This question already has an answer here:
I found this interesting paper on Arxiv devoted to explaining Einstein's field equations in simple English. The author, JC Baez, does this by considering a group of small spherical balls in gravitational field, at rest with respect to each other. He then proposes a law by which the balls would shrink in volume in presence of gravity. The idea is that when the balls shrink while still maintaining contact with each other, they must move. Also, one has to consider all the infinitely many possible ways the mutually stationary balls could initially be moving to completely understand the motion of particles in a complicated gravitational field.
Here is the law that the author describes on the rate of change of volume of such balls in gravitational field: Given a small ball of freely falling test particles initially at rest with respect to each other, the rate at which it begins to shrink is proportional to its volume times: the energy density at the center of the ball, plus the pressure in the x direction at that point, plus the pressure in the y direction, plus the pressure in the z direction.
My question: what is pressure in x (or t) direction? What is the physical meaning of this pressure? What is flow of x ( or t) momentum in x ( or t) direction? I understand x and t momentum as the component of the momentum of a body in x and t direction respectively. But what is the flow of these things in these directions? What exactly we mean by flow? What is going on?
if you do not understand my question, kindly ask to clarify or read the Arxiv paper in question.