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Gravity again

  1. Apr 16, 2004 #1

    wolram

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    gravity must be the most confounding thing to science ,we all know
    its effects, but no one knows its mechanics, what is the most up to
    date theory, theories?
     
  2. jcsd
  3. Apr 16, 2004 #2

    mathman

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    General relativity is the current theory. It is not the final theory, since ultimately it has to be reconciled with quantum theory.
     
  4. Apr 16, 2004 #3

    ZapperZ

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    Come again? NO ONE knows it's mechanics? How did you think we managed to (i) sent things to the moon, Mars, etc (ii) predict planetary/celestial motions? Just because we can't mearge it within the happy family of quantum field theory, does not mean we don't know its "mechanics". Both GR and Newton's Law of gravitation are exceedingly well-known.

    Zz.
     
  5. Apr 16, 2004 #4

    wolram

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    ---------------------------------------------------------------------
    the "laws" of gravity are well known, but can you tell me how a planet
    is kept in orbit?
     
  6. Apr 16, 2004 #5

    ZapperZ

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    If you ask the same question on other parts of physics, then you should also come to the conclusion that we know nothing about the "mechanics" of anything. So why pick on gravity only?

    Keep in mind that I'm using the word "mechanics" as applied in physics. We have the "mechanics" to describe everything we need to know about the dynamics of objects under gravitational forces. So how can you say that "NO ONE" knows about gravity? We know less about the "how" of QM wavefunction than we do about classical gravitational law. But yet, no one would say no one knows about quatum mechanics.

    Zz.
     
  7. Apr 16, 2004 #6

    wolram

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    --------------------------------------------------------------------
    but none of this tells us how mass can be converted to gravitational
    radiation", inspiraling massive body", or is the mechanism for this known?
     
  8. Apr 16, 2004 #7

    wolram

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    ------------------------------------------------------------------------
    but then why ask any question? or does science have to be so defensive?
     
  9. Apr 16, 2004 #8

    ZapperZ

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    There's a difference between "asking a question" and "making a statement". You said that no one knows about gravity. That wasn't a question. You have already determined that that is the case as of NOW. Your reason being that we don't know HOW gravity exists based on the existence of its source - mass. I then said that if you apply that to everything else, then using your logic, no knows about everything. Think about it. You said that

    "but none of this tells us how mass can be converted to gravitational
    radiation", inspiraling massive body", or is the mechanism for this known?".

    Now, apply this to "charge" and "electric field". Can you tell me "... how charge can be converted to electric field radiation, inspiraling a charged body...". If we apply your logic, then we can also get away with saying that no one knows the "mechanics" of electromagnetism, in spite of the existence of Maxwell equations, and QED, just because no one knows HOW the electric field comes about when a charge is present, or how the virtual photon field emerges out of the vacuum state when we introduce a charge.

    A "mechanics" in physics means that we have the ability to describe the dynamics of the system in question. As far as I know, we have such ablity when dealing with gravity. Is this statement under dispute?

    Zz.
     
  10. Apr 16, 2004 #9

    wolram

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    if you can tell me how mass is converted to gravitational energy
    i will be quite happy.
     
  11. Apr 16, 2004 #10

    ZapperZ

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    I can't, because it doesn't. I have never seen a mass gets "converted" into "gravitational energy", nor am I aware of an established theory that tells me that mass can get converted to gravitational "energy. If I start speculating on that, this will have to either move to the Theory Development section, or to Crank Dot Net.

    So I apologize for not being able to bring you any happiness.

    Zz.
     
  12. Apr 16, 2004 #11
    TOE - Is it possible that Gravity Waves connect with Time-Waves?


    Is it possible that gravity waves connect with time waves and the electron waves follow more of an electromagnetic or photonic law?

    I dont know the answer and if any physicists here could explain if there is such a thing as absolute time-waves? Is time as a wave function a fundamental property of super-holographic process connected with photonic energy bundles?
     
  13. Apr 17, 2004 #12

    LURCH

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    I think he's reffering to the fact that the mechanism behind the hpenomenon remains a mystery. For example, we know that ellectrical charge is caused by electrons, and electromagnetic force is carried by photons, but nobody knows if gravity is carried by a radiating partical, the "graviton".

    Nor does gravity behave as the other "forces" do. As you yourself pointed out, mass is not converted into gravitational radiation, or gravitational energy. For a force to act on objects without energy being expended is not normal. In fact, it not even believed to be possible. And this brings up a question as to wether gravity is actually a "force" (in the traditional sense) at all. Or is it a "pseudoforce", like inertia or centrifugal force (a question discussed on these Forums before)? These are sometinmes called the "inertial forces", which have their influence over objects, even though those objects remain at rest inertially.

    No, Wolram, gravity is not at all like the other forces, and not well understood. Loop Quantum Gravity and M-Theory are the two major contendors, but each attempts to propose a force-carrying "graviton", so as to reconcile GR with QC. It remains to be seen if this is the right approach or not. I personally have serious missgivings.
     
  14. Apr 17, 2004 #13

    Nereid

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    Perhaps you are thinking of binary pulsars or similar systems? From the linked page:
    "Relativity predicts that the binary system will lose energy with time as orbital energy is converted to gravitational radiation."

    "Because the binary system [PSR 1913+16 in this case, but several others have since been discovered] is losing energy, the orbits are shrinking, and someday the two stars should coalesce. Such a merger might produce strong enough gravitational radiation to be detected by instruments like the Laser Inteferometer Gravitational-Wave Observatory ..."

    If this is what you had in mind, note that the source of the gravitational radiation is not 'mass', any more than the source of electromagnetic radiation is 'charge', as Zapper Z said in an earlier post.
     
  15. Apr 17, 2004 #14

    ZapperZ

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    I think there is a misconception here that, since QFT is so successful in "describing" (not explaining) the other 3 forces, that the lack of quantized field (currently) for gravity means that we don't know a lot about gravity. This is incorrect. Gravity was the FIRST force that was very well described, even earlier than EM field. I'm not saying we have a COMPLETE and exhausted idea of it, but we certainly cannot say that "nobody knows about the "mechanics" of gravity and gravitation". That statement seems to imply that we're still cave-dwellers who still cannot predict celestial mechanics and think the earth is at the center of the universe.

    I will also say that if you continue with the "how" line of questioning, I can also ask you HOW does the photon virtual field actually emerges out of the vaccum state to mediate the EM interaction. How does something acquire charge, spin, etc? The fact that we lack those answers does NOT mean we know nothing about EM interaction. What we already have are damn accurate in describing a wealth of physical phenomena. That applies to gravity also. GR hasn't been proven wrong. And as far as I know, Newton's law of gravitation has worked even when tested up to the micrometer scale where String Theory via Arkani-Hamed et al. has predicted deviations at millimeter scale! So far the classical theories are triumphing over the more "sexy" and unfalsifiable theories. It is why I find it amusing at a statement indicating that we know nothing about gravity.

    Again, even in the worst case scenario that there are no such thing as "gravitons", and that gravity cannot be mearged with QFT, is this such a bad thing? Would this continue to result in people thinking that we know NOTHING about gravity, despite of our ability to accurately describe the dynamics of a system using Newtoninan/GR picture?

    Zz.
     
  16. Apr 17, 2004 #15

    wolram

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    but the binary does loose mass, ok some of this goes into the
    EM spectrum, but apart from mass what is their to use to produce
    these gravity waves.
    as for our understanding of gravity i will say again that we know
    how it effects things, we may understand its dynamics, but we
    know nothing about its primary mechanics, be it graviton or
    curved spacetime, the two are so fundamental different that
    it shows we have no concept of the cause of gravity.
    i apologies if my grasp of this is flawed.
     
    Last edited: Apr 17, 2004
  17. Apr 17, 2004 #16
    That made me laugh so much.
     
  18. Apr 17, 2004 #17

    Nereid

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    "mechanics" = "mechanism"?

    More completely:

    [wolfram:]"we know how it effects things, we may understand its dynamics, but we know nothing about its primary mechanics, be it graviton or curved spacetime, the two are so fundamental different that it shows we have no concept of the cause of gravity."

    =

    [Nereid:]"we can make accurate predictions about how mass interacts, gravitationally; these predictions include such things as the motion of solar system bodies, the movement of stars (etc) in galaxies, all the way up to super-clusters, and including the in-spiral of massive dense objects such as neutron stars and black holes. There are no observations or experiments which are inconsistent with the predictions we make from GR. GR also predicts gravitational radiation, and the indirect effects of such radiation have been observed, just as predicted. Several 'gravitational radiation detectors' have recently been completed, or will soon be completed; their successful detection of gravitational radiation would be a further example of the extraordinary success of GR, and of our deep understanding of the nature of gravitation.

    Within GR, gravity may be understood in terms of 'curved space-time', but the best way to understand how gravity works (within GR) is via the math.

    However, GR is incompatible with QFT, and no attempt to date to develop a quantised theory of gravity (or some other TLA) has been successful, in terms of being consistent with both GR and QFT in their respective domains.

    Regarding the graviton: a quantised theory of gravity, consistent with QFT, would likely include a particle which mediates the gravitational force, in a manner analogous to how the photon mediates the EM force. However, the graviton is entirely hypothetical; there are no observations of such a particle, and GR does a perfectly good job of predicting and matching observations and experiments without it."


    {now where do I put the question mark??}
     
  19. Apr 17, 2004 #18

    wolram

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    Several 'gravitational radiation detectors' have recently been completed, or will soon be completed; their successful detection?
    i suppose, sorry im so obstropolous.
     
  20. Apr 17, 2004 #19
    In a simple way, gravitational energy is just the product of gravity force and the component of distance in the direction of that force.

    Complete understanding of gravity will come about when the concept for the origin of mass is fully undestood.

    General relativity made the connection between mass density tensor (or energy momentum tensor) to the structure of spacetime curvature. GR does not say what mass really is. It just says there is a connection to spacetime.

    Quantum gravity attributed mass to the scalar Higgs field. The quanta of this field is the Higgs bosons if they exist at all. If these bosons are found then the theory is vindicated.
     
  21. Apr 17, 2004 #20

    LURCH

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    Yes, if gravity waves are discovered, they will be a radiative force. But that is not a measurement of gravity or the gravitational field itself, only the gravity waves that occur as a result of changes within that field. A binary system of two massive objects orbitting one another will give off gravity waves, losing energy and therefore mass in the process. But a single massive object at rest (with a mass equal to that of the two objects in the binary system) will exert as much gravitational influence on sarounding bodies as does the binary system, without loss of mass, or emission of waves.
     
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