While viewing a recent lecture on Einstein’s Field Equations, the presenter made the association between the various quantities like mass, energy, momentum and pressure directly to the four dimensions of spacetime. Depending on which derivation of the FE he was explaining, he would make an association, for example “the time-time left side of the equation is associated with the mass quantity on the right side of the equation.” (I may have the wrong associations here). Other examples associated time-Xspace dimensions with something like momentum. I understand that there are ten equations that derive from the core FE, because out of the 16 possible, 6 of them are repetitive leaving 10. And I understand that there are many contributions to the stress-energy tensor side of the equation, but I have to admit I was a little surprised to see him associating different types of quantities with different combinations of physical dimensions. My question is this; is this association arbitrary, or are they somehow important on a physical level? Is there some laymen explanation that can show why these associations are being made? Is there some deep meaning behind these connections that wasn’t obvious before Einstein? I was always a little baffled as to why pressure was important in the FE because I always just interpreted it as a form of energy and why was it being high-lighted and singled out while something like chemical energy wasn’t, perhaps this is why. I realize this is tremendously complicated, but I was wondering if there was some physical insight that connected dimensions and physical quantities in these specific ways.