High Quality Content by WIKIPEDIA articles! In theoretical physics, a scalar-tensor theory is a theory that includes both a scalar field and a tensor field to represent a certain interaction. For example, the Brans-Dicke theory of gravitation uses both a scalar field and a tensor field to mediate the gravitational interaction.Modern physics tries to derive all physical theories from as few principles as possible. In this way, newtonian mechanics as well as quantum mechanics are derived from the Hamilton's principle of least action. In this approach, the behavior of a system is not described via forces, but by functions which describe the energy of the system. Most important are the energetic quantities known as the Hamilton function (or Hamiltonian) and the Lagrange function (or Lagrangian). Their derivatives in space are known as Hamiltonian or Hamilton density and Lagrangian or Lagrange density. Going to these quantities leads to the field theories.