# Why is momentum density a source of gravity in General Relativity?

Momentum density in the $x$, $y$ and $z$ directions are components of the Stress-Energy Tensor in Einstein's Field Equations. Yet, how did Einstein come up with that conclusion? Is it possible to demonstrate why momentum should curve spacetime without as much mathematics? Also, why do the $T_{01}$, $T_{02}$, $T_{03}$, $T_{10}$, $T_{20}$ and $T_{30}$ components of the Stress-Energy Tensor refer to momentum density? I'd love to read an intuitive explanation to why the flow of energy from the time dimension to the $x$ direction, for example, represents momentum. Thanks!

Momentum occurs in the first row as part of the way that the stress-energy tensor expresses $f=ma$. The space derivative of stress (the other three rows) gives $f$. The time derivative of momentum gives $ma$. We get $f=ma$ when the sum of the corresponding derivatives add to zero.