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Question on Planck units

  1. Feb 12, 2013 #1

    Planck units are defined here

    Length, mass and time are defined via G, c and [itex]\hbar[/itex] and do not involve kB;
    But temperature contains all G, c and [itex]\hbar[/itex] and kB;

    Perhaps this is okay; just seems "uneven" that kB appears with a zero power in the definitions of length, mass and time but with nonzero power in the definition of temperature.

    It seems a bit "ad hoc"... Would anyone have a comment on these definition choices?
  2. jcsd
  3. Feb 12, 2013 #2


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    Staff: Mentor

    Mass, space and time are connected both via gravity and via quantum theory. This is not true for temperature. It is just (directly) related to energy.

    It is not a choice. If you set all those constants to 1, you have no choice how to get planck units (apart from constant prefactors).
  4. Feb 12, 2013 #3
    The reason I am asking is I read this thread (thread) on Planck units.

    It says that c, G and [itex]\hbar[/itex] form a "vector basis" (1,0,0) (0,1,0) and (0,0,1) where adding vectors corresponds to multiplication of units.

    It is interesting that adding kB happens only for temperature, as if temperature has a "4-dimensional" unit, while L, T, M are less-dimensional. Wondering what the implications of this could be.
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