Train

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© David Kaplan.

A left-handed electron is seen from a stopped train (left). If the train starts moving faster than the electron, it will appear to be moving the other way, and thus will appear right-handed. If the electron had zero mass, it would move at the speed of light, and thus no train would be able to overtake it. In that case, the left-handed and right-handed electrons would be truly distinct particles. (Unit: 2)