The Montauk Project is the answer to season 5
Oh boy, this will be a long one... sorry if it's rambly, as I'm ill and wanted to get this out. Most likely will contain 'spoilers' for the new content, so read at your own risk!
Let's start with the significance of the dates. The thing the Duffers never explained, even though they said they would.
The Montauk Project explains that time travel is possible by way of a 'vortex' that connects two points in time together. This vortex is also referred to as 'a time tunnel', that is also resembled by a spiral:
'Duncan would start off sitting in a chair. Then the transmitter would be turned on.... He would then be directed to concentrate on an opening in time from say, 1980 (then the current time) to 1990. At this point, a "hole" or time portal would appear right in the center of the Delta T antenna - you could walk through the portal from 1980 to 1990. There was an opening that you could look into. It looked like a circular corridor with a light at the other end. The time door would remain as long as Duncan would concentrate on 1990 and 1980.'
Not dissimilar from El concentrating and opening gates with her mind.... and the description of the time tunnel feels eerily similar too:
Will is building tunnels... time tunnels... (I'll get to this part later, bear with me)
A 'master vortex' connects two time points that match up with the Earth's 'natural biorhythm', which is twenty years in the MP book. Multiples of this biorhythm (i.e. 40 years), create even more potent witnesses (think of these witnesses as 'tethers' between two points in time):
These 'master vortexes' and time tunnels connect via the date. Take for example the Philadelphia Experiment, which took place on 12th August 1943. This time point is linked to another experiment taking place 40 years later, the Montauk Project, on 12th August 1983.
These master vortexes/time tunnels connect via those major dates, that form a bridge point or an anchor point.
Still following?
In Stranger Things, the natural biorhythm which is chosen is 12 years. This, of course, represents Vecna/Henry's clock obsession, and the 12-hour cycle. But like we saw above, the most potent witnesses are multiples of the natural biorhythm, so 24 years.
So how does this tie into the show?
We have two significant 'master vortexes':
Will and Henry's birthdays:
22nd March 1947 (Henry is 12 in 1959, as established in season 4) and 22nd March 1971 (Will is 12 when he goes missing in 1983)
There are 24 years between their birthdays - two multiples of 12, creating a master vortex.
Will and Henry both being aged 12 when they experience severe trauma also plays into this biorhythmic cycle.
Secondly - there are also 24 years between 6th November 1959 (when Joyce's play take place in TFS), and 6th November 1983 (when Will gets taken).
A graphical representation of the two 24-year master vortexes:
The people from the Philadelphia Experiment are able to travel between 12th August 1943 and 12th August 1983 as these are tethered by the master vortex.
The reason why the UD is stuck on 6th November 1983 is because it is, in fact, a bridge point. As explained above, the only way to travel two points in time that aren't in the master vortex, is by first jumping to the bridge point, and using that point to jump to other times. The Upside Down is stuck on 6th November 1983 while the world moves along - not because it's a wormhole, but because it's one end of the master vortex. The Upside Down stays on 6th November 1983 because you need it as a bridge to jump to other points in time on the Rightside Up.
Both Will and Henry were 12 years old when they experienced severe trauma. There were 12 children taken. It all ties to the 12-year biorhythm.
Still following?
So, using the master vortex as the anchor point, Will/Henry can jump between two times/timelines.
The book explains quite well what the exotic matter is supposed to represent, as well as what the wall in the UD is supposed to be too, and how new timelines are created.
By way of analogy, let's look at 'Einstein's donut' (fun fact: this is Einstein's bagel in Everything Everywhere, if you will):
Einstein's donut consists of a sphere inside a sphere: Sphere A vs Sphere B. Sphere A is a softball, whereas Sphere B is hollow, like a basketball.
Sphere A is dynamic. The area between Sphere A and Sphere B is not dynamic at all. This area consists of a sea of time particles, which are static, at rest:
If you haven't guessed it, Sphere A represents 'time as we know it', e.g. the timeline/timeloop our characters are currently in - the exotic matter.
The static space between sphere A and sphere B is the Upside Down, which is stuck in time, at rest, filed with time particles (or the UD dust particles). The 'wall' that Hopper/El bump into is the wall of Sphere B, the hollow basketball which contains the time particles.
See how they constantly draw the 'wall' as directly encircling the exotic matter, which is right in the middle? These are Spheres A and B - a sphere within a sphere.
In other words, two concentric circles:
Remember above, that the time tunnel that it resembled a spiral? Here, Robin is directly contrasting 'the normal' status quo - the two concentric circles, which have now turned into an open spiral.
How are new timelines/timeloops created? By putting stress of the wall of Sphere A:
Note that the original loop cannot be obliterated or denied.
Moreover, after creating time loops (ie parallel realities), it is also possible to enter alternate time lines/time loops a sometime against one's innate will:
However, entering different time loops leads to memory blocks, despite seemingly having all the normal memories from one timeline:
To regain memories, one must look for clues and hints in the normal time track to confirm that the alternate time track did in fact exist.
How does one get over this memory blocks, and gain consciousness of other time loops/time lines?
The author in the MP project regains their memories when they came into contact with Delta T antenna - an antenna which had an inter-dimensional effect on the nature of time itself.
This antenna, also called a receiver:
The receivers were capable of tuning in on the projects from any other space and time in our universe. He also believed this receiver was the main witness from Montauk to the USS Eldridge.
Remember - one witness is the 20-year time jump (in MP), and 24-years in ST. The main witness between Montauk and Philadelphia is the receiver. The receiver is capable of tuning into any particular point in time.
Who is labelled as the receiver?
There's a reason that Henry kidnapped Will specifically. He is the main witness - he is the one that is able to tune into any point in space and time.
I think that both Henry/Will are foils, stuck in two timeloops, whose memories are blending together. Henry needs Will, as the receiver, to tune into a different timeloop - which one, I'm not sure. Perhaps he's trying to enter a different loop, one where November 6th didn't happen - one where Henry never enters the cycle of trauma that changes the course of his life forever.
Referring to the 12 children as vessels to recreate/mimic the original USS Eldridge vessel. He needs Will as a receiver in order to fracture the timeline/create a new timeline/change time itself. Vecna alsos needed him originally as a builder - to create the (time) tunnels with his mind. The need for 12 children aligns with the 12-year biorhythm.
The original 'vessel' was the USS eldridge - literally a ship/vessel that is used to travel between 1943 and 1983:
Anyway, just reading the source material for the 'Montauk Pilot' really put everything into perspective.
Even the original pilot script had hints towards to time travel, and had sequences that were later incorporated into the Nina Project in Season 4.
In the original pilot script, the scientists replay the initial demogorgon attack on a video tape, that they rewind, and there are hints within the script that they 'go back in time', 'back to the beginning of our story'.
Snippets did sneak into the 1x1 script, where there's a flashback to Joyce and Will in Castle Byers:
In the MP book, they also look at video tapes depicting footage experienced by the time travellers:
Duncan in the MP book, who had the ability to create time portals with his mind (like El and her gates), also accidentally created a portal that let through a scary creature (analogous to the demogorgon):
Since the Montauk project was probably the biggest inspiration for ST as a show, it is hard to ignore the similarities here, especially as the Duffers mentioned having 'too many ideas for Season 2' that they couldn't fit into one season.
Almost everything aligns - loss of memory, blending of timelines etc
Our main characters are stuck on the wrong timetrack- it's pretty much confirmed in the MP book
They need to find a way out, and unlock the repressed memories - the authors tell us that one way to lift the memory block is to look for clues and hints that the other time track did in fact exist.
Again, I think that this part - looking for clues and hints, will be within the interactive content. We will need to lift the memory block and uncover the repressed memories (like El did in the Nina project).
The new content, the actual second time track will be its own separate beast...
