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Effects of Space-Time

  1. Nov 21, 2008 #1
    Hypothetical thought experiment;
    What would happen if you created a very long cylindrical material, eg 100 000km long and took it into deep space. Here you rotate one side of it so the tube is spinning in a large circle with radius 100 000km, ignoring friction and gravity.

    Once it is spinning, the outer edge will spin faster that the inside as it has a greater distance to travel and is doing this in the same time etc.

    If the object was sped up to travel 50-99% the speed of light*, time and space would bend in order for the outer edge to speed up to allow the edge to spin faster, without exceeding the speed of light.

    I suppose that the mass of the outer side would increase while time would slow down, and the object would begin spiraling in on itself, in the same way spaghetification occurs in a black hole, however im not entirely sure

    What would appear to occur for a person with a frame of reference;
    a. A stationary observer
    b. An observer standing on the inner edge of the circle
    c. A person on the outside edge of the circle (Moving closer to the speed of light)

    Also, i know that to move an object near the speed of light requires and infinite amount of energy, however technically your not moving the outer edge, but instead it is being pulled along by the particle bonds. Would you then still require vast amounts of energy in order to increase the objects speed?

    This isn't a homework question, it's just something i thought of and has puzzled me for a fair few days, as well as those around me. So if you guys could help with some input, it'd be great.

    * The speed, whilst still a relatively high value, may not be necessary to answer the question as long as the outer edge of the object is moving faster at near speed of light.
    Last edited: Nov 21, 2008
  2. jcsd
  3. Nov 21, 2008 #2
    The co-valent bonding between atoms of the material would be ripped-apart far below the speed you suggest, resulting in a catastrophic fragmentation.
    In other words, this "experiment" is impossible.
  4. Nov 21, 2008 #3

    Like i said, thought experiment, its the same sort of question as holding a mirror up while traveling at the speed of light, and seeing whether or not you would have a reflection.
    If the material wasn't ripped apart and could withstand the forces, what would happen then?
  5. Nov 22, 2008 #4
    Well, a very special condition would occur. The system would "lock". That is, further acceleration becomes impossible, as if an invisible, impenetrable wall is encountered.
  6. Nov 22, 2008 #5
    It's rather interesting that you should bring this subject up.

    The "C-Lock effect", as I call it, has tremendous potential, but as yet I have not been able to devise an actual, real-world physical experiment to express and validate the phenomenon.

    Too much structural stability problems. I've come to the tentative conclusion that a macroscopic C-Lock is not possible.
    Last edited: Nov 22, 2008
  7. Nov 22, 2008 #6

    George Jones

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    There is no such as a rigid body in relativity.
    Can you give a reference for this?
  8. Nov 22, 2008 #7
    An infinitely long rotating cylinder is a hypothetical model for time travel via frame dragging...
  9. Nov 22, 2008 #8
    George, I coined the term. As far as I know there is no specific reference to it apart from my own.
  10. Nov 22, 2008 #9


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    It's not at all the same sort of question. There's nothing in relativity that suggests that the mirror would break or anything like that, but the existence of an absolutely rigid body is in direct violation of the postulates of relativity.

    You'd have to be more specific. You haven't even explained how you intend to get it rotating. Are you pushing every part of it at once, or are you applying a force to the stationary end of it? I'm also curious about if you really want this treated as a GR problem (including the curvature and frame dragging effects caused by the motion of a massive cylinder) when there's enough interesting stuff going on in the SR version (mass--->0) of this problem.

    The upper bound on the set of possible angular velocities is different for a point near the "outside" endpoint of the cylinder than for a point near the "inside" endpoint. So it seems clear that if the cylinder is turned by gently applying torque to a small part near the "inside" endpoint, the cylinder will bend. The initial bend would propagate through the cylinder and straighten it out, if it wasn't for the relativistic effects. What relativity adds is that it will get more and more bent as the angular velocity of the part near the inside endpoint is increased. The internal forces in the cylinder will also make it more and more difficult to increase the angular velocity further.
    Last edited: Nov 22, 2008
  11. Nov 22, 2008 #10


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    I think that cylinder is supposed to rotate around it's symmetry axis, and this one is meant to be rotated in different way. (But I'm not 100% sure about either statement).
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