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Mass, Velocity and Special Relativity

  1. Jan 6, 2012 #1
    Is mass an ultimate product of velocity? If we were somehow able to take the Earth out of it's orbit about the Sun and then somehow slow it such that it was no longer rotating with the rest of the Milky Way would its mass decrease?

    There are a million factors here but universal expansion aside, and assuming this were possible does the "math" say this would in fact happen?
  2. jcsd
  3. Jan 6, 2012 #2


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    Gold Member

    No. Relativistic mass increase is an effect seen by an observer in a different reference frame (i.e. relative motion wrt Earth). Anyone in the same reference frame as Earth measures Earth's mass as unchanging.

    So, whether zooming around the galaxy or floating out in the void, we measure Earth's mass to be the same.

    Now, that being said, someone sitting on A. Centauri would measure Earth's mass differently - at first it was stationary wrt A.Centauri when they were both in the galaxy, but when Earth is transported to the void, it now has relative velocity, and A.Centauri would measure an increase in Earth
    's mass.

    Note that "moving" versus "stationary" is completely arbitrary. It has nothing to do with whether Earth is moving or whether A.Centauri is the one moving. You'd see the exact same thing if you left Earth where it was and instead passed by it in a rocket at the same speed.
    Last edited: Jan 6, 2012
  4. Jan 6, 2012 #3
    Well that settles that, thanks for the quick reply!
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