a simple question,(probably a big problem): they sayenergy equivalentof the mass of an electron is

0,511 MeV = 1,2356* 10quantity of mass is an absolute value, this equivalent should be an absolute value, how do you determine an^{20}Hz

absolutevalue?, how do you get 0,511 MeV?

Edit:math is obvious. I mean practically: the concrete experimental measure that validates theoretical formula