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Scalar (physics)
In physics, a scalar is a one-dimensional physical quantity, i.e. one that can be described by a single real number (sometimes signed, often with units), in other words a scalar is a physical quantity that only has magnitude but no direction, unlike (or as a special case of) vectors, tensors, etc. which are described by several numbers which characterize magnitude and direction and a vector can be defined as a physical quantity that has magnitude and direction. Formally, a scalar is unchanged by coordinate system rotations or reflections (in Newtonian mechanics), or by Lorentz transformations or space-time translations (in relativity). A related concept is a pseudoscalar, which is invariant under proper rotations but (like a pseudovector) flips sign under improper rotations. The concept of a scalar in physics is essentially the same as in mathematics. A physical scalar field is one type of more general fields, like vector fields, spinor fields, and tensor fields.
An example of a scalar quantity is temperature: the temperature at a given point is a single number. Velocity, on the other hand, is a vector quantity: velocity in three-dimensional space is specified by three values; in a Cartesian coordinate system the values are the speeds relative to each coordinate axis. The associated fields describe the temperature and velocity in each point of some space. Considering the norms of the velocity vectors results in a scalar field of the speeds in each point of the space.
==Physical quantity==
(詳細はquantity is expressed as the product of a numerical value and a physical unit, not merely a number. The quantity does not depend on the unit (e.g. for distance, 1 km is the same as 1000 m), although the number depends on the unit. Thus, following the example of distance, the quantity does not depend on the length of the base vectors of the coordinate system. Also, other changes of the coordinate system may affect the formula for computing the scalar (for example, the Euclidean formula for distance in terms of coordinates relies on the basis being orthonormal), but not the scalar itself. In this sense, physical distance deviates from the definition of metric in not being just a real number; however it satisfies all other properties. The same applies for other physical quantities which are not dimensionless. Direction does not apply to scalars; they are specified by magnitude or quantity alone.
抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』
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