Once a theory has shown to have a good predictive capability for the outcome of experiments, and there is also an equation available to describe the variables in a mathematical format, then this becomes a powerful tool to investigate varying values of variables that will satisfy the equality.
However, this can turn into a black spot.
A theory can become so accepted, that there is little doubt anymore about the validity of the equations. The equations set the variables for technical applications of the phenomena, and a technological contraption is a practical outcome. There is no need to declare all assumptions time and time over again made to develop the original mathematical equation. It is the realm of engineers to study and deliver the practical applications of the knowledge and construct machines, contraptions, ships, but also roads and dikes.
Engineers know very well that they have limited control to set the operational conditions. In the development phase, a set of specifications describes what performance is to be expected and under what conditions. They know their conceptions eventually degrade and will fail. They stipulate proper use and training of the operator. The operator maintains the usability by repair, replacement of parts and components until such time that maintaining functionality becomes too costly. An engineer must always anticipate on degrading conditions under use.
In Particle Physics we work the other way around. To study the fundamental particles, we try to decompose atoms and nuclei with all means. We try to do this with very complicated proton smashing machines because the composite particles are extremely stable.
In particle physics, an elementary particle or fundamental particle is a particle whose substructure (domain of the bigger structure which shares the similar characteristics of the domain) is unknown. Thus it is unknown whether it is composed of other particles. Known elementary particles include the fundamental fermions (quarks, leptons, antiquarks, and antileptons), which generally are “matter particles” and “antimatter particles,” as well as the fundamental bosons (gauge bosons and Higgs boson), which generally are “force particles” that mediate interactions among fermions. A particle containing two or more elementary particles is a composite particle.
It is remarkable that for some particles it is unknown whether they are constructs or elementary particles. We know that protons and neutrons are composite particles, but it is utmost difficult to demolish these constructs. They are almost indestructible. Some parts are declared to be fundamental, like the electron, because we have no idea how a substructure could exits, given the phenomena we attribute to an electron, like an electric charge. Some scientists state nowadays that we must model it as a kind of a cloud. If so, when the electron is in the discussion, a proper declaration is required for the underlying assumptions as taken into account: an electron is a point particle or a cloud or whatever.
Unfortunately, the electron as a point particle became an accepted starting assumption without further declaration.