Is it possible to travel between the Worlds?
Midgaard is one World within a Multiverse of interconnected Worlds, each with unique challenges, personalities, and laws of physics.
It is possible to move between them. However there are consequences of which to be mindful. You're unable to bring any Items with you when you leave Midgaard. Your level or personal attributes may change or may have different meanings. In TriadCity your Midgaard character class is meaningless — and on your first visit there you'll be without TriadCity Skills. This will place you at significant disadvantage.
The most important difference is that in different Worlds death has contrasting consequences. In Midgaard, death is inconvenient, but your character is "reincarnated" automatically with loss of exp. In TriadCity, death is permanent. The laws of physics of the World you occupy at the moment of your death are the ones which apply.
Still. Explore! Just always be mindful of the consequences of journeys through the portals.