As all pervasive, stationary fields, do the QFT fields (various Quark, electron, EM, etc.) constitute the modern equivalent of the traditional ether concept?

I'm taking a course on general relativity and I'm confused about something in my course notes. First there is an explanation about a curved 2D plane where it is shown how curvature can be defined by using parallel transport along a closed circuit. This yields one scalar value that characterizes t...

We know that, the speed of light in vacuum is independent of reference frame. I read the reason for this fact from Why and how is the speed of light in vacuum constant, i.e., independent of reference frame?. Similarly, is the speed of light in a medium of refractive index $n \neq 1$ also independ...