# Torsion tensor derivation

• A
hi, I looked up torsion tensor derivation on 2 different books, and encountered 2 different situations, so my mind has been confused. For the first image, I could totally understand how torsion tensor was derived, but for the second image although there are similar things, I can not make a connection between 2 images. For instance, in first image torsion tensor is between r1 and r2 points, but this torsion tensor does not correspond to torsion tensor in second image, because instead there is a [u,v] vector. Besides, as you can see "sr1" is the parallel transported vector according to first image and resembles the vector "uR" ( appearance of these 2 vectors is same I think), but
is parallel transported vector according to second image ( it is really parallel to
vector, but for the first image the parallel transported vector ("sr1" ) is not really parallel to " pq " vector. To sum up, Could you please help me understand these 2 different situation, and express or explain the truth???

FIRST IMAGE

SECOND IMAGE

Related Special and General Relativity News on Phys.org
hi, I looked up torsion tensor derivation on 2 different books, and encountered 2 different situations, so my mind has been confused. For the first image, I could totally understand how torsion tensor was derived, but for the second image although there are similar things, I can not make a connection between 2 images. For instance, in first image torsion tensor is between r1 and r2 points, but this torsion tensor does not correspond to torsion tensor in second image, because instead there is a [u,v] vector. Besides, as you can see "sr1" is the parallel transported vector according to first image and resembles the vector "uR" ( appearance of these 2 vectors is same I think), but View attachment 101110 is parallel transported vector according to second image ( it is really parallel to View attachment 101111 vector, but for the first image the parallel transported vector ("sr1" ) is not really parallel to " pq " vector. To sum up, Could you please help me understand these 2 different situation, and express or explain the truth???

FIRST IMAGE

View attachment 101107

SECOND IMAGE

View attachment 101108
Why anyone can help me?? Is it a though question or long question ??

hi, I looked up torsion tensor derivation on 2 different books, and encountered 2 different situations, so my mind has been confused. For the first image, I could totally understand how torsion tensor was derived, but for the second image although there are similar things, I can not make a connection between 2 images. For instance, in first image torsion tensor is between r1 and r2 points, but this torsion tensor does not correspond to torsion tensor in second image, because instead there is a [u,v] vector. Besides, as you can see "sr1" is the parallel transported vector according to first image and resembles the vector "uR" ( appearance of these 2 vectors is same I think), but View attachment 101110 is parallel transported vector according to second image ( it is really parallel to View attachment 101111 vector, but for the first image the parallel transported vector ("sr1" ) is not really parallel to " pq " vector. To sum up, Could you please help me understand these 2 different situation, and express or explain the truth???

FIRST IMAGE

View attachment 101107

SECOND IMAGE

View attachment 101108
hi, I looked up torsion tensor derivation on 2 different books, and encountered 2 different situations, so my mind has been confused. For the first image, I could totally understand how torsion tensor was derived, but for the second image although there are similar things, I can not make a connection between 2 images. For instance, in first image torsion tensor is between r1 and r2 points, but this torsion tensor does not correspond to torsion tensor in second image, because instead there is a [u,v] vector. Besides, as you can see "sr1" is the parallel transported vector according to first image and resembles the vector "uR" ( appearance of these 2 vectors is same I think), but View attachment 101110 is parallel transported vector according to second image ( it is really parallel to View attachment 101111 vector, but for the first image the parallel transported vector ("sr1" ) is not really parallel to " pq " vector. To sum up, Could you please help me understand these 2 different situation, and express or explain the truth???

FIRST IMAGE

View attachment 101107

SECOND IMAGE

View attachment 101108
Why can anyone help me?? Is it a though question or long ??

I don't really understand your question, I'm afraid; the two diagrams depict the exact same thing, the only difference is that the various vectors have different labels. In both cases, torsion is demonstrated as being the failure of the parallelogram to be closed.

Ibix
2020 Award
I think all this is correct, but do be a bit wary. Still learning this myself.

The second image is comparing and contrasting two similar definitions: "You draw a parallelogram and the failure to close is the torsion" and "You draw a parallelogram and the failure to close is the commutator". The inner parallelogram is the same as your first diagram. The outer parallelogram is something different but confusingly similar.

The inner parallelogram's top and right sides are formed by parallel transporting the bottom and left sides along the left and bottom sides respectively - the failure to close is the torsion. The outer parallelogram's top and right sides are the values of the vector field at R and Q respectively - the failure to close is the commutator.

The difference between the two top sides (i.e. the difference between the parallel transported vector and the "local" value) is the covariant derivative in the direction of the left side. Similarly for the two right sides.

Markus Hanke they do not depict the same thing, look at the parallel transported vectors. The parallel transported vector "sr1" in first image should correspond to
vector not
vector in the second image. By the way Ibix, how are these 2 different derivations correct ?? I mean torsion definition must be unique, but there are 2 different meanings for torsion in these images. How can we have 2 different torsion definition ?????? If we say that torsion definition is the the failure of the closure of the parallelogram made up of the small displacement vectors and their parallel transports, we can not use that definition for second image, because in second image we should not have
or
vectors as a parallel transported vectors

Markus Hanke they do not depict the same thing, look at the parallel transported vectors. The parallel transported vector "sr1" in first image should correspond to View attachment 101127 vector not
vector in the second image. By the way Ibix, how are these 2 different derivations correct ?? I mean torsion definition must be unique, but there are 2 different meanings for torsion in these images. How can we have 2 different torsion definition ?????? If we say that torsion definition is the the failure of the closure of the parallelogram made up of the small displacement vectors and their parallel transports, we can not use that definition for second image, because in second image we should not have
or View attachment 101128 vectors as a parallel transported vectors
Guys, I have been waiting for a long time, and I have not received an answer. I really want to make you sure that I can not make any progress in this topic. I got stuck, and I want to proceed to other tremendous, astonishing topics in relativity. I will appreciate, and be so pleased if you return me...........

Orodruin
Staff Emeritus
Homework Helper
Gold Member
Markus Hanke they do not depict the same thing, look at the parallel transported vectors. The parallel transported vector "sr1" in first image should correspond to View attachment 101127 vector not
vector in the second image.
No, this is incorrect, sr1 corresponds to ##u_R^\parallel##. The vectors ##u_R##/##v_Q## are not parallel transported along the parallelogram sides.

haushofer
No, this is incorrect, sr1 corresponds to ##u_R^\parallel##. The vectors ##u_R##/##v_Q## are not parallel transported along the parallelogram sides.
Thank you for your return "Orodruin" ,but if
or
are not parallel transported vectors, for what do they stand??? also What is the meaning of
and
vector ???

Orodruin
Staff Emeritus
Homework Helper
Gold Member
haushofer
The first one seems to come from Nakahara.

The vector fields at the specific points.

Transporting a vector field from some point to a point Q is different from simply evaluating it in point Q. Is that your confusion?
Thank you for your informing "Orodruin". As " haushofer" guessed, first image belongs to "Nakahara", whereas second image belongs to "F.W. Hehl and Yu.N. Obukhov". I consider the problem is: There may be a parallel transportation problem of vectors. For example, parallel transported vector "sr1" in first image do not parallel to "pq" vector, but if we pass to the second image we can examine that parallel transported vector
is totally parallel to
vector, and this situation makes a confusion. Nevertheless, "Orodruin" expressed that sr1 corresponds to
. The vectors
, and
are not parallel transported along the parallelogram sides. After this explanation I have one more question: if
or
are not parallel transported vectors, for what do they stand??? also What is the meaning of
and
vector ???

Orodruin
Staff Emeritus
Homework Helper
Gold Member
Thank you for your informing "Orodruin". As " haushofer" guessed, first image belongs to "Nakahara", whereas second image belongs to "F.W. Hehl and Yu.N. Obukhov". I consider the problem is: There may be a parallel transportation problem of vectors. For example, parallel transported vector "sr1" in first image do not parallel to "pq" vector, but if we pass to the second image we can examine that parallel transported vector
is totally parallel to View attachment 101142 vector, and this situation makes a confusion. Nevertheless, "Orodruin" expressed that sr1 corresponds to
. The vectors
, and
are not parallel transported along the parallelogram sides. After this explanation I have one more question: if
or
are not parallel transported vectors, for what do they stand??? also What is the meaning of
and
vector ???

haushofer
haushofer
I have a hard time guessing what the problem is, since we have answered your questions already. Why don't you agree with post #8?

hi, I have been digging up torsion tensor, and I would like to share 2 different images below. At the first image, I suppose that
is the difference of directional derivatives, because at the second image this type of representation corresponds to difference of directional derivatives. If we come to the first image we can say that
=
+
-(
+
), but we are aware that as I said before
is the difference of directional derivatives according to the second image. To sum up, in accordance with the
=
+
-(
+
) formula,
+
should be a directional derivative, but in line with the second image directional derivative is not necessarily tangent to manifold, as you can see first image totally lays on the tangent place,
+
is tangent to manifold instead of being not tangent according to second image.

First image is quoted from F.W. Hehl and Yu.N. Obukhov
second image quoted from David R. Wilkins

FIRST IMAGE

SECOND IMAGE

Orodruin
Staff Emeritus
Homework Helper
Gold Member
+
should be a directional derivative
No, it should not. These are two completely different vectors which a priori has nothing to do with each other.

Also, please use the LaTeX features of the forum rather than editing in images, it will make your posts significantly more readable.

No, it should not. These are two completely different vectors which a priori has nothing to do with each other.

Also, please use the LaTeX features of the forum rather than editing in images, it will make your posts significantly more readable.
Well thanks for your return, but why do we put this [u,v] vector (at the second image ) on the image?? How we can derive this vector?? The only derivation I know of this vector is the difference of directional derivatives, but I can not see any directional derivatives ( after " Orodruin" 's explanation ) at the first image. What kind of logic exist for that vector??

Orodruin
Staff Emeritus
Homework Helper
Gold Member
##[u,v]## is just the failure of the flows of the vector fields ##u## and ##v## to close. In general, I suggest that you do not try to see properties of a manifold in terms of an embedding as is done in your second image in #16. The properties under discussion are perfectly well defined only in terms of the properties of the manifold and affine connection itself (even without the affine connection when we just discuss the Lie bracket) without reference to an embedding in a higher-dimensional space.

##[u,v]## is just the failure of the flows of the vector fields ##u## and ##v## to close. In general, I suggest that you do not try to see properties of a manifold in terms of an embedding as is done in your second image in #16. The properties under discussion are perfectly well defined only in terms of the properties of the manifold and affine connection itself (even without the affine connection when we just discuss the Lie bracket) without reference to an embedding in a higher-dimensional space.
##[u,v]## is just the failure of the flows of the vector fields ##u## and ##v## to close. In general, I suggest that you do not try to see properties of a manifold in terms of an embedding as is done in your second image in #16. The properties under discussion are perfectly well defined only in terms of the properties of the manifold and affine connection itself (even without the affine connection when we just discuss the Lie bracket) without reference to an embedding in a higher-dimensional space.
Do you mean that [u,v] is not the difference of directional derivatives?? If not, how do we calculate the value of it??

Orodruin
Staff Emeritus
Homework Helper
Gold Member
Do you mean that [u,v] is not the difference of directional derivatives?? If not, how do we calculate the value of it??
If you use "directional derivative" to mean "as seen embedded into a Cartesian space" then no. The Lie bracket ##[u,v]## is the commutator between the vector fields ##u## and ##v##, which you can easily show is a vector field in itself. You can easily show that it is related to the failure of flows to commute.

If you use "directional derivative" to mean "as seen embedded into a Cartesian space" then no. The Lie bracket ##[u,v]## is the commutator between the vector fields ##u## and ##v##, which you can easily show is a vector field in itself. You can easily show that it is related to the failure of flows to commute.
Well, Could you give some specific examples or remind me how to calculate the [u,v] commutator. For instance, If I want to find a torsion tensor, I need covariant derivatives, and [u,v] commutator, but I know the formula of how to calculate the covariant derivative whereas I can not remember the formulation of commutator. What kind of proof may make a relation between the failure of flows and commutator?? I am asking because I want to visualize the concept using some proofs.

stevendaryl
Staff Emeritus
Well, Could you give some specific examples or remind me how to calculate the [u,v] commutator. For instance, If I want to find a torsion tensor, I need covariant derivatives, and [u,v] commutator, but I know the formula of how to calculate the covariant derivative whereas I can not remember the formulation of commutator. What kind of proof may make a relation between the failure of flows and commutator?? I am asking because I want to visualize the concept using some proofs.
I hope I don't screw this up, but here's my understanding:

If you have a scalar function $\phi$, and a tangent vector $u$, then the meaning of $u(\phi)$ is the rate of change of $\phi$ along the tangent $u$. In terms of coordinates, it's $u(\phi) = \sum_j u^j \frac{\partial \phi}{\partial x^j}$.

The meaning of $[u,v]$ is that tangent vector $w$ such that $w(\phi) = u(v(\phi)) - v(u(\phi))$

In terms of coordinates, $[u,v] = w \Rightarrow w^j = \sum_i (u^i \frac{\partial v^j}{\partial x^i} - v^i \frac{\partial u^j}{\partial x^i})$

stevendaryl
Staff Emeritus
In terms of coordinates, $[u,v] = w \Rightarrow w^j = \sum_i (u^i \frac{\partial v^j}{\partial x^i} - v^i \frac{\partial u^j}{\partial x^i})$
Just for clarification of what I wrote, the commutator $[u,v]$ only makes sense if $u$ and $v$ are vector fields, not vectors. A vector field assigns a vector to each point in your space. In terms of components, $u^i$ is a function of position.
Just for clarification of what I wrote, the commutator $[u,v]$ only makes sense if $u$ and $v$ are vector fields, not vectors. A vector field assigns a vector to each point in your space. In terms of components, $u^i$ is a function of position.