Exactly, speed of light in the vacuum (
c) is the hard limit, only mass-less objects can move that fast (e.g. photons). The refraction index (
n) tells you how fast light moves in a medium. It is defined as
n=c/v, where
v is the speed of light in the medium. That's why (and also because of internal reflections) the speed of light in the fiber optics is not quite reaching
c [0], and with inter-satellite communication Starlink may provide lower latency than the one using landline fiber optics channels. For air refraction index is very close to 1 [1], but not quite, at 10km altitude
n=1.0001, which is 99.99% of
c. For example a proton with an energy of 1TeV (Tera electron-volt, 1x10^12 eV) is moving 99.999956% speed of light in vacuum [2], and we see protons up to 5x10^19 eV reaching 99.99999999999999999998% of
c [3]. Even "low" energy protons (tens of GeV) hitting molecules of air can kick out electrons, which, due to being lighter, can move faster than the initial protons and emit Cherenkov radiation.
[0] https://www.quora.com/What-is-precisely-the-speed-of-light-i...
[1] https://www.researchgate.net/profile/Dmytro_Vasylyev/publica...
[2] https://uspas.fnal.gov/materials/10MIT/Review_of_Relativity....
[3] https://en.wikipedia.org/wiki/Greisen–Zatsepin–Kuzmin_limit
