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Gravity: real force or artefact of acceleration?

  1. Mar 17, 2008 #1
    The main question is really: can you consider a reference frame at rest w.r.t. a non-rotating massive object to be an inertial frame? I'd say you can't, as you would experience an acceleration in that frame and, according to the equivalence principle, you hence might as well be in an accelerating frame, which would be non-inertial - and as such you would be equating an inertial frame to a non-inertial frame, which would seem nonsensical.

    Others have said that you can very well treat the frame at rest w.r.t. a massive object as an inertial frame, when you just treat the gravitational force as a real force instead of an artifact of an accelerating reference frame.

    This might not seem like a problem at all (but merely like two alternative interpretations), but I think there is a definitive difference once you start considering the problem of a charge in a gravitational field (which has been talked about on these forums already I believe), and whether or not it radiates - as opposed to a uniformly accelerating charge.

    So, what do you people think? At rest w.r.t. a massive object, are you in an inertial reference frame? Why, or why not?
  2. jcsd
  3. Mar 17, 2008 #2


    Staff: Mentor

    I like a practical and experimentally consistent definition of inertial frames: an inertial frame is a reference frame where an ideal accelerometer (acceleration and rotation type) that is at rest in the frame registers no acceleration. Under such a definition a frame at rest wrt a non-rotating massive object is non-inertial.
  4. Mar 18, 2008 #3
    Tidal gravity is real and not simply an artifact of acceleration. To understand what tidal gravity is, you simply go ta freefalling frame, and observe what happens to test particles.

    e.g. on if we have two test particles of the same mass and drop them from a height, and observe them in a freefalling frame, we will observe the two particles come closer together over time (since both particles are attracted to the centre of the earth). The vertical acceleration is simply an artifact of us being stopped from falling by the earth's surface.

    In fact, it is the tidal gravity that is related to the curvature tensor.
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