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Why are mass, length, and energy relative?

  1. Jul 23, 2012 #1
    I understand why time is relative, (thanks to Einsteins thought experiments) but why are length, energy, and mass relative. I can see that if mass is relative so would energy be (and vice versa) but I cannot understand why either are. Same with length. Any help?
  2. jcsd
  3. Jul 23, 2012 #2


    Staff: Mentor

    Mass isn't relative, or rather, the mass that modern physicists use is the invariant mass which, as the name suggests, is not relative.

    Energy is relative even in Newtonian physics, particularly kinetic energy.

    Given that time is relative then length must be relative also otherwise c could not be invariant.
  4. Jul 24, 2012 #3
    You mentioned the thought experiment for time. If you're speaking of the 'light clock' thought experiment, then there is a direct analogue for length contraction. Imagine positioning a rigid rod on a moving rocket ship. Then, observers on the ship shine a light from below the rod to a mirror above. The length of the rod can be calculated in this frame by multiplying the speed of light by the time elapsed from the shining of the light until the impact with the mirror. Observers floating in empty space at rest can also do this calculation. However, due to the relativity of time, they both record different values for the elapsed time. Hence, they calculate different lengths.

    Keep in mind that you can derive the formula for length correction without making any reference to time dilation, but I used it for my explanation because you already understand TD.

    Regarding relativistic energy and mass, read this:

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