Editing Multiverse (section)

==== Level III: Many-worlds interpretation of quantum mechanics ====
[[File:Schroedingers cat film.svg|thumb|[[Schrödinger's cat]] in the many-worlds interpretation, where a branching of the universe occurs through a superposition of two quantum mechanical states]]
[[Hugh Everett III]]'s [[many-worlds interpretation]] (MWI) is one of several mainstream [[interpretations of quantum mechanics]].

In brief, one aspect of quantum mechanics is that certain observations cannot be predicted absolutely. Instead, there is a range of possible observations, each with a different [[probability]]. According to the MWI, each of these possible observations corresponds to a different "world" within the [[Universal wavefunction]], with each world as real as ours. Suppose a six-sided dice is thrown and that the result of the throw corresponds to [[observable]] quantum mechanics. All six possible ways the dice can fall correspond to six different worlds. In the case of the Schrödinger's cat thought experiment, [[Schrödinger's cat#Many-worlds interpretation and consistent histories|both outcomes would be "real" in at least one "world"]].

Tegmark argues that a Level III multiverse does not contain more possibilities in the Hubble volume than a Level I or Level II multiverse. In effect, all the different worlds created by "splits" in a Level III multiverse with the same physical constants can be found in some Hubble volume in a Level I multiverse. Tegmark writes that, "The only difference between Level I and Level III is where your [[doppelgänger]]s reside. In Level I they live elsewhere in good old three-dimensional space. In Level III they live on another quantum branch in infinite-dimensional [[Hilbert space]]."

Similarly, all Level II bubble universes with different physical constants can, in effect, be found as "worlds" created by "splits" at the moment of spontaneous symmetry breaking in a Level III multiverse.<ref name="TegmarkPUstaple"/> According to [[Yasunori Nomura]],<ref name="Nomura"/> [[Raphael Bousso]], and [[Leonard Susskind]],<ref name="BoussoSusskind"/> this is because global spacetime appearing in the (eternally) inflating multiverse is a redundant concept. This implies that the multiverses of Levels I, II, and III are, in fact, the same thing. This hypothesis is referred to as "Multiverse = Quantum Many Worlds". According to [[Yasunori Nomura]], this quantum multiverse is static, and time is a simple illusion.<ref>{{Cite journal |arxiv = 1205.5550|doi = 10.1103/PhysRevD.86.083505|title = Static quantum multiverse|journal = Physical Review D|volume = 86|issue = 8|pages = 083505|year = 2012|last1 = Nomura|first1 = Yasunori|last2 = Johnson|first2 = Matthew C.|last3 = Mortlock|first3 = Daniel J.|last4 = Peiris|first4 = Hiranya V.|bibcode = 2012PhRvD..86h3505N|s2cid = 119207079}}</ref>

Another version of the many-worlds idea is [[H. Dieter Zeh]]'s [[many-minds interpretation|''many-minds'' interpretation]].