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

Warping of Space-time, Confusion about the illustration

  1. Nov 28, 2008 #1
    I am not a scientist, so my apologies if this question is naïve, but I have read a bit on the subject.

    Anyway, I was recently watching a documentary on Einstein and the standard illustration of Space-time being warped was presented. A massive ball was placed on the trampoline of “space-time” and a smaller ball rolling past was drawn in to the void created. Spinning like water down a drain, circling and slowly closing into the hole created.

    My question is; “Don’t you need “gravity” to draw the smaller ball downward into the void created?” The illustration makes sense here on earth but it seems to assume the whole set is subject to the downward force of gravity. I assume this illustration would not hold true in weightless space, the ball would cruise right over the hole created.

    Can the definition of gravity include gravity? Is there a larger gravity present for this set I am ignorant of? Maybe I haven’t read enough.

    I would appreciate any replys / thoughts.

    T. Mulhall
  2. jcsd
  3. Nov 29, 2008 #2
    no the definition of gravity cant include gravity. I think the idea is that what appear to us to be long range forces are in fact at a deep enough level the result of purely short range interactions.
    Last edited: Nov 29, 2008
  4. Nov 29, 2008 #3


    User Avatar

    Staff: Mentor

    This is intended only as a crude analogy to the way spacetime curvature works in general relativity. All analogies fall apart when you look at them too closely, and this one falls apart sooner than most, as you've discovered. :smile:

    Actually, an external "super-gravity" isn't really necessary for part of the curved-sheet analogy. Imagine the sheet is curved somehow to begin with, and that objects are confined somehow to move along the sheet. Then if an object approaches a "dimple" along a grazing path, it will be "deflected" by the dimple.

    Explaining how the dimple appears in the sheet is another matter, and this is where the usual analogy come up short.
  5. Nov 29, 2008 #4


    User Avatar
    Science Advisor

    Right, the visual is just to illustrate that the presence of matter "dents" space. How it does this is not known. In the annalogy, the curvature of the surface is gravity, and the gravity making the surface curve could be seen as the "unknown" cause behind it all.
  6. Nov 29, 2008 #5
    hahaha ......another matter.

    I like the dimple on a sheet image. As long as I remind myself that the image is rotated and spun so that it is three dimensional image. And then, time ( persistence of vision ? Duration? ) is added to watch the changes in attraction or repulsion of everything in response to everything else.
  7. Nov 29, 2008 #6
    Thanks for the replys. Althought some of it is already over my head. :smile: I will read on.
  8. Dec 1, 2008 #7
    Gravity is an illusion. There is no actual force that draws matter to earth. Objects appear to fall to the earth because space-time is warped and in order for matter to be inertial, it has to accelerate towards the earth! In other words: When you are in deep space stationary, you are obligated to the passage of time, but not in motion and thereby remain inertial. But in the presence of a gravitational field (space-time warp), in order to be inertial, you must appear (to others on earth) to accelerate towards earth. You can not distinguish between a free fall towards earth with all your senses shut off (barring the differential gravity between head and toes) and floating in deep space. They are the same. Only when you come into contact with earth (electrical force) one feels the injury. So, in a gravitational field, clocks run at different rates; ie as observed from earth (ground), clocks in a tower run slowly by a very infinitesimal order than ground clocks. This is what time warp! But space and time are related. So space must also be warped. This is the space-time warp.
  9. Dec 1, 2008 #8

    Jonathan Scott

    User Avatar
    Gold Member

    The "trampoline" or "stretchy sheet" analogy can at least be used to illustrate the curvature of space-time in a different way. Imagine a toy car or similar rolling on the surface, following a locally straight path. Overall, it will be deflected a bit by a dip (although not enough to make it follow a circle, as that would require vertical sides to the dip). This is very similar to the curvature of space around a central mass.

    The overall curvature of space-time effectively also has a component which accelerates objects at rest as in Newtonian gravity, so that objects which think they are moving in a straight line in the time direction (that is, with constant velocity) will actually be accelerated towards the central mass.

    Note that the word "curvature" has multiple meanings, in that things can for example be curved like the surface of a ball, or like the surface of a cone made of flat paper. As a loose and somewhat inaccurate analogy in fewer dimensions, masses create local curvature like part of the surface of a ball, but around them the curvature of the gravitational field is more like the curvature of a cone, which can be seen as locally flat.
  10. Dec 1, 2008 #9


    User Avatar

    TMulHall, I'm with you, none of the replies (so far) came close to answering your question (one I have asked myself many times). I think Alfi's answer "how the dimple appears in the sheet is another matter" hits the nail on the head. The bottom line is, no one really knows what gravity is, or for that matter, whether space is really warped or not. It seems to me that everyone just goes along with the General Relativity/Big Bang theory and makes more and more modifications to this theory to "explain" the things that don't fit in. If I were an expert, which I'm very far from being, I'd be embarrased by this situation. For another viewpoint on this, see www.metaresearch.org which may, or may not, make more sense to you. At this point, I'm just very interested but also very confused -------- but it's lots of fun trying to figure it all out.

  11. Dec 1, 2008 #10
    The above statements are very misleading, at the least, and need some contect for beginners to understand what you mean.

    Gravity is NOT an illusion. That's a crazy statement. It is real. In calssical Newtonian physics, which is very accurate for most purposes especially most gravitational situations, gravity is a FORCE GMm/r^2.... It's a very good way to solve many,many problems.

    Gravity as a result of curved spacetime is a fundamental insight from general relativity; it does not deny gravity as a force but rather goes an additional step in explaining HOW the attraction occurs. Overall, relativity fine tunes classical Newtonian at extremes of speed and gravity....but it is no the final word either. Neither relativity nor Quantum gravity work in the most extremes of the universe...lack holes and the big bang...Neither can deal with apparent singularities.
  12. Dec 1, 2008 #11
    I disagree: The actual situation is that other theories have been tried and found inadequate. String Theory can't do the job, at least not yet, nor can any of many quantum theories, nor can loop quantum gravity, nor twistor theory...We are essentially at a rather extended impasse...but all have offered insights and valuable clues...

    is a lot closer to the mark, But no one is "embarassed" because science is constantly improving and being refined. It is true that in the past twenty years ( or 25 or 30 or whatever) I don't think anything has rivaled the circa 1920's acceptance, understanding, and experimental verifications made in relativity and quantum physics...

    Nothing else is precisely understood either...not time, not energy, not space, not forces...nobody knows exactly how any are born nor how they die...

    "We know a lot, we underastand little."
  13. Dec 2, 2008 #12
    Gravity is NOT an illusion. That's a crazy statement. It is real. In calssical Newtonian physics, which is very accurate for most purposes especially most gravitational situations, gravity is a FORCE GMm/r^2.... It's a very good way to solve many,many problems.

    [vgopalak]: Whether gravity as a force is a "Reality" or illusion depends upon how you look at. Gravity can as well be modelled as a force of attraction and Newtonian model fits perfectly except strong gravity situations.

    Gravity as a result of curved spacetime is a fundamental insight from general relativity; it does not deny gravity as a force but rather goes an additional step in explaining HOW the attraction occurs

    [vgopalak]: I have to disagree with you. GR is to dispel the Newtonian picture where space and time is absolute. GR does not talk about gravity as a force. It emphasizes that the space-time curvature is what makes matter fall to earth. If GR does not deny gravity, Why do we need a theory in place of Newtonian? GR merely replaces Netwtonian with a different question. "Why does matter curve the fabric of space-time?"
  14. Dec 2, 2008 #13


    User Avatar
    Science Advisor
    Gold Member

  15. Dec 6, 2008 #14
    Could it be that gravity is not really a fundamental force? What if gravity is a process and the process creates a force? Similar to wind being a force. Wind is created due to a process, and the process creates a force that could be used in wind turbines. The process for gravity could be a large body losing or dispersing energy outwards, which causes spacetime to become curved due to the different density levels of energy or density levels of the waves that are propagating outwards from a large mass, which causes objects to "gravitate" or attract towards each other due to the different densities in spacetime. I am sure most of you may have a similar idea of this, but when you come right down to it, gravity is not really a force, but a process that creates a force.

    According to jnorman regarding gravitons in another thread "I think that the observations of lensing indicate that a quantum theory of gravity, based on gravitons interacting with every particle including individual photons, cannot be correct. for graviton interaction to be correct, a photon would have to follow a polygonal path as it passes a massive body, each segment of the polygon being, perhaps, of planck length. mind-boggling, as if my mind isnt boggled enough already..."

    So, if it is possible that the theorized graviton is not responsible for the force of gravity, then what is? It looks to me that gravity is a process of at least one of the other fundamental forces interacting with energy/mass and curved spacetime. "Just a thought"

    "The day you stop thinking is the day you die"
  16. Dec 10, 2008 #15
Know someone interested in this topic? Share this thread via Reddit, Google+, Twitter, or Facebook

Have something to add?

Similar Discussions: Warping of Space-time, Confusion about the illustration
  1. Curved/warped space time (Replies: 20)

  2. Space-Time Warping (Replies: 3)