This is something I originally posted on the discussion page for Limbo. I stated I had a few problems with the mechanics of the thing. Here is what I wrote:
Something doesn't quite feel fight.
And what I mean by that is the different methods of going into Limbo. I seem to have a problem with the time dilation.
Indeed, Yusef did say that it was impossible to go into three dream layers, but they pulled it off in the movie under sedation. Obviously it is possible. That wasn't why he said it wasn't. The reason he said it was impossible was because the dreams would be so unstable that they were susceptable to collapse. You could actually get to three levels, you just couldn't stay there for long. So entering manually through regular layering is ruled out.
I have no problems with Saito or Cobb entering Limbo together, as they both died on Level 1. The one that is the concern at the moment is how they enter through Fischer.
Now, as opposed to Saito (who dies of both a gunshot wound to the lung and drowning on top of that) - who dies in a way where he can't possibly be brought back, Fischer is able to be revived by a defibrilator. Due to this state, the fact he wasn't quite dead, this would account for the normal time dilation. He was borderlining on Limbo, but wasn't quite past the point on no return (like Saito was.) (The your mind makes it real aspect certainly attibutes to this, though passing the body as actually aslepp doesn't quite expain hwo they pronounced him dead.) This is also why Cobb and Ariadne generated Limbo when they entered the dream, because it was Fischer's dream. But Fischer is "dead", so we have instant Limbo. Ariadne and Cobb still alive in the upper layers at this point, so time dilation is at a normal 20x.
And did Cobb necessarily stay in Limbo for 50 years? Seems kind of off as well, as he would have more than likely found Saito in that time with that method. Backtrack to level 1, where Saito is dead and Cobb is alive. Him going in after would also explain the age difference between the two. My theory is that Cobb stayed in the Limbo-esqe level after Ariadne and Fischer left. Saito might have been there, but was too far away. Because he entered before Saito did, it is now his dream. So when he is kicked, the Limbo-city becomes his own level 2. Then he is kicked like he was with the bathtub at the beginning (the water would drown him in Limbo). However, he doesn't have enough time to escape (and he doesn't want to - he has to go get Saito), so he dies on level 1. Note (again) this is after Saito's death, so what is moments for Cobb is a lifetime for Saito.
Now for something different. I had a bit of a problem with manually entering Limbo with going down levels. Let's say that level 3 is the absolute limit you can go down. It is a fact that Cobb and mal stayed in Limbo for 50 years, and it is also fact that 20x is the standard for time dilation. 50 years equals about 18260 days. A level up that would mean 912.5 days. Two levels up that's 45.625 days. And three levels up that is 2.28 days. That equals about 54 hours under. That means in Reality, they went 2 days without food, water, the need to go... you know. I don't think they would have done that, and their kids would have noticed if they were out for that long. And besides, that amount of time is nearly 7x the amount people spend sleeping, on average. And you don't spend all 8 hours dreaming. Granted, a PASIV device can have one dream for 8 hours.
Also, that doesn't account for the infinite time dilation Limbo supposed to have. Yet the entire manually in the scenario in the article, and as a result the time dilation simply must be normal.
I propose a theory for their experiments gone wrong, and this can be applied to shared dreams in general. I won't go into the details of my ziz-zag theory just yet (that itself is for another time, and is for a different dream mechanic anyways.), but it says that no person can be the dreamer (or host) of more than one dream at a time. This would explain why Nash doesn't go down into the castle with them (he is already dreaming the apartment), Authur doesn't go to the fortress with them (he is dreaming the hotel), or Yusef doesn't go under with them (he hosts the rainy city).
What the theory stipulates is that if someone were to try to host more than one dream, the dream would instead change. You can't have more than one dream at once when alone, right? If done with multiple levels, a dream could potentially become the 1st and 3rd layer at the same time. If you kick yourself out of the 3rd layer and reach the second layer, no problem. Kick yourself from the second layer... you reach a dream that is both the first and third layer, again.
Because there is a third layer under the second layer, the second layer cannot collapse. This makes exiting the dream entirely impossible. If the third and first layer were to hypothetically collapse, then the second layer would have to collapse. Or if the second layer collapsed, presumably the third layer would collapse, which would destroy the first layer as a result. But where would such a collapse generate from when there isn't even a way to put one in motion?
Cobb and Mal, however, kept going forward. My theory is, that they pushed this paradox so far that the whole thing eventually collapsed and they wound up in an actual infinite time-dilation Limbo as they were supposed to be. Because it is already hard to believe they could get past 2 layers unaffected with just the two of them, because both would have to be a dream at some point (I am absolutely certain they never hooked up projections to the machine). If two is the limit, like Yusef said, then they couldn't have reached a place of infinite time dilation (or enough time dilation to account for the 50 years).
But, if anything, these are my thoughts on the matter. Whether I'm right or I'm wrong is subjective at this point. I'll leave it to others to decide.
EDIT: I just remembered why I started typing in the first place. Saito and Fischer both reach Limbo through death, but Saito ages way more considerably than Fischer does (at least, from what I can tell). This has inconsistant time dilation. That's all.