The idea of parallel universes have been proposed by Giordano Bruno in 1600. He burned at the stake for heresy. Nowadays, physicists are talking about parallel universes as a mean to explain the universe in its whole. In fact, it put the problem more far because instead of finding a theory of one universe, the new objective become to find a theory about the multiverse which include all the parallel universes. The universe will become one among many and the whole universe will be called: “multiverse”.
Why people think about more than one universe? Does it seems obvious? In fact, it is a common practice to think about parallel universe in which you did a different choice in the past, creating a completely new environment (or new world). In mathematics it wouldn’t be called parallel in this case but “divergent universe”. It is true that the divergent universe will then be parallel to the original universe because they won’t interact each other after their separation. But what is the point to talking about many universes if we aren’t able to interact with them. Does it complicate the theory? The Occam’s Razor principle said that if theories coexist explaining the world, we must choose the most simple. But, new discovery in Quantum physics make scientists think that a parallel universe theory could simplify the whole universe theory. So we should choose this theory even if we can’t demonstrate right now that these parallel universes really exist.
In Quantum Physics, particles (which composed the matter) are impossible to measure accurately because the measurement influenced them. It is true that the measurement technologies and materials are also composed of matter and thus interact with the measured matter. The only possible way to know what the state before the measurement affects them, is to predict in which way the measurement will affect the matter. Scientists draw probabilistic schema in order to guess the real state of the particles. Chance seems to govern the world. It is not good for scientist s who want to know the universe law.
Some get the idea to think about parallel universe where each possible states of a particle are in each of these universe. The measurement we make correspond to our own universe. The other universe which have different initial conditions or even different physics law behave differently and thus the measurement show another state in their specific cases.