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Physics is the science of nature in the broadest sense. The term “physical” is derived from the Latin neuter plural physica, which in turn derives from the greek τὰ φυσικά, or “natural things” and φύσις, “nature”. The purpose of physics is the study of natural phenomena, that is, of all the events that can be described or quantified through appropriate physical quantities, in order to establish the principles and laws that govern interactions among the same items and account for their mutual variations. This objective is achieved through the rigorous application of the scientific method, whose ultimate goal is to provide a simplified diagram, or model, of the phenomenon described. The set of principles and laws of physics relative to a certain class of observed phenomena define a deductive theory of physics, coherent and relatively self-consistent, typically built from experimental induction. Time and space are the fundamental quantities of physics, together with mass, temperature, amount of substance, current intensity, and light intensity: all sizes of physics are attributable to the latter. The unit of time is the second, while the meter is the fundamental unit of space and is defined as the distance traveled by light in vacuum during a time interval. Before the twentieth century the concepts of space and time were considered absolute and independent. After the advent of the theory of physical relativity of Einstein I had to change opinion: the lengths and time intervals measured by two observers moving relative to each other, may be more or less dilated or contracted, while there is a ‘ entity, the Minkowski interval, which is invariant, and if measured by both observers provides the same result; this entity is constituted by the 3 spatial coordinates plus a fourth, that of time, that make this object belonging to a 4-dimensional space. In so doing, the space and time are no longer two fixed amount and independent of each other, but are related to each other and form a single, new base on which to operate, the space-time.