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

What is the fabric of space time really like?

  1. Aug 16, 2014 #1
    Is space time like a bowling ball on a trampoline or does it surround all matter like water does a submerged object?
     
  2. jcsd
  3. Aug 16, 2014 #2

    Dale

    Staff: Mentor

    Neither analogy is very good. I think that it is probably best to learn about spacetime directly rather than by poor analogies to other things which do not reflect most of the properties of spacetime.
     
  4. Aug 16, 2014 #3
    i was just using those as example because it is most seen and worded as space time being one dimensional and the water one was just an example of it being surrounding and 3d that surrounds the matter.
     
  5. Aug 16, 2014 #4

    phinds

    User Avatar
    Gold Member
    2016 Award

    As Dale said, the analogies are seriously flawed. Space-time isn't a THING at all, it is a framework in which matter and energy exist. It's just like length. It isn't a thing, but you can measure the relative positions of things in it.
     
  6. Aug 16, 2014 #5
    what i was trying to refer to was the concept of the gravity of matter bending space time and how it would be, since most diagrams show it as only bending space/time on one side.
     
  7. Aug 17, 2014 #6

    Nugatory

    User Avatar

    Staff: Mentor

    Those diagrams are seriously misleading.
    Here's a recent thread you may find helpful: https://www.physicsforums.com/showthread.php?t=760793 and pay particular attention to the video in #3 of that thread.
     
  8. Aug 17, 2014 #7

    Drakkith

    User Avatar

    Staff: Mentor

    Actually it's a mix of both. The "bending" of spacetime is like the trampoline, but this happens in three dimensions like the water surrounding a submerged object. The best picture I've seen is a 3d grid of lines that bend when passing by a massive object.
     
  9. Aug 17, 2014 #8

    timmdeeg

    User Avatar
    Gold Member

    In my opinion any static picture might be misleading as the function of time being part of "space-time" is missing. I therefore prefer to identify the action of gravity with the motion of neighbouring inertial bodies. If their geodesics are bent towards or away from each other (due to relative acceleration) then the space-time in this region is curved, otherwise it is flat.
    In this sense you can use the trampoline picture by watching how the distance between some balls which are rolling towards the center develops over time.
     
    Last edited: Aug 17, 2014
  10. Aug 17, 2014 #9

    A.T.

    User Avatar
    Science Advisor
    Gold Member

    If time is a dimension of the diagram, then the diagram itself can be static and yet show the development over time. That is the whole point of a space-time diagram. But it still can be helpful to animate a space-time diagram, to show the progress of time:

    https://www.youtube.com/watch?v=jlTVIMOix3I

    https://www.youtube.com/watch?v=DdC0QN6f3G4

    The trampoline is not a space-time diagram, just a space diagram. But the worldlines of free fallers are geodesics in space-time, not space.
     
  11. Aug 17, 2014 #10

    Chronos

    User Avatar
    Science Advisor
    Gold Member

    The problem with spacetime diagrams is it is very difficult to depict time distortion. That is the missing key to understanding gravity. I see A.T. beat me to the punch.
     
  12. Aug 17, 2014 #11

    Dale

    Staff: Mentor

    The mathematical language used to describe spacetime is called Riemannian geometry.

    Riemannian geometry is used to describe geometry on spaces (called manifolds) that may be curved, such as the 2D surface of a sphere. It describes the curved manifold geometry entirely from quantities that can be measured within the manifold, e.g. on the 2D surface of a sphere without reference to the 3rd dimension.

    In Riemannian geometry spacetime is a 4D curved manifold with a "signature" that leads to 3 dimensions of space and one dimension of time.
     
  13. Aug 17, 2014 #12

    timmdeeg

    User Avatar
    Gold Member

    That's right. In case my wording wasn't correct, kindly let me know.
     
  14. Aug 17, 2014 #13
    Every time I see someone use the phrase fabric of spacetime it makes me ill. As stated above, there's no such "thing" as the "fabric" of spacetime. The term "fabric" is a term used as an analogy to allow one to visualize something which is curving.

    It makes people suspicious when they hear physicists say that gravity is curving the "fabric" of spacetime when there's no "thing" to be curved. At least it was that way for me.
     
  15. Aug 18, 2014 #14

    tom.stoer

    User Avatar
    Science Advisor

    But in every theory of (quantum) gravity there is a "mathematical entity" like a 4-manifold, a Hilbert space of spin networks etc.
     
  16. Aug 18, 2014 #15
    Yes. That's quite right. Spacetime is very much a mathematical construct, a manifold. It's just not something which is made of paper and fig leaves or any other kind of matter exotic or otherwise.
     
  17. Aug 18, 2014 #16

    tom.stoer

    User Avatar
    Science Advisor

    I fully agree
     
Know someone interested in this topic? Share this thread via Reddit, Google+, Twitter, or Facebook




Similar Discussions: What is the fabric of space time really like?
  1. Space Time Fabric (Replies: 8)

  2. Space-time fabric (Replies: 3)

Loading...