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Recently I came across three questions about black holes and gravity. Below are the questions and my initial attempts at answers.1.) How does gravity get out of a black hole?It doesn’t. Gravity is not a force, it is a feature of the geometry of spacetime. Spacetime is influenced by the presence of the mass of the black hole; not by anything that has to escape from it.2.) Why is the speed of gravity restricted to the speed of light?Gravity does not travel. What travels is information about the presence/nature of the mass in question. Exchange of information is limited to “c”.3.) What is opposing the black hole so that the black hole gravity does not go to infinity (What limits the collapse)?If the centre of a black hole is a singularity, this is defined as a point where spacetime curvature is infinite, so gravity is infinite. Obviously, spacetime could not become more curved, and the area of infinite curvature must be infinitesimally small, so it is self limiting.I feel sure these attempted answers are by no means the "last word", and I would appreciate comments.

If you think in terms of a decreasing trend where less and less gravity results due to the decreasing amounts of energy in each successive generation there would be no runaway effect.

Quote from: JeffreyIf you think in terms of a decreasing trend where less and less gravity results due to the decreasing amounts of energy in each successive generation there would be no runaway effect. That makes sense to me, but are we sure that energy would decrease in successive generations, and if it did, would that necessarily bring the process to a halt?

In the case of a black hole that forms from the collapse of a star, is the gravity at the event horizon greater than the gravity at the surface of the original star?

how does a situation in which two black holes are approaching each other differ from one in which two stars are approaching each other?

Quote from: BillSIn the case of a black hole that forms from the collapse of a star, is the gravity at the event horizon greater than the gravity at the surface of the original star?The gravitational acceleration at the event horizon will be far greater than the gravitational acceleration at the surface of the original star. My reasoning:Let's take a star with radius R mIf we call the gravitational acceleration at the surface of this star A m/s^{2}.If the entire mass of that star collapses into a black hole, the gravitational acceleration at distance R from the center of the black hole will still be A m/s^{2}Let's call the Schwarzchild radius of the black hole S m.The gravitational acceleration at the surface of the black hole will be A(R/S)^{2}....And the tidal forces grow even faster than an inverse square law... Quotehow does a situation in which two black holes are approaching each other differ from one in which two stars are approaching each other?Stars in close binary orbits often suck loosely-held gas from the less-dense star, and dump it onto the denser star. This atmospheric stripping starts well before their distance is equal to the sum of their radii. This frequently causes a ...sorry, you cannot view external links. To see them, please REGISTER or LOGIN. But the stars are still at a fairly large distance when their stellar atmospheres merge.Black holes have a much smaller radius than their parent stars, so they can approach to much closer distances, with much faster orbital velocities, but with no transfer of gas between them. They merge when their event horizons touch, which is at a relatively small distance.

Since density drops with increase of mass we can arrive at a situation where g < 1 at the event horizons of very large black holes.

Gravity is an interaction between waves of light in the infrred spectrum, its all around us everywhere in space, There is a big mystery around what light is and how it travels, I explain that under TOE fun. Also this also clears up how light waves interact, The slit experiment caused a lot of confusion my theory explains it clearly and simply []

Quote from: jeffreyHSince density drops with increase of mass we can arrive at a situation where g < 1 at the event horizons of very large black holes.Spaghettification (ie acceleration due to gravity differing significantly between your head and your feet) is inevitable when approaching a stellar-mass black hole.However, the rate of change of gravitational acceleration is much gentler near a supermassive black hole, and I understand that reaching the event horizon in one piece is theoretically possible (provided that you don't get minced and/or fried by an accretion disk)....One of the largest known black hole candidates is in the ...sorry, you cannot view external links. To see them, please REGISTER or LOGIN, with an estimated mass of 21 billion solar masses, and a Shwarzchild radius r_{s}≈6x10^{13}m. If my quick calculation is correct, the acceleration due to gravity at the Schwarzchild radius is about 10 times Earth-normal (as seen by a distant observer).So I suggest that it is unlikely that you can find a black hole in our part of the universe that has a gravitational attraction as low as Earth's surface 10m/s at r_{s}.

Comparisons between Schwarzschild and Kerr black holes is interesting, but is there any evidence for non-rotating BHs anywhere in the Universe? Is it even possible, under the constraints of gravity, for a body to form by accretion without rotating?

Hi Mordeth, good to meet a fellow nit-picker. []I should have said: Is it even possible, under the constraints of gravity, for a body that may later collapse to a BH, to form by accretion without rotating?

Consider a cloud of dust. If all particles of the cloud were evenly distributed and the cloud was a perfect sphere with no angular momentum, then it could collapse via accretion into something that is non-rotating.