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If light was infinitely fast, we could just send a light signal from Earth to the planet. But I was wondering, if we made a perfectly non-elastic rope as long as the distance between earth and the far-away planet. Could somebody pulling the rope from the earth send an instant signal to a person holding the end of the rope on the far-away planet? Will the rope side on the other planet move at the exact same moment as the rope side on the Earth? Of course, we should ignore the obvious factors making this means of communication impossible -_-

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    $\begingroup$ There is a relativistic limit on how rigid an object could be. Think about how the object(rod or string) is constructed; it's a bunch of atoms and molecules. How do atoms sense the motion(let's say pulling, pushing, juggling, ...) of their neighbours? They sense it via oscillations in the electromagnetic fields, which in turn can't propagate faster than speed of light. $\endgroup$
    – Ali
    Apr 17, 2014 at 17:04

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All mater we see and touch and manipulate macroscopically is composed of atoms, molecules and solids which are held together by intermolecular forces. All these forces are electromagnetic, i.e. any disturbance in the end of one end of a string cannot be transmitted faster than the velocity of light, and usually transmission is much slower ( acoustic, vibrations). Therefore there cannot be any instantaneous transmission with any length string as you imagine.

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