To the best of my knowledge, General Relativity considers Momentum to be as fundamental a thing as Energy. However, in day-to-day descriptions of Things in Physics, Momentum always seems to be dependent upon something else (such as Mass in motion). It seems to me that if Momentum was truly a Fundamental thing, it should be able to exist independently of anything else (like a piece of Energy can exist independently of other things, in the form of a photon). Has any work been done along the lines of attempting to figure out whether or not Momentum can exist in some independent form? Or, to rephrase that Question, has any work been done to prove that Momentum must always exist as a dependency of something else (like Mass in motion)? I only know of one place in Physics where formally-published papers have had a chance of touching upon an Answer to that Question, and those papers discuss a hypothetical thing called "negative mass". Basically, consider this simple hypothetical interaction between two particles: (m)(v)---> (poof!) <---(-m)(-v) If the masses and velocities are equal and opposite, then Zero mass will be left over after the event (the process was called "nullification" by Dr. Robert L. Forward), and there also will be Zero kinetic energy left over. However, *all* the Momentum will be left over ...! What *form* could that Momentum possess? Unfortunately, so far as I know, none of the papers on negative mass ever got around to considering that question. What can you folks tell me? Thanks in advance!