The essential assumption I make for the essence of physics in science fiction is something I call the 'Fundamental Jack'. Here's a picture of it -

 

The basic premise is that there are three main forces at work that define how the universe exists, and these are divided into weak and strong versions. The effects of interaction between the prongs are observable by sapient beings. A desired change in one prong can be affected by appropriate alteration of the others. The forces are;

Strong Time - This represents the tendency of things to go forwards in time.

Weak Time - This represents the tendency of things to go backwards in time.

Strong Matter - This represents the tendency of sub-atomic particles to behave as mass.

Weak Matter - This represents the tendency of sub atomic particles to move towards each other.

Strong Energy - This represents the hyper environment.

Weak Energy - This represents all other directly observable energy forms, such as magnetism, electricity and radiant energy.

The strong matter force is what makes the gravy of the universe to form atoms, and molecules, working up towards bricks, hutts and planets. The weak matter force is essentially gravity.