Avengers 4 Theory: How Memory, B.A.R.F. And The Quantum Realm Explain Time Travel
1. Solving The Time Travel Problem
The one thing that Marvel have to really address with time travel is that it effectively ruins all sense of lasting impact once it's been introduced. As Doctor Strange confirmed, if you mess with time there will be consequences, because you've messed with the natural order of things. They can't just introduce a narrative explanation for time travel in the Quantum Realm and leave the loophole open or anyone could exploit it and change everything again and only Kevin Feige should have the power to reboot things frivolously.
But Quantum Energy being the power source for time travel fixes the problem. That's the only way they can possibly limit it and they MUST have a limit imposed. And it would be incredibly easy to suggest that Quantum Energy is finite - or that it needs to be "restocked" over a long time - in order for the time travel in Avengers 4 to be a "one-time only" sort of deal. Otherwise we'd just end up living a constant Doctor Strange-like infinite loop as the Avengers tried to Groundhog Day their way to victory.
Since that's unlikely, this has to be the answer.
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