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How Do We Know Gravity Is A Force?

  1. Sep 20, 2007 #1
    This is my first post here, and I hope it's not too stupid. This question has obsessed me for a while now.

    Why is it necessary to quantize gravity? I am NOT a scientist, but I am fascinated by physics, and gravity seems to me to be a side effect of mass and space-time.

    I don't understand why gravity is considered a "force" that must be quantized, rather than a result of mass itself.

  2. jcsd
  3. Sep 20, 2007 #2


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    Einstein thought that gravitational attraction (and inertial effects) were the result of matter's interaction with the local space in which it is embedded. He expressed this several times in the 1920's. He tried (and failed) for the rest of his life to uncover the mechanics by which this attraction could be modeled, and he was quite exasperated that the GR field equations were elevated to a status that he felt was uncalled-for compared to the gravitational effects they were intended to explain. Einstein was a pragmatist, and he felt that observable results ought to drive our perception of theory.

    Michael Strauss (science spokesperson of the SDSS team) might be a good person to ask about the properties of quasars, and their redshifts, and metallicites, since the SDSS observations constrain any standard model of cosmology model out of existence.
  4. Sep 20, 2007 #3

    Maybe another question worth asking is how sure physicists are that the graviton exists. If they are quite sure there is a Graviton, I suppose that would suggest gravity is a force.
  5. Sep 20, 2007 #4
    It's a simple matter: QM says how small light things behave, GR says how big heavy things behave, and neither existing theory tells us how small heavy things behave. So we're looking to combine the theories into a better one.
  6. Sep 20, 2007 #5
    Oh, like neutron stars or compacted things like black holes. Okay, thank you for the reply. I will think about what you are saying.
  7. Sep 21, 2007 #6
    Who told you that it is necessary to quantize gravity? That's a strong statement in light of the fact that there is no complete quantum theory of gravity yet.

    What really puzzle's me is that the title you gave to this thread is How Do We Know Gravity Is A Force? but the question you really asked is about gravity being quantized. Which is it? Or do you want to address both questions seperately and distinct? (e.g. answer the question in the title without the quantized thing since that means discussing quantum mechanics and this is a forum on classical relativity?)

    Last edited: Sep 21, 2007
  8. Sep 21, 2007 #7
    Why quantize gravity?

    Because it affects particles, and particles are subject to quantum uncertainty. If you consider a very small scale, the change in momentum must be due to a quantum kick, not a truly continuous action. Anything that affects the motion of the quantum object behaves that way, at least as far as the quantum object is concerned. If gravity turns out to be totally different then bosonic forces, then the graviton will at the very least show up as a quasi-particle, like plasmons and phonons do.

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