The law of conservation of energy provides one of the basic keys to understanding the universe. The fundamental tenet of this law is that the total mass-energy of the universe is constant; however, energy can be transferred in many ways. Historically, scientists have treated the law of conservation of matter and energy separately. All energy can be classified as either kinetic or potential. When work is done on or by a system, the energy of the system changes. This relationship is known as the work-energy theorem.

Energy may be transferred by matter or by waves. Waves transfer energy without transferring mass. Most of the information scientists gather about the universe is derived by detecting and analyzing waves. This process has been enhanced through the use of digital analysis. Types of waves include mechanical and electromagnetic. All waves have the same characteristics and exhibit certain behaviors, subject to the constraints of conservation of energy.

Note: The use of e.g. denotes examples which may be used for in-depth study. The terms for example and such as denote material which is testable. Items in parentheses denote further definition of the word(s) preceding the item and are testable.

Note: Items with *asterisks* require quantitative treatment per the Reference Table for Physics. Asterisks following individual words refer to the preceding word or phrase only; asterisks appearing after the final period of a sentence refer to all concepts or ideas presented in the sentence.

Fundamental forces govern all the interactions of the universe. The interaction of masses is determined by the gravitational force; the interaction of charges is determined by the electro-weak force; the interaction between particles in the nucleus is controlled by the strong force. Changes in the motion of an object require a force. Newton's laws can be used to explain and predict the motion of an object.

On the atomic level, the quantum nature of the fundamental forces becomes evident. Models of the atom have been developed to incorporate wave-particle duality, quantization, and the conservation laws. These models have been modified to reflect new observations; they continue to evolve.

Everyday experiences are manifestations of patterns that repeat themselves from the subnuclear to the cosmic level. Models that are used at each level reflect these patterns. The future development of physics is likely to be derived from these realms.