Killing Equation: Why Set g_μν,ρ V^ρ = 0?

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Discussion Overview

The discussion revolves around the Killing Equation in the context of differential geometry, specifically addressing the term ##g_{\mu \nu, \rho} V^\rho## where ##V## is the Killing Vector. Participants explore the reasoning behind setting this term to zero and the implications of such a decision in the derivation process.

Discussion Character

  • Technical explanation, Debate/contested

Main Points Raised

  • One participant questions the motivation for setting the term ##g_{\mu \nu, \rho} V^\rho## to zero, suggesting that there is no clear justification for demanding the derivative of the metric coefficients to vanish.
  • Another participant asks for more details regarding the derivation and the context in which the term is set to zero.
  • A participant points out that the covariant derivative of the metric is zero, implying that the term might be misinterpreted.
  • One participant shares their derivation, which includes the term ##g_{\sigma \rho, \kappa} V^\kappa##, indicating a more complex relationship than initially presented.
  • There is a discussion about the completeness of the derivation shared by one participant, with another emphasizing the importance of including all relevant steps in the derivation process.
  • A participant acknowledges a mistake in their reasoning related to the relationship between the metric and the Killing Vector, suggesting a misunderstanding in their earlier statements.

Areas of Agreement / Disagreement

Participants express differing views on the necessity and justification for setting the term to zero, indicating that the discussion remains unresolved with multiple competing perspectives on the derivation process.

Contextual Notes

There are indications of missing assumptions and incomplete derivations, particularly regarding the handling of the metric and the Killing Vector, which contribute to the ongoing debate.

kent davidge
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When deriving the Killing Equation, one ends up with a term ##g_{\mu \nu, \rho} V^\rho## where ##V## is the Killing Vector. This terms is often set to zero. Why?
I see no motivation for demanding that the derivative of the metric coefficients vanish.
 
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Can you give more details?
 
martinbn said:
Can you give more details?
What kind of details? I tried to put everything into the opening post.
 
kent davidge said:
What kind of details? I tried to put everything into the opening post.
You did not though. You said ”This term is usually set to zero. Why?” without any reference to a text actually doing so or reciting an actual derivation doing so.
 
Are you sure that they didn't write ##g_{\mu \nu;\rho} V^\rho##? The covariant derivative (indicated by semicolon, rather than comma) of ##g## is zero.
 
stevendaryl said:
Are you sure that they didn't write ##g_{\mu \nu;\rho} V^\rho##?
It's me. I'm deriving it. However I get ##\mathcal L g _{\sigma \rho} = \nabla_\sigma V_\rho + \nabla_\rho V_\sigma + 2 V_\mu \Gamma^\mu{}_{\sigma \rho} + g_{\sigma \rho, \kappa} V^\kappa##.
 
kent davidge said:
It's me. I'm deriving it. However I get ##\mathcal L g _{\sigma \rho} = \nabla_\sigma V_\rho + \nabla_\rho V_\sigma + 2 V_\mu \Gamma^\mu{}_{\sigma \rho} + g_{\sigma \rho, \kappa} V^\kappa##.
So why are you withholding the rest of your derivation?
 
Orodruin said:
So why are you withholding the rest of your derivation?
because I was not wanting to bore you with it. but never mind, i found my fault. i was equating ##g_{\mu \nu} V^\nu{}_{, \ \rho} = V_{\mu, \ \rho}## right away.
 
kent davidge said:
because I was not wanting to bore you with it

As you see, this is not a good strategy. When you ask a question about something you did, you need to include all of what you did. You can't assume that just the part you think you need to include is relevant, because if you already knew what was relevant and what was not, you wouldn't have made a mistake and you wouldn't have needed to post a question in the first place. Please keep that in mind for the future.

Since you say you have found your mistake, this thread is closed.
 
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