SpaceThis article is about the idea of space. For the space beyond Earths atmosphere (as in astronomy and spaceships), see outer space. For all other uses, see space (disambiguation).The idea of space has been of interest for philosophers and scientists for much of human history. The term is used somewhat differently in different fields of study, hence it is difficult to provide an uncontroversial and clear definition outside of specific defined contexts. Disagreement also exists on whether space itself can be measured or is part of the measuring system. (See Space in philosophy.) Science considers space to be a fundamental quantity (a quantity which can not be defined via other quantities because other quantities like force and energy are already defined via space). Thus an operational definition is used in which the procedure of measurement of space intervals (distances) and the units of measurement are defined.In philosophySpace has a range of definitions: * One view of space is that it is part of the fundamental structure of the universe, a set of dimensions in which objects are separated and located, have size and shape, and through which they can move. * A contrasting view is that space is part of a fundamental abstract mathematical conceptual framework (together with time and number) within which we compare and quantify the distance between objects, their sizes, their shapes, and their speeds. In this view, space does not refer to any kind of entity that is a container that objects move through.These opposing views are relevant also to definitions of time. Space is typically described as having three dimensions, see Three-dimensional space and that three numbers are needed to specify the size of any object and/or its location with respect to another location. Modern physics does not treat space and time as independent dimensions, but treats both as features of space-time a conception that challenges intuitive notions of distance and time.An issue of philosophical debate is whether space is an ontological entity itself, or simply a conceptual framework humans need to think (and talk) about the world. Another way to frame this is to ask, Can space itself be measured, or is space part of the measurement system? The same debate applies also to time, and an important formulation in both areas was given by Immanuel Kant.In his Critique of Pure Reason, Kant described space as an a priori intuition that (together with another a priori intuition, time) allows us to comprehend sensual experience. Kant referred to such intuitions as noumena and as things in themselves. In Kants view, neither space nor time are conceived of as substances, but rather both are elements of a systematic framework we use to structure our experience. Spatial measurements are used to quantify how far apart objects are, and temporal measurements are used to quantify how far apart events occur. However, these measurements are applied by our minds to categorize what we sense and are not an inherent part of the thing in itself.Schopenhauer, in the preface to his On the Will in Nature, stated that space is the condition of the possibility of juxtaposition. This is in accordance with Kants understanding of space as a form in the mind of an observing subject.Similar philosophical questions concerning space include: Is space absolute or purely relational? Does space have one correct geometry, or is the geometry of space just a convention? Historical positions in these debates have been taken by Isaac Newton (space is absolute), Gottfried Leibniz (space is relational), and Henri Poincar (spatial geometry is a convention). Two important thought-experiments connected with these questions are: Newtons bucket argument and Poincars sphere-world.In physicsSpace is one of the few fundamental quantities in physics, meaning that it cannot be defined via other quantities because there is nothing more fundamental known at present. Thus, similar to the definition of other fundamental quantities (like time and mass), space is defined via measurement. Currently, the standard space interval, called a standard meter or simply meter, is defined as the distance traveled by light in a vacuum during a time interval of exactly 1/299,792,458 of a second. This definition coupled with present definition of the second is based on the special theory of relativity, that our space-time is a Minkowski space.Before Einsteins work on relativistic physics, time and space were viewed as independent dimensions. Einsteins discoveries have shown that due to relativity of motion our space and time can be mathematically combined into one symmetric object space-time. (Distances in space or in time separately are not invariant versus Lorentz coordinate transformations, but distances in Minkowski space-time are which justifies the name).However, time and space dimensions should not be viewed as exa