# What is gravity? **URL:** \ **Category:** Great Debates\ **Created:** [February 25, 2006, 7:02pm UTC](https://boards.straightdope.com/t/what-is-gravity/346087 "2006-02-25T19:02:02Z")\ **Posts on this page:** 20\ **Page:** 2
**Author:** ![Qadgop\_the\_Mercotan](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/qadgop_the_mercotan/32/83_2.png) [@Qadgop\_the\_Mercotan](https://boards.straightdope.com/u/Qadgop_the_Mercotan)\ **Post date:** [February 25, 2006, 9:33pm UTC](https://boards.straightdope.com/t/what-is-gravity/346087/21 "2006-02-25T21:33:35Z")
> [@ritchpope](#): > > the object extends beyond the space which it occupies. Do you have a cite for this assertion? ---
**Author:** ![AndrewL](https://avatars.discourse-cdn.com/v4/letter/a/3d9bf3/32.png) [@AndrewL](https://boards.straightdope.com/u/AndrewL)\ **Post date:** [February 25, 2006, 9:46pm UTC](https://boards.straightdope.com/t/what-is-gravity/346087/22 "2006-02-25T21:46:27Z")
How does your theory account for the observed inverse-suare force of gravity as observed in planetary orbits and the paths of space probes? And how does it account for tidal forces? ---
**Author:** ![Contrapuntal](https://avatars.discourse-cdn.com/v4/letter/c/e274bd/32.png) [@Contrapuntal](https://boards.straightdope.com/u/Contrapuntal)\ **Post date:** [February 25, 2006, 10:01pm UTC](https://boards.straightdope.com/t/what-is-gravity/346087/23 "2006-02-25T22:01:15Z")
> [@ritchpope](#): > > When objects meet gravity is caused because the object extends beyond the space which it occupies. Yeahbut subjects actually contract from the space which they occupy, so everything cancels out. So there is no gravity. Only _gravitas_. Which is sadly lacking in the OP. ---
**Author:** ![Liberal](https://avatars.discourse-cdn.com/v4/letter/l/848f3c/32.png) [@Liberal](https://boards.straightdope.com/u/Liberal)\ **Post date:** [February 25, 2006, 10:22pm UTC](https://boards.straightdope.com/t/what-is-gravity/346087/24 "2006-02-25T22:22:53Z")
> [@ritchpope](#): > > It’s hard to explain unless I draw a picture. Fair enough. Please use alphabetical characters and numbers along with signs from algebra and the calculus in your portrait. ---
**Author:** ![BrainGlutton](https://avatars.discourse-cdn.com/v4/letter/b/82dd89/32.png) [@BrainGlutton](https://boards.straightdope.com/u/BrainGlutton)\ **Post date:** [February 26, 2006, 12:04am UTC](https://boards.straightdope.com/t/what-is-gravity/346087/25 "2006-02-26T00:04:17Z")
> [@ritchpope](#): > > The molecules of an object extend beyond what is immediately tangible and the invisible extent of these objects can interlock with one another. Extend how far? Gravity operates across interplanetary or interstellar distances. ---
**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:** [February 26, 2006, 12:14am UTC](https://boards.straightdope.com/t/what-is-gravity/346087/26 "2006-02-26T00:14:04Z")
> [@Liberal](#): > > I thought Einstein settled this question: gravity is the curvature of spacetime, isn’t it? To address a serious point in an otherwise perposterously nonsensical thread, General Relativity does posit that gravitational attraction is the result of a curvatuve of spacetime, which is caused by the presence of energy bound into the form of mass. This begs the question of what exactly the hell is this “spacetime” business. It’s all well and good to talk about bedsheets and trampolines, but it is made of some kind of aetherious fabric, or a field of fundamental energy, or 800 thread count pure Egyptian cotton, or what? The answer to that is, quite simply, we have no idea. Many models have been proposed, some of which seem to work mathematically but have no qualifiable physical instantiation. Since we can only infer its existance and properties from how it acts upon measurable objects, our ability to grok the nature of spacetime is very limited. All we know about spacetime is that it is some kind of plenum upon which forces act, or fields are propogated, or energy packets are transmitted, or something. The model of curved space–which can be defined independent of GR, by the way–is a highly successful one when describing GR and “regular” gravitational interactons on a macro scale, but not nearly so good when it comes to working out the mechanics of subatomic and fundamental particle interactions, hence the massive disconnect between GR and quantum mechanics. It’s enough to drive one to drink, and say, that sounds like a pretty good idea right now. See you at the bar. Stranger ---
**Author:** ![ultrafilter](https://avatars.discourse-cdn.com/v4/letter/u/3d9bf3/32.png) [@ultrafilter](https://boards.straightdope.com/u/ultrafilter)\ **Post date:** [February 26, 2006, 12:59am UTC](https://boards.straightdope.com/t/what-is-gravity/346087/27 "2006-02-26T00:59:57Z")
> [@Stranger On A Train](#): > > General Relativity does posit that gravitational attraction is the result of a curvatuve of spacetime, which is caused by the presence of energy bound into the form of mass. Not quite so simple. GR also works with flat spacetime and objects that curve in the presence of large masses. ---
**Author:** ![Liberal](https://avatars.discourse-cdn.com/v4/letter/l/848f3c/32.png) [@Liberal](https://boards.straightdope.com/u/Liberal)\ **Post date:** [February 26, 2006, 1:09am UTC](https://boards.straightdope.com/t/what-is-gravity/346087/28 "2006-02-26T01:09:16Z")
> [@ultrafilter](#): > > Not quite so simple. GR also works with flat spacetime and objects that curve in the presence of large masses. But that’s just the application of SR to GR, isn’t it? I mean, it’s really just a question of frame relation as to whether object O is shape S with respect to plane P, or whether plane P is shape S with respect to object O, right? ---
**Author:** ![rwjefferson](https://avatars.discourse-cdn.com/v4/letter/r/a6a055/32.png) [@rwjefferson](https://boards.straightdope.com/u/rwjefferson)\ **Post date:** [February 26, 2006, 1:28am UTC](https://boards.straightdope.com/t/what-is-gravity/346087/29 "2006-02-26T01:28:23Z")
> [@ritchpope](#): > > Gravity is a result of cohesion and adhesion. Consider gravity as the flow of spacetime into [mass](http://antwrp.gsfc.nasa.gov/apod/ap060219.html). Think of it as as high pressure flowing flowing into [low](http://www.redorbit.com/images/images-of-the-day/img/11378/tropical_cyclone_boloetse/index.html) raised a dimension. r~ ---
**Author:** ![dotchan](https://avatars.discourse-cdn.com/v4/letter/d/9d8465/32.png) [@dotchan](https://boards.straightdope.com/u/dotchan)\ **Post date:** [February 26, 2006, 1:29am UTC](https://boards.straightdope.com/t/what-is-gravity/346087/30 "2006-02-26T01:29:10Z")
Einstein’s theory seems to be a tautology, though. If the objects are following curves in space-time, how does an object “know” it should be traveling towards a more massive object? How does mass cause space-time to bend, anyway? And why? (BTW, I once read an interesting theory that gravity is what keeps the uncertainty principle from working on a larger scale; once an object gets big enough, the wave function “collapses” so that the probability of said object existing in multiple places at once becomes vanishingly small.) ---
**Author:** ![Tevildo](https://avatars.discourse-cdn.com/v4/letter/t/b9bd4f/32.png) [@Tevildo](https://boards.straightdope.com/u/Tevildo)\ **Post date:** [February 26, 2006, 1:43am UTC](https://boards.straightdope.com/t/what-is-gravity/346087/31 "2006-02-26T01:43:19Z")
> [@dotchan](#): > > Einstein’s theory seems to be a tautology, though. > > If the objects are following curves in space-time, how does an object “know” it should be traveling towards a more massive object? > > How does mass cause space-time to bend, anyway? And why? As I understand it (and I am not a physicist), objects will follow the path of “least action” - that is, the path that has the least energy expenditure associated with it. (Action, I seem to remember, is energy x time, but I may be wrong). In Newtonian space, the path of least action between two points, in the absence of a gravitational field, is a straight line. Einstein’s theory - as I understand it - is that the presence of other masses causes this line to become non-straight, so that objects follow the elliptical/parabolic paths of Newtonian gravity. ---
**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:** [February 26, 2006, 2:21am UTC](https://boards.straightdope.com/t/what-is-gravity/346087/32 "2006-02-26T02:21:37Z")
> [@dotchan](#): > > (BTW, I once read an interesting theory that gravity is what keeps the uncertainty principle from working on a larger scale; once an object gets big enough, the wave function “collapses” so that the probability of said object existing in multiple places at once becomes vanishingly small.) I’m not conversant with the theory you allude to, but the reason that large objects–that is to say, collections of fundamental particles–don’t appear to display quantum mechanical behavior is because they represent an aggregate of all “wave functions” of the particles that make them up; because it is unlikely that all particles (in a normal body) would be in alignment, the overall body appears discrete; an electron may be toddling around halfway across the galactic disk for a split-second, but in the scheme of things it’ll never be missed. The likelyhood that all particles in an entire bowling ball will suddenly disappear is literaly infinitesmal, unlikely to occur in the lifespan of the Universe. Wave function collapse is a completely different, and frankly poorly understood, phenomenon, to the point that there are religious battles today between physicists as to what the term means and how it occurs which makes the whole Windows vs. Mac controversy look like a disagreement over which fork to use with the salad. There are large arrays of particles that can behave in a quantum mechanical fashion; see [superfluids](http://en.wikipedia.org/wiki/Superfluid). Stranger ---
**Author:** ![Epimetheus](https://avatars.discourse-cdn.com/v4/letter/e/c4cdca/32.png) [@Epimetheus](https://boards.straightdope.com/u/Epimetheus)\ **Post date:** [February 26, 2006, 3:08am UTC](https://boards.straightdope.com/t/what-is-gravity/346087/33 "2006-02-26T03:08:01Z")
> [@Stranger On A Train](#): > > This begs the question of what exactly the hell is this “spacetime” business. It’s all well and good to talk about bedsheets and trampolines, but it is made of some kind of aetherious fabric, or a field of fundamental energy, or 800 thread count pure Egyptian cotton, or what? Well, In my experience whatever it is, it isn’t very comfortable. And is full of all kinds of static electricity. ---
**Author:** ![ParentalAdvisory](https://avatars.discourse-cdn.com/v4/letter/p/53a042/32.png) [@ParentalAdvisory](https://boards.straightdope.com/u/ParentalAdvisory)\ **Post date:** [February 26, 2006, 10:20am UTC](https://boards.straightdope.com/t/what-is-gravity/346087/34 "2006-02-26T10:20:17Z")
> [@ritchpope](#): > > Think of it like the wind. You can feel it but you cannot see it. Though with the right tools air can be seen. Slight hijack. Are you sure about this? I mean, you can dispel smoke into the air and see the smoke move around, as it’s manipulated by the air. Therefore proving air exists… But I don’t see how you can actually see the air when doing this. What tool can be used to “see” air? ---
**Author:** ![Der\_Trihs](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/der_trihs/32/233_2.png) [@Der\_Trihs](https://boards.straightdope.com/u/Der_Trihs)\ **Post date:** [February 26, 2006, 10:55am UTC](https://boards.straightdope.com/t/what-is-gravity/346087/35 "2006-02-26T10:55:06Z")
> [@ParentalAdvisory](#): > > Slight hijack. Are you sure about this? I mean, you can dispel smoke into the air and see the smoke move around, as it’s manipulated by the air. Therefore proving air exists… But I don’t see how you can actually see the air when doing this. What tool can be used to “see” air? Heat shimmer or really powerful shock waves ( like from fuel-air explosives; you can see the shockwave easily ) might count. You could also say that the blue sky counts as seeing air, I suppose. ---
**Author:** ![Liberal](https://avatars.discourse-cdn.com/v4/letter/l/848f3c/32.png) [@Liberal](https://boards.straightdope.com/u/Liberal)\ **Post date:** [February 26, 2006, 4:58pm UTC](https://boards.straightdope.com/t/what-is-gravity/346087/36 "2006-02-26T16:58:08Z")
Gravity is like the wind isn’t a theory; it’s a metaphor. ---
**Author:** ![RM\_Mentock](https://avatars.discourse-cdn.com/v4/letter/r/e274bd/32.png) [@RM\_Mentock](https://boards.straightdope.com/u/RM_Mentock)\ **Post date:** [February 26, 2006, 5:14pm UTC](https://boards.straightdope.com/t/what-is-gravity/346087/37 "2006-02-26T17:14:15Z")
> [@dotchan](#): > > Einstein’s theory seems to be a tautology, though. A tautology? I wouldn’t say that (I’m resisting quoting from the princess bride here). Einstein’s general relativity may turn out to be a way of describing the interconnected effects of various particles. We know light is not made up of rays, but ray-tracing calculations are very successful in predicting the path of light. Similarly, gravity may be the result of particle interaction–perhaps in a way that could be described as adhesion or cohesion. ---
**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:** [February 26, 2006, 5:25pm UTC](https://boards.straightdope.com/t/what-is-gravity/346087/38 "2006-02-26T17:25:13Z")
> [@Liberal](#): > > Gravity is like the wind isn’t a theory; it’s a metaphor. Although I don’t have the text in front of me and cannot quote verbatim, I believe that Heinlein (in the guise of Jubal Harshaw) once wrote “Analogy is even slipperier than logic,” as a means of reasoning out a problem. By sufficiently torturing the language, one can “prove” nearly anything by analogy or metaphor. In logical analysis, this type of reasoning is called a Fallacy of False Analogy; attempting to relate two manifestly unlike phenomena by comparing superficial similarities. One can see the result of this type of reasoning in the so-call wave/particle duality ; because fundamental particles behave with the properties of both waves and particles there is an underlying assumption by many that there is some kind of paradox, when the truth is that fundamental “particles” are neither waves or little bits of stuff, but something entirely different and outside the bounds of normal experience. As for the OP’s “theory”…no, sorry, I refuse to even go there. **John Mace** has already made the appropriate challenges and for his troubles received only complaint of his focus on “semantics” in rebuttal. Instead, I’ll turn out another Heinlein quote: “If it can’t be expressed in figures, it is not science; it is opinion.” Stranger ---
**Author:** ![Liberal](https://avatars.discourse-cdn.com/v4/letter/l/848f3c/32.png) [@Liberal](https://boards.straightdope.com/u/Liberal)\ **Post date:** [February 26, 2006, 5:27pm UTC](https://boards.straightdope.com/t/what-is-gravity/346087/39 "2006-02-26T17:27:56Z")
If his theory _were_ a tautology, then it would be true by definition. I think Dotchan meant that it was circular. But then, that’s the case with every empirical theory (although relativity was developed deductively). “There is no essential distinction between scientific measures and the measures of the senses. In either case our acquaintance with the external world comes to us through material channels; the observer’s body may be regarded as part of his laboratory equipment.” — Arthur Eddington ---
**Author:** ![Liberal](https://avatars.discourse-cdn.com/v4/letter/l/848f3c/32.png) [@Liberal](https://boards.straightdope.com/u/Liberal)\ **Post date:** [February 26, 2006, 5:33pm UTC](https://boards.straightdope.com/t/what-is-gravity/346087/40 "2006-02-26T17:33:05Z")
Interesting bits about Heinlen, Stranger. Analogics is a type of logic, but is slippery as soap and must be handled properly. Also, I’d point out to Heinlen that I think he meant “measurements” rather than “figures”. Figures find their best context in the analytical statements of deductive logic, rather than the empirical statements of science. [Previous page](https://boards.straightdope.com/t/what-is-gravity/346087.md?page=1) [Next page](https://boards.straightdope.com/t/what-is-gravity/346087.md?page=3)