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Multiverse is a theory offering more than one universe.
Multiverse (or meta-universe) is the hypothetical set of multiple possible universes (including our universe) that together comprise all of physical reality. The structure of the multiverse, the nature of each universe within it and the relationship between the various constituent universes, depend on the specific multiverse hypothesis considered.
Multiverses have been hypothesized in physics, philosophy, and fiction, particularly in science fiction. The specific term "multiverse", which was originally coined by William James, was popularised by science fiction author Michael Moorcock. In these contexts, such terms as "alternate universes", "parallel universes", or "parallel worlds" may often be used.
The possibility of many universes raises various scientific, philosophical, and theological questions.
Multiverse hypotheses in physics.
According to Max Tegmark, the existence of other universes is a direct implication of cosmological observations. In a 2003 Scientific American article he presents a clear and comprehensive summary. Tegmark describes the set of related concepts which share the notion that there are universes beyond the familiar observable one, and goes on to provide a taxonomy of parallel universes organized by levels. The levels are briefly described below.
Level I: (Open multiverse) A generic prediction of cosmic inflation is an infinite ergodic universe, which, being infinite, must contain Hubble volumes realizing all initial conditions - including an identical copy of a given person about meters away.
Level II: (Andre Linde's bubble theory) In chaotic inflation, other thermalized regions may have different effective physical constants, dimensionality and particle content. (Surprisingly, this level includes Wheeler's oscillating universe theory as well.)
Level III: (Hugh Everett III's Many-worlds interpretation) An interpretation of quantum mechanics that proposes the existence of multiple universes, all of which are "identical", but exist in possibly different states. It is widely believed that Everett's interpretation (considered as a formal theory) is a conservative extension of standard quantum mechanics -- that is, as far as results expressible in the language of ordinary quantum mechanics is concerned, it leads to no new results. This, according to Tegmark, "is ironic given that this level has historically been the most controversial".
Level IV: (The ultimate "Ensemble theory" of Tegmark) Other mathematical structures give different fundamental equations of physics. This level considers "real" any hypothetical universe based on one of these structures. M-Theory might be placed here. Since this subsumes all other possible ensembles, it brings closure to the hierarchy of multiverses: there cannot be a Level V.
That the universe extends infinitely and rather uniformly in all directions is uncontroversial among physicists. (A finite universe is a minority view.)
The Theory of Relativity places a firm upper limit on the speed at which information can travel, effectively dividing this infinite space into "local" universes. Our Observable universe, for example, is a sphere centered on the Earth (centered, that is, on whoever's doing the calculating), currently about 14 billion light years in radius, called the Hubble volume. The size of the Hubble volume is directly related to the age of the universe; it grows at a rate of one light year per year, or exactly the speed of light.
Thus, there are an infinite number of regions of space the same size as our Observable universe -- an infinite number of observable universes, that is. This infinite set (which must contain, among other things, an infinite number of identical copies of you, the nearest of which is about meters away, and an equally infinite number of not-quite-identical copies) comprises the level-I multiverse.
Overtly or not, physicists often use the idea of an Open Multiverse when evaluating theories. For example, Max Tegmark writes:
Multiverse bubble theory.
Bubble theory posits an infinite number of open multiverses, each with different physical constants. (The set of bubble universes is thus a Level II multiverse.) Counterintuitively, these universes are farther away than even the farthest universe in our open multiverse, which is itself infinitely far from us.
The formation of our universe from a "bubble" of a multiverse was proposed by Andre Linde. This Bubble universe theory fits well with the widely accepted theory of inflation. The bubble universe concept involves creation of universes from the quantum foam of a "parent universe." On very small scales, the foam is frothing due to energy fluctuations. These fluctuations may create tiny bubbles and wormholes. If the energy fluctuation is not very large, a tiny bubble universe may form, experience some expansion like an inflating balloon, and then contract and disappear from existence. However, if the energy fluctuation is greater than a particular critical value, a tiny bubble universe forms from the parent universe, experiences long-term expansion, and allows matter and large-scale galactic structures to form.
According to some quantum loop gravity theorists, the Big Bang was merely the beginning of a period of expansion that followed a period of contraction. In this oscillatory universe hypothesis (originally attributable to John Wheeler), the universe undergoes an infinite series of oscillations, each beginning with a big bang and ending with a Big Crunch. After the big bang, the universe expands for a while before the gravitational attraction of matter causes it to collapse back in and undergo a Big bounce. Although the model was abandoned for a time, the theory has been revived in brane cosmology as the cyclic model.
Like Bubble Theory, this oscillatory view posits a Level-II multiverse.
Multiverse many world interpretation of quantum physics.
Hugh Everett's Many-worlds interpretation (MWI) is one of several mainstream interpretations of quantum mechanics. Other interpretations include the Copenhagen and the Consistent histories interpretations. The multiverse proposed by MWI has a shared time parameter. In most formulations, all the constituent universes are structurally identical to each other and though they have the same physical laws and values for the fundamental constants, they may exist in different states. The constituent universes are furthermore non-communicating, in the sense that no information can pass between them. The state of the entire multiverse is related to the states of the constituent universes by quantum superposition. Related are Richard Feynman's multiple histories interpretation and H. Dieter Zeh's many-minds interpretation.
A multiverse of a somewhat different kind has been envisaged within the 11-dimensional extension of string theory known as M-Theory. In M-theory our universe and others are created by collisions between membranes in an 11-dimensional space. Unlike the universes in the "quantum multiverse", these universes can have completely different laws of physics-anything may be possible.
Multiverse string landscape.
Another proposal for a multiverse in string theory has received considerable attention lately. It is called the string landscape and asserts that, roughly speaking, there are a very large number of ways to go from ten dimensional string theory down to the four-dimensional low-energy world we see, and each one of these corresponds to a radically different universe.
Arguments against multiverse theories.
Critics claim that there is a lack of empirical correlation and testability in these theories and thus they are without hard physical evidence and must therefore be considered unfalsifiable, currently outside the methodology of scientific investigation to confirm or disprove, and therefore more mathematically theoretical and metaphysical than scientific in nature.
Tegmark notes that improved measurements of the microwave background radiation and of the large-scale distribution of matter may fortify or knock down two pillars of the multiverse, that of the infinitude of space and the theory of chaotic inflation. So some facets of the theory may be testable, at least.
Multiverse: It’s bad science.
Some have argued that the job of the scientist is to provide fundamental explanations for observed phenomena, without making reference to observers. Resorting to anthropic principles constitutes a "lazy way out" of accounting for features such as the apparent fine-tuning of parameters in relation to the existence of life.
Leonard Susskind claims however that some form of multiverse is unavoidable, given the current state of physics, and that observer effects are inevitable and have to be taken into account in other sciences.
Multiverse alternate universes violate Occam's razor.
To postulate an infinity of unseen and unseeable universes just to explain the one we do see seems like a case of excess baggage carried to the extreme.
Tegmark answers:"A common feature of all four multiverse levels is that the simplest and arguably most elegant theory involves parallel universes by default. To deny the existence of those universes, one needs to complicate the theory by adding experimentally unsupported processes and ad hoc postulates: finite space, wave function collapse and ontological asymmetry. Our judgment therefore comes down to which we find more wasteful and inelegant: many worlds or many words." Thus, according to Tegmark, paradoxically the multiverse scenario is more parsimonious than that of a single universe.
Multiverse: There is only one possible universe.
It is occasionally argued that the observed universe is the unique possible universe, so that talk of "other" universes is ipso facto meaningless. Einstein raised this possibility when he wondered whether the universe could have been otherwise, or non-existent altogether. The hope is sometimes expressed that once a fully unified theory of physics is achieved, it will turn out to have a unique "solution" corresponding to the observed universe.
Multiverse measures of fine-tuning are meaningless.
The principal observational support for the multiverse hypothesis comes from the Anthropic Principle: the universe we observe is bio-friendly, or we would not be observing it. While this is a tautology, when the sensitivity of biology to the form of the laws of physics and the cosmological initial conditions is considered, it has some apparent credence; but on the other hand, many key parameters of physics do not seem to be very strongly constrained by biology.
Another criticism of the fine-tuning argument is that, as far as we know, there could be a more fundamental law under which the parameters of physics must have the values they do. Thus, given such a law, it is not improbable that the known parameters of physics fall within the life-permitting range.
Multiverses merely shift the problem up one level.
Multiverse proponents are often vague about how the parameter values are selected across the defined ensemble. If there is a "law of laws" or meta-law describing how parameter values are assigned from one universe to the next, then we have only shifted the central problems of cosmology up one level, because we need to explain where the meta-law comes from. Moreover, the set of such meta-laws is infinite, so we have merely replaced the question "why this universe?" with "why this meta-law?". There would seem to be little point in invoking an infinity of universes when it would be simpler to postulate a single universe with a single principle.
In Tegmark’s extreme multiverse theory this problem is circumvented, because in that case all possible meta-laws (or all possible unified theories) are in force and describe really-existing multiverses.
Multiverse and the fake universe problem.
Most scientists are prepared to entertain the possibility of conscious machines, and some artificial intelligence advocates even claim we are not far from producing conscious computers. It is then but a small step to the point where the engineered conscious beings inhabit a simulated world. For such beings, their "fake" universe will appear indistinguishable from reality. So should we include these simulated universes in the ensemble that constitutes the multiverse? Is it meaningful to assign equal ontological status to our own, observed, universe and universes that are virtual? If it is not then is it meaningful to assign equal ontological status to our own, observed, universe and universes that can never be observed by any sentient being?
Multiverse: Why stop there?
The last objection to the existing multiverse theories is a challenge to the criteria for defining universes. In most multiverse theories, universes are labeled by laws of physics and initial conditions. It might be objected that these terms are narrow and chauvinistic; there may be criteria for categorization that lie completely beyond the scope of human comprehension.
Multiverse hypotheses in philosophy: Anthropic principle.
The concept of other universes has been proposed to explain why our universe seems to be fine-tuned for conscious life as we experience it. If there were a large number (possibly infinite) of different physical laws (or fundamental constants) in as many universes, some of these would have laws that were suitable for stars, planets and life to exist. The anthropic principle could then be applied to conclude that we would only consciously exist in those universes which were finely-tuned for our conscious existence. Thus, while the probability might be extremely small that there is life in most of the multiverses, this scarcity of life-supporting universes does not imply design is the only explanation of our existence.
The entire range of multiverse hypotheses, with specific emphasis on Hugh Everett's many-worlds interpretation, have been criticised by proponents of intelligent design. William Dembski in particular, derides it as inflating explanatory resources without evidence or warrant, and terms such concepts "inflatons". Recent pronouncements by Church authorities suggest the Catholic Church, now appears to reject all such hypotheses as well in what may be a reversal of a long standing hands-off policy regarding the physical sciences.
Multiverse: Modal realism.
Additionally, possible worlds are a way of explaining probability, hypothetical statements and the like, and some philosophers such as David Lewis believe that all possible worlds actually exist (a position known as Modal realism). This thesis is one of the central tenets of his book.
Trans-world identity issues of multiverse.
A metaphysical issue that crops up with multiverse schema that posit infinite identical copies of any given universe is that of the notion that there can be identical objects in different possible worlds.
The problem lies in the tension between classical notions of identity and quantum indeterminacy. In short, quantum reality does not allow classical - radically mechanical - 'identities' due to the Heisenberg Uncertainty Principle. However, in an infinite set of possible universes such a correspondence is presumed to exist. The question then becomes whether one can claim a distinction between entities that vary only in terms of some arbitrary dimensional metric in de Sitter space.
Multiverse: Suggested resolutions include the possibilities that:
Trans-world identity is also considered in depth in possible worlds concepts.
Parallel universe (fiction)
The concept of the multiverse figures prominently in many science fiction and fantasy novels. For some it serves primarily as a plot device, a means to put characters into an unfamiliar situation, or a framework that usually lies in the background for continuity purposes. For others it is a major theme and focus of the work. It is sometimes used as the basis for exploring "what if" scenarios, such as in Alternate history stories. The movie "The One" (2002) starring Jet Li was one of the first movies to popularize the theory.
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