Still learning about tensors

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Discussion Overview

The discussion revolves around the covariant derivative of the metric tensor \( g_{\alpha\beta} \) in the context of general relativity. Participants seek to understand why this derivative is always zero, exploring both mathematical and physical reasoning, as well as the implications of different connections on manifolds.

Discussion Character

  • Exploratory
  • Technical explanation
  • Debate/contested
  • Mathematical reasoning

Main Points Raised

  • Some participants propose that the covariant derivative of \( g_{\alpha\beta} \) is zero due to the torsion-free condition applied to the affine connection, which preserves the lengths of vectors under parallel transport.
  • Others argue that while it is defined to be zero in general relativity for convenience, it does not have to be zero to define the geometry, and different connections could yield different results.
  • A participant expresses uncertainty about the physical reasoning behind the zero derivative and seeks clarification on terminology and notation related to covariant vectors.
  • Another participant provides a mathematical proof showing that the covariant derivative of the metric tensor vanishes when using the Levi-Civita connection, while noting that using a different affine connection without the torsion-free condition could lead to different outcomes.
  • Some participants request examples and further clarification on the mathematical steps involved in the proof, indicating a gap in understanding standard tensor notation.
  • One participant emphasizes the need for a physical interpretation of why the covariant derivative of the metric is zero, suggesting that the smoothness of spacetime may imply local flatness.

Areas of Agreement / Disagreement

Participants express a mix of agreement and disagreement regarding the reasons behind the zero covariant derivative. While some accept the mathematical proof provided, others remain uncertain and seek further clarification on both the mathematical and physical aspects.

Contextual Notes

Participants note limitations in their understanding of tensor notation and the implications of different connections on the behavior of the metric tensor. The discussion reflects varying levels of familiarity with the concepts involved.

  • #31
WannabeNewton said:
In any case, doing it on S^{2} is probably a good way of getting used to it.

You may have noticed that I have posted and deleted twice now. (and now I have edited again, for emphasis) I couldn't seem to get the notation right. But my main point was that if we define a parametric path on the surface of a sphere, (\theta(\lambda),\phi(\lambda))

and you are asked to calculate the length of a differential section of that path, \left \| \left (\frac{\mathrm{d \theta} }{\mathrm{d} \lambda},\frac{\mathrm{d} \phi }{\mathrm{d} \lambda} \right )| \right \| =\sqrt {r^2 \frac{d \theta}{d\lambda}+ r^2 \sin^2(\theta)\frac{d \phi}{d\lambda}}
you need to take into account the unit vectors; (i.e. the scale factors) how long a differential change in theta or phi actually is in the three-dimensional space.

However, if you are just asked for the differential of that path

\frac{\mathrm{d} \vec s }{\mathrm{d} \lambda}=\left (\frac{\mathrm{d \theta} }{\mathrm{d} \lambda},\frac{\mathrm{d} \phi }{\mathrm{d} \lambda} \right )

you don't need to take into account how long the unit vectors are in the theta direction or phi direction, because the coordinates of the path already take that into account.

In particular, \frac{\mathrm{d} \vec s}{\mathrm{d} r},\frac{\mathrm{d} \vec s}{\mathrm{d} \theta}, \, \mathrm{and}\, \frac{\mathrm{d} \vec s}{\mathrm{d} \phi}
should be identical to \frac{\partial \vec s}{\partial r},\frac{\partial \vec s}{\partial \theta}, \, \mathrm{and}\, \frac{\partial \vec s}{\partial \phi}.

If there is no flaw in my reasoning, this seems to directly conflict with:
d _{\alpha }\mathbf{\overrightarrow{V}} = \left (\partial _{\alpha }V^{\beta } \right )\overrightarrow{e_{\beta }} + V^{\beta }\left (\partial _{\alpha }\overrightarrow{e_{\beta }} \right )

This equation also appears in various forms as equations 1a, 1b, 2a, 2b, here: http://www.mathpages.com/rr/appendix/appendix.htm
surely there must be a simply explained flaw in either my reasoning, or my understanding of what is meant.
 
Last edited:

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