Dismiss Notice
Join Physics Forums Today!

Three dimensions - x,y,z

  1. Dec 28, 2005 #1
    Most of us know about the three dimensions - x,y,z (space), and by adding time we get a 4-dimensional space-time continum.

    Some of our "genius" friends at NYU (and other places) believe that gravity is the result of the planets (and other objects) traveling around the dimple it creates in space-time fabric.

    Anyone have any idea on the PROOF of this concept?
  2. jcsd
  3. Dec 29, 2005 #2


    User Avatar
    Science Advisor
    Gold Member

    Welcome to these Forums hotzelj!

    Yes there is lots of evidence supporting this theory, it is called Einstein's Theory of General Relativity.

    Rather than just being a conceptual model GR precisely calculates the '"dimple created in space-time fabric" by the presence of massive objects.

    There are lots and lots of good websites explaining this concept (such as here) and the observations that verify it, although you cannot finally prove such a theory, as one day a modification or alternative may be proposed that makes even more accurate predictions, just as GR made more accurate predictions than Newtonian gravity.

    The basic predictions of GR that have been verified to high accuracy are:
    The deflection of light by the Sun (first checked during a total eclipse in 1919)
    The precession of the perihelion of Mercury, (known since the 19th century but only explained by GR.)
    The time delay of a spacecraft's radio signal that passes close to the Sun.
    The radiation of gravitational wave energy away from the double pulsar PSR1913+16.
    The gravitational red shift of light, verified by an experiment by Pound and Rebka in 1960. (But note there may be different ways of interpreting this.)
    The Eotvos experiments verifying the equivalence principle, that different objects all "fall at the same rate."

    These observations and experiments have verified GR predictions to within 1% accuracy or more. Any alternative theory of gravitation must be able to repeat or better these predictions.

    Note that there is a further 'laboratory' experiment, which is being evaluated at present, it is the Gravity Probe B satellite which has measured the precession of four very accurate gyroscopes as they orbited the Earth for over a year. (You can search for GP-B on these Forums)

    Last edited: Dec 29, 2005
  4. Dec 29, 2005 #3


    User Avatar
    Staff Emeritus
    Science Advisor

    The most direct proof is the fact that clocks at a high altitude tick faster than clocks at a low altitude.

    This effect is large enough that it has to be routinely corrected for when comparing atomic clocks (the clock in Denver, Colorado, is a prime example).

    Gravitational redshift implies that space-time must be "curved" - not in the technical sense of having a non-zero curvature tensor, but rather in the sense that we cannot explain the geometry of space-time with a standard Euclidian model (we need to add metric coefficients).

    See for instance

  5. Dec 29, 2005 #4

    Einstein also wrote that observations are dependent on the frame of reference - and since we don't have another frame of reference other then our own - is it not conceivable that the current "observation" of facts are incorrect?

    I throw a baseball to you at 10 MPH, since we are both traveling at a velocity of v at time t, then to us, the ball is traveling at 10 MPH. However, if we were traveling at v+1@t then the ball is traveling slower.

    I hold that 1) we cannot change the velocity of the universe, and 2) time does not change velocity regardless of the relationship between it and any other object, and 3) our preception of facts can only be based upon our frame of reference, and 4) we cannot change our frame of reference.

    Don't you think that perhaps the billions of external and internal interactions at all levels could be affecting the clock in your example - of which I have read and heard all about before. Einstein stated that measurement is inherently inaccurate, and we are certainly not perfect - nor are our methods or instruments of measurement - or anything we create or come into contact with for that matter.

    Is it not possible that we are wrong? Perhaps we only think we know everything when in fact we know nothing. We have the most complex communications network ever built - and yet, not only does it have problems every second, it's total power is barely equal to that of the nervous system of a gnat - what makes you think that you know anything?

    And, of course, this all includes me which I could be the stupidest of us all - but we really don't know.

    There really is NO answer to my question or my theory. This is simply one of those "think about that" questions. Perhaps one day we will find out.
  6. Dec 30, 2005 #5


    User Avatar
    Staff Emeritus
    Science Advisor

    It sounds like you have definite opinions on how the universe "ought to" work. And it sounds like you've realized that if you close your eyes and disregard everything to that disagrees with your opinon of how things "should" work, based on philosphical arguments like "we can't know anything for sure", you can continue to believe whatever you like.

    This doesn't seem to me to leave a lot of room for a productive debate.
  7. Dec 30, 2005 #6


    User Avatar
    Science Advisor
    Gold Member

    It is the velocity of the observer within the universe that we are talking about.
    I'm not sure what you mean, as velocity is a comparison of distance travelled with the time taken then obviously a variation in the measurement of the time taken would change velocity
    Yes we can, by moving - or observing other objects such as the atomic particles in an accelerator - that are moving very quickly, near the speed of light.
    Yes! - But that caveat also applies to you too!


Have something to add?

Similar Discussions: Three dimensions - x,y,z
  1. Time as a dimension (Replies: 68)

  2. Closed Dimensions (Replies: 4)

  3. 2 Dimensions (Replies: 9)