# Black Holes: Where's The Gravity Come From? **URL:** \ **Category:** Factual Questions\ **Created:** [October 8, 2011, 12:52am UTC](https://boards.straightdope.com/t/black-holes-wheres-the-gravity-come-from/598961 "2011-10-08T00:52:05Z")\ **Posts on this page:** 3\ **Page:** 2
**Author:** ![naita](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/naita/32/5862_2.png) [@naita](https://boards.straightdope.com/u/naita)\ **Post date:** [October 18, 2011, 3:56pm UTC](https://boards.straightdope.com/t/black-holes-wheres-the-gravity-come-from/598961/21 "2011-10-18T15:56:48Z")
> [@Jinx](#): > > Pre-quote, paraphrased: So, anything orbiting that star (now black hole) would continue orbiting it? > That’s hard to accept when, perhaps from sci-fi, we picture them as the vacuum cleaners of space (within some range). I understand a black hole has a horizon (akin to a sphere of influence, you might say). Can one calculate the distance to where the sun’s event horizon would be (should it become a black hole)? Would Mercury be sucked in or continue to orbit\*??? > > \*This question ignores the fact that, as I understand it, the sun would first nova or supernova [thereby swallowing up Mercury if not all inner planets] and then collapse upon itself. No the black hole’s horizon should not be likened to a sphere of influence. The black hole influences its surroundings the same way a star, planet or other massive object in the universe does, by gravity and gravity reaches infinitely far. Comparing a star with a black hole of the same mass, the gravity field would look exactly the same for the black hole and the star all the way to the radius of the star, so anything orbiting the star could orbit the black whole in the same way. The difference (apart from one being a glowing ball of gas and the other being a paradoxical infinite hole in the fabric of time and space) is that you can get arbitrarily close to the black hole, while the star’s mass occupies a volume. So the gravity for the black hole gets high enough for weird stuff to start happening, up to and including trapping everything including light when you get past the Schwartzschild radius. But for weird things to start happening you have to get closer to the black hole than the radius of the original star. ---
**Author:** ![Machine\_Elf](https://avatars.discourse-cdn.com/v4/letter/m/82dd89/32.png) [@Machine\_Elf](https://boards.straightdope.com/u/Machine_Elf)\ **Post date:** [October 18, 2011, 5:22pm UTC](https://boards.straightdope.com/t/black-holes-wheres-the-gravity-come-from/598961/22 "2011-10-18T17:22:03Z")
> [@Jinx](#): > > Pre-quote, paraphrased: So, anything orbiting that star (now black hole) would continue orbiting it? > That’s hard to accept when, perhaps from sci-fi, we picture them as the vacuum cleaners of space (within some range). It’s just that that range is quite small, a lot less than the original radius of the body in question. If I understand it correctly, [Newton’s law of universal gravitation still applies:](http://en.wikipedia.org/wiki/Law_of_gravitational_attraction) F = G \* (m[sub]1[/sub]\*m[sub]2[/sub])/r[sup]2[/sup] where r is measured between the _centers_ of the two bodies in question. So once a star becomes a black hole, the masses m1 and m2 haven’t changed - it’s just that now, it’s possible for r to be a lot less than the original radius of the star. > [@Jinx](#): > > Can one calculate the distance to where the sun’s event horizon would be (should it become a black hole)? Sure. [Equation is here.](http://en.wikipedia.org/wiki/Escape_velocity) M = the mass of the sun; the only difference between “star” and “black hole” is that with the black hole, r can be very small, resulting in very large escape velocities. ---
**Author:** ![Chronos](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/chronos/32/134_2.png) [@Chronos](https://boards.straightdope.com/u/Chronos)\ **Post date:** [October 18, 2011, 6:52pm UTC](https://boards.straightdope.com/t/black-holes-wheres-the-gravity-come-from/598961/23 "2011-10-18T18:52:06Z")
Nitpick: The description of an event horizon as the location where the escape speed is c is fundamentally flawed. The calculation gives the right answer, but only because there are a number of mistakes that just happen to cancel out. I prefer to describe it as a location such that there are no paths from the inside to the outside. Any direction you point, you’re always pointing yourself closer to the center. It’s like trying to point in the direction of last Thursday-- You just can’t do it. [Previous page](https://boards.straightdope.com/t/black-holes-wheres-the-gravity-come-from/598961.md?page=1)