Kruskal Diagram Hyperbola Function

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Discussion Overview

The discussion revolves around the properties and mathematical representation of hyperbolas in the Kruskal diagram, specifically how to express these hyperbolas as functions relating spacetime and the metric tensor. The scope includes theoretical exploration and mathematical reasoning.

Discussion Character

  • Exploratory, Technical explanation, Mathematical reasoning

Main Points Raised

  • Philipp inquires about creating a function from the hyperbola in the Kruskal diagram that relates spacetime and the metric tensor.
  • Philipp seeks clarification on the specific appearance of the hyperbola referenced in a Wikipedia article.
  • Another participant explains that the hyperbola represents points with a constant "r" value and notes the properties of the Kruskal diagram, including that lightlike geodesics are straight lines.
  • A further response suggests that the equation for the hyperbola can be derived from the definitions provided in the Wikipedia article, indicating specific equations for parametrization.
  • Philipp expresses gratitude and indicates a better understanding after the responses.

Areas of Agreement / Disagreement

The discussion does not reach a consensus on the exact function form but shows participants engaging with the mathematical properties of the hyperbola and its representation in the Kruskal diagram.

Contextual Notes

Participants reference the Wikipedia article for definitions and equations, indicating a reliance on external sources for clarity on the topic. There may be assumptions regarding familiarity with the Kruskal diagram and its properties that are not explicitly stated.

philipp2020
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hi

If I understand it correctly, the hyperbolas in the kruskal diagram define locations with the same space time.

Now my question is, how can I make a function out of the hyperbola solved for spacetime on one side and gravity/metric tensor on the other side?

Thank you very much for an answer

Philipp
 
Last edited:
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I'm still not sure what you are asking, even after the clarification.

The significance of the hyperbola is that they are a plot of all points with a constant "r" value on the Kruskal diagram.

The Kruskal diagram is a set of coordinates with some useful properties - one particularly useful set of properties is that all lightlike geodesics will be straight lines on the diagram.

The particulars of the conversion from Schwarzschild coordinates to Kruskal coordinates was given in the wiki.

The metric tensor in Kruskal coordinates was also given in the wiki.
 
You want the equation of the hyperbola r = const in terms of Kruskal coordinates? Isn't it just what's given in the Definition section of your Wikipedia page, setting r = constant. Parametrically in the first two equations, V = ... and U = ..., or in the fifth equation, V2 - U2 = ...
 
yes thanks very much for the answer. I think I understand it now.
 

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