Light from celestial objects is old. In the case of galaxies, it's millions of years old. It seems plausible to me that light might show signs of its age.
I was surprised that a Google search only turned up one study in this area: Measurement of the speed of light from extraterrestrial sources. It looked at the speed of light from several bright stars: Aldebaran, Capella, and Vega. The results showed that the speeds were different!
My question is, have there been other studies by physicists that looked at old light versus new? It would be so interesting to view in an interferometer light that is a million years old. I can think of many other tests, and I'm sure physicists could think of more. Why hasn't this been done or has it? Could we probably find an age marker by looking closely at the old light?
Best Answer
Light does not "experience" time, the concept "age" does not apply to light in a meaningful way (with respect to human experience). [As background; recall clocks slow for objects as they near the speed of "light" reaching a theoretical 0 if full light speed were attainable.] A thought experiment clock on a photon would therefore stand still. A photon's source does have an "age" in the traditional (human experience) sense, and it is standard that we say the light is as old as it's source. That "age" does not then carry with it the traditional effect of aging.
While the light source ages in a traditional fashion and may in fact be completely burned out though we can observe it today from our distant position in space, any photon from an object no matter how old the source is in no way different than a newly created photon presuming it is the same wavelength. As I view it you could not discern the "age" of light without knowledge of its source, because light is in reality timeless.