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Falling object at c

  1. Jul 3, 2003 #1
    Gedenke experiment: Falling object upto c?


    I would like to know the relativistic position expression for a falling object that could reach the speed of light.


    P.S.: Suppose the only things, that exist in the universe, are a BIG attractive mass and the attracted "object".
    Last edited by a moderator: Jul 4, 2003
  2. jcsd
  3. Jul 4, 2003 #2
    Nothing with mass could be accelerated to c.

    Even if it were to go into a black hole, starting an infinite distance away, once it crosses the event horizon, it is in a different spacetime.
  4. Jul 4, 2003 #3
    Gedenke experiment: Falling object upto c?

    Bonjour Brad,

    I shall agree with your mass issue.

    Suppose, isolated in the universe , a small energetic object, with zero initial relative speed, somewhere far-far-far away from a non-negligeable gravitationnally attractive accumulation of energy (both with mass equivalence (m & M) for gravitational purpose ).

    "m" shall be accelerated by "M", following gravitational effect, right?

    While m's speed is within non-relativistic speed, I got no problem to express acceleration, speed and position equations.

    What would be these equations when relativistic speed's effect is non-negligeable? (Should I ask this in Theoretical forum?)

    P.S.: ..........................................................
    Last edited by a moderator: Jul 4, 2003
  5. Jul 4, 2003 #4
    For relativistic versions of those, some tinkering with the equations here: http://www.wikipedia.org/wiki/Relativistic_equation

    might help you to get them.

    Also, http://math.ucr.edu/home/baez/physics/Relativity/SR/rocket.html [Broken]

    note that it has the equations you are looking for.
    Last edited by a moderator: May 1, 2017
  6. Jul 6, 2003 #5


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    Welcome to Physics Forum, Imagine! :smile:
  7. Jul 7, 2003 #6
    Phobos, merci pour votre mot de bienvenue.

    Brad, I red the rocket page. I also understand the origin of the following restriction: "The acceleration of the rocket must be measured at any given instant in a non-accelerating frame of reference travelling at the same instantaneous speed as the rocket".

    Thanks, that was exactly what I searched.
    Last edited by a moderator: May 1, 2017
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