Conceptual Physics (12th Edition)

If you were traveling with the electrons, they would be at rest with respect to you. They would just be "sitting there" and everything would appear to be normal. Each electron would have rest energy $mc^{2}$, and zero momentum. From that reference frame, the target approaches at a speed v that is very close to the speed of light. Observers in 2 frames of reference agree on the relative speed of the 2 frames.