Curvature of Space-Time: Why is Covariant Derivative Nonzero?

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Discussion Overview

The discussion revolves around the nonzero commutator of the covariant derivative of vectors in the context of curved space-time, particularly illustrated through examples on the surface of the Earth. Participants explore the implications of curvature on vector movements and their non-commutative nature.

Discussion Character

  • Exploratory
  • Conceptual clarification
  • Debate/contested

Main Points Raised

  • One participant expresses confusion about why the commutator of the covariant derivative is nonzero in curved space, contrasting it with flat space where it is zero.
  • Another participant uses the example of moving on the curved surface of the Earth to illustrate that movements in different directions do not commute.
  • Some participants attempt to visualize the concept using practical examples, such as walking from the North Pole to the equator and then west, noting the directional changes involved.
  • There is a discussion about how the curvature affects the outcome of movements, with one participant suggesting that starting positions can influence the results of the movements.
  • Several participants express difficulty in visualizing the concept and seek further clarification, indicating that the examples provided may not be universally clear.

Areas of Agreement / Disagreement

Participants generally agree on the non-commutative nature of movements on a curved surface, but there are differing interpretations and examples provided, leading to some confusion and lack of consensus on visualization and understanding.

Contextual Notes

Some participants mention specific starting points and distances that may affect the outcomes of the movements discussed, indicating that assumptions about position and scale are relevant to the examples provided.

TimeRip496
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I recently watched Susskind video on general relativity. I am unsure why the commutator of the covariant derivative of the vectors is nonzero when there is curvature. E.g. DrDsVm-DsDrVm
In flat space, that difference is zero. But why is it non zero in curved space? Someone please enlightened me!

Sorry for being so vague as I don't really know how to express this in words and how to use this forum.
 
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On the curved surface of the earth, moving one kilometer east and then one kilometer south does not take you to the same place as moving one kilometer south and one kilometer east - that is, movements in different directions do not commute.
 
Nugatory said:
On the curved surface of the earth, moving one kilometer east and then one kilometer south does not take you to the same place as moving one kilometer south and one kilometer east - that is, movements in different directions do not commute.
How? I find that difficult to visualise and I even try drawing it on my tennis ball but they do end up at the same place.
 
TimeRip496 said:
How? I find that difficult to visualise and I even try drawing it on my tennis ball but they do end up at the same place.

Suppose you are standing on the North Pole, facing south along the line of 0 degrees longitude. Your right hand is straight out from your side, pointing south along the line of 90 degrees west longitude. Now walk south along 0 degrees longitude to the equator, keeping your arm pointing in the same direction. At this point, you are at the equator, and your right hand is pointing west. Now walk west along the equator, keeping your right hand pointing in the same direction, until you reach the line of 90 degrees west longitude. Now walk north along this line of longitude until you get back to the North Pole. At this point, your right hand will be pointing south along the line of 180 degrees west longitude.
 
TimeRip496 said:
How? I find that difficult to visualise and I even try drawing it on my tennis ball but they do end up at the same place.

If you are standing at the equator and walk one kilometer east, your longitude changes by a certain amount. If you start at a point one kilometer south of the equator, that same one kilometer east will change your longitude by a larger amount, so the south-then-east walk will leave you farther to the east than the east-then-south walk.

If you still don't see it, try it with a larger distance - the effect will be very clear at, say, one-eighth the circumference.
 
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Nugatory said:
On the curved surface of the earth, moving one kilometer east and then one kilometer south does not take you to the same place as moving one kilometer south and one kilometer east
Unless you start 1/2km north of the equator. Maybe that's what the OP draws on his tennis ball.
 
Nugatory said:
If you are standing at the equator and walk one kilometer east, your longitude changes by a certain amount. If you start at a point one kilometer south of the equator, that same one kilometer east will change your longitude by a larger amount, so the south-then-east walk will leave you farther to the east than the east-then-south walk.

If you still don't see it, try it with a larger distance - the effect will be very clear at, say, one-eighth the circumference.
Thanks a lot!
 
A.T. said:
Unless you start 1/2km north of the equator.

Good point. :)
 

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