# A thought experiment with gravity. **URL:** \ **Category:** Factual Questions\ **Created:** [December 17, 2008, 4:27am UTC](https://boards.straightdope.com/t/a-thought-experiment-with-gravity/477519 "2008-12-17T04:27:31Z")\ **Posts on this page:** 11\ **Page:** 1
**Author:** ![Grrr](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/grrr/32/146_2.png) [@Grrr](https://boards.straightdope.com/u/Grrr)\ **Post date:** [December 17, 2008, 4:27am UTC](https://boards.straightdope.com/t/a-thought-experiment-with-gravity/477519/1 "2008-12-17T04:27:31Z")
A hypothetical universe. In it the only two things that exist are a planet say the size of Earth (spining on it’s axis) and a Hydrogen atom. Is there any amount of distance I can put between the planet and the atom where the atom _wouldn’t_ be affected by the planets gravitational pull? Basicaly, what I’m trying to say is: How far extending is it’s gravitational pull? And what sort of crazy equation do you come up with to figure that out? ---
**Author:** ![beowulff](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/beowulff/32/542_2.png) [@beowulff](https://boards.straightdope.com/u/beowulff)\ **Post date:** [December 17, 2008, 4:30am UTC](https://boards.straightdope.com/t/a-thought-experiment-with-gravity/477519/2 "2008-12-17T04:30:36Z")
Newton’s law for gravitational attraction is F = G((M1\*M2)/r^2). The gravitational force falls off as the square of the distance, but never reaches zero. More info here: [Randy Kobe, University of Winnipeg](http://theory.uwinnipeg.ca/physics/circ/node7.html) ---
**Author:** ![ftg](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/ftg/32/2801_2.png) [@ftg](https://boards.straightdope.com/u/ftg)\ **Post date:** [December 17, 2008, 12:32pm UTC](https://boards.straightdope.com/t/a-thought-experiment-with-gravity/477519/3 "2008-12-17T12:32:51Z")
If the atom is outside the [observable universe](http://en.wikipedia.org/wiki/Observable_universe#Size)\*, then it is separating from us by a speed faster than the speed of light. The generally accepted view is that gravity waves move at the speed of light. Ergo, the atom no longer interacts with the Earth in any way. \*Assuming the universe is at least that large. ---
**Author:** ![Little\_Nemo](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/little_nemo/32/3120_2.png) [@Little\_Nemo](https://boards.straightdope.com/u/Little_Nemo)\ **Post date:** [December 17, 2008, 1:27pm UTC](https://boards.straightdope.com/t/a-thought-experiment-with-gravity/477519/4 "2008-12-17T13:27:49Z")
To expand on what ftg wrote, the universe is about 14,000,000,000 years old. So the masses of any two objects more than 14,000,000,000 light years apart have not yet had time to interact. ---
**Author:** ![Exapno\_Mapcase](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/exapno_mapcase/32/1051_2.png) [@Exapno\_Mapcase](https://boards.straightdope.com/u/Exapno_Mapcase)\ **Post date:** [December 17, 2008, 4:49pm UTC](https://boards.straightdope.com/t/a-thought-experiment-with-gravity/477519/5 "2008-12-17T16:49:40Z")
> [@ftg](#): > > If the atom is outside the [observable universe](http://en.wikipedia.org/wiki/Observable_universe#Size)\*, then it is separating from us by a speed faster than the speed of light. The generally accepted view is that gravity waves move at the speed of light. > > Ergo, the atom no longer interacts with the Earth in any way. > > \*Assuming the universe is at least that large. But you’re referring to our expanding universe. The universe given by the OP is not ours and is not said to be expanding. The objects are apparently not moving away from one another at all. If it’s not and they’re not, then it’s correct that there is never a time when the objects do not have a non-zero gravitational effect on one another. ---
**Author:** ![Whack-a-Mole](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/whack-a-mole/32/141_2.png) [@Whack-a-Mole](https://boards.straightdope.com/u/Whack-a-Mole)\ **Post date:** [December 17, 2008, 5:03pm UTC](https://boards.straightdope.com/t/a-thought-experiment-with-gravity/477519/6 "2008-12-17T17:03:43Z")
Waiting for the Total Perspective Vortex to enter in to this: > [@](#): > > The machine produces a virtual reality model of the entire universe by means of the axiom that any piece of matter is affected by all other matter. The Vortex reconstructs the universe through computer processing of a high-resolution scan (“extrapolated matter analysis”) of a piece of fairy cake (cupcake). In the words of the Hitchhiker’s Guide, > > ``` > ...since every piece of matter in the Universe is in someway affected by every other piece of matter in the Universe, it is in theory possible to extrapolate the whole of creation - every Galaxy, every sun, every planet, their orbits, their composition, and their economic and social history from, say, one small piece of fairy cake. > > ``` > > SOURCE: [http://en.wikipedia.org/wiki/Total\_Perspective\_Vortex#Total\_Perspective\_Vortex](http://en.wikipedia.org/wiki/Total_Perspective_Vortex#Total_Perspective_Vortex) ---
**Author:** ![Exapno\_Mapcase](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/exapno_mapcase/32/1051_2.png) [@Exapno\_Mapcase](https://boards.straightdope.com/u/Exapno_Mapcase)\ **Post date:** [December 17, 2008, 5:44pm UTC](https://boards.straightdope.com/t/a-thought-experiment-with-gravity/477519/7 "2008-12-17T17:44:13Z")
True only in a Newtonian universe. Which might explain a lot about Adams’ worlds. 🙂 ---
**Author:** ![Boyo\_Jim](https://avatars.discourse-cdn.com/v4/letter/b/87869e/32.png) [@Boyo\_Jim](https://boards.straightdope.com/u/Boyo_Jim)\ **Post date:** [December 17, 2008, 6:17pm UTC](https://boards.straightdope.com/t/a-thought-experiment-with-gravity/477519/8 "2008-12-17T18:17:46Z")
Is the idea here that “our” universe is somehow emptied of everything except one big planet-sized rock and one atom of hydrogen, placed REALLY far apart? IANAphysicist, but as I understand it, the expansion of the universe is accelerating, which suggests there is some force working against gravitational attraction, anti-gravity if you will. One hypothesis of how this works involves involves anti-gravity increasing relative to the distance between objects. So it is conceivable there exists two points, where you could place two objects, and they would remain motionless in relation to each other. But gravity WOULD still be affecting the two objects – only its force would be counterbalanced by this other mystery force. ---
**Author:** ![ftg](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/ftg/32/2801_2.png) [@ftg](https://boards.straightdope.com/u/ftg)\ **Post date:** [December 17, 2008, 8:14pm UTC](https://boards.straightdope.com/t/a-thought-experiment-with-gravity/477519/9 "2008-12-17T20:14:43Z")
> [@Little\_Nemo](#): > > To expand on what ftg wrote, the universe is about 14,000,000,000 years old. So the masses of any two objects more than 14,000,000,000 light years apart have not yet had time to interact. Two errors here. 1. See the link in my post about why the 14BLY number doesn’t apply here. 2. Due to expansion, it’s actually the opposite direction. Things once in causal connection become unconnected. ---
**Author:** ![Little\_Nemo](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/little_nemo/32/3120_2.png) [@Little\_Nemo](https://boards.straightdope.com/u/Little_Nemo)\ **Post date:** [December 17, 2008, 11:15pm UTC](https://boards.straightdope.com/t/a-thought-experiment-with-gravity/477519/10 "2008-12-17T23:15:11Z")
> [@](#): > > Two errors here. 1. See the link in my post about why the 14BLY number doesn’t apply here. 2. Due to expansion, it’s actually the opposite direction. Things once in causal connection become unconnected. I’m not sure what your point is. I used the fourteen billion light-year figure because it’s a limit - any objects that far apart have not had a gravitational effect on each other. You can lower the figure down however to a local situation - two objects that are only 700,000,000 miles apart have not had any gravitational effect on each other if the two objects have existed less than a hour. The point I was making was that gravitational effects cannot occur faster than the speed of light, which is the same point you were making I believe. I’d also point out that when I wrote about objects being “more than 14,000,000,000 light years apart” I meant that they had always been that far apart not just currently. ---
**Author:** ![ftg](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/ftg/32/2801_2.png) [@ftg](https://boards.straightdope.com/u/ftg)\ **Post date:** [December 19, 2008, 8:10pm UTC](https://boards.straightdope.com/t/a-thought-experiment-with-gravity/477519/11 "2008-12-19T20:10:15Z")
> [@Little\_Nemo](#): > > I’m not sure what your point is. I used the fourteen billion light-year figure because it’s a limit - any objects that far apart have not had a gravitational effect on each other. Did you actually look at the link? Did you notice the section entitled “Misconceptions”?