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In mathematics, a tensor is a certain kind of geometrical entity which generalizes the concepts of scalar, vector and linear operator. Tensors are of importance in differential geometry, physics and engineering. Einstein's theory of general relativity is formulated completely in the language of tensors.
Examples
As a simple example, consider a ship in the water. We want to describe its response to an applied force. Force is a vector, and the ship will respond with an acceleration, which is also a vector. The acceleration will in general not be in the same direction as the force, because of the particular shape of the ship's body. However, it turns out that the relationship between force and acceleration is linear. Such a relationship is described by a tensor of type (1,1). The tensor can be represented as a matrix which when multiplied by a vector results in another vector. Just as the numbers which represent a vector will change if one changes the coordinate system, the numbers in the matrix that represents the tensor will also change when the coordinate system is changed.
In engineering, the stresses inside a rigid body or fluid are also described by a tensor; the word "tensor" is Latin for something that stretches, i.e. causes tension. If a particular surface element inside the material is singled out, the material on one side of the surface will apply a force on the other side. In general, this force will not be orthogonal to the surface, but it will depend on the orientation of the surface in a linear manner. This is described by a tensor of type (2,0), or more precisely by a tensor field of type (2,0) since the stresses may change from point to point.
Not all relationships in nature are linear, but most are differentiable and so may be approximated with sums of multilinear maps. Thus most quantities in the physical sciences can be expressed as tensors.
Approaches
There are two equivalent approaches to visualizing and working with tensors.
- The classical approach views tensors as multidimensional arrays (generalizations of matrices) of differentials. In other words, a tensor can roughly be viewed in this approach as an extension of the idea of a Jacobian. The "components" of the tensor are the indices of the array.
- The modern approach
- The modern (component-free) approach views tensors initially as abstract objects, expressing some definite type of multi-linear concept. Their well-known properties can be derived from their definitions, as linear maps or more generally; and the rules for manipulations of tensors arise as an extension of linear algebra to multilinear algebra.
In the end the same computational content is expressed, both ways.