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Why ##n^\mu T_{\mu\nu}## is called the pressure?

  1. I read in the textbook which says that, according to the usual definition the absolute value of ##n^\mu T_{\mu\nu}## is just the pressure. ##T_{\mu\nu}## is the energy-momentum tensor and ##n^\mu## is a four dimensional normal vector.
     
  2. jcsd
  3. As given I don't see that this holds in general... For example take a perfect fluid for which it can be written in rest frame:
    [itex]T_{\mu \nu} = diag( \rho, p , p , p ) [/itex]
    Then:
    [itex] n^{\mu}T_{\mu \nu} = n^{0}T_{00} + n^{ii}T_{ii} = n^{0} \rho + (n^{11}+n^{22}+n^{33}) p[/itex]

    Which doesn't have to be equal to the pressure... If though you choose [itex]n^{0}=0[/itex] (so it's not just any 4-dim unit vector) things can get better.

    Can you give your reference?
     
    Last edited: Aug 12, 2014
  4. Oh~~ yes, I forgot to say that ##n^0 = 0##, and ##n^\mu## is pure spatial. Thanks a lot!:smile:
     
  5. Orodruin

    Staff: Mentor

    Chris, your LHS has a free index and your RHS does not.

    In a general frame you would have
    $$
    n^\mu T_{\mu\nu} = n^\mu \left((\rho+p) u_\mu u_\nu - p g_{\mu\nu}\right)
    = (\rho+p) u_\nu (n \cdot u) - p n_\nu,
    $$
    where u is the 4-velocity of the fluid.
     
  6. Meir Achuz

    Meir Achuz 2,059
    Science Advisor
    Homework Helper
    Gold Member

    The association of ##n^\mu T_{\mu\nu}## with the pressure at a point follows from the fact that its integral over a closed surface equals the rate of change of momentum within the closed surface.
    There are simple examples where ##n^\mu T_{\mu\nu}## is NOT the pressure at a point on the surface. For instance the pressure on a dielectric slab due to a point charge a distance d from the slab is not ##n^\mu T_{\mu\nu}##.
     
  7. oops sorry... yes the correct form would have to be:
    [itex]n^{i} T_{i \mu} \equiv n_{i} p[/itex]
     
  8. Orodruin

    Staff: Mentor

    [itex]n^{i} T_{i \mu} \equiv n_{\color{Red}\mu} p[/itex]

    :smile:

    Assuming ##n^0 = 0## and that we are in the rest frame of the fluid.
     
  9. the n0 is zero
    and also mu=i for the expression not to be zero...

    For the rest frame yes, I just corrected the expression I gave in my previous post
     
  10. Orodruin

    Staff: Mentor

    Even if the expression is non-zero only for spatial ##\mu##, you must still have the same free indices on both sides of your equality. In your case you have i as a summation index on one side and as a free index on the other. I can guess what you mean because I know what you are aiming for, but formally it does not make sense.
     
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