I was always interested why have scientists tried to speed up electrons to the speed of light, unless they were trying to disprove the absolutness of speed of light itself, which highly unlikely was their intention. I mean, when electromagnetic device is used to speed the electron then photons are the mediators, right? Since photons move at c then I suppose it is natural to expect that as electron approach the speed of light it will be harder and harder to accelerate it further. My question is: Does indeed anything happen with the electron mass at high speeds, or is it just a convenient explanation of reduced ability to speed up electrons close to (or beyond?!?) the speed of the mediators?