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A Simple Thought Experiment

  1. Jul 10, 2004 #1
    A "Simple" Thought Experiment

    Imagine you're out in space with no gravitational influence. Now, you have in this space a rod, 1 light-minute long, of some super-ridgid material. At either end of the rod, there's an astronaut. Astronaut 1 also has a flashlight. Astronaut 1 rotates the rod on its major axis, and turns on the flashlight at the same time.
    Now here's the question: Which one does Astronaut 2 see first: the flashlight being turned on, or the rod rotating? Why?
  2. jcsd
  3. Jul 10, 2004 #2
    Assuming a small narrow ray of light pointed along the rod Ast 2 never sees the light !!!
  4. Jul 10, 2004 #3

    I should probably put up a diagram with this. The light is intense enough to be seen from that far away. It is also separated by some space (at least a meter) from the rod. Also, the rod is viewable to see if it is rotating (say it has a white dot near the edge so that you can see it turn). Both objects are within Astronaut 2's field of view. If you need further clarification, please ask.
  5. Jul 10, 2004 #4
    No Seriously -- inorder to rotate the rod A1 has to use two parts of his body maybe one arm and a leg. However we could equally assume he is just rotating himself wrt the rod -- but to point the lamp along the rod means that wrt. the rod his lamp is not moving. A2 sees the light but does not experience the rod turning.
  6. Jul 10, 2004 #5
    There is no super-rigid material under the consequence of relativity.
    I think the astronaut 2 will see the light and start rotating at the same time.
  7. Jul 10, 2004 #6
    Astronaut 2 will see the light first. The light will travel in a straight line from Astronaut 1 to Astronaut 2.

    Consider what it means for a rod to be rigid. It means that the bonds between the atoms of the material that make up the rod are very strong. As the atoms near the end of the rod are caused to move by Astronaut 1's hand, the electrons of those atoms push on the electrons of the adjacent atoms and cause those atoms to move. This is the kind of reaction that will propagate down the rod while it rotates. Even in atoms with the strongest bonds, there is inertia to be dealt with, and during the initial start of the rotation along each segment of the rod, the velocity is not at the full rotational speed.

    Perhaps by "super-rigid" you meant to imply that the accelleration of adjacent atoms would be infinite. Although that's physically impossible, the accelleration could be very high. Let's assume it's very nearly infinite for the sake of this argument.

    The light beam would still get there first. The force of rotation would be transferred to adjacent parts of the rod strongest in the radial direction. The force would be transferred down the rod's axis would be at the sine of the angle from the hand. So the force cannot be transferred efficiently along the long axis. It would have to more or less spiral along the rod.

    The force that makes the rod rigid is eletromagnetic between the atoms, and so also propagates at c (at most, presuming no real inertia). But since it must take a spiral path, the length is much longer, and so the rotation at the other end will be delayed.

    The light gets to the other end first.
  8. Jul 10, 2004 #7
    I stand to be corrected but I do not think phonons travel at "c" , but I believe according to the statement 'in space' the question is HOW is A1 goting to rotate anything -- he's not standing on some ground and remember this rod is rather long -- it's mass unquoted.
    From my view point A1 can only rotate himself wrt. the rod , but it supposes he is shining the lamp down it's length , then almost by definition A2 can eventually see the light but cannot experience any movement of the rod.
    If anything did move there is going to ne a nasty question on light velocity ( not speed) according to STR light velocity is vectorially added to it's source regarding direction - the final speed being 'c' , it raises a whole mess of questions.
    I think the question originator should explain the problem a little more precisely to be solved. Ray
  9. Jul 10, 2004 #8
    The rod is rotating, and starts doing so at the same time as the light is turned on. They are in nearly the same location, and do not affect one another. The observer is just determining whether the rod rotates on his end, or if he sees the light, first. Astronaut 1 is inconsequential to the actual problem at hand, and Astronaut 2 is merely an objective observer (the heck with quantum interaction on this scale!)
  10. Jul 10, 2004 #9


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    Staff: Mentor

    When astronaut 1 pushes the rod to start it rotating, a wave passes down the rod at the speed of sound in the rod, starting its rotation.
  11. Jul 11, 2004 #10
    Speed of sound? Not speed of light(ie. the greatest speed possible for any interaction ot be transfered?)? Er...?
  12. Jul 11, 2004 #11
    Russ is correct.
    Force manipulation of rigid(or non rigid) objects is far, far, far, far less than the speed of light.
    In fact, regardless of how "ideal" the rigid rod might be, it is simply impossible to expect force transfer to ocurr even remotely near the speed of light. This is because your rigid rod consists of rest mass.
    If you were to impart a significant impact shock force on one end of a 1-light minute length rod, quess how long it would take for the other end of the rod to be affected(even if it could)?
    Well, a Monday-Friday vacation in the Bahama's would probably be close.
    Qualification: at 100,000 mph shock wave trasversing a 1 light-minute length rod would take 4.6 days to reach the end if I have my math right.
  13. Jul 11, 2004 #12
    I see! Thanks.
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