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Yesterday (on Sunday) my friend texted me, "the mass of a photon is 10

^{-18}Ev/c

^{2}##

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.Stephanus said:Thank you very much. I haven't tought about it.

So even at 0.99999c, it might have traveled at 0.1c with respect to something (with mass)

but at 100%c, it always travel at 100%c with everything (with mass).Ibix said:Basically, yes. Although if we're being pedantic you used 0.99999c as if it were an absolute speed, and your wording can be interpreted as saying that something could be at 0.99999c at one time and c at another. I think the "it" in the second line is meant to be a different "it" from your third line, but that is not clear from what you wrote.

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

^{-18}eV/c

^{2}

Or 1.6 * 10

^{-37}joules/c

^{2}. 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?