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Free falling reference body

  1. Mar 6, 2010 #1
    Hi Guys,
    I have a problem visualising a free falling object to use as a use as a theoretical reference body. I assume that a geodesic path would be the path that such a body would follow, but surely rotational dynamics would preclude such a situation. If there were theoretically such a body, would it just carry on accelerating forever toward some theoretical gravitational centre or is there some speed limit (light?)
  2. jcsd
  3. Mar 6, 2010 #2
    hi pierre

    you donĀ“t normally choose an accelerating thing to be a frame of reference. in special relativity you choose an inertial frame of reference, which would be a frame of reference with uniform motion, not undergoing any kind of acceleration.

    one could imagine a body which is being accelerated by some (magical) constant force forever. lets say we take a spot in space to be our inertial frame of reference. this body would start up with no velocity relative to the reference frame, then the acceleration would kick in and last forever. this body would gain speed forever, although it could never reach the speed of light. if this body was at almost the speed of light it would seem to us that the acceleration results in ever smaller increases of speed, but(and that might seem strange) the mass of the object would increase, which is accounting for the ever greater kinetic energy.
  4. Mar 6, 2010 #3


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    I don't understand your problem. Why would rotational dynamics exclude free-fall of a non-rotating body ?

    There is a limit to the speed that can be attained, as measured by a distant stationary observer. This paper discusses free fall in the Schwarzschild spacetime.

    arXiv:gr-qc/0411060v2 (31 Aug 2006)
  5. Mar 6, 2010 #4
    Hi Mentz,
    Thanks for the response. Perhaps the qestion should be: is it possible to imagine an object in space that is not an integral part of some or other rotating system? Would the rotational speed not then prevent this object from being "free falling"?

    Mentz, please excuse my ignorance of the system, but how do I access this paper you ave referenced fo me above?
  6. Mar 6, 2010 #5


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    Hi Pierre,
    falling freely means that the object in question has no proper forces ( like rocket engines or solar sails) pushing it. This includes the great majority of astronomical objects, which are either orbiting something ( which is free fall) or being attracted by something else and falling towards them.

    However we can 'imagine' what happens if a test particle is released from rest a very long way from a black hole and allowed to accelerate undisturbed towards it. It's called a 'thought-experiment'.

    The paper is technical and you need to understand some GR. It can be got here

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