Is the Speed of Light Truly Constant in General Relativity?

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Discussion Overview

The discussion centers around the constancy of the speed of light in the context of General Relativity (GR) and its relationship to Special Relativity (SR). Participants explore the implications of gravitational effects on light, the definitions of speed and velocity, and the conditions under which light's speed may appear to vary.

Discussion Character

  • Debate/contested
  • Conceptual clarification
  • Technical explanation

Main Points Raised

  • Some participants assert that the speed of light is constant in GR, while others suggest that this constancy depends on how speed is measured.
  • It is proposed that while the speed (scalar) remains constant, the velocity (vector) can vary due to gravitational influences.
  • Some participants highlight that measurements of light speed can differ based on whether an observer is in free fall or undergoing proper acceleration.
  • There is a discussion about the difference between speed and velocity, with examples provided to clarify these concepts.
  • One participant notes that experimental evidence for light speed is primarily based on local measurements, raising questions about measuring light speed in curved spacetime.
  • Another participant mentions that measuring light speed in globally curved spacetime lacks a standard definition due to the absence of global "right-angled" axes.

Areas of Agreement / Disagreement

Participants express differing views on whether the speed of light is truly constant in all contexts within GR. While some agree on its constancy under specific conditions, others emphasize the complexities and variations that arise in different scenarios.

Contextual Notes

Limitations in the discussion include the dependence on definitions of speed and velocity, the conditions of measurement, and the challenges in measuring light speed in curved spacetime.

  • #31
These issues are fairly complex and explaining them accurately and unambiguously to someone who is asking a "simple" question is not so simple. Dr Greg's posts above are, I believe, precisely accurate. Rereading my own initial post, I would now change a few words, but likely the poster would not have gained any additional insights anyway...

The speed of light being constant to all observers is one of the fundamental aspects of special relativity, so it can't be different in general relativity, as it arose from special relativity.

This statement is not precisely accurate because special relativity implies constant velocity and no curvature of space (no gravity); general relativity involves curvature of space and acceleration and only simplifies to special relativity under certain circumstances...no gravity, no acceleration...
 

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