Why is the speed of light the same everywhere in the Universe?

In summary, the conversation discusses the question of why the speed of light is considered to be the same everywhere in the universe. The theories of general relativity and quantum gravity are mentioned and it is stated that experimental evidence supports the idea that the locally measured speed of light is invariant. The conversation also touches on the concept of the fine structure constant and how it can be used to test for variations in the speed of light in other locations. Finally, it is concluded that there is no evidence to suggest that the laws of physics vary from location to location, and that the current theories seem to accurately explain the observed phenomena.
  • #36
Rick16 said:
Since Newton's law of gravitation predicts that light would speed up (or decelerate less) the farther away it gets from a star, and this prediction is generally taken to be incorrect, isn't this proof enough, i.e. proof by counterexample?
Dale states in post #32 that "For Newtonian gravity, the speed of light can indeed change." Can you cite a reference that supports your claim that "this prediction is generally taken to be incorrect"?
 
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  • #37
renormalize said:
Dale states in post #32 that "For Newtonian gravity, the speed of light can indeed change." Can you cite a reference that supports your claim that "this prediction is generally taken to be incorrect"?
References can be found throughout this thread. My original question was whether the speed of light is constant everywhere in the universe, and I was told that it is. It is apparently a general consensus among physicists that the speed of light is constant, which means that physicists take the prediction of the Newtonian model to be incorrect. If it is really incorrect or just assumed to be incorrect -- that is exactly at the heart of my original question.
 
  • #38
If the constancy of the speed of light were an indisputable fact, then this fact could be used for a proof by counterexample that the Newtonian prediction is incorrect, right? If you say that the constancy of the speed of light is not enough to invalidate the Newtonian model, shouldn’t I conclude that the constancy of the speed of light is not an indisputable fact?
 
  • #39
Rick16 said:
If the constancy of the speed of light were an indisputable fact, then this fact could be used for a proof by counterexample that the Newtonian prediction is incorrect, right?
There is no doubt that the Newtonian prediction is incorrect.

We have been arguing about what exactly the Newtonian prediction is. You cannot answer that one by physical experiment. It is a question of language and logic, devoid of physical significance.
 
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  • #40
jbriggs444 said:
There is no doubt that the Newtonian prediction is incorrect.

We have been arguing about what exactly the Newtonian prediction is. You cannot answer that one by physical experiment. It is a question of language and logic, devoid of physical significance.
Okay, if I don’t have to prove that the Newtonian prediction is wrong, I thought that I would repeat my question from this morning, but I think I have found the answer myself. I have to learn to better distinguish between what happens in nature and what is part of a specific model. I will now shut up and read more books. Thank you very much.
 
  • #41
Rick16 said:
I would say it like this: The Newtonian model is correct for nonzero masses. Therefore it applies to nonzero masses. But it is not correct for zero mass. Therefore it does not apply to zero mass. What is wrong with this reasoning?
The Newtonian model is not correct for non-zero masses either. So that is not the concern. We know that the Newtonian model produces predictions about the motion of both massive and massless objects that are incorrect because they do not match experiment.

The concern is what is the prediction for a massless object, regardless of its correctness. Saying that the prediction is incorrect actually recognizes the fact that there is a prediction.

Rick16 said:
Since Newton's law of gravitation predicts that light would speed up (or decelerate less)
Again, decelerating less is not at all the same as speeding up.

Rick16 said:
this prediction is generally taken to be incorrect, isn't this proof enough, i.e. proof by counterexample?
Yes, but it proves a different thing. It proves that Newtonian gravity is not a correct theory in that respect. It does not prove that it doesn’t make a prediction.
 
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  • #42
Light bending around the gravity of the sun was observed with the understanding of Newtonian physics and reported in German by Johann Soldner in 1801 in Germany. https://en.wikisource.org/wiki/Tran...on_of_a_Light_Ray_from_its_Rectilinear_Motion

Henry Cavendish did the same in 1784, but never published it. Although slightly different assumptions on the model for the source of the light, at the 1st order approximation, they agree, both only-half the value predicted by GR and confirmed by British scientist Eddington's solar eclipse experiments in Africa during the eclipse of 1919. There was considerable unrest at the time, during WWI, which made this revelation unlikely from political bias. The accuracy of Eddington's formula of light bending were found accurate with later measurements in the 50's and subsequent eclipses. Einstein's prediction in 1915 of gravitational waves were observed in 1974 indirectly and accurately 100 years after 1915 from much stronger pulsar gravity fields. https://www.wikiwand.com/en/General_relativity Many scientists contributed to these revelations to support Einstein's Gravity equations.
https://www.wikiwand.com/en/Tests_of_general_relativity
https://www.wikiwand.com/en/Eddington_experiment#Expeditions_and_observations
 

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