Richard Feynman: "I want to emphasize that light comes in this form - particles. It is very important to know that light behaves like particles, especially for those of you who have gone to school, where you probably learned something about light behaving like waves. I'm telling you the way it does behave - like particles. You might say that it's just the photomultiplier that detects light as particles, but no, every instrument that has been designed to be sensitive enough to detect weak light has always ended up discovering the same thing: light is made of particles." https://www.amazon.com/QED-Strange-Theory-Light-Matter/dp/0691024170
If Feynman is correct, any wave-based concept of variation of the wavelength of light (e.g. https://www.youtube.com/watch?v=3mJTRXCMU6o&t=77s
) is unrealistic. It makes sense to advance the following
Axiom: The wavelength of light is invariable.
and examine the logical consequences. First of all, given the formula (frequency)=(speed of light)/(wavelength), we conclude:
Any frequency shift entails (is caused by) a proportional speed-of-light shift.
It follows that the cosmological (Hubble) redshift is due to light gradually slowing down as it travels through intergalactic space, in a non-expanding universe. The idea that vacuum slows down light has been largely discussed but only in terms of quantum gravity. The implication that the Hubble redshift might be due to speed of light decrease is persistently ignored:
"...in some quantum-gravity models, the speed of photons in gamma rays would be affected by the grainy nature of spacetime..." https://fqxi.org/community/articles/display/255
Sabine Hossenfelder: "It's an old story: Quantum fluctuations of space-time might change the travel-time of light. Light of higher frequencies would be a little faster than that of lower frequencies. Or slower, depending on the sign of an unknown constant. Either way, the spectral colors of light would run apart, or 'disperse' as they say if they don't want you to understand what they say. Such quantum gravitational effects are miniscule, but added up over long distances they can become observable. Gamma ray bursts are therefore ideal to search for evidence of such an energy-dependent speed of light." http://backreaction.blogspot.fr/2017/01/what-burst-fresh-attempt-to-see-space.html
"Some physicists, however, suggest that there might be one other cosmic factor that could influence the speed of light: quantum vacuum fluctuation. This theory holds that so-called empty spaces in the Universe aren't actually empty - they're teeming with particles that are just constantly changing from existent to non-existent states. Quantum fluctuations, therefore, could slow down the speed of light." https://www.sciencealert.com/how-much-do-we-really-know-about-the-speed-of-light
For not so distant stars slow speed of light is manifested as cosmological (Hubble) redshift.
Light from very distant stars (very slow light) is manifested as what is called Cosmic Microwave Background (CMB). (I used to regard CMB as coming from nearby vacuum fluctuations, but that was a wrong idea).
Beyond a certain distance, the star light does not reach us at all.
The invisible "very slow light" and the light that does not reach us explain the Olbers' paradox.
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