# Gravity question **URL:** \ **Category:** Factual Questions\ **Created:** [June 12, 2017, 7:14pm UTC](https://boards.straightdope.com/t/gravity-question/788796 "2017-06-12T19:14:59Z")\ **Posts on this page:** 13\ **Page:** 2
**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:** [June 13, 2017, 12:16am UTC](https://boards.straightdope.com/t/gravity-question/788796/21 "2017-06-13T00:16:58Z")
**igor frankensteen** , the link doesn’t mention, but that geoid map has a highly exaggerated vertical scale. If they hadn’t exaggerated the scale, it would look to the eye just like a sphere. ---
**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:** [June 13, 2017, 12:20am UTC](https://boards.straightdope.com/t/gravity-question/788796/22 "2017-06-13T00:20:00Z")
> [@eschereal](#): > > Logically, it seems to me that gravity could not be expressed as an effect of time dilation because the force would become repulsive. If you look at a small spherical body tangentiallly approaching a large spherical body, the side facing the large body would be moving into a slower frame of reference than the side away from the large body. Since v=d/t, the facing side would be moving faster than the far side, so one would expect the smaller body to tend to curve away from the larger body, which is the opposite of what we see happening. In almost all cases, the difference is way too small to even think about measuring. Time dilation seems to be a side-effect of gravity, not its impetus. This is not correct, and to be honest I cannot follow the train of logic you’re applying to come to this conclusion. One fundamental point is that frames of reference are defined by the observer’s motion in comparison to another frame of reference; in the case of accelerated frames of reference, the subject under greater acceleration (whether due to linear momentum transfer, constant rotation, or being within a gravitational field will be seen to experience the passage of time to be faster relative to an observer in an inertial (unaccelerated) reference frame. (The subject, of course, experience time to pass at the normal boring rate but sees the universe being shrunk in the direction of acceleration.) The time dilation experienced is not a “side-effect” of anything but rather is a fundamental relationship between the qualities we measure as space, time, and acceleration. And (as far as we can tell) it doesn’t matter what the cause of that acceleration is; we experience the same effects regardless. Stranger ---
**Author:** ![rat\_avatar](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/rat_avatar/32/255_2.png) [@rat\_avatar](https://boards.straightdope.com/u/rat_avatar)\ **Post date:** [June 13, 2017, 12:59am UTC](https://boards.straightdope.com/t/gravity-question/788796/23 "2017-06-13T00:59:06Z")
> [@Chronos](#): > > I totally missed that-- I’m going to have to keep an eye out for it the next time I see that movie. > > Mitch must like his pillows awfully thick, though: MTW is bulkier even than a CRC. Hard to detect, but it is when he is having a nightmare at the library table. Not really related to their projects but the section tabs will be very clear to anyone who has worked through it at right around the 1:00:00 mark. ---
**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:** [June 13, 2017, 1:00am UTC](https://boards.straightdope.com/t/gravity-question/788796/24 "2017-06-13T01:00:59Z")
> [@Chronos](#): > > To the OP, it’s an interesting question. It is in general the case that objects fall towards the places where time will pass more slowly. And you probably could mathematically re-cast General Relativity in those terms, while still being exactly mathematically equivalent to the theory we know. But I can say that I, personally, have never seen it actually cast that way. Just to take this point, such a description would be incapable of handling anything other than static asymptotically flat spacetimes due to the reliance on certain symmetries and asymptotic symmetries which do not generally hold in general relativity. However given that the gravitational time dilation is just a function of the g-force felt by a static observer, really showing that objects tend to be attracted to regions where time passes slower as seen by a static observer at infinity is just a matter of showing that a relativistically modified form of Newton’s law of Universal gravitation can be derived in static asymptotically flat spacetimes. ---
**Author:** ![rat\_avatar](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/rat_avatar/32/255_2.png) [@rat\_avatar](https://boards.straightdope.com/u/rat_avatar)\ **Post date:** [June 13, 2017, 2:55am UTC](https://boards.straightdope.com/t/gravity-question/788796/25 "2017-06-13T02:55:56Z")
> [@rat\_avatar](#): > > Hard to detect, but it is when he is having a nightmare at the library table. Not really related to their projects but the section tabs will be very clear to anyone who has worked through it at right around the 1:00:00 mark. I hate to add this because it is funny, I had to go and look at the movie, and try to find the page that most closely matches the movie. It looks like “18.4. Nearly Newtonian Gravitational Fields” which is on page 445 in my copy. While this is not for certain due to the angles and image quality; it is funny how apropos it is to this post, if not to the movie. (Being in track one it could be Mitch’s homework for other classes) ---
**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:** [June 13, 2017, 4:06am UTC](https://boards.straightdope.com/t/gravity-question/788796/26 "2017-06-13T04:06:52Z")
Note that Martin A. Gundersen, a Caltech alumnus and former Dabney Hovse member, was the technical advisor on the film, hence all the “DEI” references. The film is notable for having many accurate science refernces (at least by cinematic standards). “Would you prepared if gravity reversed itself? The only thing I can’t figure out is how to keep the change in my pockets.” Stranger ---
**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:** [June 13, 2017, 4:36am UTC](https://boards.straightdope.com/t/gravity-question/788796/27 "2017-06-13T04:36:23Z")
> [@Stranger\_On\_A\_Train](#): > > This is not correct, and to be honest I cannot follow the train of logic you’re applying to come to this conclusion.… The OP seemed to be musing whether GR time dilation might be driving gravity. I was merely trying to figure out how that possibly could be. It now occurs to me that my error was that _time_ dilation always accompanies a change in the other three dimensions, so my notion about moving faster should be canceled out by the change in space. ---
**Author:** ![Kevbo](https://avatars.discourse-cdn.com/v4/letter/k/e47774/32.png) [@Kevbo](https://boards.straightdope.com/u/Kevbo)\ **Post date:** [June 13, 2017, 3:37pm UTC](https://boards.straightdope.com/t/gravity-question/788796/28 "2017-06-13T15:37:33Z")
> [@Chronos](#): > > If you throw a ball straight up in the air and then catch it again, the ball is falling (i.e., accelerating downward) right from the moment it leaves your hand, even though it’s initially moving upwards. YES!! Preach it!! I have been called every version of stupid when making this exact point. It often comes up in regard to the “vomit comet” aircraft used for training astronauts: The “zero G” experience starts on the upward part of the parabolic flight profile, and continues through the apogee and decent. ---
**Author:** ![Kevbo](https://avatars.discourse-cdn.com/v4/letter/k/e47774/32.png) [@Kevbo](https://boards.straightdope.com/u/Kevbo)\ **Post date:** [June 13, 2017, 3:42pm UTC](https://boards.straightdope.com/t/gravity-question/788796/29 "2017-06-13T15:42:01Z")
> [@Stranger\_On\_A\_Train](#): > > …(MASCONS in geophysics jargon)… This doesn’t really affect terrestrial objects to a significant degree, but it definitely affects the trajectories of ballistic objects in long term orbits or requring high precision. It may be apocryphal, but I have heard of errors in early surveys due to mountains (Himalayas I think, but may have been the Andes) deflecting the plumb bobs used as a vertical reference. ---
**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:** [June 13, 2017, 4:02pm UTC](https://boards.straightdope.com/t/gravity-question/788796/30 "2017-06-13T16:02:11Z")
Well, you can define “vertical” to mean “the direction a plumb bob points”. But in that case, the error would be in assuming that all verticals are parallel to each other (or converge at the center of the Earth). ---
**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:** [June 13, 2017, 4:57pm UTC](https://boards.straightdope.com/t/gravity-question/788796/31 "2017-06-13T16:57:20Z")
Although the GR aspects of the question are inexplicably ignored by the Master (but His ways are unknowable), he speaks, briefly: [What if you fell into a tube through the earth?](http://www.straightdope.com/columns/read/154/what-if-you-fell-into-a-tube-through-the-earth) Addressed at length in: [Falling t[h]rough the center of the earth?](http://boards.straightdope.com/sdmb/showthread.php?t=213470) [THE ULTIMATE QUESTION](http://boards.straightdope.com/sdmb/showthread.php?t=37233) [If I fell to the center of the Earth](http://boards.straightdope.com/sdmb/showthread.php?t=373818) [How long to travel the Earth’s diameter? [by going through the center]](http://boards.straightdope.com/sdmb/showthread.php?t=378581) Man, am I feeling the bibliographic itch today. ---
**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:** [June 13, 2017, 6:15pm UTC](https://boards.straightdope.com/t/gravity-question/788796/32 "2017-06-13T18:15:17Z")
Aside from the fact that both questions involve gravity, I can see no connection whatsoever between that question and the one the OP is asking, nor with any other question in this thread. ---
**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:** [June 13, 2017, 6:18pm UTC](https://boards.straightdope.com/t/gravity-question/788796/33 "2017-06-13T18:18:35Z")
Center of the earth? Sphere? OK, normal to center of Earth. Gimme a break, that took 15 minutes. [Previous page](https://boards.straightdope.com/t/gravity-question/788796.md?page=1)