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

Is gravity just the temporal gradients effect

  1. Sep 19, 2005 #1
    :surprised any mass creates a temporal gradient around it.
    any object that moves in this gradient will suffer the effects of this gradient & stretch or pivot. If this object is not already on a direct course towards the objects center, it will find that the far side tries to slow down more than the near side, causing the object to become under tension and pivot towards the mass, therfore altering its course & spinning it. The course change will tend to take it more towards the mass. Is this correct? Also it would seem that planets should spin faster & faster during their orbits, but they dont. (Maybe an explanation would be that as an object moves faster, & cant pull apart or stretch or alter course, this 'energy' must be relieved as a further temporal distortion, hence why moving quickly creates more temporal distortion).
    So explaining why time is distorted & why this creates the effect of gravity,
    and could maybe account for some dark matter & some of the effect that keeps planets spinning & another component in why planets stretch at the equator apart from centrifugal force.
  2. jcsd
  3. Sep 19, 2005 #2


    User Avatar
    Staff Emeritus
    Science Advisor

    This is a bit mudlled but some elements of it are correct as an *approximation*. The theory you are describing is not General Relativity, though, but a recasting of Newtonian gravity in geometric langauge, called Newton-Cartan gravity. This theory is *not* fully relativistic.

    First off, I think we should define a "temporal gradient". There is a sense in which a mass creates a temporal gradient, this is the sense in which a clock near a large mass ticks more slowly due to gravitational time dilation than it ticks at infinity.

    The gradient exists only when we compare a clock near a mass to a reference clock that is well away from any gravitational fields. We find that the reference clock ticks faster.

    The Newton-Cartan theory, while it incorporates gravitational time dilation, does not incorporate the familiar time dilation due to motion. This is because it's *not* a relativistic theory, it's *not* general relativity. Rather, it's an approximation to Newtonian gravity.

    Newton-Cartan theory has a curved space-time, but flat spatial slices. The idea behind the theory is that objects move in "straight lines". The straight lines are calculated by means of the "geodesic equations". Unfortunately it takes quite a bit of math to describe this properly, I don't see quite how to explain them in elmentary terms, other than to say that objects move along geodesics in the curved space-time of the Newton-Cartan geometry, and that geodesics are as closed to "straight lines" as one can come with a non-Euclidian geometry.

    The familar "rubber sheet" anologies might work to some extent. You have to remember that in Newton-Cartan theory, though, all spatial slices are flat. It's only in the direction of "time" that we have curvature.

    Full General Relativity is very similar to Newton-Cartan theory, but in GR gravity space is curved and distorted as well as time. The spatial distortions are usually not important compared to the time distortions under most circumstances (which is why Newtonian gravity is almost correct), but under some circmustances like the bending of light, the precession of Mercury's perihelion, etc etc, the spatial distortions become significant.
  4. Sep 19, 2005 #3
    spacially how do grid lines bend then?

    in spatial terms only, would a reinforced ruler bend towards the earth or the other way like the grid lines on a rubber sheet? I did try to explain the effect of moving at speed on time, but not very well, as what happens when an object 'resists' being pulled apart
  5. Sep 19, 2005 #4


    User Avatar
    Staff Emeritus
    Science Advisor

    I'm not quite sure what you mean by this quesiton, sorry.

    If you are thinking about my remark about the distortion of space, I'm not thinking about it as being "bent" either towards or away from the sun.

    What I'm actually thinking about is that in the full theory of relativity, clocks tick slower in a gravitational field, but rulers get longer. One result is that locally the speed of light stays constant, as it must. Another result is that space isn't strictly Euclidean anymore.

    One can envision the distortion of space-time as its embedding in some higher dimensional manifold, but this is basically a visual aid. In relativity we don't actually go about thinking about 5 or 6 dimensional surfaces, we use a 4-dimensional metric to describe the properties of space and time at any point. The cannonical example is the Schwarzschild metric, which describes the properties of space-time near a large non-rotating mass M. See for instance:


    If all the diagonal coefficients of this metric are +/-1, and the non-diagonal coefficients are zero, space-time is flat. Otherwise, space-time (or at least the coordinate system being used) is "not-flat" or "distorted".
    Last edited: Sep 19, 2005
  6. Sep 20, 2005 #5
    so a photon takes longer due to time dialation and

    so a photon takes longer due to time dialation and the fact that the distance is streched, ie the sum of these 2 effects, rather than either 1 or the other being both valid ways to find its speed? in simple terms?
  7. Sep 20, 2005 #6
    so an object expands or contracts in spatial terms?

    in spatial terms only, an object gets longer or expands, but doesnt bend as such, is this correct? so the distance between the earth & the sun is slightly longer than newtonian physics would have as in spatial terms only, we have to follow a straight line which is stretched at both ends, like a spring being stretched just at the ends, but bending is not quite right as space isnt bent but more compressed, am i right? Thankyou I think the way I had imagined things was sort of right, with a few loose ends that you have tied up.
  8. Sep 20, 2005 #7


    User Avatar
    Staff Emeritus
    Science Advisor

    I think that's right, assuming you're thinking of something like the Shapiro effect

Share this great discussion with others via Reddit, Google+, Twitter, or Facebook