# Gravity, how far? **URL:** \ **Category:** Factual Questions\ **Created:** [January 13, 2017, 4:26am UTC](https://boards.straightdope.com/t/gravity-how-far/777012 "2017-01-13T04:26:42Z")\ **Posts on this page:** 16\ **Page:** 2
**Author:** ![LSLGuy](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/lslguy/32/5813_2.png) [@LSLGuy](https://boards.straightdope.com/u/LSLGuy)\ **Post date:** [January 14, 2017, 1:25pm UTC](https://boards.straightdope.com/t/gravity-how-far/777012/21 "2017-01-14T13:25:40Z")
> [@doubleminus](#): > > Is gravitation force continuous or quantified? i. e. if I measure the attraction between 2 masses, as they get further away from each other, would the attraction change in discontinuous steps > ? The proper word is “quantized”, not “quantified”. AFAIK the answer is “Nobody knows for sure. Einstein says ‘no’; quantum theory implies ‘yes’. A Nobel awaits the person who solves this riddle”. Gravity is a very weak force. As such the quanta, if they exist, are very, very, very, very, very, very, very, very, very, very, very small. Nope; actually they’re much, much, much, much, much, much, much, much smaller than that. So they’re hard to experimentally measure. Impossibly so as of yet. ---
**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:** [January 14, 2017, 4:47pm UTC](https://boards.straightdope.com/t/gravity-how-far/777012/22 "2017-01-14T16:47:20Z")
Note that, although gravity is certainly subject to quantum mechanics in some way, we have no idea how. It might be that there’s some smallest amount of gravitational force, and it might be that all gravitational forces are integer multiples of that smallest amount, but neither one is necessarily true. Alternately, it might be energy levels that are quantized, or distances, or all of the above, and none of them necessarily follows integer-multiple quantization. As for asking “where the information is stored”, the answer is that it’s stored (in a very complicated way) in the configuration of all of spacetime. But it’s a stretch to call that a “consciousness”. > [@](#): > > Quoth **Darren Garrison** : > > Of course, now the speed of gravity has the possibility of being empirically measurable–if we happen to pick up a gravity wave from a neutron star collision that is also close enough to be seen in some wavelength or another. Not necessarily a neutron star collision: Any event or system which can be observed both electromagnetically and gravitationally would suffice. For instance, closely-orbiting binary stars would work. Their gravitational waves are too low-frequency for instruments like LIGO to detect, but if we ever get around to building and launching LISA, it’d resolve the question within a day or so (there are some binary star systems that we already know of from their light which we know would be suitable for this purpose). ---
**Author:** ![Darren\_Garrison](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/darren_garrison/32/92_2.png) [@Darren\_Garrison](https://boards.straightdope.com/u/Darren_Garrison)\ **Post date:** [January 14, 2017, 5:03pm UTC](https://boards.straightdope.com/t/gravity-how-far/777012/23 "2017-01-14T17:03:41Z")
I’m thinking of things that you can pinpoint to a moment–see the bang on the gravity wave detector, see the bang with a telescope. ---
**Author:** ![RadicalPi](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/radicalpi/32/9806_2.png) [@RadicalPi](https://boards.straightdope.com/u/RadicalPi)\ **Post date:** [January 15, 2017, 7:19am UTC](https://boards.straightdope.com/t/gravity-how-far/777012/24 "2017-01-15T07:19:29Z")
> [@Chronos](#): > > Actually, although the speed of gravity (to the best of our ability to determine) is finite and exactly equal to the speed of light, in any system of objects that interact only gravitationally, in the small-potential limit (so no close flybys of black holes), the motions are exactly what you would expect if gravity were Newtonian and infinite in speed. Which isn’t too terribly different from what **jtur88** is describing, there. > > The catch, of course, is that gravity is not Newtonian. So, if the speed of gravity is finite, how does gravity escape from a black hole? Why does gravity of the singularity extend beyond the event horizon even if nothing else does? ---
**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:** [January 15, 2017, 10:42am UTC](https://boards.straightdope.com/t/gravity-how-far/777012/25 "2017-01-15T10:42:01Z")
> [@RadicalPi](#): > > So, if the speed of gravity is finite, how does gravity escape from a black hole? Why does gravity of the singularity extend beyond the event horizon even if nothing else does? Gravity is the cause of the black hole. How could gravity stop gravity? ---
**Author:** ![Francis\_Vaughan](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/francis_vaughan/32/3093_2.png) [@Francis\_Vaughan](https://boards.straightdope.com/u/Francis_Vaughan)\ **Post date:** [January 15, 2017, 12:10pm UTC](https://boards.straightdope.com/t/gravity-how-far/777012/26 "2017-01-15T12:10:18Z")
Gravity is just the curvature of space-time. An event horizon is simply a particular amount of curvature near a black hole. (A black hole being, by definition, the only thing that has enough mass to curve space-time enough to create an event horizon.) Space-time is curved even more inside the event horizon, and curved less outside of the event horizon. An event horizon can change - if more stuff falls into the black hole, as that stuff falls in, space can curve more - as the stuff falling in also curves space itself. So the location of the magic amount of curvature we denote as the event horizon will expand. And evaporating back holes will see space curve less, and so the location of the event horizon will move inward. The event horizon is not some magic line in space. You are not somehow safe just outside. Inside you never get out. A bit outside, it may take you half the age of the universe to get out, if you were a photon, you would be red-shifted down to the AM band. (Of course photons don’t experience time, so they don’t actually care.) Gravity doesn’t “escape” from within the event horizon, it is the thing that defines it. However the question should be - can changes to the gravitational field _within_ an event horizon propagate out past it? Imagine you have a very massive object - say a neutron star - and it is slowly falling into the back hole. As it orbits, the pair radiate some gravity waves. But at some point the decaying orbit of the neutron star crosses inside the event horizon. Does the system continue to radiate gravity waves detectable outside of the black hole? The answer seems to be no. But just like the photon just outside the horizon, this isn’t some sudden cut off. The curvature of space-time outside the event horizon is still insane, and the energy in the gravity waves is going to be red-shifted as well. Making it essentially impossible to detect anyway. Being inside the event horizon simply moves “essentially” to “actually”. The radiated gravity waves must be created when the objects are still some distance apart - enough that the curvature of space is not too dreadful. But these distances are minuscule on a cosmic scale. The black holes that created the waves the LIGO detector saw had event horizon radii of about 100km. The gravitational waves created by the merger that were actually detected would have been created when the pair were vastly further apart than a few hundred kilometres. Indeed the period of the wave seen is a clue here. ---
**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:** [January 15, 2017, 1:19pm UTC](https://boards.straightdope.com/t/gravity-how-far/777012/27 "2017-01-15T13:19:22Z")
Actually, the gravitational waves don’t cut off abruptly right at the moment the infalling object crosses the horizon. Even after that point, the black hole will “ring” like a bell, or like a disturbed soap bubble (the math for the oscillations is actually exactly the same as for a spherical bell or bubble). But the ringdown will exponentially decay, with a timescale comparable to the time it would take light to cross the hole. And the detected collisions (there have been at least two now, and maybe three) were starting from when the holes were very close together (less than ten times the size of their horizons, certainly), through the collision itself, and including the ringdown. Even with gravitational redshift, the peak emission comes right at the moment when the horizons touch and merge. ---
**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:** [January 15, 2017, 3:43pm UTC](https://boards.straightdope.com/t/gravity-how-far/777012/28 "2017-01-15T15:43:17Z")
> [@RadicalPi](#): > > So, if the speed of gravity is finite, how does gravity escape from a black hole? Why does gravity of the singularity extend beyond the event horizon even if nothing else does? What is meant by “the speed of gravity” is the propagation speed of gravitational radiation. A black hole needn’t be a source of gravitational radiation, but suppose it was -where would this radiation originate from? Often the following points will be mentioned: 1. Radiation coming directly from a a classical black hole need not originate in the event horizon. For a physical black hole the radiation could’ve been emitted by the surface of the star before it collapsed into a black hole, for an unphysical eternal black hole the source could be the white hole singularity. 2. Waves do not have exact locations in time and space, they are oscillations. Gravitational waves even more so as they are oscillations in the curvature of spacetime and curvature can always be made to vanish at a point. 3. Radiation in general is a far-field phenomena, that is the behaviour of a field when the source is sufficiently far away that it need not be considered. Radiation does not have an exact point of origin. 4. Gravitational radiation travels at c in the weak-field approximation. Near a black hole event horizon this approximation is no longer valid. NB for gravitational radiation the weak field and the far-field are not the same thing. 5. Despite the obvious analogy with other forms of radiation, gravitational radiation has a different character as it is an oscillation of spacetime itself. Whilst it is fundamentally forbidden in relativity for a particle to (locally) to exceed c, it is not fundamentally forbidden for waves in spacetime to exceed c (see the [Alcubierre drive](https://en.wikipedia.org/wiki/Alcubierre_drive)), this is related to the fact that such oscillations do not have any elements that are localizable. It may be that in the strong near-field region of a BH emitting gravitational radiation there are oscillations that in some sense travel faster than c. My take on it would be that 3) and 4) are the strongest and that you should really only talk about gravitational radiation and assign it a speed in a context where you can use the weak field to approximate the far field. ---
**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:** [January 15, 2017, 7:08pm UTC](https://boards.straightdope.com/t/gravity-how-far/777012/29 "2017-01-15T19:08:23Z")
It’s worth mentioning that the numerical relativity folks have managed to simulate an entire infall, even in the near-and-strong regime right before the merger, and their simulations do actually match the observations we have. So the behavior of the oscillations even in that very messy regime is known. Which is not to say that I personally know it, but it’s known by someone. ---
**Author:** ![Jackknifed\_Juggernaut](https://avatars.discourse-cdn.com/v4/letter/j/9de053/32.png) [@Jackknifed\_Juggernaut](https://boards.straightdope.com/u/Jackknifed_Juggernaut)\ **Post date:** [January 15, 2017, 10:27pm UTC](https://boards.straightdope.com/t/gravity-how-far/777012/30 "2017-01-15T22:27:19Z")
I’m probably taking this thread back a few orders of magnitude, but here’s my question. Can it be said that every object in the universe has some gravitational effect on all others, however minute? Or would that be every object in the observable universe since the gravity outside that zone would never reach us? That’s actually what I thought this thread was going to answer, based on the title. ---
**Author:** ![billfish678](https://avatars.discourse-cdn.com/v4/letter/b/7bcc69/32.png) [@billfish678](https://boards.straightdope.com/u/billfish678)\ **Post date:** [January 15, 2017, 10:35pm UTC](https://boards.straightdope.com/t/gravity-how-far/777012/31 "2017-01-15T22:35:26Z")
I’m just gonna throw this out. There was recent “popular” science news article…on a major net news feed…either AOL or Yahoo… Anyway, it was about a recently published “serious science shit” article…and this 50 something whatshisname was putting forward the idea gravity did NOT behave at very large distances like it did at more …well, on a cosmic scale…more modest distances… The kicker was there there was recent evidence that this whole dark matter thing may be a bunch of hooey…AND his theory (along with the mostly dead MOND theory) explained it all… ---
**Author:** ![Francis\_Vaughan](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/francis_vaughan/32/3093_2.png) [@Francis\_Vaughan](https://boards.straightdope.com/u/Francis_Vaughan)\ **Post date:** [January 16, 2017, 12:10am UTC](https://boards.straightdope.com/t/gravity-how-far/777012/32 "2017-01-16T00:10:56Z")
> [@billfish678](#): > > AND his theory (along with the mostly dead MOND theory) explained it all… Let me be the first to post the required [XKCD](http://xkcd.com/1758/). ---
**Author:** ![Trinopus](https://avatars.discourse-cdn.com/v4/letter/t/2bfe46/32.png) [@Trinopus](https://boards.straightdope.com/u/Trinopus)\ **Post date:** [January 16, 2017, 1:21am UTC](https://boards.straightdope.com/t/gravity-how-far/777012/33 "2017-01-16T01:21:05Z")
> [@Jackknifed\_Juggernaut](#): > > . . . Can it be said that every object in the universe has some gravitational effect on all others, however minute? . . . Yes…sorta. A single electron, here, is not going to exert any measureable gravitational attraction on a single electron in one of the most distant galaxies. (Oops; it actually won’t have had time to! But leave that aside, and just say "an electron in the galaxy in Andromeda.) The virtual photons that would carry that specific gravitational force would have wavelengths so long, they would rival the distance between the galaxies. In engineering terms, “Nuh-uh.” But in another sense, yes. Every electron curves space around it, in a Relativistic way, and those curves become part of the framework of the cosmos. Also, of course, it’s additive, so every electron contributes to the overall mass of the atom, the molecule, the mineral, the planet, and the galaxy. ---
**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:** [January 16, 2017, 2:19pm UTC](https://boards.straightdope.com/t/gravity-how-far/777012/34 "2017-01-16T14:19:05Z")
It’s yet another manifestation of the baldness paradox: Everyone agrees that a man with no hair at all is bald. And most would agree that if you take a bald man, and add a single hair to his head, then he’s still bald. But repeat that enough times, and you’ve got a full head of hair. The resolution, of course, is that there’s a difference between “almost nothing” and “nothing”. > [@](#): > > The virtual photons that would carry that specific gravitational force would have wavelengths so long, they would rival the distance between the galaxies. In engineering terms, “Nuh-uh.” This is true of the virtual photons (or gravitons) responsible for _any_ static electromagnetic (or gravitational) force, no matter the magnitude of the source. ---
**Author:** ![LSLGuy](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/lslguy/32/5813_2.png) [@LSLGuy](https://boards.straightdope.com/u/LSLGuy)\ **Post date:** [January 16, 2017, 3:14pm UTC](https://boards.straightdope.com/t/gravity-how-far/777012/35 "2017-01-16T15:14:15Z")
> [@Chronos](#): > > It’s yet another manifestation of the baldness paradox: Everyone agrees that a man with no hair at all is bald. And most would agree that if you take a bald man, and add a single hair to his head, then he’s still bald. But repeat that enough times, and you’ve got a full head of hair. The resolution, of course, is that there’s a difference between “almost nothing” and “nothing”. > … In computer science we sometimes joke that there are only three numbers: 0, 1, and many. All the distinctions that matter are between those three cases. And in a hefty chunk of those cases the distinction between 1 and many is somewhere between subtle and nil; it’s the difference between zero and not-zero that matters. ---
**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:** [January 16, 2017, 7:23pm UTC](https://boards.straightdope.com/t/gravity-how-far/777012/36 "2017-01-16T19:23:49Z")
> [@Chronos](#): > > It’s worth mentioning that the numerical relativity folks have managed to simulate an entire infall, even in the near-and-strong regime right before the merger, and their simulations do actually match the observations we have. So the behavior of the oscillations even in that very messy regime is known. > > Which is not to say that I personally know it, but it’s known by someone. Yes but the real problem is that the gravitational wave description relies on the far weak field regime. So you only get identifiable gravitational waves some way out of the region which contains the source. [Previous page](https://boards.straightdope.com/t/gravity-how-far/777012.md?page=1)