# Update on the speed of gravity **URL:** \ **Category:** Cecil's Columns/Staff Reports\ **Created:** [April 9, 2019, 2:37pm UTC](https://boards.straightdope.com/t/update-on-the-speed-of-gravity/832282 "2019-04-09T14:37:42Z")\ **Posts on this page:** 20\ **Page:** 2
**Author:** ![Stranger\_On\_A\_Train](https://avatars.discourse-cdn.com/v4/letter/s/13edae/32.png) [@Stranger\_On\_A\_Train](https://boards.straightdope.com/u/Stranger_On_A_Train)\ **Post date:** [April 11, 2019, 6:30pm UTC](https://boards.straightdope.com/t/update-on-the-speed-of-gravity/832282/21 "2019-04-11T18:30:36Z")
> [@Peter\_Morris](#): > > Gravity is often illustrated in terms of a rubber sheet. Place a bowling ball in the middle of the sheet, and it deforms creating a slope. Put a marble some distance from the bowling ball, it will roll down the slope. > > So, asking about the speed of gravity, isn’t that like asking about the speed of slope? The slope doesn’t have a speed, it’s just there. Or **Crafter\_Man** ’s question, what gravity waves are made of, isn’t that like asking what slope is made of? > > Would it make more sense to phrase the question in terms of how quickly the slope changes if you add or remove mass? > > It’s very likely I’m completely misunderstanding something here. The “rubber sheet” analogy is useful for visualizing the geometric effects of mass on space-time (which is fundamental to the concept of Einstein gravity) but it shouldn’t be extended to any dynamic phenomena relating to the conveyance or gravity itself. For one, it is (as far as we can tell) impossible to arbitrarily add or remove mass from space-time; doing so would create a discontinuity that would break general relativity. For another, while space-time of Einstein gravity is static, a quantum theory of gravity requires a continuous exchange of gravitons to mediate the apparent force of gravity. That exchange occurs (hypothetically, and as apparently verified by the recent measurements) at **c**. That has nothing to do with the level of acceleration of gravity (which is the slope of the fabric in the rubber sheet illustration); it is just a constant regardless of the amount of gravitons exchanged. The energy and/or number of gravitons is what controls the intensity of gravity. Stranger ---
**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:** [April 11, 2019, 8:05pm UTC](https://boards.straightdope.com/t/update-on-the-speed-of-gravity/832282/22 "2019-04-11T20:05:18Z")
I should also mention that people have in fact looked for gravitational waves at frequencies above the kilohertz range. This is for two reasons: There might in fact be some sources in the Universe that we don’t know about that produce waves in that range, and high-frequency gravitational wave detectors are much easier to build than the ones like LIGO. But to the surprise of nobody, including their builders, none of these high-frequency detectors have found anything. ---
**Author:** ![Mijin](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/mijin/32/9369_2.png) [@Mijin](https://boards.straightdope.com/u/Mijin)\ **Post date:** [April 12, 2019, 3:14am UTC](https://boards.straightdope.com/t/update-on-the-speed-of-gravity/832282/23 "2019-04-12T03:14:38Z")
> [@ftg](#): > > So all I need is some material that blocks out all incoming gravitons plus a source that produces a single graviton at a time and a device that measures said graviton and I’ll win a Nobel Prize. > > Hold my [del]beer[/del] caffeine-free diet cola. 😉 How about a partnership: I will dance around a laboratory, generating gravitational waves. I’ll leave it up to you to figure out some of the fiddly technical details like graviton shielding and detection apparatus. ---
**Author:** ![eschereal](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/eschereal/32/18939_2.png) [@eschereal](https://boards.straightdope.com/u/eschereal)\ **Post date:** [April 12, 2019, 4:09am UTC](https://boards.straightdope.com/t/update-on-the-speed-of-gravity/832282/24 "2019-04-12T04:09:28Z")
How large is a graviton, though? In QM, we study particle-wave dualities on a picosopic scale, but why should we assume that gravitons exist on a similar scale? Could they not be enormous entities? ---
**Author:** ![TubaDiva](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/tubadiva/32/507_2.png) [@TubaDiva](https://boards.straightdope.com/u/TubaDiva)\ **Post date:** [April 13, 2019, 1:11am UTC](https://boards.straightdope.com/t/update-on-the-speed-of-gravity/832282/25 "2019-04-13T01:11:24Z")
This is great! Thanks to all of you and especially you, Chronos. Jenny your humble TubaDiva Administrator ---
**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:** [April 13, 2019, 3:28pm UTC](https://boards.straightdope.com/t/update-on-the-speed-of-gravity/832282/26 "2019-04-13T15:28:37Z")
Fundamental particles don’t have size. They have two properties that can behave sort of analogously to size, namely wavelength and cross-section. The wavelength of typical gravitational waves (and hence also of the particles that make them up, to the extent that the particles can be said to have wavelength) is, as already mentioned, huge. And the cross-sections measure the degree to which they interact with various other particles: That’ll vary from one interaction to another, but we know from experiment that any cross-section involving gravitons will be very tiny indeed, dozens of orders of magnitude smaller than any of the standard fundamental particles. ---
**Author:** ![kanicbird](https://avatars.discourse-cdn.com/v4/letter/k/5f8ce5/32.png) [@kanicbird](https://boards.straightdope.com/u/kanicbird)\ **Post date:** [April 13, 2019, 3:53pm UTC](https://boards.straightdope.com/t/update-on-the-speed-of-gravity/832282/27 "2019-04-13T15:53:48Z")
Since we were looking at a black hole, if gravity waves is caused by the mass of the BH singularity (if it is a singularity) and gravity waves are part of the fabric of spacetime itself, why are we detecting light and gravity waves at the same time from the same event , as light can not escape from the BH, (or at least not directly) and gravity waves apparently can escape? ---
**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:** [April 13, 2019, 4:07pm UTC](https://boards.straightdope.com/t/update-on-the-speed-of-gravity/832282/28 "2019-04-13T16:07:48Z")
A black hole just sitting there won’t emit gravitational waves, any more than it emits light. Two black holes merging will produce gravitational waves, but that comes from the entire system, not from either one of the holes individually. And most of that system is outside of event horizons. ---
**Author:** ![eschereal](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/eschereal/32/18939_2.png) [@eschereal](https://boards.straightdope.com/u/eschereal)\ **Post date:** [April 13, 2019, 4:56pm UTC](https://boards.straightdope.com/t/update-on-the-speed-of-gravity/832282/29 "2019-04-13T16:56:20Z")
Nothing “just sits there”. Everything in space is in motion, so a black hole (or any other body) will invariably generate wave. Gravity is such a weak force, though, that the amplitude and frequency of most gravitational perturbance is effectively immeasurable. ---
**Author:** ![DPRK](https://avatars.discourse-cdn.com/v4/letter/d/4491bb/32.png) [@DPRK](https://boards.straightdope.com/u/DPRK)\ **Post date:** [April 13, 2019, 5:06pm UTC](https://boards.straightdope.com/t/update-on-the-speed-of-gravity/832282/30 "2019-04-13T17:06:30Z")
> [@eschereal](#): > > Nothing “just sits there”. Everything in space is in motion, so a black hole (or any other body) will invariably generate wave. Gravity is such a weak force, though, that the amplitude and frequency of most gravitational perturbance is effectively immeasurable. How would a hypothetical object just floating/moving through space radiate gravitational waves or gravitational “perturbance”? Of course something like a body orbiting another body is a different story. ---
**Author:** ![Triskadecamus](https://avatars.discourse-cdn.com/v4/letter/t/b19c9b/32.png) [@Triskadecamus](https://boards.straightdope.com/u/Triskadecamus)\ **Post date:** [April 13, 2019, 6:09pm UTC](https://boards.straightdope.com/t/update-on-the-speed-of-gravity/832282/31 "2019-04-13T18:09:51Z")
When two black holes collide, it is unlikely that nothing outside of the event horizon of either, or both of them will not also collide. Probably at fairly high speeds, and high temperatures. That event takes place at the same time as the gravity wave generation, and is “visible”. By recording the spectrum of the region as a whole, one can obtain fairly detailed information about the movements of the black holes without actually seeing them, because the effect the spectrum of the infalling, and colliding ordinary matter. Given the recent increase in resolving power of “virtual” collectors created from data streams from many sources, and the recently developed algorithms which can render them into images (as well as data with other information about the process) the evaluation of “when did the gravity wave generating event take place” is now increasingly likely to be exact to a more narrow margin of error than the distances of the objects themselves. The results imply closely that gravity waves travel at c. Tris * * * correction of error is invited, outrage will be tolerated, although probably ignored. ---
**Author:** ![Triskadecamus](https://avatars.discourse-cdn.com/v4/letter/t/b19c9b/32.png) [@Triskadecamus](https://boards.straightdope.com/u/Triskadecamus)\ **Post date:** [April 13, 2019, 6:14pm UTC](https://boards.straightdope.com/t/update-on-the-speed-of-gravity/832282/32 "2019-04-13T18:14:08Z")
Oh, by the way, if we build another one of these gravity wave detectors, we need a new acronym, so its name will be SLUGGO. Tris * * * Chances like this don’t come around often. ---
**Author:** ![Andy\_L](https://avatars.discourse-cdn.com/v4/letter/a/c67d28/32.png) [@Andy\_L](https://boards.straightdope.com/u/Andy_L)\ **Post date:** [April 13, 2019, 7:30pm UTC](https://boards.straightdope.com/t/update-on-the-speed-of-gravity/832282/33 "2019-04-13T19:30:18Z")
> [@eschereal](#): > > Nothing “just sits there”. Everything in space is in motion, so a black hole (or any other body) will invariably generate wave. Gravity is such a weak force, though, that the amplitude and frequency of most gravitational perturbance is effectively immeasurable. Unaccelerated motion does not generate gravitational waves. In fact, spherically or rationally symmetric acceleration won’t generate gravitational waves either, so a star pulsing in size or s Ringworld won’t generate gravitational waves either. The Earth isn’t exactly symmetrical so some tiny waves are generated by its rotation but a black hole is symmetric and won’t generate waves except while interacting with another body. ---
**Author:** ![Andy\_L](https://avatars.discourse-cdn.com/v4/letter/a/c67d28/32.png) [@Andy\_L](https://boards.straightdope.com/u/Andy_L)\ **Post date:** [April 13, 2019, 7:36pm UTC](https://boards.straightdope.com/t/update-on-the-speed-of-gravity/832282/34 "2019-04-13T19:36:21Z")
Nm ---
**Author:** ![eschereal](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/eschereal/32/18939_2.png) [@eschereal](https://boards.straightdope.com/u/eschereal)\ **Post date:** [April 13, 2019, 7:49pm UTC](https://boards.straightdope.com/t/update-on-the-speed-of-gravity/832282/35 "2019-04-13T19:49:27Z")
> [@DPRK](#): > > How would a hypothetical object just floating/moving through space radiate gravitational waves or gravitational “perturbance”? Of course something like a body orbiting another body is a different story. I picture spacetime as a sort of array of gradients. Go out to some distant point in space and place a baseball at that point with zero momentum (which is not really a meaningful value) and it will start moving “down” the gradient. The gradient is the vector sum of all the gravitational influences on a point in space. But the anchors of all of those vectors are in motion, so all the point gradients in spacetime are constantly changing. As that large body way over there moves on its path, its effect on a given point gradient will change, in a wave-like manner (or wake-like), and the change will propagate at the speed of light: you would have to adjust the influence vector behind where that body actually is, except, you can only observe that body based on what you can see, so where it was will be consistent with its gravitational effect. Gravity is a very weak force, so the waves in spacetime are immeasurable subtle for almost everything. But it does appear that they are there, based on what the gravitational wave detectors tell us about the really massive things. ---
**Author:** ![eschereal](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/eschereal/32/18939_2.png) [@eschereal](https://boards.straightdope.com/u/eschereal)\ **Post date:** [April 13, 2019, 7:53pm UTC](https://boards.straightdope.com/t/update-on-the-speed-of-gravity/832282/36 "2019-04-13T19:53:38Z")
> [@Andy\_L](#): > > Unaccelerated motion does not generate gravitational waves. … Give me a real example of unaccelerated motion. ---
**Author:** ![Triskadecamus](https://avatars.discourse-cdn.com/v4/letter/t/b19c9b/32.png) [@Triskadecamus](https://boards.straightdope.com/u/Triskadecamus)\ **Post date:** [April 13, 2019, 11:50pm UTC](https://boards.straightdope.com/t/update-on-the-speed-of-gravity/832282/37 "2019-04-13T23:50:28Z")
> [@eschereal](#): > > Give me a real example of unaccelerated motion. There was some guy who claimed that an object in motion, tends to remain in motion. That would be unaccelerated motion. Not sure how reliable that guy was, but I’ve heard him quoted a lot. Tris ---
**Author:** ![Pleonast](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/pleonast/32/1183_2.png) [@Pleonast](https://boards.straightdope.com/u/Pleonast)\ **Post date:** [April 15, 2019, 3:35pm UTC](https://boards.straightdope.com/t/update-on-the-speed-of-gravity/832282/38 "2019-04-15T15:35:50Z")
> [@eschereal](#): > > Give me a real example of unaccelerated motion. Any object following a ballistic path, including anything in free orbit. ---
**Author:** ![Triskadecamus](https://avatars.discourse-cdn.com/v4/letter/t/b19c9b/32.png) [@Triskadecamus](https://boards.straightdope.com/u/Triskadecamus)\ **Post date:** [April 15, 2019, 5:33pm UTC](https://boards.straightdope.com/t/update-on-the-speed-of-gravity/832282/39 "2019-04-15T17:33:07Z")
> [@Pleonast](#): > > Any object following a ballistic path, including anything in free orbit. An orbit is inherently accelerated. Gravitational attraction between any two massive objects with vectors that do not collide will accelerate those objects toward each other. The path resulting from those forces will cause the objects to orbit one of the foci of an ellipse. If the original vectors are large enough, the path will be parabolic, or hyperbolic. The acceleration will still apply. Tris * * * Come together. ---
**Author:** ![Pleonast](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/pleonast/32/1183_2.png) [@Pleonast](https://boards.straightdope.com/u/Pleonast)\ **Post date:** [April 15, 2019, 8:28pm UTC](https://boards.straightdope.com/t/update-on-the-speed-of-gravity/832282/40 "2019-04-15T20:28:24Z")
> [@Triskadecamus](#): > > An orbit is inherently accelerated. Gravitational attraction between any two massive objects with vectors that do not collide will accelerate those objects toward each other. The path resulting from those forces will cause the objects to orbit one of the foci of an ellipse. > > If the original vectors are large enough, the path will be parabolic, or hyperbolic. The acceleration will still apply. Yes, that is correct when applying Newtonian gravitation, but doesn’t work when discussing the speed of gravity or gravitational waves. Gravity is a fictitious force in the same sense that centrifugal forces are. It’s an artifact of the frame of reference. An object in free fall (that is a ballistic path or free orbit) is not experiencing any acceleration in their own frame of reference. This is what creates the phenomenon of “zero-gravity”. I’m not very good at explaining this; it’s been too long since I’ve studied it. Some quick links that might explain it better, [Proper Acceleration on Wikipedia](https://en.wikipedia.org/wiki/Proper_acceleration) and [Curved Space on Physics of the Universe site](https://www.physicsoftheuniverse.com/topics_relativity_curved.html). On a slightly related tangent, if a system of orbiting objects were emitting gravitational waves, then the system would be losing energy. [Previous page](https://boards.straightdope.com/t/update-on-the-speed-of-gravity/832282.md?page=1) [Next page](https://boards.straightdope.com/t/update-on-the-speed-of-gravity/832282.md?page=3)