The prefix is a bit irrelevant This is on renormalisation. How do they cancel out? Isn't it adding? So the mass experimental = m + (c2correction) so how do you cancel out the m and correction? I'm new to this area (just finished watching lectures by Richard Feynman, specifically a 4 lecture series on QED in 1979 in New Zealand). Is it that the experimental mass is already confirmed by experiment and because the initial mass isn't important because of a change of theory (pre-interaction to electron interacting with photons) we can just change the number so the mass experimental is a finite one? Can you give me an example of an experiment having already been done?