Coordinate and proper time, null geodesic

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Discussion Overview

The discussion revolves around the relationship between coordinate time and proper time for null geodesics in the context of general relativity. Participants explore the implications of a given question regarding the time taken for a null geodesic to traverse from a radial coordinate greater than one to infinity, while addressing the definitions and properties of proper time and affine parameters.

Discussion Character

  • Debate/contested
  • Technical explanation
  • Conceptual clarification

Main Points Raised

  • Some participants assert that there is no affine parameter for a null geodesic, while others argue that it is possible to find an affine parameter for any curve, but not to use arc length as the affine parameter for a null geodesic.
  • There is contention over the definition of proper time, with some stating it is only applicable to timelike observers, while others clarify that proper time can be defined for any trajectory, including null paths, but is zero along null paths.
  • Participants discuss the implications of the question posed, particularly regarding the interpretation of proper time and whether it refers to a timelike observer or a non-geodesic observer.
  • Concerns are raised about the clarity of the original question, with participants questioning the context, the metric in use, and the significance of the radial coordinate mentioned.
  • Some participants express confusion over the question's formulation, indicating that it lacks sufficient detail to provide a meaningful response.

Areas of Agreement / Disagreement

Participants do not reach a consensus on several points, particularly regarding the definitions of proper time and affine parameters for null geodesics. Multiple competing views remain on the interpretation of the original question and the assumptions that should be made.

Contextual Notes

There are limitations in the discussion regarding the assumptions made about the observer's trajectory and the specific metric being referenced. The question's ambiguity regarding the context and the observer's characteristics contributes to the uncertainty in responses.

  • #31
binbagsss said:
Does anyone really post on a forum expecting a yes or no answer ? No you expect discussion? Or what the hell are you going to learn ?

Well, if you posted to generate discussion, that's fine. There was quite a bit of discussion about the topic, so you should be happy. But then you complained that nobody answered your question, which makes it sound like you had a specific question you wanted an answer to. Nobody knows what that question is.

You're unsatisfied with the answers you have been given, but nobody knows why, and you can't tell them.
 
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  • #32
stevendaryl said:
Well, if you posted to generate discussion, that's fine. There was quite a bit of discussion about the topic, so you should be happy. But then you complained that nobody answered your question, which makes it sound like you had a specific question you wanted an answer to. Nobody knows what that question is.

You're unsatisfied with the answers you have been given, but nobody knows why, and you can't tell them.

i complained because people went astray and decided to state their general knowledge, not consise with what I had questioned, and then someone complained about me not having the background knowledge of such posts, when I hadnt asked for it !
 
  • #33
binbagsss said:
and then someone complained about me not having the background knowledge of such posts, when I hadnt asked for it !

When a question is asked a back-and-forth discussion almost always follows. Sometimes the discussion goes over my head and I have to just gean what I can from it and try to learn more about it if I'm interested. Education, formal or otherwise, is much more than learning how to answer questions. It involves understanding.
 
  • #34
Mister T said:
When a question is asked a back-and-forth discussion almost always follows. Sometimes the discussion goes over my head and I have to just gean what I can from it and try to learn more about it if I'm interested. Education, formal or otherwise, is much more than learning how to answer questions. It involves understanding.
yes, fair enough if you had pointed towards that, but at no point did ii point towards killing vector fields. what followed was someone merrily tapping away at their keyboard banging on about killing vector fields, and then a post asking ME where is my background knowledge !
 
  • #35
binbagsss said:
i complained because people went astray

There is no way that the discussion can "go astray" if there isn't a specific question to answer. So it seems to me that either you ask a specific question (and then you can complain if nobody answers it) or you can spark a general discussion, and then let it go where it will. There is no basis for complaint in the latter case.
 
  • #36
stevendaryl said:
There is no way that the discussion can "go astray" if there isn't a specific question to answer. So it seems to me that either you ask a specific question (and then you can complain if nobody answers it) or you can spark a general discussion, and then let it go where it will. There is no basis for complaint in the latter case.
erm nah. i don't think the world revolves around me. rather, if you think a discussion may be useful by taking a de-tour, you should mildly talk about the subject, or introduce it, and ask whether that person has the background knowledge for it or not, before ASSUMING they DO NOT, writing an essay about ut and getting your knickers in a twist about the fact that they don't
 
  • #37
binbagsss said:
erm nah. i don't think the world revolves around me.

Your complaints seem to suggest otherwise. You're upset that people didn't spend more time figuring out what you want from this discussion and complying with it.
 
  • #38
stevendaryl said:
Your complaints seem to suggest otherwise. You're upset that people didn't spend more time figuring out what you want from this discussion and complying with it.
u really want me to repeat myself?
no i am not.
the fact that it was complained why don't i have some desired background knowledge, when i asked nothing about it, and it was immediately assumed i did not have it.
i think you've got that, you just don't like loosing arguments.
 
  • #39
binbagsss said:
u really want me to repeat myself?

No. I told you what I want. I either want you to ask a specific question, or else let the discussion go where it will, without complaining.

Nobody has any idea what you want out of this discussion.
 
  • #40
stevendaryl said:
No. I told you what I want. I either want you to ask a specific question, or else let the discussion go where it will, without complaining.

Nobody has any idea what you want out of this discussion.
i believe they do about 6 posts ago, you can't let it go since felt the need to reply to my clarificaiton posts which i was asked for.
 
  • #41
binbagsss said:
The question comes from an exam paper from my university in the UK...
Presumably from a module on general relativity. What was, if there was one, the recommended text(s)?
 
  • #42
binbagsss said:
i believe they do about 6 posts ago, you can't let it go since felt the need to reply to my clarificaiton posts which i was asked for.

Okay. I'm dropping this discussion. Maybe we could take vote: Does anybody know what the question is?
 
Last edited:
  • #43
binbagsss said:
i believe they do about 6 posts ago

Oh, you mean your questions are all resolved now? Good, then we can stop the pointless wrangling about who understands what. Thread closed.
 

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