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Difference between spacetime and the gravitational field?

  1. Feb 16, 2016 #1
    Are spacetime and the gravitational quantum field (still hypothetical) separate entities? Would the gravitational field be more fundamental, one of the various entities from which spacetime as a whole is composed?

    Gravitons, which are believed to transmit the force of gravity, would surely be excitations of the gravitational field - however, gravity arises from the curvature of spacetime. If separate, would the curvature of spacetime simultaneously excite the gravitational field, or vice versa?

    Fields are defined by variables across different points in space and time, so, if the two were the same system, would the gravitational field not define itself (paradoxical)?

    Are there any theoretical answers to these questions; if not, would the answers be vital in unifying QFT and GR?
  2. jcsd
  3. Feb 16, 2016 #2
    They are different models of gravity. They may be considered 'unified' entities when somebody figures out to combine GR and quantum theory of gravity.

    based on past discussions in these forums nobody knows whether space and time are quantized or continuous...and some research papers claim that distinction is meaningless anyway. Do we need to worry about increments of Planck length or Planck time...depends on the model and scales of interest.

    QUOTE="R. E. Nettleton, post: 5380813, member: 574297"]If separate, would the curvature of spacetime simultaneously excite the gravitational field, or vice versa?[/QUOTE]

    Yes. Depends on your model. It is interesting that we model via field theories [as you posted] which assign continuous values to points in time and space, yet when we go to measure entities, what we find are point particle' interactions, of immessurably small dimension. In other words, the QFT of the Standard Model of particle physics, which so far excludes gravity,is a model of POINT like interactions. It is modeled on fixed spacetime backgrounds, not the dynamic 'changing, evolving] spacetimes of GR.

    I'm unsure if there is a consensus on whether fields or point particles are more fundamental. My own view is that depends on your model. If the photon [or virtual photon if you like] is the particle of the electromagnetic field, then the graviton is the particle of the gravitational field. Also keep in mind most of these models are approximations: for example the electromagnetic field varying as 1/r2 runs into problems as one approaches small distances, as r approaches zero.

    We use quanta because that is what we measure. Max Planck discovered physical action at small scales takes place in discrete steps, not continuous ones...hence the 'h' Planck's constant. Action at the sub atomic scale is quantized...that is we see interactions as multiples of 'h'.
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