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Does Gravity force exist?

  1. Aug 13, 2011 #1
    Hi all guys,
    for what I have understood from general relativity theory, gravity force is "just" a space-time distortion as a result of peculiar property of matter having mass. So it is not a real force, or saying with different words, if I'm in a lab without window, in a gravitational field, without contrains to motion there is no way for me to measure any gravitational force. I just feel the force when I try to escape from the "plain" trajectory in the distorced space-time.

    So my question is: why the phisicists are trying to unify the gravity with the other natural forces? I would say that it is ok to say there we can think a EM field across the space-time, hence we can talk about photons moving in a distorced space-time, but there is no gravitational field in the space-time it is the space-time itself, so why we look for gravitons?

    A second question related is: according to the general relativity theory the space-time is a local concept, ther is no absolute space-time reference, is this concept taken into account in the string theory? If not how can it be correct?

    sorry for my questions that could look ingenuous, I like science but I'm not physicist...
  2. jcsd
  3. Aug 13, 2011 #2
    The problem is that all attempts to model quantum physics in curved space-time have resulted in infinities, meaning that quantum mechanics and general relativity are incompatible with one another. Obviously this needs to be reconciled.

    The other thing to keep in mind is that all energy in quantum mechanics is quantized. Since gravitation can carry energy, there must be a corresponding particle called the graviton to carry gravitational waves.

    That's the hypothesis, anyway.

    AFAIK string theory assumes the same things as general relativity, although adding another six or seven dimensions. (Not that I think string theory's going anywhere...)
  4. Aug 13, 2011 #3
    Just a crazy idea what if gravity didn't carry energy like a force but rather transferred it to another form of energy.
  5. Aug 14, 2011 #4


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    Asking "what if" questions gets us nowhere fast. I don't even understand what you are suggesting. Different types of energy can already be converted to other types by gravity. A falling meteor is heated to a very high temperature, giving off all kinds of EM radiation. Part of the kinetic energy of the meteor has been transferred to radiative energy.
  6. Aug 16, 2011 #5
    What is still not clear to me is the different concept of gravity given by relativy theory respect the other forces. It is really elegant and amazing to define gravity like a space-time curvature and it is intriguing to see that this theory is much more accurate respect to the Newton's approach. But this fact should make gravity a very special entity so different in nature respect to EM forces that (as far I know) are not defined as space-time curvature, they originate in the space-time but are not explained as the effect of space-time distortion. This is not a matter of a different source (mass rather charge) or different particles involved (gravitons instead of photons). What we call "gravity" is based on a so different interpretation that I cannot just see how and why it can be unified....
    what do you think?
    Last edited: Aug 16, 2011
  7. Aug 16, 2011 #6
    Gravity converts potential energy into kinetic energy.
  8. Aug 16, 2011 #7
    I could be wrong but I don't see why you can't explain any force as a space-time distortion. Gravity was just described this way because we are working with large enough objects to have a detectable gradient in the field.

    I took a GR class online and all I came away with was a very elegant mathematical structure to explain different shaped attractive objects (specifically big and round). Nothing about WHY energy density attracts or HOW it simulates an accelerated frame of reference. These were just taken as they are with no explanation.
  9. Aug 16, 2011 #8


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    Because with gravity the EP applies. You already know what that is so you would know that we cannot, in a local neighborhood of space - time, differentiate between acceleration and free fall in a gravitational field so you can attribute gravity to the space - time curvature that occurs on the global scale and there is also the experimental fact that all objects fall at the same rate in a gravitational field so we can apply this to any arbitrary test particle (that any freely falling test particle travels on a geodesic through space - time). In the case of EM though particles with opposite charges will react differently to an EM field so the same concept is probably not nearly as trivial if at all possible. If you can recall an attempt to explain EM in a similar fashion to GR though I would be much obliged.
  10. Aug 16, 2011 #9
    ...EM field can create actractive or repulsive forces even if the space-time is flat. The prove is that a neutral body (no charge) is not influenced by EM field. So electrical charge or EM energy does not curve the spacetime like mass does, or at least I would say that it is not the spacetime distorsion responsible of the actractive/repulsive force...
  11. Aug 16, 2011 #10
    So can we explain EM force as a space-time distortion?

    I don't know what proves that the space-time is distorted, or what proves that space-time even exists. It seems to me they're just postulates, and what follows is a mathematical model based on those postulates. If space-time is distorted by gravity, where does the electroweak force fit in that picture? How do other forces exist if they don't distort space-time?
  12. Aug 16, 2011 #11
    If the system was taken to be only electric charges. For example say it was all positive charges, then you could explain it as space-time distortion if you wanted and say the particle is traveling along a geodesic. It is just a mathematical model.
  13. Aug 16, 2011 #12
    Of course, but what if we put negative and positive charges in the system, like in the real world?

    As I understand, we are using the space-time model for gravity and using a different model for the electroweak force. That doesn't make sense because they're all forces, so it's reasonable to assume they all interact with matter in similar ways. But I don't know almost anything about this, I don't even know if an EM field can cause time dilatation.
  14. Aug 16, 2011 #13
    The EM field does not cause time dilation, however fluctuations (photons) in the EM field increase the energy momentum tensor and do cause space-time dilation.

    This is why it is handy to treat gravity as space-time dilation. It is always attractive and always proportional to the total energy of the system. When working with other fields we are interested in the energy transferred through fluctuations of these fields and not the fields themselves.
  15. Aug 16, 2011 #14
    ok, I see that energy (any form of energy) can distort space time because of the equation E=mc^2, or we could say that it is the equivalent mass of the energy (E/c^2) to distort the space-time acting as a virtual mass (right?). And this sounds coherent. But again the motion of electrical charged bodies or particles is not driven by the spacetime distortion generated by EM energy, otherwise all the matter would be affected at the same way by the EM fields because all the matter is moving in the same distorted space-time.
  16. Aug 16, 2011 #15
    That's the problem that all theoretical physicists are hoping to solve right now. It has to be unified, because if we can't model the quantum universe using general relativity then we have an incomplete theory.
  17. Aug 17, 2011 #16
    But isn't it easier to first try to unify electroweak theory with general relativity? Is the space-time model consistent with the electroweak theory? Doesn't general relativity interpret a force as a space-time distortion? So how does the electroweak (or just electric or magnetic force, for sake of simplicity) force fits in that model?
  18. Aug 17, 2011 #17
    "Doesn't general relativity interpret a force as a space-time distortion? So how does the electroweak (or just electric or magnetic force, for sake of simplicity) force fits in that model? "

    that's the point. I can see two options here:
    1. the force concept is different for gravity, or as I was questioning at the beginning of this thread, the gravity has the effect of a force but it is not actually a force.

    2. all the forces should be interpreted as space-time distortion but in case of EM the distortion is not affecting the four dimensions we live in (x,y,z,t)....
  19. Aug 17, 2011 #18
    How's that string theory workin' for ya? Maybe I was a King once?

    Magnetism defies gravity. Gravity is alone without Mass. Magnetism needs no Mass, it creates Mass.
  20. Aug 17, 2011 #19
  21. Aug 22, 2011 #20
    Why not try to unify electromagnetism with gravity first. According to a research article on the laboratory production of gravitational waves, Gasperini and Venzo de Sabbatta worked out (1985) that they could be generated just using very strong static electric and magnetic fields and a light source (theoretically). In other words you don't require mass-energy in the form of 'pure mass' to produce gravity, the electromagnetism that characterises all atoms should do. This suggests that gravity is just a concentrated form of electromagnetism. Please refer to <http://www.scribd.com/doc/32331243/A-Comparative-Study-of-Electromagnetic-and-Gravitational-Fields> where I expand on this idea.
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