Conservation law

In physics, a conservation law includes several principles that state that particular measurable property of an isolated physical system does not change as the system evolves over time. If a system does not interact with its environment in any way, then certain properties of the system cannot change. In other words, the total quantity of certain quantities (such as for example energy or electric charge) remains constant over time even when these quantities are exchanged between the components of the system.

In classical physics, conservation laws govern energy, momentum, angular momentum, mass, and electric charge. In particle physics, other conservation laws apply to properties of subatomic particles that are invariant during interactions. Strong overall conservation laws are the conservation of baryon number and the conservation of lepton number. Specific quantum numbers have been assigned to the different fundamental particles, and other conservation laws are associated with those quantum numbers.

An important function of conservation laws is that they make it possible to predict the macroscopic behavior of a system without having to consider the microscopic details of the course of a physical process or chemical reaction. The study of the behavior of a physical system consisting of one or multiple bodies, subjected to the action of forces, is based on the laws of dynamics. Studying a physical system means predicting, instant by instant, how the physical quantities that characterize it change, such as its mass, its speed, etc. In certain cases, knowing at every moment the forces acting on the system and its characteristics, the task is particularly simple: it is possible, for example, determine the motion of a known mass body in free fall subject to the force of gravity, or of spring subject to elastic force.

In general, most of the forces acting in nature, in fact, are not constant over time or act for very short moments of time, and it is, therefore, necessary to have more general laws from which to deduce the trend of variable physical quantities, useful to get information about the motion of the systems.

When a physical system undergoes a transformation, it generally determines the variation of one of the physical quantities that characterize it: for example, a body at rest which is set in motion by a varied force its speed (and therefore its kinetic energy) or a substance that undergoes a chemical reaction can vary its mass. In these cases, attention is directed to the quantities which remain constant, instead of those that vary of the system under test.

Thanks to the conservation laws, which physicists have reached on the basis of the results of numerous experimental measurements, it is possible to formulate general predictions on the behavior of a system (for example, two bodies that collide, or exchange energy, or react chemically) without knowing in detail the complexity of interactions involved. Conservation laws provide a direct connection between the physical quantities that characterize the system in its initial and final states. Knowing that these quantities must be the same overall before and after the interaction, we can write equations that bind them, from which we can deduce the behavior of the system following the transformation.

Approximate conservation laws

From the philosophical point of view of science, these laws, like cyclical or periodic phenomena, assume fundamental importance in regulating the behavior of nature; being able more than other physical laws to confer order and logic with respect to the apparent disorder or chaos.

• Conservation of rest mass
• Conservation of baryon number (chiral anomaly and sphaleron)
• Conservation of lepton number (in the Standard Model)
• Conservation of flavor (violated by the weak interaction)
• Conservation of parity
• Invariance under charge conjugation
• Invariance under time reversal
• CP symmetry, the combination of charge conjugation and parity (equivalent to time-reversal if CPT holds)