For a minute I wondered if this might explain the Pioneer anomaly, but the numbers as given are off by a factor of about 24 for the anomaly.

The official speed of light is the speed of light in a vacuum (light travels more slowly through various mediums).

Is it possible that something else affects the speed of light on Earth besides the medium it’s traveling through? Something that wouldn’t affect neutrinos?

Has anyone ever measured light speed in deep space, away from any large mass?

(If I missed something obvious, let me know. Remember, I’m not a physicist.)

Hate to say it, but it’s probably true that this won’t pan out.

A Canadian neutrino enters a crowded
bar and says to everybody, “Excuse me, I have to pass through you all
to get to the bar, sorry, sorry, sorry.”

“As to relativity, I must confess that I would rather have a subject in which there would be a half dozen members of the Academy competent enough to understand at least a few words of what the speakers were saying if we had a symposium upon it. I pray to God that the progress of science will send relativity to some region of space beyond the fourth dimension, from whence it may never return to plague us.”

Letter of C.G. Abbot (Home Secretary of the National Academy of Science) to G.E. Hale (Director of Mount Wilson Observatory), 20 January 1920

Abbot and Hale were trying to come up with a speaker(s) for an upcoming symposium at the Smithsonian. Eventually, they settled on American astronomers Harlow Shapley and Heber Curtis, who debated on whether the “spiral nebulae” were relatively nearby objects in the Milky Way Galaxy, or whether they were other galaxies similar to the Milky Way at immense distances.

Both men were right about some things, and wrong about others, but the nature of the spiral nebulae was finally settled by Edwin Hubble, in 1925.

I’ve always found it amazing that the most impressive and profound things we have learned about the universe and its architecture, that it is an infinite collection of galaxies expanding away from one another, was discovered by one man, during the lifetime of some people who are still living today.