Fundamental particles, as far as our current understanding of particle physics goes, are considered indivisible or elementary particles. They cannot be further divided into smaller constituent particles. This concept is based on our current theoretical framework known as the Standard Model of particle physics.
There are a few reasons why fundamental particles are believed to be indivisible:
Experimental evidence: Numerous high-energy particle collider experiments have been conducted to probe the fundamental building blocks of matter. These experiments have consistently shown no evidence of substructure within the particles. No smaller, more fundamental particles have been detected within the ones we currently consider elementary.
Point-like behavior: Fundamental particles are often described as point-like entities, meaning they have no spatial extent. This concept arises from quantum field theory, where particles are treated as excitations of underlying fields. The mathematical formalism of the Standard Model assumes point-like particles.
Consistency with symmetries and interactions: The symmetries and interactions described by the Standard Model are based on the assumption of elementary particles. These symmetries and interactions have been extensively tested and found to be highly successful in describing the behavior of particles at high energies.
However, it's worth noting that our understanding of particle physics is not complete, and future discoveries or advancements may reveal a deeper level of substructure or new fundamental entities. For instance, in the past, particles like electrons and quarks were considered elementary until experiments revealed the existence of subatomic particles within them (such as quarks and leptons). So, while the current understanding suggests that fundamental particles cannot be divided further, science remains open to revision as we continue to explore and investigate the nature of the universe.