Of Monsters and Portals
In answering a question asked in a recent review (big thanks author carrymehome for sparking this), I found myself going into a detailed explanation of how I had reasoned out the science behind the gates/portals and how El accidentally closing one shut them all down. I've decided to post those details here as a bit of a supplemental chapter for others that may be interested in a more technical explanation of the rules I have been following through this story.
If you don't really have an interest in the technical science(fiction...so very much fiction) behind this, feel free to skip this chapter. None of the narrative will be furthered here, and the story-proper will pick back up in the next chapter.
For those sticking around, here are the rules I have been playing by, based on what we have been given in the show (S1 and S2), and details I have added in to build a more complete picture. I'm sure my framework still has more holes than a block of Swiss cheese, and I expect S3 will punch even more tunnels right through that.
Many Worlds
There are an infinite (or nearly so) number of universes, each existing as slight variations of one another. These realities all exist in the same three physical dimensions, as well as the same time dimension. In some higher dimension, these worlds are stacked in such a way that the same physical/time point in each universe is touching the same point in each of the others, allowing a connection to be made between them.
Imagine our universe is a single sheet of paper, because that's everyone's favorite tool for visualizing these sorts of things. Now imagine a second sheet of paper, nearly identical in every possible way, except for some tiny change in one corner. Stack up these two sheets so the edges all line up neatly. Now place these two pages on a stack of infinitely more, near identical pages. If you were to poke a tiny hole down through the stack, you would connect the same location in every universe. The exact contents of that location may be different in each world - you might hit Hawkins or you might wind up out in space - but you will hit the same point in physical space and time in each universe. One more tiny stretch, imagine that the stack of pages is constantly shuffling, so that each page is effectively right next to every other page.
Portals Between
Because of the nature of the infinite universes, any point in our world is immediately adjacent to that same point and time in every other. This allows a properly constructed gateway to connect any two worlds and provide passage between. Because these portals exist in the same point in physical space, passing through this doorway is instantaneous to the traveler, used to perceiving their world in three physical dimensions and a constant flow of time. In reality, they have also crossed a tremendous distance in the dimension where the worlds stack together. This last distance is what affects the effort required to join two points into an active tunnel, and will be discussed further when we talk about monsters and Eleven, who is most definitely not a monster.
Pairing Up
Creating a gateway between any two worlds starts with a potential portal. Through a perfect blending of electromagnetic fields, a portal is created, ready to be connected with another world. We saw this created in a very rough, unfocused fashion when El lost control of her powers in the lab and in a clean, refined form by Mike in the basement with the unyielding disk. By the nature of the stacked universes, this portal has technically been created in every world, but is solid and disconnected in each. At this point, the opening is still waiting to be dialed, connecting it to a particular world and allowing the traveler to pass.
This is where the vibrations of the portal come into play. Each pair of worlds has a resonant frequency that connects them together. Lets return to our stack-of-paper multiverse. Imagine we now have sheets of heavy plastic, stretched tight in a frame, in the middle of each piece of paper, and a tiny gap of air between the pages. Tapping the plastic on the top page will send a vibration down through the stack, causing each other piece of plastic to similarly vibrate. The exact frequency induced on each will be affected by the distance and number of pages between them and the originally tapped page. In this setup, exactly one other piece of plastic will wind up resonating with the exact frequency as the original, and those are the two ends that will open to one another and allow passage.
While the same frequency will always connect two worlds from either side, other frequencies will make different pairings depending on where they are dialed from (addressed, but not a spoiler, later in this story). For example, lets imagine World A and World B are paired by a frequency of 1. Similarly, World A and World C are paired by a frequency of 2. However, if one were to make a connection from World B with a frequency of 2, you might wind up at World D instead. Meanwhile, World A has to use a frequency of 2317 to reach World D. The precise rules around these pairings was at the heart of Mike's search.
Tunnels Running Deep
While Mr. Clarke may have overestimated the energy required to open a portal, it is still no an easy task. While the physical separation between the two ends of a tunnel are virtually non-existent, there is a great distance to be crossed in the higher dimension. In my opinion, El is the only one so far that has the native ability to create these connections herself. I believe the openings created by the demogorgon, and those expanded by the mind-flayer, were making use of the main tunnel already established by El; more on that in a minute. Mike, of course, worked out how to mechanically reproduce the effect, but that in no way diminishes what El is able to do. In the same way mankind has figured out how to build airplanes, that takes nothing away from birds who can achieve flight completely unassisted.
This will come into play in just a moment, when discussing the creatures of the Upside-Down, but picture the space between two portal openings as a long tunnel bored through solid rock under a mountain. Difficult to construct, it is ultimately a stable passageway once complete. However, with the right conditions, it can be collapsed upon itself.
Monsters
For all their fearsome strength and menacing face that is nothing but mouth and teeth, I think the demogorgon is ultimately a low-intelligence predator. I S1, we are shown a single creature clawing its way into Hawkins and leaving havoc in its wake. However, all we really see the creature doing is hunting its next meal. On its own, I don't believe the demogorgon has any telekinetic abilities to assist in the creation of portals to pull itself across dimensions. Instead, it is the influence of the mind-flayer on an infected host that gave the demogorgon the ability to create offshoot portals.
The mind-flayer seems to be the Upside-Down creature with the most ability when it comes to telekinetics. It exerted control over Will and the demogorgon army in S2, but even then, it appeared to struggle when it came to portals. While it managed to expand the existing portal in the lab, it was unable to bring itself across until El began working to close the gate for good.
Going back to our rock-tunnel example, I think neither the mind-flayer, nor a demogorgon under its influence, was capable of creating a full connection. Instead, they could only create short offshoots off the completed gateway already opened by El, emerging just a few miles from the original rift in the lab. If our tunnel in the mountain were several miles long, this would be the equivalent of drilling a tiny tube off to the side, connecting just inches inside the opening. And given the understanding above that portals resolve in the same place in every universe, creating this offshoot makes a matched opening on each end. It was a new one of these offshoots that El inadvertently created as she destroyed the demogorgon and wound up in the Upside-Down version of the classroom.
Given the idea that all the openings that came in went throughout S1 and S2 were just offshoots of the main gate El originally opened, I hold that closing down the actual tunnel itself would effectively close off all openings that had been branched. In the case of this story, since she collapsed the connection on her way through, all of the doorways back to Hawkins would have been closed in that same moment. Her new offshoot in the classroom, the portal in the school hallway, any that might have still be in existence in the Byers' home and the original lab opening, all would shut down together.
Inter-dimensional Photocopier
Let's call this one magic, to refer to science we just don't understand enough yet, but here is a basic idea. Going back to our example of a stack of paper, lets imagine you have a felt-tip marker and you're drawing something on the top page. As long as you keep the pen moving, the worlds below are left unaffected. However, once you bring the pen to rest, ink can begin to seep through and mark the adjacent worlds. I believe objects have to be at rest for a period of time before they copy over based on a few facts from the show. In S1E8, after the demogorgon is torched in the Byers' hallway, the bear trap is still nailed down in the hallway in our world. However, when Hopper and Joyce inspect that same spot in the Upside-Down, the trap isn't there, only the bloodstains where it pulled itself back through. On the other hand, in S2, we see cars parked on the street (presumably for several hours) and the dance decorations at the school, which have been in place long enough to mirror across.
Imperfect Portals
Portals created in the middle of walls will have a damaging effect on the solid matter around them. This leads to splintering and bowing while the portal is active. The way damaged surfaces can repair themselves after the fact, I can only explain as an artifact of the copy-over effect, perhaps enhanced by the lingering remains of the former connection.
Portals created organically, by mental will, can be subject to imperfections not found in a mechanically generated opening. If the resonance between two endpoints isn't exact, the portal may maintain a film that must be broken through. If the imperfection is small, the resistance may be broken through by hand (S2E2). If the imperfection is greater, the portal may resist nearly all force applied to it, and allow only muted sound and visible light through, acting more as a window than a passage way (S1E4).
As I said, I'm sure this is still full of plenty of holes, but it is the general framework I have been working from.