Stranger Things Season 3 ended on a major cliffhanger that left Jim Hopper’s fate up in the air and the state of the Upside Down in flux. We know that Joyce Byers turns the key that destroys the machine overloading the Russian base, effectively closing the portal to the Upside Down dimension. While Hopper seemingly dies, a clever new theory suggests he is still alive and Season 4 will use time travel to explain his return.
Fans have long speculated that Hopper is "the American" the Russians spared from the Demogorgon — an interdimensional monster that originated in the Upside Down — during Season 3’s post-credits scene. However, if you rewatch the final moments of the Season 3 finale, you’ll notice that Hopper’s “death” is never actually portrayed onscreen. It’s possible he was never sucked into the portal in the first place.
Reddit user markjhamill predicts the Upside Down is simply the world in the future. The implication is that Hopper is the unknown American and is simply displaced in time, landing “back in the past when the Russians are only starting to set up that base.” When he tries to intervene in their plans, he’s captured and held hostage.
In short, the machine acts like a tether through time, which makes a lot of sense. Time travel would certainly help explain the surprise reappearance of the Demogorgon. The creature was thought to have been destroyed and its return in Season 3 was completely baffling.
What’s more, the theorist suggests the five individuals who walk into the machine room wearing Hazmat suits are actually the present-day kids who have somehow “come back in time.” It’s possible the reason they’re so calm about what’s happening because they caused machine to overload.
The gist of the theory is that everyone’s caught in a time loop and the kids and Hopper have traveled to prevent the Russians from using the machine to begin with. It wouldn't be the first time a sci-fi-fantasy series has used time travel to get out of tough spots. It would also be very '80s, and iconic movies of the decade like Back to the Future and The Terminator both incorporated time-jumping storylines.
The idea that Stranger Things might incorporate time travel into Season 4 isn’t new. Fans have speculated that might be the case since the end of Season 3. In fact, series creators The Duffer Brothers may have even hinted at time travel during an interview, stating that next season’s storyline would “open up in terms of allowing portals into areas outside of Hawkins.” It’s possible these portals will allow for the formal introduction of time travel after all, though we’re still a ways from finding out.
Stranger Things Season 3 is currently available to stream on Netflix.