well i would like to add in the whole equestria girls time line into this, since celestia and luna are aware of the mirror, and could most likely go through it as well, they could be reverse engineering the the tech from the other world. i mean if sunset shimmer and twilight could freely move back and forth, who is to say other can't as well. since the other world does have phones and the internet, but celestia and luna haven't figure out how to reverse engineer the tech, so they introduced it into equestria. well that is my take on the subject.