Understanding Geodesic Deviation: Solving for Equations (7) to (8)

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Discussion Overview

The discussion centers around the transition from Equation (7) to Equation (8) in a document related to geodesic deviation, focusing on the mathematical steps involved in this derivation. Participants explore the implications of certain terms in the equations and how they affect the calculations, with an emphasis on the behavior of small quantities.

Discussion Character

  • Technical explanation
  • Mathematical reasoning
  • Homework-related

Main Points Raised

  • One participant requests clarification on the transition from Eq. (7) to Eq. (8) and notes encountering additional terms during their calculations.
  • Another participant suggests that the calculations are straightforward and asks for specifics on where the first participant got stuck and what extra terms they encountered.
  • It is mentioned that since x is a geodesic, there is a geodesic equation in x that vanishes, and that terms involving higher powers of a small quantity χ can be neglected.
  • A participant identifies that not only the term involving χχ but also the product of χ and a derivative of χ should be dropped, which they initially overlooked.
  • One participant expresses gratitude for the clarification and confirms their understanding of the necessary approximations in the derivation.
  • Another participant reflects on how the detail of dropping the term with χdχdT had previously caused confusion but now seems clear, and they seek further validation of their understanding by referencing another source.
  • A participant notes that the equation of geodesic deviation indicates that tidal forces are dependent on velocity.

Areas of Agreement / Disagreement

Participants generally agree on the need to drop certain terms in the derivation, but there is no consensus on the specific challenges faced by the initial poster. The discussion includes multiple perspectives on the calculations and interpretations of the equations involved.

Contextual Notes

Participants express uncertainty regarding the handling of small quantities and the implications of dropping specific terms, indicating that the derivation may depend on careful attention to these details.

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Originally posted by Arcon
Can someone take a look at

http://wps.aw.com/wps/media/objects/500/512494/supplements/Ch21.pdf

and tell me how they go from Eq. (7) to Eq. (8)? I've tried this and keep getting additional terms.

i took a look at it, and did the calculation. i thought it was pretty straightforward. where did you get stuck? what extra terms do you have?

remember that x is a geodesic. so there is a geodesic equation in x, and it therefore vanishes. and remember that χ is very small; drop any term with more than one χ in it.
 


Originally posted by lethe
i took a look at it, and did the calculation. i thought it was pretty straightforward. where did you get stuck? what extra terms do you have?

remember that x is a geodesic. so there is a geodesic equation in x, and it therefore vanishes. and remember that χ is very small; drop any term with more than one χ in it.

I fingered it out :smile:

One has to drop not only the term &chi*&chi but the term which is the product of &chi and a derivative of &chi. That was what I was missing.
 


Thank you

I believe that I've fingered it out :smile:

One has to drop not only the term χ*χ but the term which is the product of χ and a derivative of χ. That was what I was missing.

Again - thanks for the response

Arcon
 


Originally posted by lethe
i took a look at it, and did the calculation. i thought it was pretty straightforward. where did you get stuck? what extra terms do you have?

remember that x is a geodesic. so there is a geodesic equation in x, and it therefore vanishes. and remember that χ is very small; drop any term with more than one χ in it.

Seems that this small detail (drop term with χdχdT) has always tripped me up in that derivation. I guess I was just blind to it. But now that I know it then the derivation is simple.

Just to make sure I understood the approximation can you check this for me?

http://www.geocities.com/physics_world/gr/geodesic_deviation.htm

I commented on the terms to drop right after Eq. (14) and right after Eq. (15)

Thanks

I don't know how I missed this before but the equation of geodesic deviation clearly shows that tidal forces are velocity dependent!

Arcon
 

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