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What is the fabric of spacetime made of?

  1. Aug 25, 2011 #1
    I know that the rubbersheet / fabric analogies that we see on TV and in various diagrams are only a metaphor, but I'm having difficulty getting started researching what we actually know / theorise about the make-up / workings of the "fabric"?

    Could you give me any pointers to good postings, or even key words to search on, or good articles that discuss the workings of the fabric of spacetime?


    Thanks in advance,


    Noel.
     
  2. jcsd
  3. Aug 25, 2011 #2

    Pengwuino

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    Nothing makes up the "fabric" of spacetime. It's just used as a visual aid. What space-time actually is is a 4-dimensional manifold that can be "seen" by pretending it's a piece of fabric.
     
  4. Aug 25, 2011 #3

    MathematicalPhysicist

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    And the latter is just a mathematical aid.
    :-)
     
  5. Aug 25, 2011 #4
    Please correct me if I am wrong but I thought spacetime was all of reality, or that spacetime is the metric for reality. A time scaling metric for information exchange possibility.

    Is the fabric of spacetime not just the metric of GR? As in spacetime is made from interaction/information exchange and the framework of governing laws?

    I may be well off the mark and it might be a question like asking "whats outside the universe?" - one with no resonable answer. Or it might be totally measurable just not within our current understanding - I find this really interesting.
     
  6. Aug 25, 2011 #5
    Space time is made from an area and a period of observation ;p
     
  7. Aug 25, 2011 #6

    WannabeNewton

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    "Space - time" refers to the 4 - manifold; the metric is what gives it "structure" and allows one to define geodesics, curvature, parallel transport, etc. in terms of it in GR (I say in GR because not all geometric theories of gravity need to be metric theories; one can use the affine connection alone to give meaning to some of the above as well).
     
  8. Aug 25, 2011 #7
    "We may assume the existence of an aether; only we must give up ascribing a definite state of motion to it, i.e. we must by abstraction take from it the last mechanical characteristic which Lorentz had still left it. … 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." A. Einstein.
     
  9. Aug 25, 2011 #8
    Thanks all. Lots of food for thought.

    Recognising that this is theory and not ... fact (? I not sure if that is the right word), am I correct (ish) in summarising that as a mathetical manifold, the "fabric" can impact on matter (and matter can impact on it) without it consisting of particles or fields (again, forgive me if this is clumsy language, I'm trying to be ... all encompassing)?

    Regards,


    Noel.
     
  10. Aug 25, 2011 #9

    Chronos

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    Relativity quashed the notion of absolute space and time leading to the idea of a 'spacetime continuum' [also referred to as the 'fabric of space-time']. Einstein introduced the notion that space and time are merely qualities of the gravitational field that have no independent existence or meaning. For a brief discussion see this article by NASA astronomer Dr. Sten Odenwald http://www.astronomycafe.net/gravity/gravity.html
     
  11. Aug 26, 2011 #10
    Chronos, For some reason I had understood that Einstein did not completely discount all of the luminiferous aether concepts? I am still trying to get to the bottom of this.

    http://en.wikipedia.org/wiki/Luminiferous_aether


    One issue I am trying to understand is if space itself has no independant Physical properties and hence does not exist, then how can a wave propagate through such a true vaccum which has no Physical existance? Does free space itself have any physical properties or does it have no Physical existence whatsover? Is vacuum energy considered a property of space and could this provide medium through which waves can propagate?

    http://en.wikipedia.org/wiki/Vacuum_energy


    http://en.wikipedia.org/wiki/Casimir_effect :
    "However, even the vacuum has a vastly complex structure, so all calculations of quantum field theory must be made in relation to this model of the vacuum."


    I am dealing with RF propagation on a daily basis and I am baffled by free space having permitivity and permeability which is non zero if in fact free space has no Physical existence. My pcb, along which my waves propagate, has permitivity and permeability relative to free space, which apparently has no Physical existence!


    If correct, this link seems to be useful for understanding the role of space:
    http://www.astronomycafe.net/qadir/ask/a10926.html

    Is light conducted by space, or does it travel through it?
    Light has nothing to do with space. It is a property of the electromagnetic field when this field is undergoing changes. from its point of origin, this field radiates out into space in all directions. It is invisible, but can be felt by charged particles. When this field is changed, for instance by suddenly altering the position of its emitter, a kink develops in the field which propagates out into space at the speed of light. Under the right circumstances of acceleration, this kink becomes electromagnetic radiation - light. So space simply acts as a passive container for the electromagnetic field, which in turn is the 'medium' in which light exists. If there is no electromagnetic field in space, there is no light either, as in the inside of a closed, shielded box. The maximum propagation speed is the speed of the expansion of the electromagnetic field in a perfect vacuum, which is 299,792.5 kilometers/sec.
     
    Last edited: Aug 26, 2011
  12. Aug 26, 2011 #11

    Chronos

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    The vacuum is ascribed the property of permeability as a function of the ISO definition of an ampere:
    "The ampere is that constant current which, if maintained in two straight
    parallel conductors of infinite length, of negligible circular crosssection,
    and placed 1 meter apart in vacuum, would produce between
    these conductors a force equal to 2 × 10−7 newton per meter of length.
    It follows that the magnetic constant μ0, also known as the permeability of vacuum,
    is exactly 4pi x 10−7 henries per meter."

    Wiki comments that
    "Ampère's law as stated above describes a physical property of the world. However, the choices about the form of km and the value of μ0 are totally human decisions, taken by international bodies composed of representatives of the national standards organizations of all participating countries. The parameter μ0 is a measurement-system constant, not a physical constant that can be measured. It does not, in any meaningful sense, describe a physical property of the vacuum. This is why the relevant Standards Organizations prefer the name "magnetic constant", rather than any name that carries the hidden and misleading implication that μ0 describes some physical property of the vacuum."

    In "Probing the Quantum Vacuum" by Dittrich and Gies, they note:

    "During the development of quantum field theory it has become a populer point of view to consider the quantum vacuum as a medium. The manifold phenomenon which arise from the presence [or provoked absence] of virtual particles in the vacuum, such as the Lamb shift or the Casimir effect, tempt one to assign real properties to the vacuum. On the other hand, a physicist can hardly accept an establishment of this kind of 'modern ether' without reservations; it is, at least, unsatisfactory to formulate a fundamental theory with the aid of some ingredients which elude direct measurement.

    In the present volume, we do not insist on either point of view as the one and only foundation of physics, but intend to demonstrate it can be pragmatically appropriate and useful to understand certain quantum effects as peculiarities of the vacuum – both intuitively and formally.

    The formal concept of relating the full quantum theory to properties of the vacuum is given by the effective action. By integrating out the high energy degrees of freedom of the exact theory, one arrives at an effective theory of the low-energy degrees of freedom which are relevant to the physics of the vacuum."

    Is it fair to say the vacuum is imbued with measurable properties, or merely a baseline against which to compare the properties of matter, energy and interactions thereof?
     
    Last edited: Aug 26, 2011
  13. Aug 26, 2011 #12
    Thank you Chronos & Tanelorn. Very informative, but will take me sometime to digest.

    Chronos, may I highlight one part of you quote "... it is, at least, unsatisfactory to formulate a fundemental theory with the aid of some ingredients which elude direct measurement. ...". While I agree completely with the sentiment, is this not something that science does & dicusses on a daily basis in relation to many aspects of many subjects (even think about the money spent daily on things like the LHC)?

    Regards,

    Noel.
     
  14. Aug 27, 2011 #13

    Chronos

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    Science is not satisfied without testing every hypothesis.
     
  15. Aug 27, 2011 #14
    Thanks Chronos.
     
  16. Aug 27, 2011 #15
    is the fabric of spacetime really all of the various fields?
     
  17. Aug 27, 2011 #16

    Chronos

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    That is the modern view.
     
  18. Aug 31, 2011 #17
    "Is the fabric of spacetime really all of the various fields?"

    "That is the modern view."

    I used to understand it that way also. It was only whilst reading historical Physics and thinking how can a wave propagate through the non existant medium of space that I wavered.
     
  19. Sep 1, 2011 #18
    Tanelorn, when you mention "historical physics" are you talking about reading material generally, or a specific "Historical Physics" book / article?

    Regards,


    Noel.
     
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