Talk:Ricci calculus/Archive 2

Archive 1

Archive 2

"these need to be rewritten or removed"

The following was commented out by JRSpriggs:

Riemann curvature tensor $R^{\rho }{}_{\sigma \mu \nu }=dx^{\rho }(R(\partial _{\mu },\partial _{\nu })\partial _{\sigma })$ Torsion tensor $T^{c}{}_{ab}=\Gamma ^{c}{}_{ab}-\Gamma ^{c}{}_{ba}-\gamma ^{c}{}_{ab}$ which follows from $T=\nabla _{X}Y-\nabla _{Y}X-[X,Y]$ where X and Y are vector fields and [ , ] is the Lie bracket of vector fields. Levi-Civita tensor The covariant Levi-Civita tensor in an n-D metric space may be defined as the unique (up to a sign) n-form (completely antisymmetric order-n covariant tensor) that obeys the relation $\left|\epsilon _{\alpha _{1}\dots \alpha _{n}}g^{\alpha _{1}\beta _{1}}\dots g^{\alpha _{n}\beta _{n}}\epsilon _{\beta _{1}\dots \beta _{n}}\right|=n!$ The choice of sign defines an orientation in the space. The contravariant Levi-Civita tensor is an n-vector that may be defined by raising each of the indices of the corresponding covariant tensor: $\epsilon ^{\alpha _{1}\dots \alpha _{n}}=g^{\alpha _{1}\beta _{1}}\dots g^{\alpha _{n}\beta _{n}}\epsilon _{\beta _{1}\dots \beta _{n}}$

not that I agree/disagree with the commenting-out, just brought it to attention here so people can see it clearer. Unless anyone is up for rewriting these, if they're incorrect they may as well stay out of the article... F = q(E+v×B) ⇄ ∑_ic_i 10:08, 18 June 2012 (UTC)

Some specific comments

• I trust we all agree that the Riemann curvature tensor should be listed in any section listing notable tensors. The question is how we present it, and I'm not convinced that the current presentation is suitable for the article (it uses unexplained notation, for one). Another expression could be

$R^{\rho }{}_{\sigma \mu \nu }=\partial _{\mu }\Gamma ^{\rho }{}_{\nu \sigma }-\partial _{\nu }\Gamma _{\mu \sigma }^{\rho }+\Gamma ^{\rho }{}_{\mu \lambda }\Gamma ^{\lambda }{}_{\nu \sigma }-\Gamma ^{\rho }{}_{\nu \lambda }\Gamma ^{\lambda }{}_{\mu \sigma }$

• The torsion tensor is just as notable as the the Riemann curvature tensor; anyone objecting to its presentation should state why.
• The Levi-Civita tensor in my mind is even more notable, only the Levi-Civita symbol gets used so often in its place as a mathematical shortcut that we forget that there is a perfectly good tensor for the job (provided that there is a metric). Some may have issues with the characterisation given thereof, and in some cases the pseudotensor is defined instead (IMO unnecessary when the tensor is as good or better); I guess it may make sense to find a good reference and go with their definition.
• The metric tensor is not quite right (you can't use the same symbol g to operate on both covariant and contravariant basis vectors). I would prefer eliminating the unexplained "operating on" notation.

In all, I think these do belong in the article, and objections should be explained so that we all understand them and can try to accommodate them. — Quondum^☏ 12:05, 18 June 2012 (UTC)

If the expression for the Riemann curvature tensor were rewritten to use the notation of this article it would become

$R^{\rho }{}_{\sigma \mu \nu }=\mathbf {e} ^{\rho }(R(\mathbf {e} _{\mu },\mathbf {e} _{\nu })\mathbf {e} _{\sigma })$

which is not particularly informative, as I hope you will agree. I think we should provide a formula which shows how this tensor is the commutator of the covariant derivative with itself.

$A_{\nu ;\rho \sigma }-A_{\nu ;\sigma \rho }=A_{\beta }R_{\nu \rho \sigma }^{\beta }\,$

as stated on pages 20 and 21 of "General Theory of Relativity" by P. A. M. Dirac. This can be generalized to get the commutators for two covariant derivatives of arbitrary tensors.

I really do not like the torsion tensor; it has no place in Einstein's theory. There is no evidence that such a tensor actually exists. It is a figment of the imagination of those who support the Einstein–Cartan theory.

The Levi-Civita symbol#Tensor density is best understood as a tensor density, not as an ordinary tensor. JRSpriggs (talk) 16:16, 18 June 2012 (UTC)

Thank you for your clarification. I find I disagree on several points, though I suspect that each of us is a bit too fond of our own perspective – in my case, a coordinate-independent approach where possible.

Though on the Riemann curvature tensor, I agree that your expression has stripped the expression of any defining powers, as any order-4 tensor of that variance will satisfy your expression: it simply says what the components are. Though I'm not sure that I agree that this is the "notation of this article": I would like to remove any reference to the basis vectors except in a section defining the components of a tensor if possible; this goes for the section on the metric as well. You have not commented on my expression above that gives the components in terms of the Christoffel symbols, using what I would call the dominant notation of the article. I do however like your definition in terms of the commutator of the covariant derivative; my expression could be added as a derived consequence.

The torsion is generally zero. This is not the same as it being non-existent. It is thus a core tensor in all metric field theories.

On the Levi-Civita density I feel most strongly opposed, and I think I've said why. You clearly like tensor densities, though. — Quondum^☏ 17:35, 18 June 2012 (UTC)

On the subject of torsion and the related question of non-holonomic bases (Frame fields in general relativity) for the tangent bundle. These are unnecessary add-ons to the mathematical structure used in general relativity. They serve no purpose other than making the subject harder to understand. In reality, it is the irreducible representations of a group which are important. For example, second order covariant tensors which are neither symmetric (like g_αβ) nor antisymmetric (like F_αβ) do not appear because they are reducible. The Levi-Civita connection is irreducible; other connections (which include torsion) are not. Thus there is good reason (Occam's razor) to believe that torsion does not exist, beyond the mere absence of evident for it. JRSpriggs (talk) 07:51, 20 June 2012 (UTC)

To Quondum: If you think that you can get along without tensor densities, then tell me with what would you replace ${\sqrt {-g}}\,$? JRSpriggs (talk) 11:35, 21 June 2012 (UTC)

This is an inherently coordinate-dependent expression: a calibration of the local scale of the chosen coordinates against the metric. It has no other instrinsic meaning, at least as far as I can tell. In short, it is only needed to deal with non-tensors such as the Levi-Civita density; if one has only tensors (e.g. the the Levi-Civita tensor) as a starting point, its need completely vanishes — Quondum^☏ 13:08, 21 June 2012 (UTC)

Tensor densities arise unavoidably in calculating the action of a field theory. For example, see Hilbert action. JRSpriggs (talk) 23:54, 21 June 2012 (UTC)

Are they? I don't find this example compelling: it would be easily explained as the quantity $d{\mathbf {x} }^{4}$ transforming as a tensor density, not as a tensor, but by the quantity ${\sqrt {-g}}\,d{\mathbf {x} }^{4}$ being the tensor quantity. Not being familiar with actions nor differential forms I cannot comment with authority, of course, but it does have a marked resemblance to the covariant Levi-Civita tensor: the Riemannian volume form seems to be the natural quantity to use here: no tensor densities required other than to compensate for others. — Quondum^☏ 06:03, 22 June 2012 (UTC)

The Hilbert action is just $\int RdV$, where dV is the volume form generated by the metric. Not tensor densities involved. Tensor densities only become relevant if you try to formulate the Lagrangian density, which is obviously a scalar density.TR 16:42, 22 June 2012 (UTC)

It is not so: tensor densities are involved. In fact, $dV={\sqrt {-g}}\,d{\mathbf {x} }^{4}={\sqrt {-g}}\,dx^{1}\wedge \cdots \wedge dx^{4}$, $R{\sqrt {-g}}\,$ is the Lagrangian density of the Hilbert action. In this way, the Hilbert action $I=\int RdV=\int R{\sqrt {-g}}\,d{\mathbf {x} }^{4}$ is a scalar; geometrically well defined, invariant under (proper) coordinate transformations. Hence, the spacetime manifold must be oriented. Please cf. Lovelock, David (1989) [1975]. Tensors, Differential Forms, and Variational Principles. Dover. pp. 299–300. ISBN 978-0-486-65840-7. {{cite book}}: Unknown parameter |coauthors= ignored (|author= suggested) (help)Mgvongoeden (talk) 19:17, 22 June 2012 (UTC)

How does that contradict what I just said. The split $dV={\sqrt {-g}}\,d{\mathbf {x} }^{4}$ is not necessary to formulate the Hilbert action. Tensor densities only become relevant when you formulate the Lagrangian density. (Which is indicative of the fundamental non-covariantness of the used variational calculus).TR 20:56, 22 June 2012 (UTC)

Scalar densities arise unavoidably a priori in calculating the action of a field theory, for example, the Hilbert action, because in order to perform an integral on (pseudo) Riemannian manifolds one needs a volume element, i.e., a nowhere vanishing n-form $\Omega$ on $M$. In fact, on manifolds it is difficult to define integral of functions, so one prefers to define integral of n-forms. On an orientable (pseudo) Riemannian spacetime manifold with metric $g$, corresponding to an orientation of $M$, there is a uniquely determined 4-form $\Omega _{g}=dV={\sqrt {-g}}\,d{\mathbf {x} }^{4}$ which gives the orientation and which has the value $+1$ on every oriented orthonormal frame. Cf. Boothby, W.M. (1986). An introduction to differentiable manifolds and Riemannian geometry. Academic Press, Inc. p. 218. ISBN 0-12-116052-1. Just because you have this 'canonical' 4-form, a posteriori, you can 'forget' about scalar densities. We are telling the same story.Mgvongoeden (talk) 23:46, 22 June 2012 (UTC)

You can find this canonical 4-form without ever using scalar densities. (See for example J.M. Lee, Riemannian Manifolds, pg 29 and 30).TR 10:29, 23 June 2012 (UTC)

Which roughly corresponds to what I intended with "...may be defined as the unique (up to a sign) n-form that...", retained in the box above. — Quondum^☏ 12:52, 23 June 2012 (UTC)

I've encountered JRSprigg's peculiar prejudice against the torsion tensor elsewhere on Wikipedia. May I say that I do not find this prejudice helpful in building an encyclopedia. The torsion tensor has many applications to mathematics outside of general relativity. It even has applications within GR itself. Connections with torsion appear regularly in the work of people who study horizons, having no connection with JRSprigg's bogeyman of Einstein-Cartan theory. Sławomir Biały (talk) 19:39, 24 June 2012 (UTC)

Also, I find statement "There is no evidence that the torsion tensor actually exists" somewhat puzzling. Surely, the theory posits that not only does the torsion tensor exist, but that it is zero! The real question is, what evidence is there to support this much stronger assertion? (I don't know the answer. Perhaps JRSprigg's knows one.) Sławomir Biały (talk) 19:45, 24 June 2012 (UTC)

Perhaps we should rather be focusing on whether the torsion tensor should be mentioned in Ricci calculus#Notable tensors, regardless of when or whether it necessarily vanishes (and should not depend upon whether the chosen basis is holonomic). What criteria of notability should we use? Does the section belong in the article? — Quondum^☏ 20:53, 24 June 2012 (UTC)

Sławomir Biały says "the theory posits that not only does the torsion tensor exist, but that it is zero". This does not even make sense! This is like saying that GTR posits not only that The Purple People Eater tensor exists, but that it is zero. No theory should assume the existence of anything without some evidence that it has some non-trivial effect.

Not only do I reject the idea of a connection different from the Levi-Civita connection, I am also unsure that even the Levi-Civita connection exists (outside our mathematical imagination). If God has an action integral for the universe, how do we know that any covariant derivatives or Lie derivatives occur in it? All that seems clear so far is that it has something like partial derivatives and that it can be written most simply in a generally covariant form with the integrand being a scalar density. JRSpriggs (talk) 05:05, 25 June 2012 (UTC)

As Ayn Rand says, the burden of proof is on the one who asserts that something exists, not on the one who denies it. JRSpriggs (talk) 05:20, 25 June 2012 (UTC)

You seem to be getting tied up in semantic subtleties. Please substitute the term "defined" for "exists", and reconsider. — Quondum^☏ 06:40, 25 June 2012 (UTC)

Despite the bad analogy with "Purple People Eater", I think I understand your position. As soon as you have a rule for parallel transport, you have a connection. As soon as you have a connection, you have torsion (which might be zero). However, it could be that parallel transport is not something that is given a priori in the spacetime. Instead, agreeing on a parallel transport protocol if all we have is the metric might require something equivalent to setting the torsion to zero. In that way, the connection only "exists" in the sense that it defines an agreed-upon protocol for parallel transport, when the only datum that actually "exists" is the metric. So the torsion in spacetime isn't something real that can be measured, since the connection also is just a convenience. I wonder if there is a clear write-up somewhere? Sławomir Biały (talk) 16:47, 25 June 2012 (UTC)

Yes. As I see it, the metric should be used only when necessary. It is merely the potential of the gravitational field. So it should be used sparingly as the potential of the electromagnetic field is. Christoffel symbols are merely a device for keeping track of the ${\frac {\partial y^{k}}{\partial x^{m}}}\,{\frac {\partial ^{2}x^{m}}{\partial y^{i}\partial y^{j}}}\,$ terms which arise when we transform coordinates, because we need to make them cancel out when creating a scalar density to be the integrand of the action integral. JRSpriggs (talk) 18:27, 25 June 2012 (UTC)

The metric is not at all analogous to a potential. It's completely gauge invariant. Sławomir Biały (talk) 23:45, 25 June 2012 (UTC)

"The metric is not at all analogous to a potential. It's completely gauge invariant." - WTF? Are potentials nnot gauge invariant?? If so, potentials and metrics are similar? Hublolly (talk) 18:36, 9 July 2012 (UTC)

Spinors and van der Waerden notation...

Copied from here

I strongly disagree with tucking the statement of van der Waerden notation into a ref where people will not see it easily - it has been reinstated. On the contrary spinors and their notation are even more unclear and confusing than tensors, so the link should be obvious for the reader's awareness, and stated as soon as possible (why leave it to the end, even under the "see also section"?). Also that article should be expanded to be the spinor analogue of Ricci calculus (for tensors). F = q(E+v×B) ⇄ ∑_ic_i 23:11, 16 June 2012 (UTC)

Lie derivative...

is not (now) referenced. Chapter 4 in T.Frankel is about the Lie derivative but I don't recognize it... If anyone has this ref or any other then preferably add one (although it is linked, so it's not a desperate issue). F = q(E+v×B) ⇄ ∑_ic_i 09:52, 18 June 2012 (UTC)

There is now, thanks user:Mgvongoeden! In addtion to your other edits in the article. =) F = q(E+v×B) ⇄ ∑_ic_i 21:13, 19 June 2012 (UTC)

what are the Ricci identites?

In the very last section, which formulae are the Ricci identities?

• the commutator of the covariant derivative with itself

$A_{\nu ;\rho \sigma }-A_{\nu ;\sigma \rho }=A_{\beta }R_{\nu \rho \sigma }^{\beta }\,,$

• since the connection $\Gamma ^{\alpha }{}_{\beta \mu }\,$ is torsionless, which means that the torsion tensor $\Gamma ^{\lambda }{}_{\mu \nu }-\Gamma ^{\lambda }{}_{\nu \mu }\,$ vanishes.

• This can be generalized to get the commutator for two covariant derivatives of an arbitrary tensor as follows

${\begin{aligned}T^{\alpha _{1}\cdots \alpha _{r}}{}_{\beta _{1}\cdots \beta _{s};\gamma \delta }-T^{\alpha _{1}\cdots \alpha _{r}}{}_{\beta _{1}\cdots \beta _{s};\delta \gamma }=\,&-R^{\alpha _{1}}{}_{\rho \gamma \delta }T^{\rho \alpha _{2}\cdots \alpha _{r}}{}_{\beta _{1}\cdots \beta _{s}}-\cdots -R^{\alpha _{r}}{}_{\rho \gamma \delta }T^{\alpha _{1}\cdots \alpha _{r-1}\rho }{}_{\beta _{1}\cdots \beta _{s}}\\&+\,R^{\sigma }{}_{\beta _{1}\gamma \delta }T^{\alpha _{1}\cdots \alpha _{r}}{}_{\sigma \beta _{2}\cdots \beta _{s}}+\cdots +R^{\sigma }{}_{\beta _{s}\gamma \delta }T^{\alpha _{1}\cdots \alpha _{r}}{}_{\beta _{1}\cdots \beta _{s-1}\sigma }\,\end{aligned}}$

Are all or some the identities? It just vaguely says after the last one "which are often referred to as the Ricci identities. ". F = q(E+v×B)⇄ ∑_ic_i 08:39, 21 June 2012 (UTC)

Mgvongoeden wrote it in a way which made it clear that it was the last (rather complicated) formula. I was trying to make the text briefer by changing his sentence into the subordinate clause which you find ambiguous. Obviously the formula involving A is a special case of the more general formula, so it would also be a Ricci identity. JRSpriggs (talk) 11:29, 21 June 2012 (UTC)

What was I thinking, the tensor-component expression represents a number of identites, not just one... thanks for clarification. F = q(E+v×B)⇄ ∑_ic_i 11:47, 21 June 2012 (UTC)

Please, cf. the following interesting paper

• Cabras, A. (1996), "The Ricci identity for general connections", Proc. 6th Int. Conf. Conf. Differential Geometry and Applications Brno, Czech Republic August 28 - September 1, 1995: 121–126

It is a pretty good article. Mgvongoeden (talk) 15:32, 21 June 2012 (UTC)

"Fact" tagging...

Do we really need a reference for this:

"The context should prevent confusion between letters used as indices (which take integer values) and as labels for coordinates"

as tagged by TR, which I hoped would state the blatent obvious? I will not remove it, but what is the reason?

F = q(E+v×B)⇄ ∑_ic_i 21:54, 24 June 2012 (UTC)

A general statement here: the article has chosen a particular convention and is falsely presenting that as the only convention (even in relativity theory). It's one thing for the article to be internally consistent. Some conventions are necessary for this. But let's not pretend that sources all use the same conventions as we do. That's just wrong. Sławomir Biały (talk) 22:33, 24 June 2012 (UTC)

Are you referring to the space-time split convention before this italic statement? What other conventions are missing/misleading in the article?

The sentence in italic here simply refers to notation like "A_x, A_y, A_z" or "A_r, A_θ, A_φ" or whatever making use of subscripts for "x, y, z" or "r, θ, φ" etc. which are not supposed to be indices but they would look like indices to a reader, i.e. the sentence and example is to prevent the reader thinking "A_x" etc. is the same as "A_i" which actually does comprise an index for "i = 1, 2, 3".

In any case, whatever the other conventions authors use, they should be summarized wherever appropriate, since notation is what the article is about, if we should'nt pretend this article's conventions are used by everyone else. If this is what TR is referring to, then I understand the citation tag... F = q(E+v×B)⇄ ∑_ic_i 22:57, 24 June 2012 (UTC)

Short answer, you need references for everything.TR 06:15, 25 June 2012 (UTC)

I think different people might be attributing different purposes to the statement. If it is intended as a guide to interpretation of something else in the article rather than as a referenceable fact in itself, the onus of reference seems excessive. Rephrasing may make this more apparent. For example, we may point out that "authors will usually make it clear whether a subscript is intended as an index or as a label". We are simply alerting the reader to be aware of what might otherwise be confusing. To find a reference for this statement, we'd need to find a text that is an analysis of typical notations, not merely examples of what is being described. — Quondum^☏ 06:53, 25 June 2012 (UTC)

I do agree with Qoundum, not with Mr. Index Man. Boys, this is an article about Ricci calculus, not Index Notation. Some others authors use latin indices in order to denote non holonomic bases, orthonormal frames, Lorentz indices (...). Mgvongoeden (talk) 11:38, 25 June 2012 (UTC)

To TR: If that's how "simple" it is, do we need references for "circles are round" or "petrol ignites when lit" or "rain makes you wet" or "an abacus can be considered a child's toy" also?

These silly examples are no different to the triviality of the above italic statement, and no references are needed. I don't have any reference which explicitly states the same thing but surley we all have books which use "A_x, A_y, A_z" and use index notation "A_i". Quondum and Mgvongoeden seem to disagree with the tagging also... F = q(E+v×B)⇄ ∑_ic_i 15:32, 25 June 2012 (UTC)

It has been re-phrased. F = q(E+v×B)⇄ ∑_ic_i 16:17, 25 June 2012 (UTC)

The wedge product returns...

A new User:Hublolly added the wedge product back, when we have decided through all this before (now archived). =( What to do? F = q(E+v×B)⇄ ∑_ic_i 00:42, 6 July 2012 (UTC)

As my summary stated - it SHOULD be included if you have the tensor product and tensor components and Koszul connection and so on.. Please leave it. Hublolly (talk) 00:46, 6 July 2012 (UTC)

If you have a problem with including all these things - why do you keep the section? Why did you add more to it today and in the past? And why the hell did user:JRSpriggs say in the archive that these are off-topic yet allow them??? (and user:Quondum and you did get things wrong, according to archive #1.. Hublolly (talk) 00:49, 6 July 2012 (UTC)

It was decided not to include it. If you insert this content, it dismisses and defeats the discussions (please see WP:consensus). F = q(E+v×B)⇄ ∑_ic_i 00:53, 6 July 2012 (UTC)

How ironic: you *summarily removed* my efforts and when it happens to YOU - you can't even take it?? [1][2] (thouugh at least user:TimothyRias has eneogh decency to compromise [3]). :-( Reverted back. Again pretty please leave it. Hublolly (talk) 01:08, 6 July 2012 (UTC)

I should add that it's BECUASE of user:TimothyRias's comprimise that this article was created and now exists, from collaboration of others also, according to that link. It wasn't just your idea after all!? was it? See the type of thinking that gets things done and how progress is made and how we learn? Maybe you can learn from my example I lead right now - to be firm and correct in thinking and doing. How about that!? I have only just started out an hour or so ago and I'm already learning millions of times quicker than YOU. :-) Hublolly (talk) 01:33, 6 July 2012 (UTC)

And does your above non-specific and un-constructive and sarcastic comment (about the reference to index labels on round circles/lit petrol/wet rain/toy abacus) directed to user:TimothyRias himself NOT defeat WP:talk page guidelines??? Hublolly (talk) 01:35, 6 July 2012 (UTC)

Nope. And please concentrate on improving the article instead of looking for every opportunity to complain wherever someone gets it wrong. Thank you. F = q(E+v×B)⇄ ∑_ic_i 01:39, 6 July 2012 (UTC)

Hublolly, please refrain from being personally direct; direct your comments at the content of the articles exclusively, as per WP policy.
On the exterior product, I discovered through the discussion on the subject that there is more than one way to define the exterior product in terms of the tensor product – in particular, an exterior product may be defined with different scalar multiples of the fully antisymmetric tenosr product, and where two choices appear to dominate. If this product is to be included in the article, then the alternatives should be made clear. Thus, it should be stated along the lines of "An exterior product can be defined as a scalar multiple of the antisymmetrized tensor product... Common choices of the scalar are 1 and p!, where p is the number of vectors." (There are also other more complicated ways of defining an exterior product in terms of tensors, such as in geometric algebra.) And there is also the (as yet unaddressed here) problem of how to define the exterior product of arbitrary (higher-order, as well as non-antisymmetric) elements of the tensor algebra.
It was also unclear to me that this product was relevant in the contexts where the Ricci calculus was used. Notability in this context is to be established (through references). In short, the issues I have stated here should be resolved to justify inclusion. References are necessary in this instance. — Quondum^☏ 05:39, 6 July 2012 (UTC)

It’s in MTW - exactly where I said before (p.83). I asked this user if he has the reference or any other. To be fair, I can't tell anything incorrect with the equations (assuming scalar coefficient 1 not p!), and it does follow on from the tensor product admittedly nicely, and does illustrate an application of the index notation, but for reasons you point out it could potentially lead to extensive content much less on Ricci calculus and more on exterior algebra which is not helpful or purposeful at all. I reverted again for now. F = q(E+v×B)⇄ ∑_ic_i 06:46, 6 July 2012 (UTC)

It IS correct, but I see what you mean about the coefficient of 1 or p! for p vectors or covectors. But it wouldn't become an "extensive" discussion on the exterior algebra, no need for generalization just give the formulae for vectors/covectors (see what I mean)? Hublolly (talk) 07:49, 6 July 2012 (UTC)

Ok - using the reference quoted here and Quondum's words:

"An exterior product can be defined as a scalar multiple of the antisymmetrized tensor product... Common choices of the scalar are 1 and p!, where p is the number of vectors."

I will add it back.

Although why should Geometric algebra be relevent ? I notice Quondum's addiction to GA [4]query at WP maths helpdesk[5] or for that matter [6] [7] (also user:Rschwieb) in discussions on differential geometry! Hublolly (talk) 07:58, 6 July 2012 (UTC)

And please verify and reference it. My statement was off the top of my head, and may be in error. Using GA is illustrative only, in this instance of the non-uniqueness of an exterior product over the tensor algebra (a GA can be formally related to a tensor algebra). — Quondum^☏ 08:17, 6 July 2012 (UTC)

(edit conflict) Hublolly - what is it with you and your prejudice to other people's interests?? For one thing, GA is an alternative "parallel" (!) to differential geometry (say in formulating mathematical physics or purley mathematical geometry, correct me if wrong), and its perfectly allowable (and NOT decided by you, me, anyone else) for Quondum to prefer GA to DG (and Rschwieb for that matter). STOP being so nosy - please leave these editors alone. =(

I understand you would like to demonstrate application of index notation to the exterior product - at least that much is in good faith, but this article shouldn't become increasingly filled with every conceivable example of where and when index notation is applied - it is intended to be a summary that covers the main rules and notations with tensor indices; a reference point ("Appendix/glossary"?) on what all the twiddles and brackets in tensor equations mean.

By the way - there was no consensus for removing the {{fact}} template. Even though others and I seem to disagree; that was put there by someone else's decision (TimothyRias). You can't just cancel it with your opinion. That has been changed back.

I'm not sure the links you added to the see also are much help (except for exterior algebra, which should replace the section you added), but that’s a minor point I will not revert.

And do you have the book by MTW, or did you just copy what I said? If not, please find references of your own. F = q(E+v×B)⇄ ∑_ic_i 08:38, 6 July 2012 (UTC)

F=q(E+v^B): Yes I do have that book, which is why I added the ref and everything.

Quondum: I already did verify it by adding the reference.

See for yourselves in the edit history [8]. Hublolly (talk) 16:01, 6 July 2012 (UTC)

I meant a reference for the "Common choices" statement, in particular the scalar of p!, of which I'm not entirely confident. The reference you've given for the stated expression is fine (or so I assume: I don't have this book). I've seen a factor of 2 for a wedge of two vectors, that's all. — Quondum^☏ 20:08, 6 July 2012 (UTC)

In MTW, T.Frankel, and the WP article on the exterior product - there is no p! factor. For now the {{fact}} template has been added to that convention. F = q(E+v×B)⇄ ∑_ic_i 22:01, 6 July 2012 (UTC)

I don't have "prejudice to other peoples interest". I didn't cause Quondum dispair or insult did I?? If not I'm SO ssorry. Please forgive and forget. I don't have time right now but ... Hublolly (talk) 23:02, 6 July 2012 (UTC)

Tensor product ∧ inner products ∧ multilinear functions collected

Maybe we could add more on the tensor product of vectors and 1-forms as multilinear functions of other vectors and 1-forms equal to the product of inner products. E.g.

$(\mathbf {a} \otimes {\boldsymbol {\sigma }})({\boldsymbol {\lambda }},\mathbf {b} )=\langle {\boldsymbol {\sigma }},\mathbf {a} \rangle \langle \mathbf {b} ,{\boldsymbol {\lambda }}\rangle$

again in MTW - p.76. Hublolly (talk) 16:39, 6 July 2012 (UTC)

In a word - no. As said before, this is just adding extra inessential add-ons to the article, even if you write this in index notation. By the way this is wrong - the correct expression is:

$(\mathbf {a} \otimes {\boldsymbol {\sigma }})({\boldsymbol {\lambda }},\mathbf {b} )=\langle {\boldsymbol {\lambda }},\mathbf {a} \rangle \langle {\boldsymbol {\sigma }},\mathbf {b} \rangle$

and if you are going to use the book - use their notation (and preferably the extended version):

$(\mathbf {u} \otimes \mathbf {v} \otimes {\boldsymbol {\beta }}\otimes \mathbf {w} )({\boldsymbol {\sigma }},{\boldsymbol {\lambda }},\mathbf {n} ,{\boldsymbol {\xi }})=\langle {\boldsymbol {\sigma }},\mathbf {u} \rangle \langle {\boldsymbol {\lambda }},\mathbf {v} \rangle \langle {\boldsymbol {\beta }},\mathbf {n} \rangle \langle {\boldsymbol {\xi }},\mathbf {w} \rangle \,,\quad \mathbf {S} =\mathbf {u} \otimes \mathbf {v} \otimes {\boldsymbol {\beta }}\otimes \mathbf {w}$

else using our notation with letters closer to the beggining of the Latin/Greek alphabets in a more logical and easier-to-follow order (in an extended form just like you did with your added exterior products):

$(\mathbf {a} \otimes {\boldsymbol {\beta }}\otimes \mathbf {c} \cdots )({\boldsymbol {\alpha }},\mathbf {b} ,{\boldsymbol {\gamma }})=\langle {\boldsymbol {\alpha }},\mathbf {a} \rangle \langle {\boldsymbol {\beta }},\mathbf {b} \rangle \langle {\boldsymbol {\gamma }},\mathbf {c} \rangle \cdots \,,\quad \mathbf {T} =\mathbf {a} \otimes {\boldsymbol {\beta }}\otimes \mathbf {c} \cdots$

But I don't see the point for inclusion...

F = q(E+v×B)⇄ ∑_ic_i 22:01, 6 July 2012 (UTC)

As expected of you - its just always *no-no-no-no-no---* to absoluley everything I propose.. What is the point in welcoming users and then immediatley telling them NOT to edit an article they would like to IMPROVE? :-( YOu are the one that appears to have the most prejudice against what others would like to add in the article. :-(

Let me add it, and you'll see a difference. It shows nicley the "transferable" nature of components and indices (i.e. the components are just numbers and as they are multiplied together they are commutative and can be re-ordered, also the indices can be changed and re-labelled without changing meaning as it already says earlier in the article), and since a certain group of components multiplied together equals another tensor, it is consistent with recent rewrite [9] on tensors as multilinear functions.

This little addition would wrap a number of things up. :-) Hublolly (talk) 23:00, 6 July 2012 (UTC)

To some extent you have provided some level of motivation - so there is no favour to not include. Hublolly (talk) 23:14, 6 July 2012 (UTC)

No that is not what I meant. Read the post fully. F = q(E+v×B)⇄ ∑_ic_i 23:39, 6 July 2012 (UTC)

I'm going to ask once more - no more adding inessential content. If you keep this up we'll both get into trouble due to an edit war. F = q(E+v×B)⇄ ∑_ic_i 01:15, 7 July 2012 (UTC)

F=q(E+v^B) does have a point. The article is on the longish side already. So we need to avoid putting in unnecessary things and take out those which are already there. JRSpriggs (talk) 01:49, 7 July 2012 (UTC)

No he doesn't have a driven reason - he just keeps using some shitty excuse for not including anything I have to propose, and linking to irrelevent pages. Every edit I make he objects or reverts - here is his most recent edit summary:

"REVERT: "demonstrating that in addition to efficiency the notation is self-consistent."... what the hell?? so what?? of course it has to be "self-consistent", this is ALREADY illustrated throughout the article...) "

which is insulting. :-(

The added material doesn't add that much to article either, F=q(E+v^B) pointed out himself that its a neat way of applying the notation to related background [10][11] (worse still in this link - claims he DOESN'T understand tensor calculus AND is STILL editing such articles time to time, with obsession). That is my motivation for what I feel may extend the scope, instead of restricting it.

And JRSpriggs - what bits would YOU remove that were already there before I "sprang into action"?? I'm curious... the most recent edits are F=q(E+v^B)'s, yours, Quondum's and Mgvongoeden's. You latter three have added the Lie derivative - thats a pretty deep, specialized and complicated thing which should be kept in its main article, MORE complicated than the wedge and inner products: is that so relevent that it CAN'T BE DELETED???. :-( Hublolly (talk) 23:20, 7 July 2012 (UTC)

F=q(E+v^B) (directly): The reason you did not see the content may have been an interface error on my computer. I produced and inserted the contents, clicked save, it was there. Then you reverted. Then I re-reverted. Only the heading appeared, and I couldn't recover the contents back from the edit history becuase it was not there - only the title heading, so had to start from scratch with a dodgily functioning computer and internet at the time, and it eventually suceeded its way into the article before you reverted yet again. Now it IS in the article, and I politley ask you not to remove other people's contributions - you (really NOone) owns the article (how do YOU like being pointed to rules pages for once?!?). Basically a gltich may have happened. Hope that clarifies your beloved reversions... :-( Hublolly (talk) 23:31, 7 July 2012 (UTC)

ANd my "mistaken" expression above was a TYPO. I meant what you immediatley wrote after. Hublolly (talk) 23:38, 7 July 2012 (UTC)

Scope of article and content

I echo what JRSpriggs says under the above section. Additionally, I'd suggest that this and several previous discussions and disagreements between editors of this article (myself included) are, at root, really about the scope of this article. Reading the lead and the introduction, it is clear that it is really about the the tensor index notation of the Ricci calculus and its interpretation, and not about the full scope of the abstract tensors and their operations, which is covered in several related articles such as Tensor. As soon as we introduce component-free expressions such as those above, it seems we have strayed beyond the scope as per my interpretation here. Disagreement/rephrasing with my interpretation of this scope should be dealt with first (including wording of the lede and intro), rather than via specific inclusions/exclusions without agreement on the scope. — Quondum^☏ 08:20, 7 July 2012 (UTC)

To be honest the scope and content of the article was fine before Hublolly sprang into action. =( As I've already tried telling him (and said elsewhere) - this article should just concentrate on all the rules and notations of tensor index manipulation, and not cover the full theory of tensors (obviously the subject of several other articles)...

So I'm with Quondum's and JRSpriggs's opinions. F = q(E+v×B)⇄ ∑_ic_i 21:26, 7 July 2012 (UTC)

Its true I extended the section Components of tensors a bit too much... I don't mind if anyone trims/transforms/annihilates my edits from that section. =) The main intent was to show the notational correspondence between

• the function notation (tensors as functions of vectors /1-forms) and
• how the tensor components contract with the components of the vectors/1-forms,
• in doing so the calculation of tensor components (since the whole article uses components of tensors)

to tie it into other background context, but none of that is really essential either. Clearly it was a mistake which now encourages adding more off-topic stuff... =( F = q(E+v×B)⇄ ∑_ic_i 21:47, 7 July 2012 (UTC)

I have never seen the multi-indices used before, so I would be happy to see that part reduced or removed. And the stuff about basis vectors is not really necessary to someone using the Ricci calculus exclusively; it is only helpful when going back and forth between that and the abstract notation. So it should be reduced. JRSpriggs (talk) 23:46, 7 July 2012 (UTC)

Agree with reducing both sections - disagree with completley eliminating multi-index notation. It is certianly used throughout T.Frankel but I don't know how universal it actually is in the remaining literature (MTW never uses it, nor do any other books I have...). I'll have a go at rewriting anyway. =) F = q(E+v×B)⇄ ∑_ic_i 00:02, 8 July 2012 (UTC)

I deleted:

• my own insertions of the Covariant and Lie derivatives using multi-indices, there is not that much advantage since the formulae are still not very simple to use and the reader has to interpret the multi-indices missing out an index which may be confusing... (I see what you meant...)
• all the stuff I wrote on the tensor product, components, functions etc.
• and Hublolly's wedge product,
• the non-tensorial objects: Christoffel symbols (second kind) and commutator coefficients,
• the passive mention on the basis since nothing else in the article depends on them (only the now-deleted contents).

As you say - even mentioning is irrelevant to the article.

All that remains is the Notable tensors section. Is the new (older?) form of the article better now? =| F = q(E+v×B)⇄ ∑_ic_i 00:33, 8 July 2012 (UTC)

I am happy with it. Others will have to speak for themselves. JRSpriggs (talk) 02:27, 8 July 2012 (UTC)

As it stands at the moment, I think the article is looking very neat. With regard to components of tensors, a link to Tensor (intrinsic definition)#Basis or similar should be sufficient. — Quondum^☏ 07:14, 8 July 2012 (UTC)

Thanks to both for positive feedback. =) I was actually going to keep but shorten some of the tensor components/functions etc and add the same links Tensor (intrinsic definition)#Basis and Tensor #definition, but no point: obviously the article links to tensor and has the tensor template at the beginning, and that's all deleted anyway. F = q(E+v×B)⇄ ∑_ic_i 21:00, 8 July 2012 (UTC)

Great move - not only have you pulled out my stuff, but others also (even your own). Now the article has less content. :-( I will at least be making minor edits to re-organize stuff better which I hope will not be reverted. Hublolly (talk) 07:30, 9 July 2012 (UTC)

"this formula is wrong and the section was better before"

F=q(E+v^B): Could you explain how the hell this

"The metric tensor gives the length of any geodesic curve: $\Delta \lambda =\int _{\lambda _{1}}^{\lambda _{2}}{\sqrt {g_{\alpha \beta }{\frac {dx^{\alpha }}{d\lambda }}{\frac {dx^{\beta }}{d\lambda }}}}\,d\lambda$ where $\lambda$ parameterizes the path."

was wrong??? For one thing - JRSpriggs biased the section to GR. Thats fine as far as it goes, but it would be better to give it more generally (this is a maths article nit physics). Opinions?? Hublolly (talk) 08:57, 9 July 2012 (UTC)

The $\Delta \lambda$ is supposed to be the change in arc length (not change in parameter $\lambda$), since the equation calculates the length of a parameterized curve. That’s the geometric concept of the metric - it gives the notion of geometric length to space, in a completely general way which works for flat and curved spacetime alike. Also JRSprigg's version wasn't "biased to GR" since it only refers to spacetime, which is also referred to in the article. F = q(E+v×B)⇄ ∑_ic_i 09:13, 9 July 2012 (UTC)

Good to see you have finally made some constructive edits to the article, and are now more focussed on improvement than before by actually engaging in topic conversation here on the talk page. See that I didn't revert every single most other edits you made? F = q(E+v×B)⇄ ∑_ic_i 09:18, 9 July 2012 (UTC)

Ok - I removed yet another of your insertions on the tensor/exterior products since this is redundant within the scope of the section - the statement you give is by definition of these products, and that belongs to the main articles. Good work on the explanation of the tensor/scalar equation(s) though. F = q(E+v×B)⇄ ∑_ic_i 09:34, 9 July 2012 (UTC)

WTFH NO it isn't. A reader can get confused that index positions can be freely moved around like so:

$A^{i}{}_{jk}\leftrightarrow A_{j}{}^{i}{}_{k}\leftrightarrow A_{k}{}^{i}{}_{j}\leftrightarrow A_{kj}{}^{i}....$

becuase they may think the tensor product is commutative. My explaination of the tensor product fixes that by explicitly stating you CANNOT move the indices aroud - the ordering of the indices is the same as the order of the tensor multiplications. Hublolly (talk) 09:44, 9 July 2012 (UTC)

"non-linear functions"...

Hublolly: This is clearly off topic for the article - but since you insist I'll say what I like (and have to). You wrote "non-linear functions do not form a vector space". Well, vector spaces are certainly relevant to the article (indices based on their dimensionality), but how do the polynomials 1 = x⁰, x = x¹, x², x³... or trigonometric functions: 1, sin(nx), cos(nx) NOT form a vector space? These are basis elements and a linear combination of them gives a function (Taylor series? Fourier series?), even if it’s an infinite number of basis elements to give an infinite-dimensional polynomial space. Even as an example/analogy to an already insignificant section, it is useless for understanding anything because it is wrong (never mind the article itself).

ONCE MORE: Please STOP inserting irrelevant things. F = q(E+v×B)⇄ ∑_ic_i 10:43, 9 July 2012 (UTC)

You clearly do NOT even understand what "linear" means.

Here is a linear function:

$f(x)=ax+b$

which uses addition and scalar multiplication, hence it is linear and values of a and b form a vector space.

Here is the simplist non-linear function:

$f(x)=ax^{2}$

Do you know what the operation of squaring does to a number? It multiplies the number with itself twice, so a sum under a sqaure is not linear:

$f(x+y)=a(x+y)^{2}=a(x+y)(x+y)=a(x^{2}+2xy+y^{2})$

and you're telling me you did not even know that?? ?? So "how the hell" can non-linear functions form a vector space? Could you explain that one?? Please take time to review basic algebra and to read what you are doing before editing (If you can't even do basic elementary algebra, I am sincerley not suprised that you foul up so many articles with mathematical typos).Hublolly (talk) 11:06, 9 July 2012 (UTC)

You said the basis stuff is also irrelevent. So I have no idea what you mean by saying "polynomials/trig functions are basis" when they are functions anyway. Why would you introduce a basis when you can just treat the sum of the 1,x,x² ... 1, sin, cos, all as one function?? Hublolly (talk) 11:15, 9 July 2012 (UTC)

Isn't this simply a misunderstandig? There are vector spaces where (nonlinear) functions are the points, so nonlinear functions do indeed form vector spaces. However, the only morphisms of vector spaces are linear functions. So linear functions transform vector spaces.--LutzL (talk) 11:47, 9 July 2012 (UTC)

Non-linear functions as "points" - huh???? Hublolly (talk) 12:02, 9 July 2012 (UTC)

Hublolly - it might help if you actually make the effort and take the time to READ what we have to tell you instead of editing other peoples comments like you did in these: [12] (your summary: "fixes") [13] (your next summary: "more fixes"). I restored my original post during this edit [14].

I already explained everything you are asking, LutzL has now explained further details I didn't, and you still don't get it? So much for calling yourself a "progidy". When we said "basis stuff is irrelevent" - that referred to including basis vectors, basis 1-forms, and basis tensors with all the stuff that used to be there (now deleted), not to the general out-of-topic description in this section.

Thank you for your additional reply, LutzL. =) F = q(E+v×B)⇄ ∑_ic_i 12:14, 9 July 2012 (UTC)

It seems both of you are confusing the nature of functions and splattering over it with irrevent crap. Why all this talk about basis functions when you say its irrelevent yourself and when the Taylor and Fourier series are only alternative representations of a function???

The aviodance of talking about basis functions in the analogy was exactly for no obfuscation, that the contraction between the tensor and the inserted vectors and/or 1-forms IS the operation, when we think of tensors as multilinear functions. The parallel with analysis is that analytic functions have algebraic operations like +, −, ×, ÷, √ etc. Do you get the point of that little "insignificant section":

Products Whenever the tensor product (and hence exterior product) is taken between vectors and/or covectors - the order of the vectors and/or covectors in the product correspond to the order of the indices in the final tensor, regardless of the raised/lowered index position. For vectors a, b, c and covectors α, β, γ, the tensor product of them to get a tensor T is: ${\boldsymbol {\mathsf {T}}}=\mathbf {a} \otimes {\boldsymbol {\alpha }}\otimes \mathbf {b} \otimes {\boldsymbol {\beta }}\otimes {\boldsymbol {\gamma }}\otimes \mathbf {c} \cdots \ \Rightarrow \ T^{i}{}_{j}{}^{k}{}_{\ell }{}_{m}{}^{n\cdots }=a^{i}\alpha _{j}b^{k}\beta _{\ell }\gamma _{m}c^{n}\cdots$ Notational correspondance between multilinear functions and tensor contraction One aspect of tensors is that they are multilinear functions. The function notation for the tensor: ${\boldsymbol {\mathsf {H}}}(\mathbf {a} ,{\boldsymbol {\alpha }},\mathbf {b} ,{\boldsymbol {\beta }},{\boldsymbol {\gamma }},\mathbf {c} \cdots )=H_{i}{}^{j}{}_{k}{}^{\ell }{}^{m}{}_{n\cdots }a^{i}\alpha _{j}b^{k}\beta _{\ell }\gamma _{m}c^{n\cdots }\cdots$ can be identified with the operation of contraction with vectors a, b, c... and 1-forms α, β, γ... (just like in analysis, linear functions may be identified with scalar multiplication and addition: $f(x)=ax+b$ or for polynomials and other functions which are non-linear, say $f(x)=\sum _{n=0}^{N}a_{n}x^{n}$ although non-linear functions do not form vector spaces).

now? (actually there is not just ONE, but TWO - again learn some algebra please). Hublolly (talk) 12:55, 9 July 2012 (UTC)

The correct contents of your insertion, the connection between index-based and index-less tensor calculus, is already present in the tensor and tensor product pages. This page is solely concerned with an overview of the different constructions in the index notation. And of course do polynomials form a vector space over the corresponding coordinate field. Addition and scalar multiplication are well defined.--LutzL (talk) 15:05, 9 July 2012 (UTC)

Is there anything wrong with overlap and reproducing it in this article? Of course there will be overlap, that ISN'T a reason for removing it. Hublolly (talk) 18:24, 9 July 2012 (UTC)

YOu say the "connection between connection between index-based and index-less tensor calculus" - is that NOT the Ricci calculus?? According to the article lead - it says "manipulation of tensor indices", one might call this "index gymnastics", so Ricci calculus must essentially comprise expressions of tensor indices, which WAS (and IS) my inclusion. Hublolly (talk) 18:29, 9 July 2012 (UTC)

STOP.EDITING.MY.POSTS. [15]F = q(E+v×B)⇄ ∑_ic_i 20:59, 9 July 2012 (UTC)