Board Thread:Episode Discussion/@comment-24573226-20161003151422/@comment-25396609-20161004022155

the tl;dr on it is that mass and force are effectively the same thing; objects have mass because they are constantly in motion on sub-atomic levels, constantly generating force which creates a well in the fabric of space time (there's a more specific term that i can't recall off the top of my head).

as an object gains speed the force it is generating on that field increases causing its apparent mass to rise.