Need Deflection of Light Clarification

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Discussion Overview

The discussion centers on the deflection of light in the context of Newtonian gravitation and General Relativity. Participants explore the implications of light's behavior near massive objects, addressing concepts such as vector displacement, local versus global measurements of light speed, and the effects of spacetime curvature.

Discussion Character

  • Exploratory
  • Technical explanation
  • Debate/contested

Main Points Raised

  • Some participants note that both Newtonian theory and General Relativity predict a change in the direction of light near large masses, but question the implications of vector displacement on light speed.
  • It is proposed that in General Relativity, the speed of light is constant only in a local sense, with the average speed between distant points potentially differing due to spacetime curvature.
  • One participant emphasizes that vector displacement represents the difference between deflected and undeflected paths, asserting that nothing travels along this displacement vector.
  • Another participant reiterates that while locally measured speeds of light remain constant, the coordinate speed can vary in different global systems, with local measurements being more physically significant.
  • A participant expresses confusion regarding the implications of these concepts, initially assuming a luminiferous ether model.

Areas of Agreement / Disagreement

Participants express differing views on the implications of vector displacement and the nature of light's speed in various contexts. The discussion remains unresolved, with multiple competing interpretations of the concepts presented.

Contextual Notes

There are limitations in the discussion regarding the assumptions made about light's behavior and the definitions of local versus global measurements. The implications of curvature in spacetime and its effects on light speed are also not fully resolved.

Devin
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Both the Newtonian theory on gravitation and the General theory of Relativity predict a change in the direction of a beam of light as it propagates past a large mass. Regardless of the precision of the Δ velocity component (the component that can represent the vertical shift towards a mass), If you think in terms of vector displacement, then you have a resultant vector who's magnitude is greater than the speed of light. I apologize for the awful depiction. https://twitter.com/ValorAtmC/status/480107448385019904/photo/1 ... I understand that if this were true, a conflict with special relativity is created. With that being said, can somebody please point out what I'm missing?
 
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In general relativity, the statement "the speed of light is a constant c" is true only in a local sense. That is, if you could measure the speed of a pulse of light as it passes by you, using measurements that are "close enough" to your own location, you would get c, no matter where you are.

However, the "average" speed between two "distantly" separated points, computed as Δx/Δt, can be different from c, because of effects due to the curvature of spacetime.
 
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Devin said:
can somebody please point out what I'm missing?
Your vector displacement is the difference between a deflected path and an undeflected one. Nothing travels along this displacement vector.
 
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The speed of light is equal to "c" locally in any small region as measured by local clocks and local rules in any local inertial frame. However, the coordinate speed of light is not necessarily "c" in any given global coordinate system. In generalized coordinates, and in curved space-time, the coordinate speeds don't have any particular physical significance, the locally measured speeds are much more physically significant.

Some of the effects are due to the curvature of space-time, and are rather similar to how a great circle can be the shortest distance between two points on the Earth's surface, yet appear to be "curved" when plotted out on an chart. Like a great circle on the surface of the Earth, the path of light through space-time is a geodesic.
 
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jtbell said:
In general relativity, the statement "the speed of light is a constant c" is true only in a local sense. That is, if you could measure the speed of a pulse of light as it passes by you, using measurements that are "close enough" to your own location, you would get c, no matter where you are.

However, the "average" speed between two "distantly" separated points, computed as Δx/Δt, can be different from c, because of effects due to the curvature of spacetime.

Thank you for the response. I was just confused because I assumed a luminiferous ether.
 
Bill_K said:
Your vector displacement is the difference between a deflected path and an undeflected one. Nothing travels along this displacement vector.


Thank you for the response.
 

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