In the series the Upside-Down is explained by the example of a rope and an ant. So it would be a “continuation” of regular space.
- In the show it seems necessary to use gates to reach it.
Also it seems the Demogorgon can open gates as it pleases. And it is hinted that it only started after the meeting with Eleven. Let's not forget that from the expanation it takes an incredible amount of energy to pen one.
Another point is that some objects from the regular world seem to be projected into the Upside-Down - but in the other way from the Upside-Down it is possible to hear/observe(?) the regular world and also manipulate electricity.
- Lastly it seems one gate to manipulate magnetic fields. Given that portals seem to open all over the place - why don’t the others manipulate the magnetic field?
So is there a way where those observations can be explained by this theory?
I see i stated my question a bit ambiguous as answers go in the direction of explaining the upside-down. My actual intent was actually to clarify whether the 'ant on the rope' is technobabble or an actual useful description. To give an example chaos theory in jurassic park is also a valid scientific theory woven into the story - although the sections of chaos theory are actually completely disconnected from the story. Or many shows handwave quantum mechanics to explain any magic they want.
So when I read an answer I would consider this: suppose I replace the 'ant on rope' part with 'teleported to another world', 'spirit world (as in night watch - nochnoy dozor), 'from hell' or that D&D story they were refering - does the 'ant on rope' explanation offer any benefit over the 'replacement explanations'. E.g. is "the demogorgon goes along the rope through a gate from the upside-down to the real world" a better explanation than "the demogorgon goes through a gate from hell" - considering how we see things work in the show. Or even better "the demogorgon needs a portal to come along the side of the rope" vs "the demogorgon needs a portal to come from hell" (i really think the rope theory is lacking here).
The answer I am looking for is in-universe, so as @Broklynite points out children logic and knowledge applys (i suppose that would include D&D rulebooks more than real live physics). As observations i would mostly consider what we see in the show. Although characters talking about their experiences is good as well.
Since it is some time now I was hoping some new information in form of novellizations or screen caps might turn up.
I was pondering about it a little. For one it seems there is a slight indication for the upside-down theory in Els turning over the board game to point to Wills position. Besides I found it noteworthy that none of the characters that physically travelled to the upside-down uses any language even remotly pointing to the theory. I am especially curious about the research lab - they obviously are able to hold an entrance open and have gear for expeditions. They should at least have some theory. I wonder if there is ever a screen or document lying around that might shed some light.
** Update 3 **
With two more seasons out I become less confident with the answer. The teacher in season 3 explanation is a just minor plot device. I don’t recall much upside-down later on. Does it still have any relevance?