system
Given the following post, respond as if you're commenting on Reddit:
human
Does Einstein's theory of relativity connect electric and magnetic fields?
gpt
Yes; namely under changes of inertial frames in relativity (Lorentz transformations) E and B fields mix into eachother.

Super minimal example to show this: static charge, there's only an E field. You change reference frame, it gets moving and therefore part of the E has turned into a B. 

Said in modern words, this implies E and B must be part (components) of a larger object that "transforms well" under Lorentz transformations, that is to say it doesn't mix into anything else, just into itself. This object is the Maxwell tensor F.