What is Tensor: Definition and 1000 Discussions

In mathematics, a tensor is an algebraic object that describes a (multilinear) relationship between sets of algebraic objects related to a vector space. Objects that tensors may map between include vectors and scalars, and even other tensors. There are many types of tensors, including scalars and vectors (which are the simplest tensors), dual vectors, multilinear maps between vector spaces, and even some operations such as the dot product. Tensors are defined independent of any basis, although they are often referred to by their components in a basis related to a particular coordinate system.
Tensors have become important in physics because they provide a concise mathematical framework for formulating and solving physics problems in areas such as mechanics (stress, elasticity, fluid mechanics, moment of inertia, ...), electrodynamics (electromagnetic tensor, Maxwell tensor, permittivity, magnetic susceptibility, ...), or general relativity (stress–energy tensor, curvature tensor, ...) and others. In applications, it is common to study situations in which a different tensor can occur at each point of an object; for example the stress within an object may vary from one location to another. This leads to the concept of a tensor field. In some areas, tensor fields are so ubiquitous that they are often simply called "tensors".
Tullio Levi-Civita and Gregorio Ricci-Curbastro popularised tensors in 1900 - continuing the earlier work of Bernhard Riemann and Elwin Bruno Christoffel and others - as part of the absolute differential calculus. The concept enabled an alternative formulation of the intrinsic differential geometry of a manifold in the form of the Riemann curvature tensor.

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  1. Elnur Hajiyev

    A Can geodesic deviation be zero while curvature tensor is not

    I understand(or assume understand) that geodesic deviation describes how much parallel geodesics diverge/converge on manifolds while moving along these geodesic. But is not it a definition for intrinsic curvature? If it is same as Riemann curvature tensor in terms of describing curvature, why...
  2. Math Amateur

    I Tensor Product - Knapp, Chapter VI, Section 6

    I am reading Anthony W. Knapp's book: Basic Algebra in order to understand tensor products ... ... I need some help with an aspect of Theorem 6.10 in Section 6 of Chapter VI: Multilinear Algebra ... The text of Theorem 6.10 reads as follows: About midway in the above text, just at the start...
  3. Math Amateur

    MHB Tensor Product - Knapp, Chapter VI, Section 6

    I am reading Anthony W. Knapp's book: Basic Algebra in order to understand tensor products ... ... I need some help with an aspect of Theorem 6.10 in Section 6 of Chapter VI: Multilinear Algebra ... The text of Theorem 6.10 reads as follows:https://www.physicsforums.com/attachments/5391...
  4. Math Amateur

    I Proof of Existence of Tensor Product .... Cooperstein ....

    I am reading Bruce N. Coopersteins book: Advanced Linear Algebra (Second Edition) ... ... I am focused on Section 10.1 Introduction to Tensor Products ... ... I need help with the proof of Theorem 10.1 on the existence of a tensor product ... ...Theorem 10.1 reads as follows: In the above text...
  5. Math Amateur

    MHB Proof of Existence of Tensor Product .... Cooperstein ....

    I am reading Bruce N. Coopersteins book: Advanced Linear Algebra (Second Edition) ... ... I am focused on Section 10.1 Introduction to Tensor Products ... ... I need help with the proof of Theorem 10.1 on the existence of a tensor product ... ... Theorem 10.1 reads as follows:In the above...
  6. Math Amateur

    MHB The Uniqueness of a Tensor Product

    I am reading Bruce N. Coopersteins book: Advanced Linear Algebra (Second Edition) ... ... I am focused on Section 10.1 Introduction to Tensor Products ... ... I need help with the proof of Lemma 10.1 on the uniqueness of a tensor product ... ... Before proving the uniqueness (up to an...
  7. pixel

    I Calc Precession of Mercury Orbit: Stress-Energy Tensor=0?

    In the calculation of the precession of Mercury's orbit, why is the stress-energy tensor equal to 0? There is energy and momentum at the location of the planet.
  8. C

    Stress tensor for non-Newtonian fluid

    How does one setup the stress tensor for a non-Newtonian fluid? I know that for any fluid the normals should be the pressure and for a power law fluid the shear stress in the direction of flow is related by K(du/dy)^n. Does this mean that all other components are 0 for a symmetric pipe or...
  9. F

    I Difference between 'Field' (algebra) and 'Field' (geometry)

    I am trying to build up a kind of mind map of the following: Module (eg. vector space) Ring (eg Field) Linear algebra (concerning vectors and vector spaces, from what I understood) Multilinear Algebra (analogously concerning tensors and multi-linear maps) Linear maps & Multilinear maps The...
  10. M

    Schmid Tensor / Resolved Shear Stress question

    Homework Statement Calculate the magnitude of the resolved shear stress on the ## (112)## plane in the ##[11\bar{1}] ##direction in a bcc crystal. Homework Equations Tried to use: ## t^{\alpha} = \frac{1}{2} \sigma_{ij} (m_{i} n_{j} + m_{i} n_{j}) ## where m is the direction and n is the...
  11. W

    The elasticity/stiffness tensor for an isotropic materials

    Hi PF, As you may know, is the the elasticity/stiffness tensor for isotropic and homogeneous materials characterized by two independant material parameters (λ and μ) and is given by the bellow representation. C_{ijkl} = \lambda\delta_{ij}\delta_{kl} + \mu(\delta_{ik}\delta_{jl} +...
  12. S

    Tensor product of two arbitrary vectors an arbitrary tensor?

    I am trying to show that if (C^ab)(A_a)(B_b) is a scalar for arbitrary vectors A_a and B_b then C^ab is a tensor. I want to take the product of the two vectors then use the quotient rule to show that C^ab must then be a tensor. This lead to the question of whether or a not the product of two...
  13. S

    Example Application Ricci Tensor & Scalar for 3D Understanding

    Can anyone show me an example of applying the Ricci curvature tensor to something other than GR? I also ask the same for the curvature scalar. Lately I've been trying to truly increase my understanding of curvature, so that I can see exactly how solutions of the EFE's predict the existence and...
  14. Mrinmoy Naskar

    A good book for tensor notation....

    I am doing my M.sc in physics.. In my course I have classical field theory and electrodynamics... I need to learn tensor notations to understand the above subjects... Please tell me about some good introductory books to learn tensor notation to handle things in electrodynamics and classical...
  15. C

    Calculating Covariant Riemann Tensor with Diag Metric gab

    Using Ray D'Inverno's Introducing Einstein's Relativity. Ex 6.31 Pg 90. I am trying to calculate the purely covariant Riemann Tensor, Rabcd, for the metric gab=diag(ev,-eλ,-r2,-r2sin2θ) where v=v(t,r) and λ=λ(t,r). I have calculated the Christoffel Symbols and I am now attempting the...
  16. B

    Vectors, Hilbert Spaces, and Tensor Products

    If I ever say anything incorrect, please promptly correct me! The state of a system in classical mechanics is specified by point in phase space, the point giving us the position and velocity at a given instance. Could we rephrase it by saying a vector in phase space specifies the system? If...
  17. W

    Pressure tensor reduces to scalar pressure for isotropic dis

    1. Does anyone know why for an isotropic distribution function, pressure tensor reduces to a scalar pressure? For instance, for a Maxwellian distribution P=A ∫ vx vy exp-(vx2 + vy2 + vz2) dvx dvy dvz is not zero. I think everybody should realize how bogus some of the authors are. Google...
  18. D

    What is the trace of a second rank covariant tensor?

    What is the trace of a second rank tensor covariant in both indices? For a tensor covariant in one index and contravariant in another ##T^i_j##, the trace is ##T^k_k## but what is the trace for ##T_{ij}## because ##T_{kk}## is not even a tensor?
  19. DiracPool

    GR Metric Tensor Rank 2: Quadratic vs Shear Forces

    Is the metric tensor a tensor of rank two simply because the line element (or equivalent Pythagorean relation between differential distances) is "quadratic" in nature? This would be in opposition to say, the stress tensor being a tensor of rank two because it has to deal with "shear" forces. I...
  20. Alain De Vos

    Einstein Tensors and Energy-Momentum Tensors as Operators

    Can these tensor be seen as operators on two elements. So given two elements of something they produce something, for instance a scalar ?
  21. L

    Inertia tensor combination point mass-sphere

    Homework Statement hello, i want to calculate the inertia tensor of the combination of a point mass and a sphere in the object's frame, the center of mass is at the origin. The point mass remains at the surface of the phere The sphere is uniform, radius r and mass M, and the point mass has mass...
  22. W

    Construction of an affine tensor of rank 4

    Homework Statement In En the quantities Bij are the components of an affine tensor of rank 2. Construct two affine tensors each of rank 4, with components Cijkl and Dijkl for which ∑k ∑l Cijkl Bkl = Bij + Bji ∑k ∑l Dijkl Bkl = Bij - Bji are identities. Homework Equations The Attempt at a...
  23. R

    Curl of tensor * non-constant vector

    I know by definition that if T is a 2nd order tensor and v is a vector, curl(Tv)=curl(T)v but what if instead of constant vector v, I have w=grad(u), not constant but obviously an irrotational vector field. Is this still true: curl(Tw)=curl(T)w ? My guess is yes since curl(w)=0 but have no...
  24. I

    Maxwell's Equations - Tensor form 2

    Homework Statement The Lorenz gauge ∂Φ/∂t + ∇. A = 0 enables the Maxwell equations (in terms of potentials) to be written as two uncoupled equations; ∂2Φ/∂t2 - ∇2Φ = ρ 1 and ∂2A/∂t2 - ∇2A = j 2 The tensor version using the Lorenz gauge is, i am told, ∂μ∂μ Aα = jα 3 expanded this is...
  25. I

    Maxwell's Equations - Tensor form

    Homework Statement The gauge ∂tχ - A =0 enables Maxwell's equations to be written in terms of A and φ as two uncoupled second order differential equations. However, when the lorentz condition div A = 0 is applied, we are told the equation can be encapsulated as: one tensor equation ∂μFμA = jμ...
  26. CharlesJQuarra

    Einstein tensor of a gravitational source

    In section 4.4 of gravitational radiation chapter in Wald's general relativity, eq.4.4.49 shows the far-field generated by a variable mass quadrupole: \gamma_{\mu \nu}(t,r)=\frac{2}{3R} \frac{d^2 q_{\mu \nu}}{dt^2} \bigg|_{t'=t-R/c} I have the following field from a rotating binary...
  27. S

    Solving Electromagnetic Tensor & B-Field

    Hello, first off, I'm not sure if I put this question in the right place so sorry about that. Given Bi = 1/2 εijk Fjk how would you find F in terms of B? I think you multiply through by another Levi-Civita, but then I don't know what to do after that. Any help would much appreciated.
  28. EsmeeDijk

    Understanding Tensor Gradients in R3

    Homework Statement We have the following orthogonal tensor in R3: t_{ij} (x^2) = a (x^2) x_i x_j + b(x^2) \delta _{ij} x^2 + c(x^2) \epsilon_ {ijk} x_k Calculate the following quantities and simplify your expression as much as possible: \nabla _j t_{ij}(x) and \epsilon _{ijk} \nabla _i...
  29. EsmeeDijk

    Setting the trace of a tensor equal to zero

    Homework Statement A tensor t has the following components in a given orthonormal basis of R3 tij(x) = a(x2)xixj + b(x2) \deltaij x2 + c(x2) \epsilonijk xk (1) where the indices i,j,k = 1, 2, 3. We use the Einstein summation convention. We will only consider orthogonal transformations...
  30. N

    Tensor Product Functor & Einstein Eqns: Relation?

    At the risk of sounding ignorant I'd like to propose a question to someone well versed in Homological Algebra and General Relativity. I'm starting to study the tensor product functor in the context of category theory because I'm interested in possibly doing a paper on TQFT for a directed...
  31. T

    A Opposite "sides" of a surface - Differential Geometry.

    How, if at all, would differential geometry differ between the opposite "sides" of the surface in question. Simplest example: suppose you look at vectors etc on the outside of a sphere as opposed to the inside. Or a flat plane? Wouldn't one of the coordinates be essentially a mirror while...
  32. H

    Tensor & Matrix: Cartesian Vector & Transformation Rule?

    Each set of constant numbers such as ##(v_1, v_2, v_3)## are the components of a constant Cartesian vector because by rotation of coordinates they satisfy the transformation rule. Can we consider each set of constant arrays ## a_{ij};i,j=1,2,3 ## as components of a Cartesian tensor? In other...
  33. tomdodd4598

    Electromagnetic Tensor in (-+++) Convention

    Hi there, Over the last couple of weeks, I have been learning about the relativistic description of electromagnetism through Leonard Susskind's Theoretical Minimum lectures, and although I have managed to follow it, there are some parts which I am becoming increasingly confused by, not helped...
  34. P

    Are 10 dimensions related to the tensor of 3d

    I think I've read the the tensor in three dimensions has 10 elements in its matrix(?). Is this related to the 10 dimensions in some forms of string theory?
  35. A

    How optical axis is related to dielectric tensor?

    I want to know the relationship between the optical axis direction of a crystal and the dielectric constants in different directions in an anisotropic material.
  36. V

    Find Perfect Fluid & EM Tensor in Rest Frame

    we use perfect fluid which is characterized by a energy density and isotropic pressure for general forms of matter. When guessing the values of energy momentum tensor indices we can use the physical insight that they are the flux of four momentum in a constant surface of spacetime. The...
  37. Geofleur

    Example of an algebra tensor product

    On pages 67 & 68 of Hassani's mathematical physics book, he gives the following definition: "Let ## \mathcal{A} ## and ## \mathcal{B} ## be algebras. The the vector space tensor product ## \mathcal{A} \otimes \mathcal{B} ## becomes an algebra tensor product if we define the product ##...
  38. olgerm

    Tensor in D-dimensional space crosswise with 2 vectors

    I have 2 vectors ##\vec {V}=(v_1,v_2,v_3,v_4...v_D)## and ##\vec {X}=(x_1,x_2,x_3,x_4...x_D)## in D-dimensional euclidean space. I want a tensor ,which is crosswise with both of them. I think that the tensor is parallel with (D-2)-dimensional area, am I right? I do not know a lot about tensors...
  39. AwesomeTrains

    Question about the derivation of the energy momentum tensor

    Hey I'm trying to follow the derivation given here: http://lampx.tugraz.at/~hadley/ss1/studentpresentations/Bloch08.pdf Homework Statement As it says in the pdf: "Based on Noether's theorem construct the energy-momentum tensor for classical electromagnetism from the above Lagrangian. L=-1/4...
  40. ShayanJ

    Contracting Riemann tensor with itself

    In chapter 8 of Padmanabhan's "Gravitation: Foundations and Frontiers" titiled Black Holes, where he wants to explain that the horizon singularity of the Schwarzschild metric is only a coordinate singularity, he does this by trying to find a scalar built from Riemann tensor and show that its...
  41. N

    Singularity of spacetime/singularity of the riemanian tensor

    Hello, I have a question to the singularities of spacetime (where the metric tensor is infinite, but not the coordinate singularities which can be removed be a change of coordinate) It's easy to show that a singularity of the riemanian tensor scalar RαβμνRαβμν leads to a singularity of the...
  42. D

    Question about Metric Tensor: Learn Differential Geometry

    Hey, I have not done any proper differential geometry before starting general relativity (from Sean Carroll's book: space time and geometry), so excuse me if this is a stupid question. The metric tensor can be written as $$ g = g_{\mu\nu} dx^{\mu} \otimes dx^{\nu}$$ and its also written as...
  43. L

    Einstein-Cartan Theory: Dynamical Definition of Spin Tensor

    Hi, this is my first message on thi forum :D I apologize in advance for my english. I'm doing my thesis work on the theory of relativity of Einstein-Cartan. I'm following the article of Hehl of 1976; it's title is "General relativity with spin and torsion: Foundations and prospects". I can't...
  44. W

    How to get stress tensor from force

    Homework Statement I'm analyzing the landing gear of a plane for an extracurricular project. I know that the landing gear will undergo impact loading during touch down. Using what we've been taught, I converted the impact load to a static load, P that will act on the landing gear. Now I need to...
  45. Q

    What Is the Principal Moment of Inertia in Robotics?

    Hi! I am new at robotics, can you guys please help me what is the principal moment of inertia?? how to define the pose of axis about center of mass of my robotic link of a legged robot?? please guide me with some visual representation and also how to calculate Inertia Tensor?? I will really...
  46. Isaac0427

    Cosmological constant times the metric tensor

    In the EFE, what does adding Λgμν mean and why is it not included in the Einstein tensor?
  47. K

    Is the moment of inertia matrix a tensor?

    Homework Statement Is the moment of inertia matrix a tensor? Hint: the dyadic product of two vectors transforms according to the rule for second order tensors. I is the inertia matrix L is the angular momentum \omega is the angular velocity Homework Equations The transformation rule for a...
  48. D

    Demonstrate the matrix represents a 2nd order tensor

    Homework Statement Demonstrate that matrix ##T## represents a 2nd order tensor ##T = \pmatrix{ x_2^2 && -x_1x_2 \\ -x_1x_2 && x_1^2}## Homework Equations To show that something is a tensor, it must transform by ##T_{ij}' = L_{il}L_{jm}T_{lm}##. I cannot find a neat general form for ##T_{ij}##...
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