From $E=mc^2$ it seems mass and energy are interchangeable.
I can understand that mass is distinguishable, one type from another, such as protons and electrons.
But in the reverse process, how is it possible that the energy that became one particle is completely indistinguishable from the energy that became a different type of particle.
Shouldn't there be only one type of particle? does this imply that energy is some kind of composite thing with the potential to be this or that, like a stem cell in biology. or is it a simple thing?
Take an electromagnetic wave (photon) as an example (which is massless). it is completely indistinguishable from another photon of the same frequency. yet one can "condense" into one kind of particle and the other into a different kind. how is this possible if both photons were completely indistinguishable.