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The Speed of light vs a stick?

  1. Jan 12, 2010 #1

    Ok, so i posted this on another forum and got a lot of different answers. My main question is:

    Imagine this, i am somewhere between the moon and the earth and my friend John is on Pluto. In my right hand i have an unbreakable stick ( for the sake of argument ) that reached all the way to John, in my left hand i have a light-switch that i could turn off the sun with instantly.

    If i were to yank the stick at the same time as i flipped the switch for the sun, what would John notice first, the sun going out or the stick moving?
    Last edited: Jan 12, 2010
  2. jcsd
  3. Jan 12, 2010 #2


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    He would notice the sun going out first. The stick cannot transfer information as fast as the speed of light. If that doesn't make sense to you, consider that there is no such thing as a perfectly rigid stick. The stick would deform, and that deformation would travel at some velocity down the stick slower than the speed of light (Actually, the velocity would probably be the speed of sound within that stick, which would be vastly slower).
  4. Jan 13, 2010 #3
    I see, thanks for the answer.
  5. Jan 13, 2010 #4
    Hi there,

    I agree with the answer given, but something confused me.

    I thought the question was an unbreakble stick and not a rigid stick. From my understanding, the stick can be deformed, just not broke apart.

    Secondly, why the speed of sound??? Information can be transfered in solid objects through phonons, which have nothing to do with the speed of sound. But there again, I agree that the transfer velocity would be quite a bit less than the speed of light.

  6. Jan 13, 2010 #5
    They actually do. Phonons are vibrations, and the speed of sound is the speed at which vibrations get transmitted through an object.

    Not necessarily, in astrophysical objects, the speed of sound can be close to the speed of light. This is partly what causes white dwarfs to collapse into neutron stars. When the speed of sound gets close to the speed of light, the white dwarf becomes non-rigid and liquifies into a neutron star.
  7. Jan 14, 2010 #6


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    There is no such thing as an infinitely rigid stick/string. This is forbidden by GR. A stick/string compresses/stretches before the other end moves. A compression/tensile wave causes the other end to move. That wave can never travel in excess of c.
  8. Jan 14, 2010 #7


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    Okay, so the non-rigid stick answer is still valid.

    Exuse me? Phonon, by definition, travel at the speed of sound in the object.

  9. Jan 14, 2010 #8


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    There is also no such thing as an unbreakable stick. The binding energy must be finite, and therefore the bonds in the stick must be breakable. There is no such thing in nature that produces an infinite binding energy (maybe a black hole? Not sure about that, but in any case, that's not a stick...).
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