# Supermassive Black Hole Gravity Question **URL:** \ **Category:** Factual Questions\ **Created:** [July 16, 2026, 4:08pm UTC](https://boards.straightdope.com/t/supermassive-black-hole-gravity-question/1031721 "2026-07-16T16:08:42Z")\ **Posts on this page:** 7\ **Page:** 3
**Author:** ![GreysonCarlisle](https://avatars.discourse-cdn.com/v4/letter/g/a87d85/32.png) [@GreysonCarlisle](https://boards.straightdope.com/u/GreysonCarlisle)\ **Post date:** [August 4, 2026, 5:30pm UTC](https://boards.straightdope.com/t/supermassive-black-hole-gravity-question/1031721/41 "2026-08-04T17:30:26Z")
> [@Chronos](#): > > What assumptions are you making there, @GreysonCarlisle ? That’s where my math puts things. You probably have better math. Starting assumption is that our sun is somehow replaced by the black hole. ---
**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:** [August 4, 2026, 5:49pm UTC](https://boards.straightdope.com/t/supermassive-black-hole-gravity-question/1031721/42 "2026-08-04T17:49:20Z")
The math should be the same for all of us. I’m asking what you fed into the math. ---
**Author:** ![GreysonCarlisle](https://avatars.discourse-cdn.com/v4/letter/g/a87d85/32.png) [@GreysonCarlisle](https://boards.straightdope.com/u/GreysonCarlisle)\ **Post date:** [August 4, 2026, 6:13pm UTC](https://boards.straightdope.com/t/supermassive-black-hole-gravity-question/1031721/43 "2026-08-04T18:13:36Z")
Poof! The sun is replaced by the black hole. The innermost planets are immediately consumed. Jupiter and Saturn are doomed. The other bodies remain in some kind of orbit (they don’t get hoovered into the black hole in a straight line). Their speeds increase until they achieve escape velocity. For Uranus, that’s about 95,000 km/s and 75,000 km/s for Neptune. At that point, they’re flung out of the solar system and stop accelerating. ---
**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:** [August 4, 2026, 6:18pm UTC](https://boards.straightdope.com/t/supermassive-black-hole-gravity-question/1031721/44 "2026-08-04T18:18:10Z")
Of course they don’t fall in in a straight line. But the question is, are they retaining their initial, pre-hole velocity, or their initial, pre-hole paths? And there’s no way that they end up achieving escape velocity. You can’t “attain escape velocity” around an object via that object’s gravity. If you have it, you always have it. ---
**Author:** ![GreysonCarlisle](https://avatars.discourse-cdn.com/v4/letter/g/a87d85/32.png) [@GreysonCarlisle](https://boards.straightdope.com/u/GreysonCarlisle)\ **Post date:** [August 4, 2026, 6:21pm UTC](https://boards.straightdope.com/t/supermassive-black-hole-gravity-question/1031721/45 "2026-08-04T18:21:51Z")
> [@Chronos](#): > > You can’t “attain escape velocity” around an object via that object’s gravity. Then either my proposed escape velocities are wrong or my projected speeds for the bodies is. Or, since they’re the same speeds, maybe both. They started with all of their initial conditions, but all that went out the window when the black hole showed up. ---
**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:** [August 4, 2026, 6:55pm UTC](https://boards.straightdope.com/t/supermassive-black-hole-gravity-question/1031721/46 "2026-08-04T18:55:47Z")
I think that maybe what you did was find the escape speeds for those planets’ initial positions, and then checked for when they reached those speeds? But escape speed isn’t a single number: It depends on how close to the central object you are. And when those planets got closer, their escape speeds correspondingly increased, such that their speed at that instant was still below escape speed. ---
**Author:** ![GreysonCarlisle](https://avatars.discourse-cdn.com/v4/letter/g/a87d85/32.png) [@GreysonCarlisle](https://boards.straightdope.com/u/GreysonCarlisle)\ **Post date:** [August 4, 2026, 7:02pm UTC](https://boards.straightdope.com/t/supermassive-black-hole-gravity-question/1031721/47 "2026-08-04T19:02:29Z")
That makes sense. [Previous page](https://boards.straightdope.com/t/supermassive-black-hole-gravity-question/1031721.md?page=2)