Is Cobb and Mal's Dream Sequence Limbo?
There is debate whether Cobb and Mal’s dream is limbo or not. There are arguments to support both cases, of which I will present the two most important for each.
I will start by presenting the case AGAINST limbo.
First, the most obvious clue was the sequence showing both of them killing themselves on the train tracks. They ended up back on the carpet of their living room. If they were in limbo, killing themselves would not work because the only way to become unstuck in limbo was to wait for the sedative to wear off in reality. If anyone could escape limbo by killing himself, then why would limbo be so scary in the first place?
Second, Cobb and Mal’s dream was the exact same one as the “building level” dream during the heist sequence. This was the dream they stayed in for what seemed like 50 years, which they spent time building streets of houses and buildings, transforming the dream into a reality-like place where they could “live their dream”. Since I have established that the “building level” was not limbo, it simply meant that Cobb and Mal’s dream (i.e. “building level”) was also not limbo.
Now I will present the case FOR limbo.
First, Cobb and Mal spent 50 years and grew old together in that dream. That could be limbo if they had used a very strong sedative in reality and killed themselves in a “one-layer deep” dream to get there. This was possible because they were exploring dreams within dreams (i.e. a strong sedative was most likely needed). They could have gotten into limbo by accidentally killing themselves in a “one-layer deep” dream which they were too confused to be able to identify as such.
Second, Cobb and Mal’s dream was limbo because if it was a normal dream, they would need to enter at least three levels of dreams to be able to spend 50 years there, that was assuming that in reality they were plugged into the dream machine for more than 50 hours (since 10 hours in reality give 10 years in a three-level deep dream based on the heist sequence). Can the human body sustain without food and water for more than 50 hours? Still possible but dangerous. But is there a sedative strong enough to put a person to sleep for 50 hours straight while still maintaining subconscious activity? I doubt so. Even Yusuf (whom Cobb and Mal had not known at that time) could not have manufactured such a sedative since his custom-made sedative for the inception mission was considered unprecedented.
My understanding of Cobb and Mal’s dream sequences:
Nolan integrated short flashback sequences of Cobb and Mal’s dream sporadically over the film. There were two reasons for doing that. One, a full flashback sequence would have revealed a lot of information to the viewer; this would not be consistent with Inception’s labyrinth-like narrative structure. Second, and more relevantly, Nolan wanted to show that the “disjointed” flashback sequences were not necessary connected to each other (this would form the basis of my understanding).
It is not important to know whether Cobb and Mal’s dream was limbo or not. Rather, it would be more effective to understand the chronological structure of their dream. No one knows for sure what happened but from what was shown in the film and using my theory of limbo, I propose the following scenario:
Cobb and Mal were exploring the concept of dreams within dreams. Initially, it could be assumed that they tried a two-level deep dream. Later, they would venture further and explore a three-level deep dream. At that time, a sedative as strong as the one used in the heist sequence would not have been manufactured yet. So, they probably used a normal (read: mild) sedative, which was possible because they plugged themselves into the dream machine in their very own home (stable and quiet (their children were probably taken care of by Miles) enough that nothing could act as a “kick” to wake them up).
They successfully navigated through three levels of dreams on a few occasions. This was implied from the shot of their young hands clasping together as they laid on the train tracks to kill themselves. However, as time went by, they became obsessed and wanted to spend a long time in a dream because it was their escape from reality. They could also build the reality that they envisioned in their dream. One day, they set up the dream machine in such a way that they would be in sedation for 50 hours in reality. They entered three levels of dream in which the deepest was the dream that they built i.e. “building level”, spending 50 years there. They “lived their dream” and grew old together.
Mal had a totem, the spinning top, which would remind her that no matter how real her dream was or how long she spent in her dream, she was after all still in a dream. Because she wanted to forget that, she hid her totem in her safe. Cobb later found her safe, broke into it, and left the top spinning inside, planting the idea into Mal that her world was just a dream and that to get back to reality, she had to kill herself.
Now with both of them old, Cobb persuaded Mal that it was time to kill themselves to get back to reality. Why Cobb did not want to wait for the timer of the dream machine to count to zero was because if that happened, the involuntary waking up in reality would become very frustrating for Mal. It would have been literally “a dream cut short” for her. Therefore, voluntarily killing themselves to wake up in reality would be considerably less frustrating.
They got to the train tracks and lay down, theirs hands clasping together. A shot of their aged hands in the film explained this. The train ran over them, and they eventually woke up in reality. Now, returning to reality after 50 years in a dream (state of mind limbo i.e. 1SOT) was very hard to accept for Mal. The idea that Cobb implanted into her also began to manifest in her mind (this was how Cobb discovered that inception was possible). Soon, she became disillusioned with reality and believed that it was a dream, and that to get back to her perceived reality, she would have to kill herself.
On their wedding anniversary, Mal called Cobb to a hotel room, where he found her sitting on a window ledge, waiting to jump to her death. She wanted Cobb to join her, but the latter refused. She eventually jumped and killed herself.
**From my proposed scenario, you can tell that Cobb and Mal’s dream was 1SOT limbo and not 2SOT limbo.