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[power meters] run backward when the "power" direction reversed.
(Does this make power a (pseudo?) vector?)
The stress-energy tensor, aka energy-momentum tensor, T_{ab}, where
a,b go from 0 to 3. This tells you everything about what energy and
momentum are doing at your given point of spacetime, as follows:
T_{ab} is the flow in the "a" direction of momentum in the "b"
direction!
To understand this, remember that a,b=0,1,2,3 correspond to t,x,y,
and z respectively. Also, remember that "energy" is the same as
"momentum in the time direction", and that "density" is the same as
"flow in the time direction". Thus the top row of the stress-energy
tensor keeps track of the density of energy --- that's T_{00} --- and
the density of momentum in the x,y, and z directions --- those are
T_{01}, T_{02}, and T_{03} respectively. On the other hand, T_{10},
T_{20}, and T_{30} represent the flow of energy in the x, y and z
directions, respectively. The other entries keep track of the flow of
spatial momentum in various spatial directions. For example, T_{12}
keeps track of the flow in the x direction of momentum in the y
direction.
"density" is the same as
"flow in the time direction".
"energy" is the same as
"momentum in the time direction",
T_{ab} is the flow in the "a" direction of momentum in the "b"
direction