The actions of energy/matter are presently understood by quantum physicists in terms of elementary (fundamental) particles. According to current quantum theory, every observed interaction of elementary particles and the large-scale behavior of matter throughout the universe is attributed to four fundamental forces:
- Strong Interaction (interaction between quarks)
- Electromagnetism (interaction between particles with charge/magnetism)
- Weak Interaction (interaction that changes quark types)
- Gravitation (interaction between particles with mass)
Strong interaction is by far the strongest of the forces, followed by the electromagnetism, weak interaction, and gravitation.
Since the early 1970s, these fundamental forces (except gravitation) have been interpreted as manifestations of a single, underlying, “unified” force. This interpretation — known as the “standard model” of interaction — suggests that the particles do not interact directly, but rather manifest virtual particles that mediate the actual interactions: Nucleons (strong interaction), photons (electromagnetism), and weak gauge bosons (weak interaction).
Standard model does not include the effects of gravitational interactions because gravitation (inherited from classical physics) cannot be renormalized. In order to unify gravitation with the other three fundamental forces, the theory of quantum gravity emerged. It postulates that the existence of a hypothetical particle (the graviton) is the mediating element in gravitation interactions.