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Problem With Einstein's Model

  1. Nov 18, 2013 #1
    Hi, I'm new posting to these forums, so I'm not sure if this is in the right place. Anyway, here it goes:

    I've done a lot of research (on my own and in college; academic articles, educational videos, etc.) about astrophysics, and I've always been fascinated by the concept of gravity. Gravity is what makes life flow; it is the causation of all macro-events (so we think), and is still one of the greatest mysteries to puzzle mankind. As we know, the Newtonian view of gravity doesn't really work. Right now, though, we think that Einstein's general relativity model does. However, after spending hours looking over it, it doesn't quite seem to fit to me(or at least it's not polished). I certainly agree that the general concept is sound (with rifts in space-time being the causation for gravity, force, perception of time etc.), and that there is something out there that mass alters, but I've always wondered about space-time's mechanical properties.

    Space-time is often assimilated to a fabric that can be folded and dimpled by a mass--but I think that this is a very narrow view. In order to make an indentation, would the mass not have to make an impact in space-time, thus already exhibiting the properties that space-time supposedly gives it? In essence, how is a mass--let's say a planet--supposed to warp this fabric in a way that implies a force, if it's force is inherent in the warp in space time? Even further, why is space-time represented as a single fabric, a two-dimensional entity? What tells an object to travel along this indented, two-dimensional path in a three-dimensional world?

    Does anyone with more knowledge on this subject have any input? I am aware that I have no foundation on which to assert my argument, but I've always found these questions/observations puzzling.
  2. jcsd
  3. Nov 19, 2013 #2


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    "Hours", huh? You think you can acquire an understanding of GR in a matter of hours?? It takes a lot longer than that to achieve even a basic understanding.

    The curvature of 4D spacetime is determined by the energy-momentum tensor of matter fields, via Einstein's field equations.

    It's not. Spacetime is 4-dimensional (3 space and 1 time).

    Small test particles travel along geodesics in the curved spacetime, just as they follow a straight line in a flat spacetime. (A geodesic is a path of shortest length.) This can be derived from the Einstein's field equations.

    Input? Well, yes -- you really need to try and study some proper textbooks. This forum is intended for discussions at graduate-level and higher. Also, questions on General Relativity should be asked in the Relativity Forum (surprise!).
  4. Nov 19, 2013 #3


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    I don't know what you mean by this. There are four "fundamental forces", gravity, electro-magnetism, the strong nuclear force, and the weak nuclear force. They are equally important to life or any other property of the universe. (Actually gravity is the weakest of them.)

  5. Nov 19, 2013 #4


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    It sounds very much as if you've been misled by the so-called "rubber sheet" pictures in the popular press, which shows a two-dimensional surface with an indented dimple and the mass at the bottom of the dimple.

    That's not a good starting point for understanding GR. You might try searching this forum for the really excellent animation done by user A.T. to explain how curvature really works.
  6. Nov 19, 2013 #5
    Thanks. Sorry, I definitely put this in the wrong forum. I have very little knowledge of high-level astrophysics, and was just putting out a few things I questioned. Wasn't trying to rewrite the laws of physics lol.
  7. Nov 19, 2013 #6
    You don't seem to know much about relativity, and there is nothing wrong with that.

    Space and time are related. They are the two sides of the same coin. A massive object like a planet curves the space time and objects just follow a straight line. Technically gravity isn't a force. Objects in orbit are just following a straight line. They can't know that the space is curved
  8. Nov 19, 2013 #7
    John Wheeler said

    "Matter tells spacetime how to curve, and spacetime tells matter how to move."

    Spacetime curvature IS gravity....three dimensions of space and one of time.

    Try reading here for a bit more of an introduction:


    and try here for some background on spacetime


    You should note that while relativity is focused on three dimensions of space and one of time, other theories include many more dimensions....
  9. Nov 20, 2013 #8


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    Because humans can't even visualize flat 4-dimensional manifolds, yet alone curved ones. You reduce the dimensions of the manifold, to visualize its curvature by embedding it in a higher dimensional flat manifold. Since flat 3D is the limit for intuitive human understanding, 2D is the intuitive limit for curved manifolds.

    When no forces act on the object, it follows a straight line in distorted space-time. This animation shows this better than the indented rubber sheet you are referring to.

  10. Nov 20, 2013 #9


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    Inertia. Inertial means that in the absence of an external force an object travels along a geodesic. In flat spacetime this means travelling in a straight line at a constant velocity, but when the spacetime is curved these "straight" lines (geodesics) become more complicated.

    One of the great theoretical accomplishments of GR is to unify inertia and gravity. Prior to Einstein it was recognized that the passive gravitational mass was equal to inertial mass, but it was not known why. Afterwards, it became clear that they must be equal since gravitation is inertia in curved spacetime.
  11. Nov 25, 2013 #10
    Hi rubi 32. Its ok to be apprehensive about any theory - If Einstein were alive today, he would likely agree - historically, Einstein's thinking changed over the years. In 1916 he was convinced the universe was static and closed. As first published the theory couldn't work because a static universe would collapse due to gravity. Moreover, the theory is incomplete even today as it does predict the gravitational constant - G is inserted from the measured value, and it is G that tells space and time how to obey. In spite of these shortcomings, the theory is considered an epic of theoretical physics, it makes predictions that have been verified and could not be understood or even appreciated prior to General Relativity. Over the years other theories of gravity have been proposed based upon Mach's principle, scalar-tensor and like. A good book that ties history and physics together at an undergrad level is Ed Harrison's book, "Cosmology, the Science of the Universe.
    Last edited: Nov 25, 2013
  12. Nov 26, 2013 #11
    I think this is supposed to say .." as it does NOT predict the gravitational constant"
    which is a common occurrence in many of our theories.

    GR also seems to be incomplete , to fail, at points of extreme gravity, extreme curvature, such as the center of black holes at at the Big Bang. At these places, infinities are predicted yet noinfinites have ever been observed.
  13. Nov 26, 2013 #12


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    Do you know of ANY theory in physics that's "complete"?

  14. Nov 26, 2013 #13


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    This isn't a limitation of the theory (although there are limitations such as that mentioned by Naty1). It is an artifact of our system of units. We can set G to any value we like by simple choice of units, and frequently we choose to set it to 1.

    See: http://math.ucr.edu/home/baez/constants.html
  15. Nov 26, 2013 #14
    Good point Zapper - no theory is ever complete - I recall a statement from Hawking years back. We have two theories of gravity and neither can predict its strength, nor do we as yet have a theory to explain the magnitude of the electron charge.

    I guess I will have to get to work on it
  16. Nov 26, 2013 #15


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    Then please note that the "incompleteness" of GR is NOT a "problem" here, the way you had written earlier, since every theory in physics can be considered as incomplete. Practically every single theory in physics have to use values that are experimentally derived, and not from First Principle calculations.

  17. Nov 27, 2013 #16
    That is true - there are no "first principle calculations" and that is the basis for my observation - the so called constants have dimensional units that relate their properties to one another. General Relativity is descriptive at the point where functionality is needed. Einstein did the best that was possible with the knowledge of the day. As in Special Relativity, He turned the problem into a postulate - a century later there is still no satisfactory explanation as to how inert matter can distort static space.

    "What I cannot create, I do not understand" Written by Richard Feynman in the corner of his office blackboard at Caltech where it remained for more than 8 years

    My object in commenting to this o.p. and others who have ventured to inquire with a criticism of a standard theory that nobody really understands (Like why G has the value we measure in relation to the value of something else we also measure) is to blunt some of the harsh criticism heaped upon new posters ... I sometimes get into trouble for this.
  18. Nov 27, 2013 #17


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    Again, dimensionful universal constants like G do not represent any limitation of any theory. They are simply artifacts of our system of units. There can never be any "first principle calculations" because they don't come from physics at all. They come from our chosen system of units. That is all.

    That simply isn't correct. It isn't a criticism of any theory, and we understand completely. G has the value it does because we use the units we use.
  19. Nov 27, 2013 #18
    What about when we compare the force of gravity to something like the strong nuclear force at short range. The ratio is then independent of the units used and as far as I know there is no way to predict why the dimensionless constants have the values they have.
  20. Nov 27, 2013 #19


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    Yes, that is correct. The dimensionless fundamental constants of a theory do reflect a limitation of the theory. I don't expect that there will ever be a complete theory of everything without any dimensionless constants, so I think it is a limitation that we will have to just accept. But nonetheless, it is an actual limitation of the theory.
    Last edited: Nov 27, 2013
  21. Nov 27, 2013 #20


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    All the other forces have dimensionless coupling constants. Gravity does not. You can use G to form the Planck units, but nothing that is dimensionless.

    Ratios such as the above that seem to characterize the strength of gravity actually do not. For example, comparison of the electrostatic force between two particles to the gravitational force between them only tells you about the particles themselves, namely their charge to mass ratio.
  22. Nov 27, 2013 #21
    I suspected I might of chosen a bad example :). Does the lack of a dimensionless coupling constant for gravity, have anything to do with the problems of forming a GUT?
  23. Nov 27, 2013 #22
    You miss the point of that observation - of course the numerical value of G depends upon the units - but in the context of my statement it makes no difference if you express c in units of meters per second or furlongs per fortnight - the statement that G has certain a certain value refers to its relation to other things we measure - as the post above (#18) points out.
    Last edited: Nov 27, 2013
  24. Nov 27, 2013 #23


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    No, it doesn't, as this points out:

    G and c and h have no meaning beyond the system of units chosen. They are entirely dependent on the system of units chosen and provide no information whatsoever as to the properties of the universe itself or the outcomes of any physical measurements. Here is an interesting exercise that I went through and posted on the topic:

    G is only one piece of information, one parameter. Because it depends on your system of units it clearly tells you about your system of units. Since it is only one piece of information, once you have used that information to tell you about your units there is no additional information left over to tell you anything about the universe or physics.
  25. Nov 27, 2013 #24


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    And as #20 above points out, these relationships don't tell you anything at all about G.
  26. Nov 27, 2013 #25
    This is a response to post 13 and 17. First, I do not read the John Baez paper the way you have interpreted it....IT covers a lot of subjects very briefly - and some of the statements can be taken in ways that might appear to be something different that what John is saying -

    In order to avoid going too far astray for the issue raised by the OP, its worth clarifying what is meant by the gravitational constant. It is simply a coefficient - it could be a long term variable as many have suspected including Robert Dicke and P Dirac and other notables. For the purpose of discussing the significance of the dimensionality - whether it changes or not is not a critical shortcoming of GR. But the dimensionality itself is a big clue to the nature of G. Whatever units you choose - G boils down to volumetric acceleration per unit mass. If you use mks units this is expressed as meters^3 per sec^2 per kgm. Once you know the volumetric accel you can calculate G. One way to estimate the volumetric acceleration is to use the relationship which Robert Dicke claimed to be the ratio between inertial and gravitational mass, namely GM/Rc^2 = 1 within the limits of experimental error. From this calculate G based upon R, the mass of the universe M, and the velocity of light.

    The dimensionality of G is revealing - it tells us something about the universe. In fact it tells us a lot more than what I have illustrated here - but that is neither proper or necessary to make my point. This is lost by equating c = 1, G = 1 ect ... it throws away valuable information which can be is useful in these types of puzzles. Of course, one can argue all this is a coincidence -
    Last edited: Nov 27, 2013
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