How does CMBR differ from ether (SR self study -- Question from W. Rindler) Hello everyone First of all, apologies for a previous post which was against forum rules. I was unaware that I had violated the rules by posting a link to a paper that wasn't from a proper peer-reviewed scientific journal (something I discovered much later). Anyway, I have the following question, which is actually from Wolfgang Rindler's book on Special Relativity. This isn't homework (I'm teaching SR to myself). My random thoughts for each part... Part 1: Unless light is known to interact with such a fluid, slowing it down or speeding it up, such a fluid will not affect special relativity. Part 2: CMBR itself comprises of photons, whereas ether (nonexistent) is comprised of ___ (?). CMBR is itself radiation that travels at light speed, whereas ether is static. I know this may sound silly, but I wanted to discuss this part out here. I know CMBR isn't ether. But what really is the difference, formally? Also, what is the significance of this question? Thanks and cheers.