Impossibility of alternative realities because we always make the decision we believe is best (which has nothing to do with the absolute best decision)
To understand why alternative realities are impossible, we must look at the mechanics of how a choice is actually made. The impossibility of alternative realities rests entirely on the fact that any decision-maker will always evaluate their available options and select what they believe is the single best possible decision.
This process has absolutely nothing to do with making the absolute best decision. Making the absolute best decision would require a person to be omniscient, knowing exactly how every choice would turn out. Because humans are not omniscient, our choices are strictly limited by our present state—specifically, our prior ability, knowledge, perceptions, memory, and reasoning available at the exact moment the decision is made.
Here is how this subjective belief inherently eliminates alternative realities by funneling a person toward a single, inevitable decision:
Decisions come from a limited set of known options: You can only choose from the options you are aware of. For instance, in a game of chess, a beginner only knows a small set of possible moves, while a grandmaster knows thousands. However, the size of the set does not matter; in both cases, the player evaluates their specific set of known moves and selects the single move they believe is the best one.
The belief strictly dictates the choice: Even if we assume a person has absolute “free will” that operates completely outside the laws of physics, they will still always evaluate their options and choose the one they think is best. It is logically impossible for a person to evaluate their options and intentionally select one they believe is a bad or worse move. Whatever motivation they have for choosing an option instantly makes it the “best” option in their mind. For example, if a chess player is playing against a dictator who will kill them if they win, they will intentionally lose the game. Losing the game becomes what they believe is the best move because it achieves their primary goal of staying alive.
A single belief produces a single outcome: Because a person will always—without exception—choose the single option they calculate to be best based on their specific knowledge at that exact second, only one possible choice is ever allowed.
Ultimately, because the subjective belief itself acts as a strict filter that can only ever produce one single decision, the individual will always behave in a completely deterministic way. Because you will always execute the one option you believe is best in that moment, you could never have chosen differently, which means that alternative realities are logically impossible.