Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

Equivalence Principle questions

  1. Jun 3, 2013 #1
    Sorry for some probably very basic questions, but here goes.

    If gravity equals acceleration, how is the Earth's gravity defined by acceleration?

    If an accelerating body distorts spacetime, as described in Einstein's thought experiment about a light beam shined through an accelerating box, how is this equivalent to gravity in terms of what's going on outside the box? Put another way, is an accelerating object's equivalence to gravity only extremely local to the object?

    If a large body's distortion of spacetime is propagated by gravitons, as some have theorized, do accelerating bodies also produce gravitons? Einstein's thought experiment suggests to me that the answer is no, because the spacetime distortion seems to be a function of geometry, not force. But then, whence gravitons?

    --Thanks for indulging a non-physicist (but a curious one!)
  2. jcsd
  3. Jun 3, 2013 #2


    Staff: Mentor

    That's not what the equivalence principle says. A better statement of the EP would be something like this: Being at rest in a gravitational field is locally equivalent to being at rest in an accelerating rocket. There is no implication that "gravity equals acceleration" in all respects.

    It doesn't.

    This thought experiment doesn't say acceleration distorts spacetime. It just says that if the box is accelerating, an observer at rest relative to the box will see a light beam passing through the box "fall" downward. Spacetime is flat throughout; there's no distortion of it.

    It isn't.

    The EP is local, yes. See above.

    The easy answer is no, but the strictly correct answer is "it depends", because some types of accelerations can produce gravitational waves. A rocket accelerating in a straight line does not produce any gravitational waves, but other accelerating objects might.
  4. Jun 3, 2013 #3


    User Avatar
    Science Advisor

    It doesn't? Sure it does! If Capt Kirk throws lead bricks out the hatch, not only does the Enterprise accelerate, but the third time derivative of the quadrupole moment of the ship + bricks system is nonzero, and in lowest order this quantity is the source of gravitational waves.
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook