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The curvature cause by gravity implies absolute space?

  1. Mar 18, 2006 #1
    if gravity is a property of geometry of spacetime then does it imply that space is an entity in itself as newton hypothesized, i mean if space is curved by a mass of a body and therefore creativg gravity around a particular radius then doesn't it imply that space isn't relative to masses as the relativists (as leibnitz and mach depicted) thought and it's also a key figure in nature (as an entity in itself).
    ?
     
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  3. Mar 20, 2006 #2

    Ich

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  4. Mar 20, 2006 #3

    robphy

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    Not to put words in AE's mouth... but...
    from http://www.tu-harburg.de/rzt/rzt/it/Ether.html (Einstein 1920)
    I'd prefer it if his last paragraph was written (suggested edits in red)
    "Recapitulating, we may say that according to the general theory of relativity spacetime is endowed with physical qualities; in this sense, therefore, there exists an ether. According to the general theory of relativity spacetime without ether is unthinkable; for in such spacetime there not only would be no propagation of light, but also no possibility of existence for standards of space and time (measuring-rods and clocks), nor therefore any space-time intervals in the physical sense. But this ether may not be thought of as endowed with the quality characteristic of ponderable inedia, as consisting of parts which may be tracked through time. The idea of motion may not be applied to it."

    I think it's fair to say that, by 1920, these subtleties might not have been completely ironed out.

    My $0.02.
     
  5. Mar 20, 2006 #4

    Ich

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    Agreed. I mean, it was he who showed that space and time merge.
     
  6. Mar 20, 2006 #5

    russ_watters

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    They way people generally say it around here is that GR provides for a "new aether", which is different from the classical lumiferous ether and does not "imply absolute space".
     
  7. Mar 20, 2006 #6

    robphy

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    In my opinion, I think Minkowski deserves a signficant part of the credit for this "merging" of space and time. Certainly, Einstein (and others) set up the stage... but it was Minkowski (probably influenced by Felix Klein) that nailed down the geometrical structure of spacetime [which even Einstein didn't fully appreciate (or apparently like) until he was in the middle of formulating General Relativity].
     
  8. Mar 21, 2006 #7

    Ich

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    Yep, those "mere shadows". Not exactly Einstein´s way of calling things.
     
  9. Mar 21, 2006 #8
    Einstein's 1920 speech is more than the last paragraph - it reflects a change in attitude - Einstein's ether of 1920 has many of the characteristics we would associate with a substantive medium, except that of motion. Minkowski's space-time follows directly from the invariance of the interval - he presented a graphical description, but no new physics.
     
  10. Mar 21, 2006 #9

    robphy

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    I agree with what you say about Einstein.

    Likewise...
    Minkowski's 1908 speech is more than a graphical description - it reflects a change in attitude - Minkowski's spacetime of 1908 has many of the characteristics we would associate with a 4-dimensional spacetime manifold, without its dynamics [via the Einstein field equations]. The point of Minkowski work is more than a "graphical description" (akin to a mere plot of some functions)... it's the emphasis on geometrical structures on a new object: the four-dimensional spacetime continuum [i.e. a particular 4-manifold with a noneuclidean metric]. Indeed, a coordinate system and functional relationships within that coordinate system can only take one so far. [pun intended]

    Certainly, if all one were doing is Special Relativity, Minkowski's work could be regarded as a merely dotting an i. But on the road to General Relativity, one needs more than non-cartesian coordinate systems... one needs the geometrical structure of a more general 4-manifold.

    So, I could agree that Minkowski provides no new physics... but Minkowski certainly provides a new geometrical (and historically important and fruitful) way to think about physics.

    Some interesting reading
    http://www-history.mcs.st-andrews.ac.uk/Biographies/Minkowski.html
    http://www.univ-nancy2.fr/DepPhilo/walter/papers/einstd7.pdf
     
  11. Mar 21, 2006 #10

    Ian

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    Loop quantum gravity,
    Think of space-time (the vacuum) as a compressible fluid that surrounds and permeates all solid matter.
    Think of all solid matter as porous to the vacuum to a degree determined by density.
    Example; A steel ball is solid to us but at the atomic level there is a great deal of 'space' between the atoms and also between the electrons and nuclei of the atoms. This 'space' in between the particles is filled by the vacuum just as the 'space' between the sun and earth is also filled with the vacuum. Therefore a steel ball is porous to the vacuum.
    A lead ball is less porous than steel because the same volume of lead contains more particles per volume and therefore the compression of the vacuum within lead is greater that in steel.
    The refraction of light as it passes from a solid to a gas is due to the condition of the vacuum within the solid being different to that of the gas. The density of the matter determines the condition of the vacuum within any substance, even the sun earth system.
    To understand this fully have a look at the flat-sheet solid ball analogy of how matter distorts space-time and correct it - yes that's right, correct it and express the distortion mathematically (there's a challenge for you)
     
  12. Mar 21, 2006 #11

    russ_watters

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    Be careful relating space-time to a fluid - it implies an absolute reference frame (the reference frame of the fluid).
     
  13. Mar 21, 2006 #12
    robphy - yes - I would agree that Minkowski's work is highly significant as a precursor to GR. Einstein depreciated it initially -then came to embrace it.
     
  14. Mar 22, 2006 #13
    Since most of this discussion is going into the direction of a discussion about the existence of an "aether", a new one that must not be compaired with the old one ... I would suggest to re consider the basis discussion of the general relativity for which one could (at the limit) consider that a coordinates system can be attached to any event. In this sense the set of all events (the differernt histories) is associated to a set of coordinates systems that is the "new aether"... but no more poetry in this representation: only mathematics...
     
  15. Mar 3, 2007 #14
    but if space itself is curved, then it's curved with respect to some object.
    if space were empty would the hypothesized aether still exist?
    i feel that this new/old term is another name for absolute spacetime.
    when i mean absolute, i mean that it has a "life" of its own, i.e if there werent any matter in the cosmos then it would still exist.
    and that is quite perplexing if this assumption is true cause then by the big bang theory matter and space started together, and if also space-time is an entity then what are its physical qualities, does it have mass?! or something like that, i mean usually in our life we see matter bend or curved.
    but now also space time which is our reference frame, can change properties according to bodies in it.
     
  16. Mar 3, 2007 #15

    russ_watters

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    Does a magnetic field have mass?
     
  17. Mar 3, 2007 #16
    obviously not, but is spacetime a field?
     
  18. Mar 3, 2007 #17

    pervect

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    Depends on what you mean by a field. It's not too uncommon to regard the metric tensor as a field, in the sense that it's a tensor field and extremizes an action via a Lagrangian.
     
  19. Mar 3, 2007 #18
    Why not - it has energy - and the energy is proportinal to the square of the velocity of the electric charge - so the field can be analogized as the kinetic energy of an effective mass
     
  20. Mar 4, 2007 #19
    well, my definition of a field, to which i think also russ meant, was the classical meaning i.e, the force acted on a mass or a charge.
    such as the definitons of electric field E=F/q and gravity field.
    i havent yet learned of tensors and their applications but i reackon this semester i'll know more about them.
     
  21. Mar 4, 2007 #20

    pervect

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    We can measure the electric force on a charge by comparing the motion of a charged particle to a reference, uncharged particle.

    Unfortunately, there is no way to similarly measure the gravitational force on a particle without a reference "gravitationally uncharged" particle, which doesn't exist as far as we know.

    There's a section on MTW on this, I don't recall the exact chapter, but it's probably not worthwhile to dig it up unless you have the textbook in question ("Gravitation" by Misner, Thorne, Wheeler).
     
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