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Relativity question

  1. Jan 29, 2009 #1
    Suppose we have a two giant ideal springs very far apart from each other such that each spring can be used to launch a spaceship X to relativistic speeds. So a spaceship is launched using spring A to relativistic speeds, it reaches spring B which it compresses and is thus relaunched when spring B relaxes and the cycle thus continues.

    Now say there is a person inside spaceship X who shines a flashlight out of a window of the spaceship onto say a large number of solar panels lined up along the route from spring A to B. Whenever the spaceship accelerates (decelerates) he switches off the flashlight.

    Now from the perspective of the solar panels, since the spaceship X is traveling at relativistic speeds, say 1 minute of the shining of the flashlight relative to the spaceship corresponds to 10 years relative to the solar panels.
    How is a 1 minute burn of a flashlight able to generate enough energy to generate electricity for 10 years? Is it because the chemical bond strengths change at relativistic speeds?
  2. jcsd
  3. Jan 30, 2009 #2


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    I think everything balances up without resort to the proper times of the two observers. The energy transferred from the light to the panels is proportional to (intensity x time). Measured in each frame the energy will be the same if the transverse Doppler effect is taken into account.

    I haven't got time now to do the calculation.

    Naively ( light frame quantitities are primed) L is the length of the panel, t is clock time and v is the velocity of the light emitter relative to the panels.

    in the panel frame : t = L/v, I=I'*gamma
    in the light frame : t'=gamma*L/v, I'=I'


    Looks a bit too easy, but given that light energy is proportional to frequency, it could be right.
    Last edited: Jan 30, 2009
  4. Jan 31, 2009 #3
    sid: interesting question!! Good perspective!!

    Yet I did not get the spring reasoning....why not just a space ship passing by at relatvistic speed?? Does the spring continuing cycle have any effect...I don't see any.
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