Concept of ‘World’ and ‘Universe’
The concept of world and common attitude shared by human beings differ due to the variations of the perception based on religious beliefs. Even the concept of world in MWI is somewhat different with several features.As we all know that each object in this world is made up of elementary particles such as protons, neutrons, electrons etc. Consequently, the world and hence the universe can be explained as the totality of these particles. But in MWI, such microscopic elementary particles might be in a superposition.
For an example, the probability of an individual radioactive element’s atom that will decay is indicated by quantum theory but within the ranges of probabilities it is hard to tell the precise time of the particular decay that will take place. Think that you have some atoms of radioactive elements which have a decaying chance of 50% in two hours. Then in two hours, 50% of those particular atoms will decay. Until the measurement is made for a given atom, there is no possible way to tell whether it is decayed or not according to traditional quantum theory (Copenhagen interpretation). In fact you need to treat the atoms if it is in a superposition of states: both decayed and not decayed. If it is so, then MWI concludes that there should be two universes in existence: one in which the atom is decayed and one which is not decayed.
If we measure a quantum object, it causes an actual split of the universe according to Everett. Here, literally the universe is duplicated by splitting each universe to accommodate for each possible outcome. For another example, think of an object’s wave function which is of both a particle and wave form. There are two possible outcomes if we measure this particular object as it can be either measured as a particle or a wave. So, a physicist in one universe finds out that the object can be measured in particle form. The same physicist in another universe measures the same object as a wave. This explains how one particle can be measured in several states.
Even if an action has several outcomes then the universe splits when that particular action is taken and it also happens when the chosen action is not put into action too. It means that if your pet is in a death situation as a possible outcome, then in a parallel universe to our one, your pet is dead.
In short in MWI, each time when a random event takes place, there is a split of universe between the various available options. A different outcome of that particular event is contained on each separate version of the universe. This results a creation of infinite number of quantum universes. In MWI, for each possible outcome of an action, the universe splits and it is an instantaneous process called as decohesion. This can be further explained using choose your own adventure story books. Here rather than choosing specific alternatives such as exploring the castle, reading old treasure map, find the treasure, fight with the enemy etc., the universe splits into copies so that each particular action is taken. When the universe splits, the particular person is unaware of his existence in other versions of universe. That means for example, the hero who found the treasure ends up living happily is completely unaware of his other version that entered the castle who now faces death and vise versa.
In fact, Everett postulated that all isolated systems evolve according to Schrödinger equation. In MWI, the universe is treated as an isolated system where there is continuous existence in a superposition of multiple states.
Concept of ‘I’
This particular personal identity concept seems to be ambiguous to ordinary living beings. How can a person be aware of his other selves’ existence and death in parallel universes? How do we ever know MWI is either a fantasy or a correct theory?
Myths of MWI
In late 1990s from a thought experiment which was an imagined experiment used to prove or disprove theoretically called quantum suicide gave an assurance that the MWI is theoretically possible. It was proposed by Max Tegmark in 1997.In quantum suicide, there is a man with a gun. When he pulls the trigger there are two possible outcomes: the gun either fires or not. Even the man either lives or dies as a result.
Each time the trigger is pulled, the universe splits to accommodate the possible outcomes. When the man dies, the universe is unable to split based on pulling off the trigger. The possible outcome for death is reduced to one continued death. Comparatively, with life there are two remaining chances: the man dies or continues living. When the man pulls the trigger and the universe splits in two versions. However the version where man lives will be unaware that in the other split universe he has died. He will continue to live and will again have the chance to pull the trigger. Each time he pulls the trigger, the universe will split again with the living version of the man continuing on an on while being unaware of all of his deaths that occur in parallel universes. In other words, he can exist indefinitely. This is what is known as quantum immortality.
If so, all of people who have attempted to kill themselves can be immortal? According to MWI, in some parallel universe they are. We are observers of another person’s suicide and this does not apply to us as the universal splitting is not dependent on our life and death situation. As observers we are subjected to probability. For example, when the gun went off in one particular universe in which version we witness the suicide, we are stuck with that particular outcome. Let us think that we continue to shoot the man but the universe will remain in a single state. Finally, once the man is dead, the number of possible outcomes for shooting the dead man is reduced to one.
Actually, this thought experiment renewed the interest of MWI which was highlighted for many years under arguments and debates in the scientific community. Yet, MWI is not only the theory that aids to explain neither our universe nor the suggestion of the existence of parallel worlds. MWI itself stands in contradictions with Copenhagen interpretation too. Hence it is not the most accepted quantum theory among physicists but it has become popular  over the years.
- Everett, H., (1957) ‘Relative State Formulation of quantum mechanics’, Review of Modern Physics 29, pp. 454-462
- Parfit, D., (1986) Reasons and Persons, New York: Oxford University Press
- Tegmark, M., (1997) ‘The Interpretation of Quantum Mechanics: Many Worlds or Many Words?’, Fundamental Problems in Quantum Theory, eds Rubin, M.H and Shih, Y.H, Wiley