Here are some arguments pro and con. The question that scientist are asking is not if there are parallel universes, but whether there are 0, 1, 2, 3 or 4 levels of multiverses. Cosmology observations support Level I by pointing to a at infinite space with ergodic matter distribution, and Level I plus inflation elegantly eliminates the initial condition problem. Level II is supported by the success of inflation theory in explaining cosmological observations, and it can explain apparent ne-tuning of physical parameters. Level III is supported by both experimental and theoretical evidence for unitarity, and explains the apparent quantum randomness that bothered Einstein so much without abandoning causality from the bird perspective. Level IV explains Wigner's unreasonable e ectiveness of mathematics for describing physics and answers the question "why these equations, not others?".
The principal arguments against parallel universes are that they are wasteful and weird. The first argument is that multiverse theories are vulnerable to Ockham's razor, since they postulate the existence of other worlds that we can never observe. Why should nature be so ontologically wasteful and indulge in such opulence as to contain an infinity of different worlds? But this argument can actually argue for a multiverse. When we feel that nature is wasteful, what precisely are we disturbed about her wasting? Certainly not "space", since the standard at universe model with its in nite volume draws no such objections. Certainly not "mass" or "atoms" either, for the same reason once you have wasted an in nite amount of something, who cares if you waste some more?
Rather, it is probably the apparent reduction in simplicity that appears disturbing, the quantity of information necessary to specify all these unseen worlds. However, as is discussed in more detail in Tegmark (1996), an entire ensemble is often much simpler than one of its members. For instance, the algorithmic information content of a generic integer n is of order log2 n (Chaitin 1987), the number of bits required to write it out in binary. Nonetheless, the set of all integers 1; 2; 3; ... can be generated by quite a trivial computer program, so the algorithmic complexity of the whole set is smaller than that of a generic member. Similarly, the set of all perfect fluid solutions to the Einstein eld equations has a smaller algorithmic complexity than a generic particular solution, since the former is speci ed simply by giving a few equations and the latter requires the speci cation of vast amounts of initial data on some hypersurface. The apparent information content rises when people restrict their attention to one particular element in an ensemble, thus losing the symmetry and simplicity that was inherent in the totality of all elements taken together. In this sense, the higher level multiverses have less algorithmic complexity.
Going from our universe to the Level I multiverse eliminates the need to specify initial conditions, upgrading to Level II eliminates the need to specify physical constants and the Level IV multiverse of all mathematical structures has essentially no algorithmic complexity at all. Since it is merely in the frog perspective, in the subjective perceptions of observers, that this opulence of information and complexity is really there, a multiverse theory is arguably more economical than one endowing only a single ensemble element with physical existence (Tegmark 1996).
The second common complaint about multiverses is that they are weird. This objection is aesthetic rather than scientific, and really only makes sense in the Aristotelian world view. In the Platonic paradigm, one might expect observers to complain that the correct TOE was weird if the bird perspective was sufficiently different from the frog perspective, and there is every indication that this is the case for us. The perceived weirdness is hardly surprising, since evolution provided us with intuition only for the everyday physics that had survival value for our distant ancestors.
Thanks to clever inventions, humans have glimpsed slightly more than the frog perspective of their normal inside view, and sure enough, they have encountered bizarre phenomena whenever departing from human scales in any way: at high speeds (time slows down), on small scales (quantum particles can be at several places at once), on large scales (black holes), at low temperatures (liquid Helium can ow upward), at high temperatures (colliding particles can change identity), and large already accepted that the frog and bird perspectivesare very different, a prevalent modern view of quantum eld theory is that the standard model is merely an effective theory, a low-energy limit of a yet to be discovered theory that is even more removed from our cozy classical concepts (involving strings in 10 dimensions, for example).
The principal arguments against parallel universes are that they are wasteful and weird. The first argument is that multiverse theories are vulnerable to Ockham's razor, since they postulate the existence of other worlds that we can never observe. Why should nature be so ontologically wasteful and indulge in such opulence as to contain an infinity of different worlds? But this argument can actually argue for a multiverse. When we feel that nature is wasteful, what precisely are we disturbed about her wasting? Certainly not "space", since the standard at universe model with its in nite volume draws no such objections. Certainly not "mass" or "atoms" either, for the same reason once you have wasted an in nite amount of something, who cares if you waste some more?
Rather, it is probably the apparent reduction in simplicity that appears disturbing, the quantity of information necessary to specify all these unseen worlds. However, as is discussed in more detail in Tegmark (1996), an entire ensemble is often much simpler than one of its members. For instance, the algorithmic information content of a generic integer n is of order log2 n (Chaitin 1987), the number of bits required to write it out in binary. Nonetheless, the set of all integers 1; 2; 3; ... can be generated by quite a trivial computer program, so the algorithmic complexity of the whole set is smaller than that of a generic member. Similarly, the set of all perfect fluid solutions to the Einstein eld equations has a smaller algorithmic complexity than a generic particular solution, since the former is speci ed simply by giving a few equations and the latter requires the speci cation of vast amounts of initial data on some hypersurface. The apparent information content rises when people restrict their attention to one particular element in an ensemble, thus losing the symmetry and simplicity that was inherent in the totality of all elements taken together. In this sense, the higher level multiverses have less algorithmic complexity.
Going from our universe to the Level I multiverse eliminates the need to specify initial conditions, upgrading to Level II eliminates the need to specify physical constants and the Level IV multiverse of all mathematical structures has essentially no algorithmic complexity at all. Since it is merely in the frog perspective, in the subjective perceptions of observers, that this opulence of information and complexity is really there, a multiverse theory is arguably more economical than one endowing only a single ensemble element with physical existence (Tegmark 1996).
The second common complaint about multiverses is that they are weird. This objection is aesthetic rather than scientific, and really only makes sense in the Aristotelian world view. In the Platonic paradigm, one might expect observers to complain that the correct TOE was weird if the bird perspective was sufficiently different from the frog perspective, and there is every indication that this is the case for us. The perceived weirdness is hardly surprising, since evolution provided us with intuition only for the everyday physics that had survival value for our distant ancestors.
Thanks to clever inventions, humans have glimpsed slightly more than the frog perspective of their normal inside view, and sure enough, they have encountered bizarre phenomena whenever departing from human scales in any way: at high speeds (time slows down), on small scales (quantum particles can be at several places at once), on large scales (black holes), at low temperatures (liquid Helium can ow upward), at high temperatures (colliding particles can change identity), and large already accepted that the frog and bird perspectivesare very different, a prevalent modern view of quantum eld theory is that the standard model is merely an effective theory, a low-energy limit of a yet to be discovered theory that is even more removed from our cozy classical concepts (involving strings in 10 dimensions, for example).
