Hugh Everett's MWI According to Hugh Everett's many worlds interpretation, all the possibilities of one action can happen at the same time in other parallel universes, so how come we can't see these worlds? now I bumped into something called the quantum decoherence but I can't seem to get how this decoherence work?
 A: To "see another World" would require doing a measurement that involves (partially) reversing the effect that led to the split. In practice this is impossible to realize because the observer is a macroscopic object itself and it will decohere very fast. Decoherence means that  the system becomes correlated with the environment and that poses a big problem here because you would have to reverse the effects of all this. This is similar to why entropy always increases and gives rise to irreversible phenomena on the macroscopic scale.
But you can still contemplate thought experiments and demonstrate how one can perform such measurements in principle. David Deutsch has proposed the following thought experiment to prove the existence of parallel World. Suppose that we create a virtual observer inside a quantum computer. The observer prepares a spin polarized in the x-direction. He then measures the z-component of the spin. The z-component can be found to be 1/2 or -1/2 with equal probability.
According to the MWI, there are two Worlds were both possibilities are realized. Suppose that we then reverse the act of measurement, but such that the observer will keep the memory of having performed the measurement (a complete reversal would necesaarily mean that the observer's memory has been restored to what it was initially). Now it's easy to show that this can be realized by a unitary transform, so it is an operation that can in principle be performed on a quantum computer.
In that final state, the spin is restored in the orgina state where it is polarized in the x-direction and the observer can verify that this is the case by doing additional measurements on it. However the observer also knows that he had measured the z-component of the spin before, but if only one World really exists after the measurement, then the initial state of the spin could never have been restored by that unitary transform.
