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How's energy stated in special relativity.?

  1. May 26, 2012 #1
    suppose we have a frame which light beam of energy=h(nu)..where nu is frequency with inverse time period..now what is this time period measured as..? We get different time period's from different frame and hence different energies..which frame's time period is taken and did his violate conservation of energy
     
  2. jcsd
  3. May 26, 2012 #2

    Dale

    Staff: Mentor

    You are confusing conservation with invariance. When a quantity is the same at all points in time then it is called conserved. When a quantity is the same in all different reference frames then it is called invariant. They are two completely different concepts.

    Energy is conserved. Energy is not invariant.
     
  4. May 27, 2012 #3

    Ger

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    Another take: Speed of light is the same in any frame of reference. The light beam will cover the same amount of ground in the same time (doing work) with the same end result. Energy is conserved.
     
  5. May 27, 2012 #4
    regarding

    What work is done transporting a photon from one point to another?? none. Once light is emitted in a vacuum it does not change color...it does not change energy. Furthermore in an expanding universe such as ours, cosmological distance [expansion] also changes over time.

    "We get different time periods..."

    What about the 'Doppler shift' of electromagnetic waves, as in a radar speed trap? We infer speed from observed frequency shift. But photon signal frequency and wavelength can be viewed as fixed... Doppler shift is a particular explanation of redshift, based on the relative velocity of source and detector, with a particular relativistic formula. The frequency of the signal itself remains fixed coming and going; It's the the relative motion between the emitter and absorber [the Doppler effect] that causes the effect, not anything to do with photons themselves.
     
  6. May 27, 2012 #5

    Ger

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    Energy is capability to do work. Measuring is doing some work. For the measuring of the light source in another frame moving along another geodesic line, the ray of light has to cross. That will give doppler shifts doppler broadening of the lines. Total energy content as measured will be the same.

    From the book:

    4. Tests of Time Dilation and Transverse Doppler Effect

    The Doppler effect is the observed variation in frequency of a source when it is observed by a detector that is moving relative to the source. This effect is most pronounced when the source is moving directly toward or away from the detector, and in pre-relativity physics its value was zero for transverse motion (motion perpendicular to the source-detector line). In SR there is a non-zero Doppler effect for transverse motion, due to the relative time dilation of the source as seen by the detector. Measurements of Doppler shifts for sources moving with velocities approaching c can test the validity of SR's prediction for such observations, which differs significantly from classical predictions; the experiments support SR and are in complete disagreement with non-relativistic predictions.
     
  7. May 27, 2012 #6
    What is all this supposed to show??
     
  8. May 27, 2012 #7

    Dale

    Staff: Mentor

    Naty1 is correct. A photon doesn't do any work simply by moving from A to B at c. It does work by being absorbed.
    The same as what? The total energy content will not be the same in different reference frames. It will be the same from one time to a later time in the same reference frame. Energy is conserved, it is not frame invariant.
     
  9. Jun 1, 2012 #8
    It was only the middle sentence that we are taking issue with; we agree energy IS conserved in special relativity. The middle sentence is wrong on two counts: no work is done; and over vast [cosmological] distances, light does NOT cover the same 'ground' [distance] in the same time due to varying rates of cosmological distance.

    For adit's benefit, a way to think about a simple case: think of an emitter and receiver of electromagnetic radiation, perhaps a few miles apart: Say one is moving with respect to the other; the measured frequency at each will be different. You can tell this is a measurement issue by then having the emitter and receiver stationary with respect to each other and taking another set of measurements: voila, the emitted and received frequency are now measured to be identical.
     
  10. Jun 3, 2012 #9
    so whats the time period of frequency taken..?i guess this not answered..is that proper time's time period or diluted time period(from other frame).?
     
  11. Jun 3, 2012 #10

    Nugatory

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    Staff: Mentor

    You can use any frame's time. The frequency and hence the energy you measure will vary according to which frame you choose (that's why we say it's not invariant), but no matter which frame you choose, energy will be conserved and the relationship between energy and frequency will hold.
     
  12. Jun 3, 2012 #11

    Dale

    Staff: Mentor

    As Nugatory said, take frame A, measure the frequency fA in that frame, calculate EA=hfA, that is the energy in A. Now take another frame B, measure the frequency fB in that frame, calculate EB=hfB, that is the energy in B. In general fA≠fB so EA≠EB.
     
  13. Jun 4, 2012 #12
    A massive object as seen from different frames has different energies due to different speeds, a massless photon has different energies as seen from different frames not due to different speeds, but different frequencies.
     
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