Ron Maimon wrote:
"Quarks are probably not made of anything more fundamental. The idea that everything has to be made of something else is not true. Light is not made of anything else, neither is gravity. That atoms had internal stuff going on was obvious, because they are electrically neutral, and yet scatter light at definite magic frequencies. Neutrons and protons betrayed their non-elementary structure because of their magnetic moments and too-strong scattering at short distances. It is usually obvious when a particle is composite.
The quarks, on the other hand, along with the electrons, light, gravity, and the gluons and W and Z bosons, are perfectly elementary, in the sense that their interactions are described well by a renormalizable quantum field theory. If they are not elementary, it is probably at a scale where they are revealed to be a string theory excitation, a quantum black hole.
Models of composite standard model fermions were interesting because they could explain the phenomenon of generations, the repeating standard model families. But string theory gives a much more natural explanation of generations, in terms of the geometry of the compactification. There is no real motivation for substructure, even though people speculate.
Here is a thought experiment concerning, "Light is not made of anything else, neither is gravity".
In the beginning is it possible there was nothing but gravity (singularity) and the antimatter of gravity is light?
The light somehow encapsulated gravity. The thought experiment concludes there is a massive singularity in the center of the Universe and an Infinite singularity outside the universe forever.