Interlude
The portal to travel between dimensions is indeed a mysterious thing. There are a very limited few in the world who believe in the existence of alternate dimensions and fewer still who have an inkling of a possible theory as to how to open a portal and only one with the true knowledge and ability to open the portal and guards said knowledge fiercely.
It was said by those who built the portal that traveling through it will take you to any parallel world and but not to the time you intend it to take you to.
You could leave one world in the year 1990 and end up in another world in the year 1889 or 2089 or any random year. It's just the way the portal works and has no explanation as to why it works that way.
It is said that as soon as one enters the portal he or she immediately appears in another dimension but that is not true.
When a being enters the portal, its body is disintegrated into un-countable number of particles which drift through the dimension between worlds until the entrance to another is found.
It could be years, it could be minutes or even decades but you will never know as your conscious ceases to exist once you enter the portal. You are simply another particle in space.
Your conscious mind may not exist but your subconscious does. And the moment it finds an exit, the body reforms and comes out as if no time has passed.
Even if multiple beings enter the portal at the same time, there is no guarantee that they will end up in the same dimension. It is only pure luck if they do end up together.
But traveling between worlds is very dangerous because it upsets the balance of energy in the both worlds as a soul is lost in one and gained in another. Depending on the power the soul holds the balance of power in the worlds shifts too.
Though it is strange... for when the person travels back to his own world, he shall arrive at the exact same time at which he left. He shall not have aged even a second regardless of the time spent in the other world and shall remain the same as they were when they left.
That it why it is most curious as to why Harry James Potter and Lillian Mary Austin were not back in their own world and why they still seemed to be in another world.
.
.
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Unless they were already back and didn't want to be found...?