think about a paradox, then some people may start to appreciate einstein's special relativity.


imagine such a scenario:

a stream of postive charges are moving upward at constant speed; meanwhile, a single positive charge is moving upward at the same constant speed.


if we stand on the ground, we find the stream of moving charges form a current, which generates a meganetic field; and we find the single charge cuts the megnetic field and experinces a lorentz force, so, the charge will be attracted towards the stream of charges.


if we sit on an inertial coordinates moving in parallel at the same speed as the streamed chrages, we find the single charge always has the same upward speed as us and cuts no magnetic field at all; so we conclude, the single charge receives no attraction at all.


but we shall only get one answer.


this is a paradox.


please explain it.


we you are a physics student who studied special relativity but never heard of this paradox, then it's pretty tragic.



with einstein's relativity, you can easily defunct this paradox.



from this example, you may realize his theory addresses:

the retardation in coordinates tranformation