We know very few details about Avengers 4, but one thing we can bank on is that the Quantum Realm is going to play a huge role. Presumably, that’s how the Avengers are going to bring back the fallen heroes. The idea of the Quantum Realm was first introduced in Ant-Man, but a new theory suggests it was introduced long before we met Hank Pym.
A new theory presented by a Reddit user reveals the Quantum Realm was first established in the MCU in the unlikeliest of movies: Thor: The Dark World. What is considered to be the low point of the MCU ended up giving us one of its biggest contributors.
During the movie, about thirty minutes in, Jane Foster has a conversation with an Asgardian doctor called Eir. Foster identifies the “quantum field generator” that’s being used to examine her and asks if it “transfer[s] molecular energy from one place to another.” Eir responds that in Asgard, it is called a “Soul Forge.”
With hot words like quantum and soul being thrown around, it’s easy to make the connection with the Quantum Realm and the Soul Stone. The conversation between Foster and Eir may confirm that there is a connection between the “quantum field” and the soul, neatly connecting with the Avengers’ use of the Quantum Realm to somehow retrieve the souls of the dead characters in Avengers 4, which they’re rumored to be in the Soul Stone. After all, they can be transferred from one place to another, according to Eir.
This is really going into the weeds, but it makes a surprising amount of sense. As is standard, take the theory with a grain of salt. It could just be a spectacular coincidence, or it could be the first bread crumb the MCU dropped in the development of the Quantum Realm.