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Is there a limit on real inertial frames?

  1. Sep 14, 2008 #1
    Hi guys,

    I initially posted this here ...


    ... but I see by the forum rules I should have posted it independently.

    First of all, am I correct in understanding that a) it is impossible for an observer to observe anything accelerating to c? b) all the equations concerning this are relativistic except c) acceleration which does have a real affect of slowing the "clock" on any accelerated object?

    If any of this is wrong, please correct me.

    The problem for me is this: regardless of how an observer sees a moving object, even if the apparently relative speeds approaching c, the observed is still experiencing time normally. As the observed object appears to approach c the relative time is slowing.

    But this is just relative; it is not a real effect. The object being observed still is experiencing normal time (processes) passing even if they are observed to be almost stopping or stopped by an outside observer.

    To explain my question let me paint a theoretical picture. A rocket is observed passing at just below c (relatively speaking). The hand of the pilot is poised to accelerate but because of time dilation, it is practically frozen (moving so slowly that the observer would die before any movement could be detected).

    If the pilot's and rocket's process were somehow maintaining their normal rate to an observer (as things would appear if they were in the same inertial frame) as the rocket's speed was relatively approaching c, the rocket's acceleration would be exponentially increasing.

    But that is exactly what is happening from the point of view of the pilot. From her point of view, it takes two seconds to move the throttle fully forward, even the the observer dies, along with several of his suns, before he can experience any of it.

    So, my question is: what has happened to this pilot relative to the first observer (other than the observer and his universe dies before the hand reaches the throttle) once she has moved the throttle fully forward and the rocket accelerated away?

    Is my question clear?
  2. jcsd
  3. Sep 14, 2008 #2


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    The fact that the rocket can't reach c is not solely due to time dilation, in fact an observer on board the rocket could be accelerating at a constant rate (constant G-force as felt on board the rocket) forever according to her own clock and still never see measure the outside universe to be moving at c in the rocket's inertial rest frame at any given moment (equations concerning a rocket accelerating at a constant rate can be found on the relativistic rocket page). This has something to do with the way velocity addition works in SR--see my post #4 on this thread.
  4. Sep 15, 2008 #3
    relativistic effects are 'real'. objects that are moving really are length contracted, time dilated, and really do experience loss of simultaneity.
  5. Sep 15, 2008 #4
    You say "objects that are moving really are length contracted, time dilated, and really do experience loss of simultaneity". What are they "moving" relative to?
  6. Sep 15, 2008 #5

    George Jones

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    A particular inertial reference frame. And I think one has to take care with what is meant by "real."

    These things are "real" in the sense that given a pair of events, the spatial and temporal separations between the events are frame dependent. A moving watch, however, doesn't notice anything special happening to the way it keeps time, i.e., it doesn't notice any physical change in itself.
  7. Sep 15, 2008 #6
    Okay, thanks everyone. I got it.
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