Dear PF Forum, Yesterday (on Sunday) my friend texted me, "the mass of a photon is 10-18 Ev/c2## If my number is correct. And I also I check it in Wiki, and it's true. Now what I want to ask is this first. I Understand, my wording is confusing. Now, I'll rearrange my question for a new question. A: If something travels at 0.99999c wrt (A), than it can travel 0.1c wrt (B). It CANNOT travels at c execpt wrt light. B: If something ELSE travels at c wrt object with mass, it will always travel at c wrt anything, even against light. Is A and B true? I think so. I just want to rearrange my questions and get confirmations. Now about this photon, it seems that it's not massless, It's 10-18eV/c2 Or 1.6 * 10-37 joules/c2. Or around 0.18 * 10-53 kg. C. Does proton ALWAYS travels at c against something? It has mass no matter how tiny does it, right? D. If photon represents light, than light doesn't always travel at c? I think C and D can contradict A and B E. I read in wiki that light doesn't always travel at c. c is the universal constant where maximum speed is allowed in this universe. Or perhaps 10-53 kg is such too small number that we can ignore its mass? I check the mass of photon/proton is around 10-30, so anything below 10-30 mass of a proton can be considered massless?