Reading some time travel stories which caused me to think about the issue of time again. [Below has nothing about time travel] We often heard that the future is unknown or not fixed, while the past is permanent. For example From some threads around the forum in the physics section, some mentioned about that the things we saw is a fraction of a second in the past. In quantum mechanics, there is something known as the Many Worlds Interpretation, which (If I recalled properly) that for every event, the universes splits into many and each of these represent the various different outcomes and that they cannot interact with each other (because of something known as quantum decoherence, which I'm not really that familiar) My question is, since in the perspective of a more distant past, what we known as 'past' is the future. If there are multiple possible futures, why there cannot be multiple possible past leading to the same future or present? In short: Why we only observe one past, but not mutiple in both directions?