Let's imagine two equal charges with the same sign which travel parallel and in the same direction at the speed of light.
For an external observer. they would continue parallel while travelling at this speed.
For an observer who was moving with them, the charges would separate indefinitely.
This famous paradox, called by the physicists of the 19th century the "moving magnet and conductor problem", together with the results of the Michelson-Morley experiment were the bases on which Einstein's Principle of Relativity was founded.

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