Is the Speed of Light Really Constant?

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    Accelerate Photons
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Discussion Overview

The discussion centers around the constancy of the speed of light, particularly in the context of general relativity (GR) and the behavior of photons in curved spacetime. Participants explore whether photons can be considered to accelerate when following curved trajectories and how their paths are influenced by gravitational fields.

Discussion Character

  • Debate/contested
  • Technical explanation
  • Conceptual clarification

Main Points Raised

  • One participant questions if photons can be considered to accelerate in a relative manner when following curved trajectories.
  • Another participant clarifies that in a vacuum, photons travel along null geodesics, which are considered 'straight lines' in curved spacetime, and argues that photons do not accelerate despite their paths being deflected by gravitational masses.
  • A different participant expresses skepticism about the agreement on the theory, suggesting that it remains contentious.
  • Another participant asserts that according to GR, light must follow a null geodesic, and while it may appear to vary in velocity from a stationary frame due to phenomena like gravitational lensing, it does not actually change speed.

Areas of Agreement / Disagreement

Participants express differing views on whether photons can be said to accelerate and how their paths are interpreted in the context of general relativity. There is no consensus on the implications of these interpretations.

Contextual Notes

Participants reference specific phenomena such as gravitational lensing and historical observations related to the verification of general relativity, indicating that the discussion involves complex interpretations of light behavior in gravitational fields.

Loren Booda
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If photons can follow a curved trajectory, may they be considered to accelerate in a (generally) relative manner?
 
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In a vacuum photons travel along geodesics, to be specific null-geodsics, which can be considered 'straight lines'. If the presence of an intermediary mass causes a light ray to be deflected, the path of the photon is still 'straight', it just happens to be along a surface of space-time that itself is curved by the presence of the gravitating mass.

So in this case the photon is not said to 'accelerate'.

Garth
 
It's a theory, I don't think anyone can agree on it.

Check out this link http://van.hep.uiuc.edu/van/qa/section/Light_and_Sound/Properties_of_Light/20040223072254.htm
 
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I believe that in GR the definition of a null Geodesic is the path traveled by light. So in that sense it is impossible for light to NOT follow a null geodesic. If you were able to travel with a photon you would never observe it to vary in its velocity. However if we consider our frame stationary and observe the path of a photon it can appear to vary its velocity. This is observed and called a gravitational lens. Also it was the supposed observation of the variation of the position of star near the limb of the sun during a solar eclipse which was the first verification of GR.
 

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