Since most 3D games are modeled as another world, you need a physics engine to define the physical rules of the game.
Mathematics Knowledge
Mathematics is the language of physics, so you need to be competent in it to write the physics component of the game engine.
Calculus: Differentiation, integration, differential equations, approximations, numerical methods
Linear Algebra: Vectors, matrices
Programming Knowledge
To program a physics engine, one requires strong logic and algorithm design skills.
Other Knowledge
Of course since your objective is to model the universe, knowledge of classical
