## I.

There are four fundamental interactions in Nature–the electromagnetism, the gravity, the strong and the weak interactions. The current route of the unification of the four interactions is under the following assumption:

there is only one interaction of Nature, which, under different energy conditions, degenerates to the four interactions we observe.

Mathematically, this assumption translates to searching for unification through a large symmetry group.

Our first viewpoint is that such an assumption is imaginary and non-falsifiable. We have no reason to believe its validity, and consequently, any unification theory built upon this imaginary assumption may not reflect Nature. In fact, this assumption breaks the principle of representation invariance (PRI), discovered by (Ma-Wang, 2012), which is simply a logic requirement.

## II.

Nature tells us that when several interactions are present in a given physical system, each interaction obeys its own symmetry. These interactions are affecting each other in the physical system, and the coupling is inevitably necessary. In other words, Nature suggests us that

the coupling is the essence and a natural route for the unification of the four fundamental interactions.

This route of unification is the reflection of Nature, and is directly built upon observable natural phenomena, instead of on an imaginary assumption in the above traditional route of unification.

This is the route of unification we adopt in the PID unified field theory we have recently developed; see the previous blog  and a recent book.

## III.

Now we introduce briefly the main ideas of the PID unified field theory.

1. Given a physical system, involving all four interactions, the goal of a unified field theory is to derive physical laws of the system:
• a unified field theory is to derive field equations coupling the interactions and dynamics of the system, and
• the field equations should be derived based on a few fundamental principles.
2. Our PID unified field theory is based on the following principles:
3. The symmetry principles play a decisive role in determining laws of Nature. In other words, symmetry principles dictate the actions of the interactions.With the actions at our disposal, PID gives rise to the field equations of the interacting physical system coupling different interactions and subsystems.The PRI ensures the sources of the interaction, the mass charge for gravity, the electric charge for electromagnetic interaction, the weak charge for the weak interaction and the strong charge for the strong interaction.In deriving the field equations above, the coupling principle of symmetry breaking (PSB) validates the symmetry-breaking of certain subsystems in the coupling.
4. The unified (coupling) field equations offer solutions to a few challenging problems, which include the dark matter and dark energy phenomena, sources of interactions, asymptotic freedom and the quark confinement, formulas for the weak and strong interaction potentials and forces, and the nucleon interaction potential.

Tian Ma and Shouhong Wang