The 1970s introduced string theory, grand unification, supersymmetry, and the naturalness argument. Problems of naturalness arise in influential field theories. All matter fields need to be coupled to the gravitational field. Naturalism does not accomplish that. Gravity is universal. All forms of matter and energy act as sources of gravity. Matter does not interfere with gravity. Gravity couples to all states of matter, including matter with no rest mass, such as the electromagnetic field. Gravity acts through the stress-energy-momentum tensor of matter. The weak equivalence principle states that all objects respond to gravity in the same way, regardless of their shape. We have failed to quantize gravity.
A scalar field illustrates how failures of naturalness manifest in the renormalization group flow for unnatural couplings. Charge determines the amount of force acting on a particle; its mass determines the inertial resistance to that force. We can now infer the fifth force after discovering the Higgs particle. The Higgs field induces this newly discovered force between two particles.
As electrons, quarks, and other particles move through space, they interact with Higgs bosons; they gain mass.
The coupling of gravity with the electromagnetic field and the fields for electrons, neutrinos, quarks, and pressure. Time goes slower in a gravitational field. The stronger the gravitational field, the slower the time moves. We need a quantum theory of gravity to connect to the quantum theories of everything else. A graviton is a gauge boson.
Gravity is the force by which a planet or other body draws objects toward its center. All things with mass or energy, including planets, stars, galaxies, and even light, move toward one another.
Since we expect gravity to couple weakly and minimally to all matter fields, this way, spacetime would not remain free of matter and radiation forever.
The two naturalness problems are symptoms of a deeper issue. They generate the solutions from the fundamental level of physics.
