system
Given the following post, respond as if you're commenting on Reddit:
human
[Particle Physics] Can a particle have internal energy stored as anything OTHER than mass?

A question states that a particle splits in two pieces that go away from each other at high velocity. According to definitions and properties of particles in particle physics, is it necessary that the energy to accelerate those two pieces comes from mass (i.e. E = mc\^2)? Or is it possible for there to be internal energy stored within the original particle in a form OTHER than mass?

This is not a homework question - I'm just wondering because, in my current course on modern physics, these problems always assume that the energy to accelerate the pieces comes from mass and nothing else.
gpt
Internal energy, the way you are using it, IS mass. Mass is energy that you cannot get rid of by changing to any reference frame, ie exists in the rest frame if the system. 


A hotter object has (trivially) more mass. A fuel, chemical or nuclear, has more mass than its products. Two massless photons, that aren't traveling parallel, have a mass as a system. A mirrored box with photons in it would have a mass. Even within a composite particle like a proton, electrostatic repulsion and the storing nuclear force  effect the mass. Actually, most of the mass of a proton is the strong nuclear force binding energy. It weighs more than the quarks it's comprised of.