Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

B Space Dilation Interaction Between Two Masses

  1. May 13, 2017 #1
    Edit: Disclaimer, this post contains faulty ideas

    I'm a 'pedestrian' to physics trying to wrap my mind around various concepts of relativity, and one of those is a visualization of two, say, planets, say each starting at rest. By my understanding, their mass causes space to relatively compact/condense more and more closer to the planet. But of course if they're close enough to each other, they'll begin to accelerate towards each other. My conception of this was that, even though a given planet's gravitational 'field' of compacted spacetime did not cause it to move in any direction, the interaction of the two compacting 'fields' had a compounding effect in each other's (distant) spacetime vicinity, essentially multiplying off each other's manipulation of space such that it causes them to move through the space surrounding them. Or causing the space to flow past them and out from in between them, if it's relative in that way. Is this an accurate conceptualization of General Relativity/spacetime/gravity, and specifically the mechanism by which said two planets would accelerate towards one another through what we'd observe as 'space'? It's a concept I feel very compelled towards, in trying to cultivate my overall understanding of spacetime and such, but I haven't been able to find satisfactory results through searches to quell my curiosity.
     
    Last edited: May 13, 2017
  2. jcsd
  3. May 13, 2017 #2
    I like the "rubber sheet" analogy the best for visualizing gravity wells...
     
  4. May 13, 2017 #3

    Dale

    Staff: Mentor

    I don't recognize any relationship between the math of GR and what you wrote. Do you have a specific reference that discusses the "compression" idea or the "compounding" effect? It might help for us to read those sources and determine what they intended to convey.
     
  5. May 13, 2017 #4

    phinds

    User Avatar
    Gold Member
    2016 Award

    There is no such thing. You have to say at rest relative to WHAT? If you mean at rest relative to each other, that's fine, but you have to be specific.

    (1)Space is not a thing that can "compact/condense" and in any event, (2) you mean spacetime, not space.


    That happen regardless of whether they are "close" or not.

    Again, it doesn't.

    The gravitational attraction between the two requires the existence of both of them so I'm not clear what you mean here.

    Space doesn't "flow". It isn't a thing that can do that sort of thing

    see comments above
    Keep reading. It can be confusing at first.

    Also, I should mention that your title makes no sense. There is no such thing as "space dilation". Things get farther apart but that's not any kind of dilation of space. Google "metric expansion"
     
  6. May 13, 2017 #5
    I'm aware of that but I didn't think it addressed my queries about the precise nature of the gravity/spacetime interactions.
     
  7. May 13, 2017 #6

    phinds

    User Avatar
    Gold Member
    2016 Award

    It won't. It's an analogy, and like most analogies it has its flaws.
     
  8. May 13, 2017 #7

    Dale

    Staff: Mentor

    Almost nobody else here likes it, but to each his own.
     
  9. May 13, 2017 #8
    What are 'gravity wells' if not contraction of spacetime? Is the 'curvature' concept something else entirely? Do you have a visual/spacial concept of the 'curvature', or do you basically rely on a very functional/abstract conceptualization?

    And gravity is just mass (or energy) manipulating/distorting spacetime, so by what mechanism does that cause two planets to accelerate through spacetime towards each other? If not by... effectively reducing the amount of space between them, or something. Isn't the 'gravitational attraction' idea Neutonian and inaccurate?
     
    Last edited: May 13, 2017
  10. May 13, 2017 #9
    No, it was my attempt to make the 'gravity wells'/manipulation of spacetime/curvature concept more comprehensive and understandable, based on basic info/explanations, but it seems it's not right.
     
  11. May 13, 2017 #10

    Dale

    Staff: Mentor

    Yeah, I am sorry to be the bearer of bad news, but if this is your personal explanation then I would discard it entirely and go back to learning the existing material first before trying to simplify it. I think that pursuing a personal analogy like this will be a bigger and bigger obstacle to really learning the theory the longer that you spend on it.
     
  12. May 13, 2017 #11

    phinds

    User Avatar
    Gold Member
    2016 Award

    Things in spacetime, absent any external force (and gravity is not a force in GR) travel on straight lines, but these "straight lines" are more formally called "geodesics" and they are defined by the geometry of spacetime and the geometry of spacetime is affected by mass. That's the simple way of saying that massive bodies "attract" each other by affecting the geometry of spacetime and creating straight lines towards each other, not by doing anything to "space" which, again, is not a thing, it's just a component of spacetime.
     
  13. May 13, 2017 #12

    PeroK

    User Avatar
    Science Advisor
    Homework Helper
    Gold Member

    That's definitely the best way to visualise a rubber sheet, where the balls move down under the influence of gravity. But, if the shape of the rubber sheet is gravity, then why don't the balls move up the sheet or stay where they are? What is the force pushing the balls down the sheet or stopping them moving wherever they like?

    If you took that curved sheet onto the space station, the balls would no pay no attention to its shape!
     
  14. May 13, 2017 #13
    On the space station, the sheet would be almost perfectly flat, unlike here on Earth where the "spacetime" sheet has a curve under it's own weight! Spacetime doesn't warp itself!
     
  15. May 13, 2017 #14

    PeroK

    User Avatar
    Science Advisor
    Homework Helper
    Gold Member

    It wouldn't be flat if you pushed a mass down into it.

    Anywhere, how is time involved in the rubber sheet analogy? Isn't the rubber sheet just a curvature of space?
     
  16. May 13, 2017 #15
    I think the "compounding" you were trying to describe is when two masses approach each other the combined well is deeper, but as two masses circle closer some of the "warping" is dissipated as gravitational waves...
    Time is still time... a two dimension plus time representation uses the third spatial dimension (that we see) as a representation of the amount of "warp" which has to be imagined as a fourth spatial dimension in three dimensions plus time, also known as "spacetime".
     
  17. May 13, 2017 #16

    PeroK

    User Avatar
    Science Advisor
    Homework Helper
    Gold Member

    Well, we'll have to disagree. The rubber sheet analogy, in my opinion, is highly misleading for many reasons - not least because it cannot represent the curvature of time at all (only of space) and it clearly represents gravity as a force pushing things "down" along a curve. That is Newtonian gravity, not General Relativity! Without Newtonian gravity in the picture, there is no reason for a ball to move down the sheet rather than up the sheet - or to stay where it is.
     
  18. May 13, 2017 #17
    To someone who might think gravity just "instantly pulls things together at a consistent rate", it should certainly show them how masses accelerate towards each other, with the force increasing as they approach...
    Does general relativity include the warping of time?
     
  19. May 13, 2017 #18

    phinds

    User Avatar
    Gold Member
    2016 Award

    What do you mean by "the warping of time"?
     
  20. May 13, 2017 #19
    That's why I asked...
     
  21. May 13, 2017 #20

    PeroK

    User Avatar
    Science Advisor
    Homework Helper
    Gold Member

    A particle moves in GR not because of a force but by following the Lagrangian principle of trying to maximise the spacetime distance it travels. In the absence of gravity, this reduces to constant velocity (in a straight line).

    Mass curves spacetime in the sense that the distance between points in spacetime depends on the mass and the spatial "distance" from the mass. The curvature due to a spherical mass like the Earth or the Sun affects both the time and space coordinates.

    This is hard to visualise, I agree, and it takes some mathematics to go from the equations of spacetime curvature to the familiar equations of motion - which are very similar to the Newtonian equations.

    Personally, I feel you have to bite the Lagrangian bullet when presenting GR and at least give some explanation of why things move. The problem is that the naive student or lay-person will accept the rubber sheet because it is so close to Newtonian gravity. Whereas, the more perceptive student will ask: "but, if there is no force, why should a particle choose one path over another"? Things only follow a shape if they are constrained by that shape (like a ball attached to a wire) or a force compels them (like gravity on a rollercoaster). And, since particles are clearly not attached to spacetime and there is no force, why do they follow the paths they do?

    In short, the problem with the rubber sheet analogy is that it stops you really thinking about why things move in the absence of a force.
     
    Last edited: May 13, 2017
Know someone interested in this topic? Share this thread via Reddit, Google+, Twitter, or Facebook

Have something to add?
Draft saved Draft deleted