needed. The paper press-released in TFA the spinning object is a black hole, which lets one get much closer to the centre of mass (because a BH is compact compared to a planet) and because can spin much faster than a planet (which would tend to fly apart), which lets the LT effect be stronger. It also contemplates a large accretion disc in which LT torques can create pronounced observable effects. The singularity is not relevant to the analysis (it is not even mentioned in the paper). The horizon is barely relevant: towards the horizon they reduce the resolution of the adaptive mesh they calculate with and justify doing so in a couple of ways -- most of the interesting physics are at least 2R and mostly above 4R, where R := r_g which is the Kerr equivalent of the Schwarzschild radius r_s.