Measuring the Speed of Light from Distant Sources: How Do We Know It's Constant?

Prometheus
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Does anyone know how the speed of light from sources outside the galaxy is measured, such that we are able to recognize that even from these sources the speed of light is constant?
 
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Why would you think the speed of light would depend on the location of the light source? As far as we can tell, whether the light source is a flashlight sitting two feet away from you or the sun 93 million miles away, the speed of light is c.
 
Gza, you skipped past the question right into the assumption you thought was behind it [I am guilty of that all too often!] Prometheus: That proposition has been tested by examining light sources from different objects at great distances, using mirrors and interferometers, then comparing them to local sources. There is no measurable diffence in 'c'. A number of experiments, some very recent, have been performed to explore your very good question. I will give references if you like.
 
Chronos said:
A number of experiments, some very recent, have been performed to explore your very good question. I will give references if you like.
I would appreciate references.
 
http://www.student.oulu.fi/~ktikkane/c-const.html of speed of light from very distant objects. The proceedure looks valid to me.

Keep on chuggin !

Vern
 
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