Measuring Coordinates in Strong Gravity: Schwarzschild Metric

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Discussion Overview

The discussion centers around the measurement of coordinates in the context of the Schwarzschild metric, particularly in regions of strong gravitational influence, such as near the event horizon. Participants explore the implications of curvature on the interpretation of the radial coordinate and the validity of certain definitions and measurements in these extreme conditions.

Discussion Character

  • Conceptual clarification
  • Debate/contested
  • Mathematical reasoning

Main Points Raised

  • One participant notes that the Schwarzschild metric describes an asymptotically flat spacetime, suggesting that the ##r## coordinate can be interpreted as a distance from the center when far from the event horizon, but questions arise about its meaning close to the event horizon.
  • Another participant emphasizes that coordinates are defined rather than measured, proposing that measurements can be related to defined coordinates through specific relationships.
  • A participant proposes a method for defining and measuring the ##r## coordinate as the integral of proper length along a specific curve, questioning the validity of this approach inside the event horizon.
  • A later reply affirms the proposed method, indicating that it is equivalent to measuring area in the context of spherical symmetry, but does not clarify the implications of this inside the event horizon.

Areas of Agreement / Disagreement

Participants express differing views on the nature of coordinate measurement in strong gravity, with some agreeing on the equivalence of certain definitions while others raise questions about their validity in extreme conditions. The discussion remains unresolved regarding the implications of these measurements inside the event horizon.

Contextual Notes

There are limitations regarding the assumptions made about the relationship between measurements and coordinates, particularly in the context of strong gravitational fields and the behavior of the Schwarzschild metric near the event horizon.

victorvmotti
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We know that Schwarzschild metric describes an asymptotically flat spacetime. This means that far away from the event horizon we can safely interpret the ##r## coordinate as distance from the center.

But when close enough to the event horizon the curvature becomes significant and our common sense of ##r## breaks.

The question is that what is understood as measurement of the coordinates near very strong gravity?
 
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Coordinates are defined, not measured. You can of course define them such that they have some defined relationship to measurements. In that case you perform the measurements, apply the defined relationship and obtain the coordinate.
 
Is it reasonable to define and "measure" the ##r## coordinate like this:

##r## is in the Schwarzschild metric the integral of the proper length along the curve $$dt = dr = d\theta = 0$$ for ##\phi## from ##0## to ##2\pi## divided by ##2\pi##?

Is this definition and measurement valid inside the event horizon as well?
 
Yes. Although usually it is stated in terms of measuring the area, but with spherical symmetry what you wrote is equivalent
 
Last edited:

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