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. binbagsss

    Quick question, index notation, alternating tensor.

    Q) I am using index notation to show that ε^{0123}=-1 given that ε_{0123}=1. The soluton is: ε^{0123}=g^{00}g^{11}g^{22}g^{33}ε_{0123}=-ε_{0123} where g_{\alpha\beta} is the metric tensor. I am struggling to understand the last equality. Many thanks for any assistance.
  2. binbagsss

    Moment of Inertia tensor - displaced axes theorem:

    Ok, so the system consists of two massive spheres, m1 and m2, of radii a and b respectively, connected by a massless rod of length R, as seen in the diagram attached. The question is to calculate the moment of inertia tensor. Sol: Set the origin at the centre of mass . So that we are in...
  3. M

    Stress-Energy Tensor from Lagrangian: Technical Question II

    This thread is supposed to be a continuation of the discussion of this thread: (1) https://www.physicsforums.com/showthread.php?t=88570. The previous thread was closed but there was a lot of things I did not understand. This is also somewhat related to a recent thread I created: (2)...
  4. M

    Stress-energy tensor for electromagnetic field with interaction term

    First of all, I'm not sure if this thread belongs here or at the "Special & General Relativity" sub-forum, if I posted at the wrong place please move it. Homework Statement I encountered this problem working in my master's degree. I need to find the stress-energy tensor of the following...
  5. G

    Einstein tensor fully written out

    Hi, Does somebody know a link where the Einstein tensor is fully written out, i.e. only containing the metric and its derivatives? I'm just wondering how much is actually hidden in the notation.
  6. E

    Tensor Notation and derivatives

    Hi folks. Hope that you can help me. I have an equation, that has been rewritten, and i don't see how: has been rewritten to: Can someone explain me how? Or can someone just tell me if this is correct in tensor notation: σij,jζui = (σijζui),j really hope, that...
  7. P

    Variation of the action using tensor algebra.

    Homework Statement Hi, I have a problem calculating the variation of the action using tensor algebra because two derivative indices are causing some problem. Homework Equations Generally you have the action S = \int L(A^{\mu}, A^{\mu}_{\;,\nu}, x^{\mu})d^4x where: A ^{\mu}=...
  8. B

    [Electromagnetism,optics]How to attack a problem of dielectric tensor?

    Hi, I am currently making an effort to solve a boundary value problem of electromagnetic field. The problem is as follows: The region ##y<0## is vacuum. The region ##y \geq 0## is filled with material with ##\mu=\mu_0## and dielectric tensor ## \left( \begin{array}{ccc} \alpha & i\beta &...
  9. skate_nerd

    MHB Proving vector calculus identities w/ tensor notation

    I have an vector calculus identity to prove and I need to use vector notation to do it. The identity is $$\vec{\nabla}(fg)=f\vec{\nabla}{g}+g\vec{\nabla}{f}$$ I tried starting with the left side by writing $\vec{\nabla}(fg)=\nabla_j(fg)$. Now I look and that and it really looks like there is...
  10. V

    Tensor Contraction: Is it Always Information Subset?

    Hello Everyone, I came here with a question and hope you can shed some light. We know that Ricci tensor which is a contraction of Riemann tensor contains a subset of information as contained by Riemann tensor. In 3-D infact they contain the same information. I was wondering is it always...
  11. S

    Derivation of energy-stress tensor in GR

    Would there be a direct proof of the energy-stress tensor of general relativity? My lecturer only provides me with a simplified proof - 1. Guess the form of the tensor in special relativity in co-moving frame (ρ+p)uμuv+pημv Note that the pη00 term cancels the p in u0u0, to simplify the...
  12. N

    Tensor Derivatives Homework Help

    Homework Statement Hi I am reading about some fluid mechanics, when suddenly I read saw that someone took the derivate of a tensor. It is in this thesis, on page 26 eq. (70). It is the final equality I can't understand. So the author is taking the derivate \partial_{x_{\alpha}}...
  13. N

    How to construct stress-energy tensor for a system?

    Given a particular system, how would one construct the stress-energy tensor? I was reading Mallett's paper and the stress-energy given for an infinitely long circulating cylinder of light is of the form T_{\mu\nu}=\epsilon \eta_\mu \eta_\nu where \eta_\mu=(\eta_0,0,\eta_2,0) and ε is the energy...
  14. Sudharaka

    MHB What is the annihilator of a tensor in vector space V?

    Hi everyone, :) This is a question I don't understand at all. What is the annihilator in this context? Hope you can help me out with this. Problem: Find the annihilator of the tensor \(e_1\wedge e_2+e_3\wedge e_4\) in \(V=\left<e_1,\,e_2,\,e_3,\,e_4\right>\).
  15. Sudharaka

    MHB Structure Tensor of Matrices

    Hi everyone, :) Here's is a question I have trouble understanding. Hope you can help me out. :) Specifically what is meant by the structure tensor and how is it computed when given a \(2\times 2\) triangular matrix? Problem: Write the structure tensor for the algebra \(A\) of traingular...
  16. C

    Projection tensor in from (m+n) dim down on n-dim

    Suppose that we have an (n+m)-dimensional tangent space ##T_p^{n+m}## which we decompose into the direct sum of two tangent spaces ##T_p^{n+m} = T_p^n \oplus T_p^m##. We have a coordinate basis in some region of the manifold ##\left\{\partial_{\mu}\right\}_{\mu=1}^{n+m}## from which we want to...
  17. TrickyDicky

    Is the Maxwell stress tensor a true stress?

    Should it be added to the Cauchy stress to calculate a "total stress", or it doesn't have such a physical interpretation as a surface force(EM field force is usually considered more of a "body force")? Certainly when the MST was first derived before aether theories were made superfluous by...
  18. Sudharaka

    MHB Canonical Isomorphism and Tensor Products

    Hi everyone, :) Here's a problem that I have trouble understanding. Specifically I am not quite getting what it means by the expression \(\alpha (t)(v)\). Hope somebody can help me to improve my understanding. :) Problem: Let \(\alpha\) be the canonical isomorphism from \(V^*\otimes V\) to...
  19. Q

    Transpose of a Tensor Identity

    My textbook (regarding continuum mechanics) has the following identity that is supposed to be true for all tensors: a\cdotTb = b\cdotTTa But I don't get the same result for both sides when I work it out. For each side, I'm doing the dot product last. For example, I compute Tb first and...
  20. Jalo

    Archived Find the vortices of a square after a transformation given by a tensor

    Homework Statement Given a square and the respective distension tensor, ε, find the position on his vortices after the transformation. ε = 0.1...0.25 ...0.25...0.1 Homework Equations The Attempt at a Solution I got kind of lost in this question. I started thinking that maybe...
  21. Sudharaka

    MHB Understanding Tensors: Finding the Value of a Tensor

    Hi everyone, :) Here's a problem that I recently encountered and want to get an hint on how to solve. :) Problem: Find the value \(F(v,\,f)\) of the tensor \(F=e^1\otimes e_2+e^2\otimes (e_1+3e_3)\in T_{1}^{1}(V)\), where \(v=e_1+5e_2+4e_3\), \(f=e^1+e^2+e^3\).
  22. 7

    What is a Tensor? Exploring Vectors & Multiplying Tensors

    I am not sure if this a right place to ask what is a tensor. I already asked about vectors in Math section, but I think a tensor has more to do with physics that mathematics, so I came here. I am reading A Zee's book Einstein Gravity that students have fear of tensors. I also think that...
  23. S

    MHB Solving for $F(v,f)$ in Tensor $F$

    Hello everyone Here is the problem: Find the value $F(v,f)$ of the tensor $F=e^1\otimes e_2 +e^2\otimes(e_1+3e_3)\in T^1_1(V)$ where $v=e_1+5e_2+4e_3, f=e^1+e^2+e^3$ Does $e^1\otimes e_2=0$ in this problem?Thanks
  24. C

    Projection of the Riemann tensor formula.

    Suppose we are given two projection operators H' and H'' such that H' + H'' = 1, i.e. that any vector can be written as V = V' + V'' = (H' + H'') V. I'm trying to prove the formula $$R(X',Y'')Z' \cdot V'' = (Z' \cdot (\nabla'_{X'}B') + \left<X'\cdot B', Z' \cdot B'\right>)(Y'', V'') + (V''...
  25. J

    Tensor Decomposition: Exploring Linear Decomposition Methods

    Given a vector \vec{r}=\begin{bmatrix} x\\ y \end{bmatrix} It's possible to decompose it linearly, so: \vec{r}=x\hat{i}+y\hat{j} So, how would the linear decomposition of a tensor? Thx!
  26. N

    Levi-Civita Tensor & Group Theory: Symmetry?

    I have been trying to think about the Levi-Civita tensor in the context of Group Theory. Is there a group that it is symmetric to? I'm sorry if this is a double post but I don't think my original identical post submitted correctly. Thanks, Nate
  27. wavepart7cle

    Einstein's applications of tensor calculus

    Hey everyone, I recently learned that my certified genius weird-uncle-who-lives-at-home (IQ over 200 something, legitimate 'genius') or WULAH for short, passes his spare time by lounging around his place and doing tensor calculus. I've done some calc in 3d in college and I know that's commonly...
  28. G

    What does vanishing Ricci tensor signify ?

    Are Ricci flat manifolds analogous to flat space-time ? Further for Ricci flat manifolds does the Riemann tensor vanish ?
  29. V

    Using parallel propagator to derive Riemann tensor in Sean Carroll's

    Hello all, In Carroll's there is a brief mention of how to get an idea about the curvature tensor using two infinitesimal vectors. Exercise 7 in Chapter 3 asks to compute the components of Riemann tensor by using the series expression for the parallel propagator. Can anyone please provide a...
  30. H

    Coordinate and dual basis vectors and metric tensor

    I have been reading an introductory book to General Relativity by H Hobson. I have been following it step by step and now I am stuck. It is stated in the book that: "It is straightforward to show that the coordinate and dual basis vectors themselves are related... "ea = gabeb ..." I have...
  31. D

    Pauli matrices and the Levi-Civita tensor : commutation relations

    Homework Statement Whats up guys! I've got this question typed up in Word cos I reckon its faster: http://imageshack.com/a/img5/2286/br30.jpg Homework Equations I don't know of any The Attempt at a Solution I don't know where to start! can u guys help me out please? Thanks!
  32. D

    Proving properties of the Levi-Civita tensor

    Homework Statement Hey everyone, So I've got to prove a couple of equations to do with the Levi-Civita tensor. So we've been given: \epsilon_{ijk}=-\epsilon_{jik}=-\epsilon_{ikj} We need to prove the following: (1) \epsilon_{ijk}=-\epsilon_{kji} (2)...
  33. T

    Why does loose tensor tympani cause hypoacusis?

    If the tensor tympany is loose, the ear drum is also loose. But then, the ear drum will vibrate even by weak sound waves, which will cause increased vibration of malleus, incus, and stapes. Isn't that supposed to cause hyperacusis instead?
  34. M

    Understanding the Cauchy Stress Tensor: A Guide for Beginners

    Hello, I am not sure what the first indice in the cauchy stress tensor indicates For example, σ_xy means that the stress in the y direction, but does x mean the cross sectional area is normal to the x direction?
  35. Philosophaie

    What is the value of Q in the equation for the Kerr-Newman Metric Tensor?

    Our galaxy is rotating and is charged therefore the choice for the metric is the Kerr-Newman Metric. I want to solve for the Kerr-Newman Metric Tensor. There are a few questions. 1-What is the value for Q in the equation: ##r_Q^2=\frac{Q^2*G}{4*\pi*\epsilon_0*c^4}## where ##G=6.674E-20...
  36. D

    Why is the inertia tensor calculated about a point instead of an axis?

    Generally, when we talk about moment of inertia, we talk about rotation and inherently, we talk about moment of inertia about an axis. But when we talk about inertia tensor, we calculate about a point. Is there a reason for this difference? Am I missing something? I am new to tensors.
  37. S

    Integrals featuring the laplacian and a tensor

    Ok, so I'd like some advice on doing integrals that involve a laplacian and a tensor for example =\int\frac{\delta}{\delta A_{\mu}}\frac{1}{4M^{2}}(\partial_{\rho}A_{\sigma}-\partial_{\sigma}A_{\rho})\frac{\partial^{2}}{\partial x^{2}}(\partial^{\rho}A^{\sigma}-\partial^{\sigma}A^{\rho}) where...
  38. Philosophaie

    Metric Tensor of the Reissner–Nordström Metric

    I am looking for the Metric Tensor of the Reissner–Nordström Metric.g_{μv} I have searched the web: Wiki and Bing but I can not find the metric tensor derivations. Thanks in advance!
  39. F

    Tensor Notation for Triple Scalar Product Squared

    Homework Statement Hi all, Here's the problem: Prove, in tensor notation, that the triple scalar product of (A x B), (B x C), and (C x A), is equal to the square of the triple scalar product of A, B, and C. Homework Equations The Attempt at a Solution I started by looking at the triple...
  40. fluidistic

    Density of energy from the stress-energy tensor

    Homework Statement Hi guys, I would like to show that if ##t^\mu## is a temporal vector then ##t^\mu t^\nu T_{\mu\nu}## is the density of energy of the EM field measured by an observer with velocity ##t^\mu##. And that it is greater or equal to 0. Density of energy is proportional to...
  41. fluidistic

    Trace of the stress-energy tensor

    0. Homework Statement Hi guys, I must show that the trace of the stress energy tensor is zero. The definition of it is ##T^{\mu \nu }=\frac{1}{4\pi} \left ( F^{\mu \sigma } F^{\nu \rho} \eta _{\sigma \rho}-\frac{1}{4} \eta ^{\mu \nu } F^{\sigma \rho} F_{\sigma \rho} \right )##. 1. The...
  42. T

    Magnetic field from vector potential function using tensor notation

    Homework Statement We will see (in Chap. 5) that the magnetic field can be derived from a vector potential function as follows: B = ∇×A Show that, in the special case of a uniform magnetic field B_{0} , one possible vector potential function is A = \frac{1}{2}B_{0}×r MUST USE TENSOR NOTATIONm...
  43. Ibix

    Validity of Tensor Expressions in (a)-(d)

    Another trivial question from me. Homework Statement Which (if any) of the following are valid tensor expressions: (a)A^\alpha+B_\alpha (b)R^\alpha{}_\beta A^\beta+B^\alpha=0 (c)R_{\alpha\beta}=T_\gamma (d)A_{\alpha\beta}=B_{\beta\alpha} Homework Equations Nothing relevant -...
  44. Mandelbroth

    Can the tensor product be visualized as a machine for processing vectors?

    "Seeing" Tensor Products Is there a way to "visualize" the tensor product of two (or ##n##) vectors/tensors/algebras/etc.? I'm having a lot of trouble making the tensor product feel intuitive. I know its properties, and I can usually apply it without too much of a problem, but it does not...
  45. E

    Tensor equation in Dirac's 1975 book

    Dirac has equation 3.4 as: x^{\lambda}_{,\mu}x^{\mu}_{,\nu}=g^{\lambda}_{\nu} Shouldn't that have a 4 on the right side? x^{\lambda}_{,\mu}x^{\mu}_{,\nu}=(4?)g^{\lambda}_{\nu}
  46. skate_nerd

    MHB Tensor notation for vector product proofs

    I am new to tensor notation, but have known how to work with vector calculus for a while now. I understand for the most part how the Levi-Civita and Kronecker Delta symbol work with Einstein summation convention. However there are a few things I'm iffy about. For example, I have a problem where...
  47. TheFerruccio

    Prove the following tensor identity

    I am back again, with more tensor questions. I am getting better at this, but it is still a tough challenge of pattern recognition. Problem Statement Prove the following identity is true, using indicial notation: \nabla\times(\nabla \vec{v})^T = \nabla(\nabla\times\vec{v}) Attempt at...
  48. D

    Maxwell stress tensor in different coordinate system

    Hi guys, I would like to know if the answer given to this thread is correct https://www.physicsforums.com/showthread.php?t=457405 I got the same doubt, is the expression for the tensor given in cartesian coordinates or is it general to any orthogonal coordinate system? Thanks in advance
  49. C

    Construct electromagnetic stress-energy tensor for a non-flat metric

    Hi, I am having problems in constructing a stress-energy tensor representing a constant magnetic field Bz in the \hat{z} direction. The coordinate system is a cylindric {t,r,z,\varphi}. The metric signature is (+,-,-,-). I ended with the following mixed stress-energy tensor: Is this...
  50. C

    Definition of the extrinsic-curvature tensor.

    Some define the extrinsic curvature tensor as $$K_{\mu \nu} = h^{\ \ \ \sigma}_\nu h^{\ \ \ \lambda}_\nu \nabla_\sigma n_\lambda.$$ From the expression it seems like the index of the covariant derivative in can be any spacetime index. However, does it makes sense to ask what the...
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