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B Question about gravity

  1. Oct 28, 2016 #1
    Based on this website and the image that is portrayed it shows a dip in space going downward. I'm just curious as to how are these images made if there is no sense of direction in space? Shouldn't gravity be warped around the planet and not dipped downward, and if so, why are we informed with these types of images?



    http://spaceplace.nasa.gov/what-is-gravity/en/
     
  2. jcsd
  3. Oct 28, 2016 #2

    phinds

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    These pop-science depictions are a gross oversimplification of what's going on, as is absolutely typical of such presentations. Never think you are learning actual science from pop-science presentations. Yes, spacetime IS warped all around the planet but in a way that really can't be shown in 2D.

    In fact, the whole "warping" statement is a bit of a misnomer since it is an attempt to use an English language word to describe something that really can only be properly described in the language of physics, which is math.
     
  4. Oct 28, 2016 #3
    Thank you on the clarification.
     
  5. Oct 28, 2016 #4

    David Lewis

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    In a simplified 2D model of curved spacetime, if you had a frictionless table with a depression in the center, a marble would roll downhill toward the center, or enter an elliptical orbit around the center as if there were a distance-dependent force attracting it.
     
    Last edited: Oct 28, 2016
  6. Oct 28, 2016 #5

    phinds

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    (1) Only if the object was spherical, or similar, whereas in actual space-time that geometric characteristic is irrelevant
    (2) You are only re-enforcing the flawed analogy, not helping the OP to understand why it is flawed.

    @ItchyFungus, what really happens is that when properly described. there is no "warping" at all and light doesn't bend around dense objects. Yes, that is a correct description when viewed through the lens of Euclidean geometry but the fallacy is that spacetime doesn't USE Euclidean geometry, it uses Riemann geometry and objects follow straight lines (formally called "geodesics") in that geometry.
     
  7. Oct 29, 2016 #6
    Will you say this video is a pretty well explanation of geodesics as you previously explained?



    I understand why they use Riemann geometry because there is no mass in space as oppose to planet or star, so it makes sense why they use two different types of geometry to figure it out.
     
  8. Oct 29, 2016 #7
    I would assume for a more accurate video they will have to show a planet bending spacetime around it then showing how an object interferes with it. The reason I find this personally important is because it helps make sense on why there is a tug and pull with the Earth and the Moon, or any other planet or star in the galaxy.
     
  9. Oct 29, 2016 #8

    Fervent Freyja

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    What is gravitational lensing then? Delivering the OP a wrong statement that contradicts GR doesn't help.

    I recall coming across a video somewhere that addressed this very common misrepresentation. Nothing you have wrote was even brought up, but oddly, what David Lewis posted was perfectly in line with the accurate representation that the video attempted to put across!

    Since you are insisting that a contributing member should, for some reason, abide by your judgement alone when posting, then I will suggest this: Disrespecting contributing members in front of young people isn't being a good role model. It's bad taste and devalues their post. That should be addressed privately if that bothersome.
     
  10. Oct 29, 2016 #9

    phinds

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    for some reason my computer is acting up this morning and I can't get any sound from the video so only watched the first minute or so but from what I saw it seems to be correctly representing what happens, although I can't comment on their descriptions since I couldn't hear anything.

    Space, as far as I am aware, is Euclidean. It is SPACETIME that follows Riemann geometry and it does so everywhere, not just out in space, and it does so BECAUSE there is mass in the universe, not because there is "no mass in space". I think your thought process on this is at least somewhat correct though because you are realizing intuitively that geodesics look Euclidean when far away from mass and only when near massive objects does it become clear that the geodesics diverge sharply from Euclidean because of the mass.

    This is sort of shown in the first minute of that video. Unfortunately, the way the video shows the Geodesics uses Euclidean geometry which I think somewhat confuses the issue. That is, the lines shown in the first minute of the video do show the way geodesics look relative to Euclidean geometry but because they do so USING Euclidean geometry, that could be a bit misleading.

    Someone once posted here on PF a depiction of a good "3D" representation of geodesics near a massive body but I don't have a link unfortunately. That one also used Euclidean geometry in the depiction but because it showed the "warping" all around the planet it seemed a much more accurate representation than the simple "rubber sheet" model that you first asked about. I hope someone who reads this will know the one I'm referring to and have a link to it.
     
    Last edited: Oct 29, 2016
  11. Oct 29, 2016 #10

    phinds

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    I'm not sure what you mean by "an object interfering with it" so I can't comment on that.

    One of the things I think you are missing, and this is something that I definitely missed when I first learned this stuff, is that in General Relativity, there isn't really the "tug and pull" of the kind you describe. Gravity is not a force, it is geometry. That is, things move in space near massive objects, in the absence of any force, along geodesics NOT because a force makes them do so but in fact because there IS no force and they are following their natural inclination to follow a geodesic.

    So, weird as it sounds, in GR, a apple falls to the ground not because of a gravitational force but because the spacetime geometry of the Earth/apple means they have a natural path towards each other.

    Here's a thread that explores this some:
    https://www.physicsforums.com/threads/general-relativity-is-gravity-a-force.382066/
     
  12. Oct 29, 2016 #11

    phinds

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    Gravitational lensing is in fact an excellent example of my exact point. Gravitational lensing would not happen in Newtonian gravity and a Euclidean universe because light is massless. In GR, light has no mass but it has momentum because of the spacetime tensor (/ Riemann tensor) and so it follows a Riemann geodesic, not a Euclidean straight line.
     
  13. Oct 29, 2016 #12

    phinds

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    @ItchyFungus, one thing I want to add to all this.

    Consider this statement:

    Actual spacetime follows General Relativity and thus massless photons follow a geodesic which, when looked at from the point of view of Euclidean geometry are "bent" near massive objects because they do not follow Euclidean straight lines but rather follow Riemann straight lines (the"geodesics").

    Now that is an accurate statement but who wants to SAY that all the time when it is so much easier to just say "mass bends light"?

    My point is that you will hear even serious physicists use this kind of shorthand description of things. The problem with pop-sci presentation is not so much that they use shorthand descriptions, it's that they don't TELL you it's shorthand, much less do they explain what it is shorthand FOR, and this is further complicated by the fact that the shorthand usually lends itself to misinterpretation/misunderstanding by people who don't know better.
     
    Last edited: Oct 29, 2016
  14. Oct 29, 2016 #13
    With all do respect please pardon my grammar, especially in physics.

    When I mean "tug and pull" I mean the pulling between two planets or a star. For instance if the Moon pulls the earth from one side and the Sun is exactly on the opposite side it makes sea levels slightly rise. I'm trying to figure out how this is happening intuitively. You see I could be the type of person who studies science, but if I can't imagine it, it doesn't process well.

    For the 2d images, perhaps they can tell us it's a shorthand, but then show us an actual 3d image and explain what how " riemann geometry" works.

    Now what I imagine on why there is a pull between atoms, molecules, and objects is because everything is vibrating. I would think even when particles combine to make an atom that is then labeled as an element they have a different vibrational state then when they are separate. So everything, is attracting one another due to different vibrational levels. The more elements combine and form molecules along with structures there are different levels of vibrations, then you have massive forms of vibrations due to everything reacting with one another in which we call planets, and stars.

    That's as close as it gets for me to understand intuitively on how anything connects and relates in the universe. So in this sense, gravity is the relation between all particles frequency vibrations which refers to the "tugging and pulling".
     
    Last edited: Oct 29, 2016
  15. Oct 29, 2016 #14
    I agree. For teaching, I believe in simplicity. Even when searching for a problem, however, when applied I know there is a lot of information going on which is useless when presenting an idea to the masses.
     
  16. Oct 29, 2016 #15
    Also, @phinds there is no sound to the video, it's purely visual. Also what will be interesting is when someone portrays a 3d image of a planet bending space around it, it will show the gap when an object falls into that gap it becomes part of the orbit. The gap after doing research is labeled to the atmospheres if I'm correct. I guess this is how they visualized Earth bending space because of the atmospheric levels. Once an object breaks out of the exospheric level then it's sent off into space.

    This is my source by the way.

    http://www.nasa.gov/mission_pages/sunearth/science/atmosphere-layers2.html
     
  17. Oct 29, 2016 #16

    phinds

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    Exactly. There IS NO "tug and pull", it's all the geometry of spacetime. "tug and pull" are forces and gravity is not a force in General Relativity. Personally, I find this very hard to "visualize" but that does not change the fact that it is correct.


    No, that is not correct either. You need to read up on the strong force, the weak force, and the electromagnetic force. Vibration has nothing to do with it.
     
  18. Oct 29, 2016 #17

    phinds

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    I don't know how you reached that conclusion but it is not correct. The geometry of spacetime is indifferent to the existence of atmosphere. For example, there is a geodesic that runs from the center of the moon to the center of the Earth.
     
  19. Oct 29, 2016 #18

    phinds

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    "useless" only if you don't care that "the masses" are getting an incorrect picture of scientific phenomena. Simplicity is great but too much simplicity is often very misleading or just downright wrong.
     
  20. Oct 29, 2016 #19

    phinds

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    @ItchyFungus, I hope you are not offended that I keep telling you you are wrong. My intent is to help you learn, not make you feel bad.
     
  21. Oct 29, 2016 #20
    Interesting.

    I appreciate all of your input. This has been on my mind for a few years now and you've helped in clarifying it.

    I also have to look up on the forces as you explained earlier.

    I'm not exactly sure what words they use for vibrating particles, but when I mean vibration I mean the state of the particle. I think it's frequency? Basically in my mind on a quantum level everything is static like and connected.
     
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