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A multiverse (or meta-universe) is the hypothetical set of multiple possible universes (including our universe) that together comprise all of reality. The different universes within a multiverse are sometimes called parallel universes. 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 cosmology, physics, astronomy, philosophy, theology, and fiction, particularly in science fiction and fantasy. The specific term "multiverse," which was coined by William James, (James, William, The Will to Believe, 1895; and earlier in 1895, as cited in OED's new 2003 entry for "multiverse": "1895 W. JAMES in Internat. Jrnl. Ethics 6 10) Visible nature is all plasticity and indifference, a multiverse, as one might call it, and not a universe." was popularized by science fiction author Michael Moorcock.

The possibility of many universes raises various scientific and philosophical questions.

Multiverse hypotheses in physics

Laura Mersini-Houghton claims that the WMAP cold spot may provide testable empirical evidence for a parallel universe within the multiverse. According to Max Tegmark, (Scientific American, 2003, May, Parallel Universes) the existence of other universes is a direct implication of cosmological observations. 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. (Parallel Universes [1])


In order to clarify terminology, George Ellis, U. Kirchner and W. R. Stoeger recommend using the term "the Universe" for the theoretical model of the whole of the causally connected spacetime in which we live, universe domain for the observable universe or a similar part of the same space-time, "universe" for a general space-time, either our own "Universe" or another one disconnected from our own, multiverse for a set of disconnected space-times, and multi-domain universe to refer to a model of the whole of a single connected space-time in the sense of chaotic inflation models.

The levels according to Tegmark's classification and using Ellis, Koechner and Stoeger's terminology are briefly described below.

Multi-domain universes (Ellis, Koechner and Stoeger sense):

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.

Universes with different physical constants

Level II: (Andrei 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.)

Multiverses (Ellis, Koechner and Stoeger sense)

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 are concerned, it leads to no new results. This, according to Tegmark, "is ironic given that this level has historically been the most controversial". In September 2007 David Deutsch presented what is considered a proof of the many-worlds interpretation., Parallel universes exist - study, Sept 23 2007

Ultimate ensemble

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. Since this subsumes all other possible ensembles, it brings closure to the hierarchy of multiverses: there cannot be a Level V. The question is open whether or not scientists will subdivide Level IV in the future.

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.) Counter-intuitively, these universes are farther away than even the farthest universe in our open multiverse.

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 cosmic 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.

Many worlds 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, although in Everett's formulation they may potentially affect each other. Tegmark, Max, [2] The Interpretation of Quantum Mechanics: Many Worlds or Many Words?], 1998. To quote: "What Everett does NOT postulate: "At certain magic instances, the world undergoes some sort of metaphysical 'split' into two branches that subsequently never interact." This is not only a misrepresentation of the MWI, but also inconsistent with the Everett postulate, since the subsequent time evolution could in principle make the two terms...interfere. According to the MWI, there is, was and always will be only one wavefunction, and only decoherence calculations, not postulates, can tell us when it is a good approximation to treat two terms as non-interacting."</ref> through quantum interference. Deutsch, David, David Deutsch's Many Worlds, Frontiers, 1998. The state of the entire multiverse is related to the states of the constituent universes by quantum superposition, and is described by a single universal wavefunction. Related are Richard Feynman's multiple histories interpretation and H. Dieter Zeh's many-minds interpretation.

Many worlds interpretation cannot explain the apparently Fine-tuned universe. The physical constants of all the "many worlds" are the same. Many worlds interpretation can, however explain the apparent improbability of a planet like Earth existing. See Rare Earth hypothesis. If the Many worlds interpretation is true there are so many copies of our universe that the existence of at least one planet like Earth is not surprising.


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 different laws of physics.

String landscape

The string landscape theory asserts that a different universe exists for each of the very large ensemble of solutions generated when ten dimensional string theory is reduced to the four-dimensional low-energy world we see.

Criticisms of multiverse theories

Non-scientific claims

Critics claim that these theories lack empirical correlation and testability, and without hard physical evidence are unfalsifiable; outside the methodology of scientific investigation to confirm or disprove.

Bad science

Some have argued that the job of a 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.

Occam's Razor

To postulate an infinity of unseen and unseeable universes just to explain the one we do see seems superficially contrary to Occam's Razor.

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." [3] Thus, according to Tegmark, paradoxically the multiverse scenario is more parsimonious than that of a single universe.

David Lewis, however, draws a distinction between qualitative and quantitative excess. Postulating extra universe just like our own does not increase the number of kinds of things there are, and thus there is only qualitative invarience.

One unique universe

It is sometimes 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{. This possibility is also expressed in theories such as determinism and chaos theory. The hope is sometimes expressed that once a grand unified theory of everything is achieved, it will turn out to have a unique "solution" corresponding to the observed universe.


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 it only shifts the central problems of cosmology up one level, because of the need to explain where the meta-law comes from. Moreover, the set of such meta-laws is infinite, so it merely replaces the question "why this universe?" with "why this meta-law?". Rather than invoking an infinite number of universes, it would be simpler to postulate a single universe with a single principle. There is no reason to assume that any meta-law is fine-tuned. It is unknown whether or not any meta-law would have to be in a narrow range to allow at least one universe like ours. We do not know enough about any meta-law to say whether it needs to be fine-tuned or can be coarsely-tuned.

Tegmark maintains that in his multiverse theory this problem is circumvented because all possible meta-laws (or all possible unified theories) are in force and describe really-existing multiverses. However, his ultimate ensemble is still restricted to mathematical (or mathematically describable) laws, processes, and structures. If it is in any way possible for something non-mathematical to exist, his ensemble is not ultimate, and relies on a contingent meta-law law excluding the non-mathematical from actual existence.

Other objections

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".[4]

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 intelligent design as the only explanation of our existence.

Critics of this argument (Steven Jay Gould, Richard Dawkins and many others) point out that the cause and the effect have been reversed by those who claim that the universe seems to be fine-tuned for our benefit. Dr. Gould compared it to claiming that sausages were originally made long and narrow so that they would fit modern hotdog buns, or that humans evolved fingernails so that fingernail polish would be invented. Critics cite the vast store of evolutionary evidence which shows that life is perfectly and naturally tuned to the universe it arose in. Fossil, genetic and other biological evidence abundantly supports the observation that life adapts to physics, not the other way around.

The paleophysicist Caroline Miller writes: "The Anthropic Principle is based on the underlying belief that the universe was created for our benefit. Unfortunately for its adherents, all of the reality-based evidence at our disposal contradicts this belief. In a non-anthropocentric universe, there is no need for multiple universes or supernatural entities to explain life as we know it."

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 exist, and are just as real as the actual world (a position known as modal realism).

Trans-world identity

A metaphysical issue that crops up in 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. According to the counterpart theory of David Lewis, the objects should be regarded as similar rather than identical.<ref>Deutsch, Harry, "Relative Identity", The Stanford Encyclopedia of Philosophy (Summer '02), Edward N. Zalta (ed.)</ref><ref>Paul B. Kantor "The Interpretation of Cultures and Possible Worlds", 1 October 2002</ref>

Virtual realities as a multiverse

Some scientists entertain the possibility of creating artificial 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 reality. For such beings, their "fake" universe will appear indistinguishable from reality.

Multiverse hypotheses in religions around the globe

Christianity, Judaism and Islam all incorporate parts of this idea to explain the existence of three separate realities (Heaven, Hell and Earth) all existing simultaneously. The Christian Bible, Islamic Koran and Hebrew Torah all cite examples of heaven, hell and earth existing since the beginning of recorded time.

Hindu universes

The earliest known records describing the concept of a multiverse are found in ancient Hindu cosmology, in texts such as the Puranas. They expressed the idea of an infinite number of universes, each with its own gods, inhabitants and planets, and an infinite cycle of births, deaths, and rebirths of a universe, with each cycle lasting 8.4 billion years. The belief is too that the number of universes are infinite.<ref>Carl Sagan, Placido P D'Souza (1980s). Hindu cosmology's time-scale for the universe is in consonance with modern science.; Dick Teresi (2002). Lost Discoveries : The Ancient Roots of Modern Science – from the Babylonians to the Maya.</ref>

Fictional multiverses

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 TV show Sliders from the 1990's first popularized the concept through the TV entertainment medium. The film The One (2002) starring Jet Li carried the same idea to the action film medium. The popular MYST computer game franchise uses concepts of describing a world and then linking to that world, which is part of a multiverse of infinite possible and concurrently existing universes, matching the descriptions. Also, the computer game Legacy of Kain: Soul Reaver and its sequels feature two distinct parallel universes. The protagonist, Raziel, is capable of existing in both the Material Realm, or normal reality, and the Spectral Realm, a dark and distorted version of the former with its own physics and properties. The Michael Crichton novel Timeline featured a method for what appeared to be time travel by traveling to parallel universes that are identical except for the moment of their birth, thus rendering off-set yet parallel time.

The DC Universe, famous home of Batman and Superman, uses the multiverse as the basis for their universe. This is in part to help deal with their 67 year history, beginning in The Flash issue #123. In the 1980s DC published the ever popular Crisis on Infinite Earths which detailed a breakdown of the Multiverse at the hands of the Anti-Monitor. The television series Star Trek has many times gone into parallel "Mirror" universes, and Stargate SG-1 has postulated parallel universes. The Anime series Bokurano is based on a multiverse. In the BBC television show Doctor Who, the Doctor travels between parallel universes and is parted indefinitely from his companion Rose Tyler when she becomes trapped within one. The fantasy trilogy His Dark Materials by Philip Pullman is set in multiple parallel universes.


Criticism of Many worlds in one

Parallel universes exist - study

See also

External links