Is everything made of massless particles? Photons have no mass. Yet they interact gravitationally, as all energy does, with other energetic and massive particles. This means that if you put multiple photons in a system, you get something that appears to have mass, even though none of the constituent particles do have mass.
That makes me wonder:
Is mass really a fundamental part of reality? Couldn't it be that massive particles (protons/neutrons/electrons) are just composed of massless particles like photons knotted up, confined to a small area and whizzing around in very tight orbits? So everything is, in a sense, massless?
The search for and discovery of the Higgs Boson suggests to me, in my limited understanding, that scientists believe mass is a fundamental property that some particles have. And also that mass is fundamentally different than other types of energy (though conversion is possible). Does all of this preclude a system like I describe?
 A: It's certainly possible for a particle's mass to come partially from kinetic energy of massless particles; for example, about half of a proton's mass is the kinetic energy of its gluons. But the kind of mass that fundamental particles have, the kind that comes from the Higgs mechanism, doesn't appear to be of that kind. Maybe someday we will discover that it is. (This would be the case if string theory turns out to be correct, for instance.)
By the way, scientists do not believe that mass is fundamentally different from energy. Mass is just one type of energy.
A: This is a very interesting question. It turns out that in the standard model before symmetry breaking the only particle with a mass term is the Higgs boson itself (and possibly the neutrinos) The fermions and weak gauge bosons acquire mass through the Higgs mechanism and more mass in nature is generated by the confinement of quarks inside the nucleons, but there is a real sense in which everything in the standard model + gravity is made of massless particles except the Higgs and possibly neutrinos.
This is significant because it means that the standard model almost has conformal invariance. This is invariance under a space-time symmetry which can change distance scales locally so long as all angles remain fixed. Only the Higgs mass term breaks the symmetry. It has been suggested that the standard model could have exact conformal invariance in classical form with the Higgs term generated dynamically in the quantum theory, but this is not a highly regarded idea yet.
