User blog comment:FairyTailFan7/Sting's Attacks Are Light Speed?/@comment-24367018-20150819160648

Bringing this sort of theory and mechanics into an anime basically introduces Quantum Physics into the question, which isn't really worth doing in my honest opinion. Saying that Natsu is faster than light would mean that every time he reacted he be time travelling backwards.

As well as that, it'd be similar to saying that every character ever that uses lightning moves as fast as electricity, which is about 0.3m/s... And that clearly ain't true. :/