Physics Asked on January 19, 2021
There are some points in this wikipedia chapter. Main equation is:
$$ T^{alpha beta} , = left(rho + {p over c^2}right)u^{alpha}u^{beta} + p g^{alpha beta} $$
where $c$ is explicit.
The one for the trace is:
$$T = 3p – rho c^2$$
that seems contradictory with:
$$T^{alphabeta} = left( begin{matrix}
rho & 0 & 0 & 0
0 & p & 0 & 0
0 & 0 & p & 0
0 & 0 & 0 & p
end{matrix} right)$$
with trace $3p+rho$ (difference in sign and value of last term).
The expression for the four-velocity:
$$u^{alpha} = (1, 0, 0, 0)$$
is not the usual one $(c, 0, 0, 0)$.
Finally, the metric:
$$g^{alphabeta} , = left( begin{matrix}
– c^{-2} & 0 & 0 & 0
0 & 1 & 0 & 0
0 & 0 & 1 & 0
0 & 0 & 0 & 1
end{matrix} right)$$
also with explicit $c$, it is also not the usual:
$$left( begin{matrix}
-1 & 0 & 0 & 0
0 & 1 & 0 & 0
0 & 0 & 1 & 0
0 & 0 & 0 & 1
end{matrix} right)$$
Are the wikipedia equations in this chapter using a coherent notation ? If yes, how to explain the previous points ?
The trace is ${T^mu}_mu$ not $T^{mumu}$, so the minus sign from lowering lowering the $mu=0$ index accounts for the sign difference. Also $c=1$ for most people, so the he wiki may not be consistent, but getting right is easy.
Looking at the Wiki, it is $g^{munu}$nthat has the $c^{-2}$, and to lower the index you need $g_{munu}$ which has the $-c^2$, so the wiki article is consistent.
Correct answer by mike stone on January 19, 2021
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