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I GR as a Graded Time Dilation Field in Euclidean Space?

  1. Mar 11, 2014 #1
    The title says it all, really. Are we able to describe GR in terms of a Graded Time Dilation Field in Euclidean space?

    From here we can see that light curvature can be analogously described via a material with a graded index refraction, so my question is really whether or not the following is is capable of encompassing GR:

    [tex] t_0 = t_f \sqrt {1 - \frac{r_0}{r}}

    \frac{t_f}{t_0} = \frac{1}{\sqrt{1 - \frac{r_0}{r}}} = [/tex]analogy to "n" in optical medium
     
  2. jcsd
  3. Mar 11, 2014 #2

    phyzguy

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    The metric tensor is a symmetric tensor with 10 degrees of freedom. Some of these are removed by the physical constraints of the theory, but it is certainly not possible to describe GR with a single scalar field as you propose.
     
  4. Mar 11, 2014 #3

    Dale

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    As phyzguy said, the answer is no, there are not enough degrees of freedom in a scalar field.

    Note that the result from the paper you cited applies specifically to a "static spherically symmetrical gravitational field". This particular spacetime has reduced degrees of freedom due to the assumed symmetry, so it can be represented by a scalar field whereas general spacetimes cannot.
     
  5. Mar 11, 2014 #4

    Bill_K

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    A few things this "theory" does not describe correctly:

    • cosmological solutions
    • gravitational waves
    • black holes
    • perihelion precession
     
  6. Mar 11, 2014 #5
    Yes! I was formulating a response to phyzguy along these lines before I read your response, DaleSpam. I'd like to understand "where" the analogy breaks down. It's difficult for me to extend the analogy into anything other than a gravitational field (e.g. time dilation caused by relative motion, etc), but I'm left wondering if the pure simplicity of a 3-D Euclidean space could possibly be reduced this far; dimensional symmetry, zero curvature...I thought perhaps the excess degrees of freedom could be eliminated...but I don't know which is of course why I'm asking. :smile:

    Hi Bill_K!
    I would say that the analogy could be extended to some or all of these in addition to DaleSpam's rotating masses, etc, by replacing the calculated GR curvature with the time dilation field. Note that the "formula" I give for calculating this field is derived from the GR time dilation itself.
     
  7. Mar 11, 2014 #6

    WannabeNewton

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    How would you predict frame dragging using this theory?
     
  8. Mar 11, 2014 #7

    PAllen

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    NO, it is derived from one solution of GR that is static and has perfect spherical symmetry. There is no part of the universe that exactly meets these conditions. In any realistic solution, curvature cannot be represented by a single function of position and time. This is a mathematical theorem, not subject to debate or interpretation. Specifically, curvature of 2-surface can be represented by a scalar. For more than 2 dimensions, it cannot.
     
  9. Mar 12, 2014 #8

    Dale

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    In the vacuum region I think it breaks down as soon as you break spherical symmetry. In the matter region it breaks down as soon as you break spherical symmetry, or have a non-static distribution of matter, or have a matter field which is not a perfect dust.

    A rotating mass is not spherically symmetric, it is at best axisymmetric.
     
  10. Mar 12, 2014 #9

    phyzguy

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    For example, consider the case of two orbiting neutron stars, losing energy due to gravitational radiation and spiraling in to form a black hole. We know they lose energy due to gravitational radiation (see Hulse-Taylor binary or this paper). There is no way to describe a situation like this with a single scalar field. As Bill_K said, it will not even describe gravitational waves.
     
  11. Mar 12, 2014 #10
    Hi PAllen! I appreciate what you're saying but the OP calls for Euclidean space with zero curvature. We model EM activity as a field in flat 3-D space, yes? Restricting the conversation solely to time dilation caused by gravity could we do the same thing here?

    I not claiming that you or others are wrong on this point, I just want to verify that everyone is stripping time out from the geometry before denouncing the idea.
     
  12. Mar 12, 2014 #11

    PeterDonis

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    No, it doesn't. The OP is asking about "GR", in general.

    This leaves out a lot of "GR", since there are many scenarios covered by "GR" that do not even have a meaningful concept of "time dilation caused by gravity". If you want to restrict consideration only to those scenarios that do, then you need to ask about something more restricted than "GR". (Which then leads to the question, why would you be restricting yourself to just those scenarios?)
     
  13. Mar 12, 2014 #12
    The reason is: baby steps. I'm just exploring the idea. Could you give me some examples of scenarios covered by GR in which "time dilation caused by gravity" does not apply? Are you referring to time dilation caused by relative motion? Or something else?
     
  14. Mar 12, 2014 #13

    PAllen

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    Any situation that isn't stationary. That is, anything more complex than an isolated body in an unchanging state. It really is only the case of isolated, unchanging, body for which you could define a time dilation field. In all other cases, GR does not specify gravitational time dilation. That is, the whole notion of gravitational time dilation is not a general feature of GR; it is a specific derived feature applicable only in the most restricted cases.
     
  15. Mar 12, 2014 #14

    PeterDonis

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    Previous posters have already listed a number of them, but to summarize:

    (1) The concept of "time dilation due to gravity" only applies if the spacetime is stationary, which basically means that one can find a family of worldlines in the spacetime along which the geometry does not change with time. (The technical way to say this is that the spacetime has a timelike Killing vector field.)

    Examples of spacetimes that are not stationary: any spacetime in which objects have orbits that are not perfectly circular (such as the planets in the solar system, including effects like the perihelion precession of Mercury); any spacetime where gravitational waves are emitted (such as binary pulsars); black holes (because the region inside the event horizon is not stationary even if the region outside the horizon is); cosmology (because the universe is expanding).

    (2) The concept of "time dilation due to gravity" is only *sufficient* to describe all the effects of gravity if the spacetime is static, which means that there is a family of spacelike hypersurfaces that are orthogonal to the family of worldlines along which the geometry does not change with time. The orthogonality property is necessary for us to be able to view the spacelike hypersurfaces as "space at an instant of time".

    The primary example of a spacetime that is stationary but not static is any spacetime in which the central object is rotating. The rotation causes effects such as "frame dragging" that cannot be modeled by treating gravity as a scalar field. So even though there is a meaningful concept of "time dilation due to gravity", that by itself is not sufficient to describe all the effects of gravity for a rotating source.
     
  16. Mar 12, 2014 #15
    If we could account for motion-induced time dilation in addition to gravity-induced time dilation would you consider the theory more complete? Or are you suggesting that there are additional elements of GR which would not be accounted for?
     
  17. Mar 12, 2014 #16

    PeterDonis

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    This wouldn't change anything. "Motion-induced time dilation" can't be modeled as a scalar field anyway; it's not an invariant, it's a frame-dependent quantity, so it's not even the same kind of thing as "gravity-induced time dilation" to begin with.
     
  18. Mar 12, 2014 #17

    WannabeNewton

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    Could you account for all of electromagnetic theory using only a scalar electric potential? Could a scalar potential describe the circulating magnetic fields generated by currents and the circulating electric fields generated by time-varying magnetic fields?
     
  19. Mar 12, 2014 #18

    PAllen

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    How about 6, which is the actual number required. Then just call it the metric tensor after specification of arbitrary coordinate conditions.
     
  20. Mar 12, 2014 #19
    PAllen, you've already made the journey through GR (I presume), but don't you agree that teaching gravity as a time dilation field analogous to an EM field in a flat Euclidean space would be...more aesthetically pleasing if nothing else? Why single out gravity as the only force determining the geometry of our surroundings?
     
  21. Mar 12, 2014 #20

    PAllen

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    You don't have to call it geometry. You can treat it as fields on an abstract background. Weinberg, among others, has worked on expressing it this way. The point is, that if you want it to match the predictions of GR or spin-2 field theory, you need 6 functions not one or two.
     
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