Physics is the branch of science whose goal is to understand and describe the fundamental laws governing all natural phenomena. Such description is required in modern physics to be primarily mathematical, with natural language playing an auxiliary role. This allows physical theories to make precise, quantitative and unambiguous predictions, which can be tested against experimental results. The scientific method is at the foundation of physics: agreement with experimental data is the ultimate test for the validity of any theory; and no theory, no matter how authoritative, can be said to have been proven true once and forever, since the possibility always exists of new experimental data falsifying it.
Physical theories can be classified as fundamental, or phenomenological (often called effective, especially in the context of Quantum field theory). A fundamental theory about a physical system is, at its core, a mathematical model whose features mirror the supposed real physical and causal structure of the considered system; in such a theory, relevant objects, events and interactions existing in reality are believed to have a direct counterpart in the mathematical description. An effective theory, by contrast, describes the observed behavior of the system, or of some quantity relative to it, through a mathematical model which does not, or only partially, capture the true physical and causal features of the system. Some theories are declaredly created as phenomenological, and regarded as provisional in wait for further insight; others, previously believed fundamental, are understood as effective after being superseded by a new fundamental theory on the same subject through crucial experiments. Often, in these cases, older theories are shown to be derived from the new one through some restrictive assumption or in some special limit;
A trend towards generalization has been a major driving force in the evolution of physics: the requirement of economy in the introduction of different theoretical concepts (see Occam's razor) has led physicists to search, and often discover, unifying theories which made previous distinctions obsolete and introduced more general and profound ideas. Important concepts in the framework of modern physics are, among others, energy, interaction, field, space, time and symmetry. Also, the philosophical view of reductionism, the belief that the behaviour of complex systems can be understood, at least in principle, by knowing the laws governing the evolution of their elementary components, has been of great historical influence, leading physicists to attach high importance to theories and experimental data regarding the smallest and most fundamental physical entities that, at any given time, technology allowed to identify. Recently, reductionism has been challenged by authoritable emergentist positions, but is still the majority view among physicists. Considering the above principal directions, one can understand why a theory of everything, which would include general laws completely describing the features and interactions of the elementary components of the universe in an unifying picture, is thought by many to be the greatest, maybe final, goal of physics.
Research in physics is traditionally divided in two main branchs: experimental physics, which deals with the design and realization of experiments aimed at assessing the validity of existing theories or investigating poorly explored phenomena; and theoretical physics, consisting in the invention of mathematical frameworks describing new theories; in the derivation and solution of mathematical models representing physical systems according to existing frameworks, and sometimes in the discussion of purely hypothetical models, which are believed to have no direct corrispondence with reality, but may still give useful insight. Today, computational physics is sometimes regarded as the "third branch" of physics, intermediate between theoretical and experimental, given its immense value in connecting theory to experiment, as well as in all fields of applied physics.