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Rest mass time dependent?

  1. Jun 30, 2015 #1

    ChrisVer

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    I was wondering... can the rest mass of an object be time dependent? Like in a scenario where the body is losing mass?

    (Sorry I meant for a title "rest mass time dependent?")
     
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  3. Jun 30, 2015 #2

    Orodruin

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    Only if the object is losing mass by somehow radiating it away.
     
  4. Jun 30, 2015 #3

    HallsofIvy

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    Or if you chop pieces off with a knife!
     
  5. Jun 30, 2015 #4

    Dale

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    Last edited: Jun 30, 2015
  6. Jun 30, 2015 #5

    Matterwave

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    I think the usual application of this scenario is the rocket problem. :)
     
  7. Jun 30, 2015 #6
    Well, this is the relativity subforum, where rest mass is synonymous with energy. What process doesn't change the energy?

    The energy of a tree changes if I start walking to the left.
     
  8. Jun 30, 2015 #7

    PeterDonis

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    No, it isn't. Energy is one component of the 4-momentum vector, and is frame-dependent. Rest mass is the invariant length of the 4-momentum vector, and is not frame-dependent.
     
  9. Jul 1, 2015 #8

    Ibix

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    Using older terminology than Peter, you're talking about relativistic mass (a concept that has largely fallen out of favour, by the way), not rest mass.
     
  10. Jul 1, 2015 #9

    strangerep

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    My $0.02 is that one should consider the dynamical group for the entire system. E.g., in a rocket, matter is ejected in one direction to change the rocket's velocity in the opposite direction. The rocket thus feels acceleration, so one must be cautious about naively applying the Poincare group to the rocket in isolation, and hence should also be cautious about the "invariant mass" Casimir of the Poincare group. Rather, one should probably decompose into com and relative coordinates, and find the full dynamical group applicable to the system with com dofs factored out. Then look at the Casimirs of that group applicable to the relative motion...
     
  11. Jul 1, 2015 #10
    You love the word "no", Peter. No, no, no, no no. Intrinsic mass is normally associated with the norm of P.
     
  12. Jul 1, 2015 #11
    - The energy of a tree is just as little affected by your change of choice of reference system as the amount of gasoline in a car is affected by your choice of using gallons or liters. Changing the observation (assuming no interaction) can not affect what is observed.
    - Your choice of reference system is free, it does not depend on your state of motion (compare GPS)
     
  13. Jul 1, 2015 #12

    strangerep

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    What's your problem, mate? Peter is not the one who made an incorrect statement. You did, in post #6, i.e.,
    That's just plain wrong. The word "no" is entirely appropriate in an answer to correct an error.

    If by "P" you mean 4-momentum, then,... PeterD said essentially the same thing. But that's different from your incorrect statement that "rest mass is synonymous with energy".
    In modern parlance, "Intrinsic mass", "invariant mass", and "rest mass" are synonymous (though I prefer "invariant mass"). Cf. this Wiki page.
     
  14. Jul 1, 2015 #13

    ChrisVer

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    Yes I had the moving rocket in mind.
    However I don't understand what you meant in the quoted message
     
  15. Jul 1, 2015 #14

    strangerep

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    Uh oh. To answer that properly needs a lot more than a mere "$0.02 worth", but I don't have much spare time at the moment. Sorry. :oldfrown:
     
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