Is the Speed of Light Truly Constant in General Relativity?

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SUMMARY

The speed of light is constant in General Relativity (GR) for all observers, as established by the principles of Special Relativity. While the speed (a scalar quantity) remains constant, the velocity (a vector quantity) can vary depending on the observer's frame of reference, especially in the presence of gravitational fields. Local measurements of light speed yield consistent results, but measuring light speed in globally curved spacetime is complex and lacks a standard definition. Thus, the speed of light remains a fundamental aspect of both Special and General Relativity, despite the complexities introduced by gravitational effects.

PREREQUISITES
  • Understanding of Special Relativity principles
  • Familiarity with vector and scalar quantities
  • Knowledge of gravitational effects on light
  • Basic concepts of spacetime curvature
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  • Study the implications of gravitational time dilation on light speed
  • Explore the Shapiro time delay effect in curved spacetime
  • Investigate the relationship between acceleration and light speed measurements
  • Learn about the mathematical framework of General Relativity
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Physicists, students of relativity, and anyone interested in the fundamental principles of light behavior in gravitational fields.

  • #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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