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Speed of force

  1. Jan 1, 2004 #1


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    Hai friends..
    We are getting enough data on speed of gravity.But what about
    the speed of other mechanical forces.
    When I am pulling one side of a long rop how much time will
    it take to reach the othe side .I don't know, may be the speed of wave.
    I think if I pulls an inextensible light weight rope
    i can move it towards me with out a time waste.Then that makes problems.If I tied one side of the rop to a massive object like sun and started pulling indeed i can pull it without any difficulty atleast for 8 minutes.If I am pulling with half the speed of light
    it will be get extended to millions of km.So what is the solution to this.
  2. jcsd
  3. Jan 1, 2004 #2


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    When you work with ideal models - inextensible cords, perfectly elastic billiard balls and all - you pretty much assume the "speed of force" is infinite. But in the real world force can't be transmitted any faster than the atomic structures can adapt to it. If you push on a metal rod, the force is transmitted to the other end by the adaptations of the rod's electronic structures. And than can't go any faster than the speed of sound in that metal.
  4. Jan 4, 2004 #3


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    In relativity there cannot be any rigid objects (or "inextensible rope") for exactly the reasons you give.
  5. Jan 7, 2004 #4
    Isn't it C?

    I thought a photon exerts a force, so wouldn't it be C?

    Anyway, I just spotted a post about data on the speed of gravity, which is something I'm interested in because of its relevance to string theory models, and I'm new to these boards, so this might be a naive observation: If gravity is an artifact of "string" mechanics, then it is "inextensible rope" and the speed of force is infinite; however, if gravity has a speed, such as light seems to, this would cause serious problems in the model of the universe now because it would imply that gravity is transmitted by something that has mass: e.g., the Graviton.

    There is one good argument for the concept that gravity is rigid, and that's by examining quark pairs whereby you can separate the pairs, affect one, and produce instantaneous effect on its counterpart -- even faster than C. The concept exists in physics. It has also been theorized that this effect can be produced on a macro level by utilizing the Kazimir effect to "pair" large superconducting plates in vacuum and then physically separating the plates, thereby producing a working model for the trekkie "subspace communicator."
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