# How far does gravity reach? **URL:** \ **Category:** Factual Questions\ **Created:** [May 7, 2013, 6:17pm UTC](https://boards.straightdope.com/t/how-far-does-gravity-reach/657726 "2013-05-07T18:17:23Z")\ **Posts on this page:** 11\ **Page:** 3
**Author:** ![AaronX](https://avatars.discourse-cdn.com/v4/letter/a/7bcc69/32.png) [@AaronX](https://boards.straightdope.com/u/AaronX)\ **Post date:** [May 13, 2013, 4:25am UTC](https://boards.straightdope.com/t/how-far-does-gravity-reach/657726/41 "2013-05-13T04:25:47Z")
> [@toofs](#): > > In regards to the specks at each end of an otherwise empty universe, I have read that our universe is expanding away from the point of the big bang at an ever increasing rate. Wouldn’t gravity’s infinite reach ultimately slow the rate of expansion, eventually causing all matter in the universe to reverse direction and meet once again at the point of origin? Is there a conflict between the theory of the infinite reach of gravity and the observation that the speed of the big bang expansion is increasing? This conflict is exactly why we think there’s dark energy. You can start reading at [Cosmological constant - Wikipedia](http://en.wikipedia.org/wiki/Cosmological_constant) From what I understand, this is done to make the equations fit the observations, but has no theoretical basis. ---
**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:** [May 13, 2013, 1:27pm UTC](https://boards.straightdope.com/t/how-far-does-gravity-reach/657726/42 "2013-05-13T13:27:04Z")
One might rather say that it does have a theoretical basis, but the theoretical basis it has is worse than none at all. Most models of particle physics predict the existence of a vacuum energy which would have the correct qualitative properties to be the dark energy. However, if you attempt to use those particle models to estimate the strength of the vacuum energy, you get a number that’s 120 orders of magnitude higher than our observations. Note: Not a factor of 120, 120 orders of magnitude. Frankly, it’s rather embarrassing. ---
**Author:** ![billfish678](https://avatars.discourse-cdn.com/v4/letter/b/7bcc69/32.png) [@billfish678](https://boards.straightdope.com/u/billfish678)\ **Post date:** [May 13, 2013, 1:36pm UTC](https://boards.straightdope.com/t/how-far-does-gravity-reach/657726/43 "2013-05-13T13:36:05Z")
And to think I got points taken off on a thermodynamics test because I put down 273.12 rather than 273.13. ---
**Author:** ![Leo\_Bloom](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/leo_bloom/32/10377_2.png) [@Leo\_Bloom](https://boards.straightdope.com/u/Leo_Bloom)\ **Post date:** [May 13, 2013, 1:55pm UTC](https://boards.straightdope.com/t/how-far-does-gravity-reach/657726/44 "2013-05-13T13:55:50Z")
> [@Asympotically\_fat](#): > > …Gravity’s reach is thought to be infinite, though nobody has been to infinity to report back as of yet. > > In Newtonian physics, gravitational field strength gets less the further away from the source, but never goes to zero… This has got to be one of the best username/post combos. More than just pointing out when **Leo Bloom** mentions Leo Bloom, because of… Riffing the username (which I just missed doing first because (damn!) I wanted to read the thread first): > [@drewtwo99](#): > > What about yo’ mama? > > Yo mama’s so fat, her gravitational pull extends infinitely at the speed of light. > [@Frazzled](#): > > Yo mama so fat she attracts galaxies in _every_ dimension! ---
**Author:** ![Really\_Not\_All\_That\_Bright](https://avatars.discourse-cdn.com/v4/letter/r/e8c25b/32.png) [@Really\_Not\_All\_That\_Bright](https://boards.straightdope.com/u/Really_Not_All_That_Bright)\ **Post date:** [May 13, 2013, 3:08pm UTC](https://boards.straightdope.com/t/how-far-does-gravity-reach/657726/45 "2013-05-13T15:08:45Z")
> [@Czarcasm](#): > > Would a compass on the moon point towards the Earth? > [@Chronos](#): > > Compasses react to magnetic fields, not gravitational. I’m pretty sure that on the Moon, the dominant magnetic field is either some trace of small field the Moon itself has, or the Sun’s. And even if the Earth’s area of magnetic influence did extend out that far, the direction of the field there would be mostly perpendicular to the line from the Moon to the Earth. > [@President\_Johnny\_Gentle](#): > > According to NASA, the moon spends [six days each month](http://www.nasa.gov/topics/moonmars/features/magnetotail_080416.html) within influence of the Earth’s magnetic field (the time around a fool moon, when the moon enters the tail of the field.) They hypothesize that it could be strong enough to cause moon dust to hover. The moon doesn’t have a dipolar field; its magnetic field is essentially a series of small fields generated by ferrous deposits in the crust. For reasons that are not well understood, its liquid core doesn’t generated an overarching field. ---
**Author:** ![ralph124c](https://avatars.discourse-cdn.com/v4/letter/r/8797f3/32.png) [@ralph124c](https://boards.straightdope.com/u/ralph124c)\ **Post date:** [May 13, 2013, 3:29pm UTC](https://boards.straightdope.com/t/how-far-does-gravity-reach/657726/46 "2013-05-13T15:29:12Z")
As the universe expands, the local density of matter decreases-yet, all the matter in the universe has the same center of mass that it did billions of years ago. If the universe is infinite, the average gravitational attraction will continually decrease-and things will tent to move apart-does this apply to stars and planets bound in galaxies as well? ---
**Author:** ![Really\_Not\_All\_That\_Bright](https://avatars.discourse-cdn.com/v4/letter/r/e8c25b/32.png) [@Really\_Not\_All\_That\_Bright](https://boards.straightdope.com/u/Really_Not_All_That_Bright)\ **Post date:** [May 13, 2013, 3:58pm UTC](https://boards.straightdope.com/t/how-far-does-gravity-reach/657726/47 "2013-05-13T15:58:52Z")
No. Individual galaxies are not expanding, as far as we can tell. Having said all that, eventually all the stars in the Milky Way will have too little mass to maintain our current gravitational bindings, and bits and pieces of our galaxy will start flying off into the distance. ---
**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:** [May 13, 2013, 5:32pm UTC](https://boards.straightdope.com/t/how-far-does-gravity-reach/657726/48 "2013-05-13T17:32:21Z")
> [@](#): > > Having said all that, eventually all the stars in the Milky Way will have too little mass to maintain our current gravitational bindings… How would that happen? And there are actually some models where the strength of the dark energy (whatever it is) increases with time, such that eventually, it does become strong enough to rip apart galaxies, planets, and subatomic particles. Such a scenario is called the “Big Rip”, and it’s one of the more potentially imminent end-of-the-Universe scenarios. Such models are not particularly mainstream, however, since the data are still consistent with the simplest models where the dark energy has constant strength. ---
**Author:** ![Asympotically\_fat](https://avatars.discourse-cdn.com/v4/letter/a/e47c2d/32.png) [@Asympotically\_fat](https://boards.straightdope.com/u/Asympotically_fat)\ **Post date:** [May 13, 2013, 5:54pm UTC](https://boards.straightdope.com/t/how-far-does-gravity-reach/657726/49 "2013-05-13T17:54:17Z")
I’m not really sure of the long-term dynamics of a galaxy: I’d guess over time stars would ‘evaporate’ from a galaxy (i.e. the exchange of energy over time between stars will allow the most energetic stars to escape the galaxy) over a very long time period leaving eventually just a central supermassive black hole, which over an even longer period of time will evaporate itself (by a different process - i.e. the Hawking process). ---
**Author:** ![Inigo\_Montoya](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/inigo_montoya/32/124_2.png) [@Inigo\_Montoya](https://boards.straightdope.com/u/Inigo_Montoya)\ **Post date:** [May 13, 2013, 7:37pm UTC](https://boards.straightdope.com/t/how-far-does-gravity-reach/657726/50 "2013-05-13T19:37:24Z")
> [@Colibri](#): > > nearly infinite Is that even a functionally meaningful phrase? I mean, there’s “vast” and “lots” but as infinity is by definition immeasurable, wouldn’t “nearly inifinite” mean essentially the same as “laughably not even close to inifinite”? Yes, that’s somewhat of a joke–but still, do phrases like “∞-1” or “∞/12” ever come up in math? Seems like it’d be easier to express it is “∞”. ---
**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:** [May 13, 2013, 8:40pm UTC](https://boards.straightdope.com/t/how-far-does-gravity-reach/657726/51 "2013-05-13T20:40:51Z")
It depends on the context. Roughly speaking, you can describe X as “nearly infinite” in the same set of situations that you can describe 1/X as “nearly zero”. That is to say, X is much larger than any other relevant quantities in the problem with the same units, or 1/X is much smaller than any other relevant quantities with the same units. Of course, how much is “much” depends on your required level of precision. [Previous page](https://boards.straightdope.com/t/how-far-does-gravity-reach/657726.md?page=2)