The multiverse is a hypothetical group of multiple universes. Together, these universes comprise everything that exists: the entirety of space, time, matter, energy, information, and the physical laws and

constants Constant or The Constant may refer to: Mathematics * Constant (mathematics), a non-varying value * Mathematical constant, a special number that arises naturally in mathematics, such as or Other concepts * Control variable or scientific const ...

that describe them. The different universes within the multiverse are called "parallel universes", "other universes", "alternate universes", or "many worlds".

History of the concept

According to some, the idea of infinite worlds was first suggested by the pre-Socratic Greek philosopher

Anaximander Anaximander (; grc-gre, Ἀναξίμανδρος ''Anaximandros''; ) was a pre-Socratic Greek philosopher who lived in Miletus,"Anaximander" in ''Chambers's Encyclopædia''. London: George Newnes, 1961, Vol. 1, p. 403. a city of Ionia (in moder ...

in the sixth century BCE. However, there is debate as to whether he believed in multiple worlds, and if he did, whether those worlds were co-existent or successive. The first to whom we can definitively attribute the concept of innumerable worlds are the Ancient Greek Atomists, beginning with Leucippus and Democritus in the 5th century BCE, followed by Epicurus (341-270 BCE) and Lucretius (1st century BCE). In the third century BCE, the philosopher

Chrysippus Chrysippus of Soli (; grc-gre, Χρύσιππος ὁ Σολεύς, ; ) was a Greek Stoic philosopher. He was a native of Soli, Cilicia, but moved to Athens as a young man, where he became a pupil of the Stoic philosopher Cleanthes. When Clean ...

suggested that the world eternally expired and regenerated, effectively suggesting the existence of multiple universes across time. The concept of multiple universes became more defined in the Middle Ages. The American philosopher and psychologist William James used the term "multiverse" in 1895, but in a different context. The concept first appeared in the modern scientific context in the course of the debate between Boltzmann and Zermelo in 1895. In Dublin in 1952, Erwin Schrödinger gave a lecture in which he jocularly warned his audience that what he was about to say might "seem lunatic". He said that when his equations seemed to describe several different histories, these were "not alternatives, but all really happen simultaneously". This sort of duality is called " superposition". In his 1930 autobiography ''My Early Life'',

Winston Churchill Sir Winston Leonard Spencer Churchill (30 November 187424 January 1965) was a British statesman, soldier, and writer who served as Prime Minister of the United Kingdom twice, from 1940 to 1945 Winston Churchill in the Second World War, dur ...

cited the theory when explaining his preference for "believing whatever I want to believe":

Certainly nothing could be more repulsive to both our minds and feelings than the spectacle of thousands of millions of universes – for that is what they say it comes to now – all knocking about together for ever without any rational or good purpose behind them.

The term was first used in fiction in September 1961 in the DC comic book titled Flash of Two Worlds (Flash Volume 1 #123) by

Carmine Infantino Carmine Michael Infantino (; May 24, 1925 – April 4, 2013) was an American comics artist and editing, editor, primarily for DC Comics, during the late 1950s and early 1960s period known as the Silver Age of Comic Books. Among his character creat ...

and Gardner Fox. In the story, Flash meets with his duplicate version of another Earth (Earth-2) and another Flash (Flash-2). The term was first used in fiction in its current physics context by Michael Moorcock in his 1963 SF Adventures novella ''The Sundered Worlds'' (part of his Eternal Champion series). (see Multiverse (Michael Moorcock))

Brief explanation

Multiple universes have been hypothesized in cosmology, physics, astronomy, religion,

philosophy Philosophy (from , ) is the systematized study of general and fundamental questions, such as those about existence, reason, knowledge, values, mind, and language. Such questions are often posed as problems to be studied or resolved. Some ...

, transpersonal psychology, music, and all kinds of literature, particularly in science fiction,

comic books A comic book, also called comicbook, comic magazine or (in the United Kingdom and Ireland) simply comic, is a publication that consists of comics art in the form of sequential juxtaposed panels that represent individual scenes. Panels are of ...

and fantasy. In these contexts, parallel universes are also called "alternate universes", "quantum universes", "interpenetrating dimensions", "parallel universes", "parallel dimensions", "parallel worlds", "parallel realities", "quantum realities", "alternate realities", " alternate timelines", "alternate dimensions" and "dimensional planes". The physics community has debated the various multiverse theories over time. Prominent physicists are divided about whether any other universes exist outside of our own. Some physicists say the multiverse is not a legitimate topic of scientific inquiry. Concerns have been raised about whether attempts to exempt the multiverse from experimental verification could erode public confidence in science and ultimately damage the study of fundamental physics. Some have argued that the multiverse is a philosophical notion rather than a scientific hypothesis because it cannot be empirically falsified. The ability to disprove a theory by means of scientific experiment is a critical criterion of the accepted scientific method. Paul Steinhardt has famously argued that no experiment can rule out a theory if the theory provides for all possible outcomes. In 2007, Nobel laureate Steven Weinberg suggested that if the multiverse existed, "the hope of finding a rational explanation for the precise values of quark masses and other constants of the standard model that we observe in our

Big Bang The Big Bang event is a physical theory that describes how the universe expanded from an initial state of high density and temperature. Various cosmological models of the Big Bang explain the evolution of the observable universe from the ...

is doomed, for their values would be an accident of the particular part of the multiverse in which we live."

Search for evidence

Around 2010, scientists such as Stephen M. Feeney analyzed

Wilkinson Microwave Anisotropy Probe The Wilkinson Microwave Anisotropy Probe (WMAP), originally known as the Microwave Anisotropy Probe (MAP and Explorer 80), was a NASA spacecraft operating from 2001 to 2010 which measured temperature differences across the sky in the cosmic mic ...

(WMAP) data and claimed to find evidence suggesting that this universe collided with other (parallel) universes in the distant past. However, a more thorough analysis of data from the WMAP and from the

Planck satellite ''Planck'' was a space observatory operated by the European Space Agency (ESA) from 2009 to 2013, which mapped the anisotropies of the cosmic microwave background (CMB) at microwave and infrared frequencies, with high sensitivity and small angu ...

, which has a resolution three times higher than WMAP, did not reveal any statistically significant evidence of such a

bubble universe Eternal inflation is a hypothetical inflationary universe model, which is itself an outgrowth or extension of the Big Bang theory. According to eternal inflation, the inflationary phase of the universe's expansion lasts forever throughout most of ...

collision.. In addition, there was no evidence of any gravitational pull of other universes on ours.

Proponents and skeptics

Modern proponents of one or more of the multiverse hypotheses include Don Page, Brian Greene, Max Tegmark, Alan Guth,

Andrei Linde Andrei Dmitriyevich Linde (russian: Андре́й Дми́триевич Ли́нде; born March 2, 1948) is a Russian-American theoretical physicist and the Harald Trap Friis Professor of Physics at Stanford University. Linde is one of the ...

, Michio Kaku, David Deutsch, Leonard Susskind, Alexander Vilenkin, Yasunori Nomura, Raj Pathria, Laura Mersini-Houghton, Neil deGrasse Tyson, Sean Carroll and Stephen Hawking. Scientists who are generally skeptical of the multiverse hypothesis include David Gross, Paul Steinhardt, Anna Ijjas, Abraham Loeb, David Spergel, Neil Turok,

Viatcheslav Mukhanov Viatcheslav Fyodorovich Mukhanov (russian: Вячесла́в Фёдорович Муха́нов; born October 2, 1956) is a Soviet/ Russian born German theoretical physicist and cosmologist. He is best known for the theory of Quantum Origin ...

Michael S. Turner Michael S. Turner (born July 29, 1949) is an American theoretical cosmologist who coined the term ''dark energy'' in 1998. He is the Rauner Distinguished Service Professor Emeritus of Physics at the University of Chicago, having previously serv ...

Roger Penrose Sir Roger Penrose (born 8 August 1931) is an English mathematician, mathematical physicist, philosopher of science and Nobel Laureate in Physics. He is Emeritus Rouse Ball Professor of Mathematics in the University of Oxford, an emeritus fello ...

, George Ellis,

Joe Silk Joseph Ivor Silk Fellow of the Royal Society, FRS (born 3 December 1942) is a British-American astrophysicist. He was the Savilian Chair of Astronomy at the University of Oxford from 1999 to September 2011. He is an Emeritus Fellow of New C ...

Carlo Rovelli Carlo Rovelli (born May 3, 1956) is an Italian theoretical physicist and writer who has worked in Italy, the United States and, since 2000, in France. He is also currently a Distinguished Visiting Research Chair at the Perimeter Institute, and c ...

Adam Frank Adam Frank (born August 1, 1962) is an American physicist, astronomer, and writer. His scientific research has focused on computational astrophysics with an emphasis on star formation and late stages of stellar evolution. His work includes studi ...

, Marcelo Gleiser,

Jim Baggott James Edward Baggott (born 2 March 1957) is a British science writer living in Reading, Berkshire, England who writes about science, philosophy and science history. Baggott is the author of nine books, including ''Farewell to Reality: How Modern ...

and Paul Davies.

Arguments against multiverse theories

In his 2003 ''New York Times'' opinion piece, "A Brief History of the Multiverse", author and cosmologist Paul Davies offered a variety of arguments that multiverse theories are non-scientific: George Ellis, writing in August 2011, provided a criticism of the multiverse, and pointed out that it is not a traditional scientific theory. He accepts that the multiverse is thought to exist far beyond the cosmological horizon. He emphasized that it is theorized to be so far away that it is unlikely any evidence will ever be found. Ellis also explained that some theorists do not believe the lack of

empirical Empirical evidence for a proposition is evidence, i.e. what supports or counters this proposition, that is constituted by or accessible to sense experience or experimental procedure. Empirical evidence is of central importance to the sciences and ...

testability and falsifiability is a major concern, but he is opposed to that line of thinking: Ellis says that scientists have proposed the idea of the multiverse as a way of explaining the nature of existence. He points out that it ultimately leaves those questions unresolved because it is a metaphysical issue that cannot be resolved by empirical science. He argues that observational testing is at the core of science and should not be abandoned: Philosopher Philip Goff argues that the inference of a multiverse to explain the apparent fine-tuning of the universe is an example of

Inverse Gambler's Fallacy The inverse gambler's fallacy, named by philosopher Ian Hacking, is a formal fallacy of Bayesian inference which is an inverse of the better known gambler's fallacy. It is the fallacy of concluding, on the basis of an unlikely outcome of a rando ...

Types

Max Tegmark and Brian Greene have devised classification schemes for the various theoretical types of multiverses and universes that they might comprise.

Max Tegmark's four levels

Cosmologist Max Tegmark has provided a taxonomy of universes beyond the familiar observable universe. The four levels of Tegmark's classification are arranged such that subsequent levels can be understood to encompass and expand upon previous levels. They are briefly described below.

Level I: An extension of our universe

A prediction of

cosmic inflation In physical cosmology, cosmic inflation, cosmological inflation, or just inflation, is a theory of exponential expansion of space in the early universe. The inflationary epoch lasted from seconds after the conjectured Big Bang singularity ...

is the existence of an infinite

ergodic In mathematics, ergodicity expresses the idea that a point of a moving system, either a dynamical system or a stochastic process, will eventually visit all parts of the space that the system moves in, in a uniform and random sense. This implies tha ...

universe, which, being infinite, must contain

Hubble volume In cosmology, a Hubble volume (named for the astronomer Edwin Hubble) or Hubble sphere, subluminal sphere, causal sphere and sphere of causality is a spherical region of the observable universe surrounding an observer beyond which objects recede ...

s realizing all initial conditions. Accordingly, an infinite universe will contain an infinite number of Hubble volumes, all having the same physical laws and

physical constant A physical constant, sometimes fundamental physical constant or universal constant, is a physical quantity that is generally believed to be both universal in nature and have constant value in time. It is contrasted with a mathematical constant, ...

s. In regard to configurations such as the distribution of matter, almost all will differ from our Hubble volume. However, because there are infinitely many, far beyond the cosmological horizon, there will eventually be Hubble volumes with similar, and even identical, configurations. Tegmark estimates that an identical volume to ours should be about 10^10¹¹⁵ meters away from us. Given infinite space, there would, in fact, be an infinite number of Hubble volumes identical to ours in the universe."Parallel universes. Not just a staple of science fiction, other universes are a direct implication of cosmological observations.", Tegmark M., Sci Am. 2003 May;288(5):40–51. This follows directly from the cosmological principle, wherein it is assumed that our Hubble volume is not special or unique.

Level II: Universes with different physical constants

In the eternal inflation theory, which is a variant of the

theory, the multiverse or space as a whole is stretching and will continue doing so forever, but some regions of space stop stretching and form distinct bubbles (like gas pockets in a loaf of rising bread). Such bubbles are embryonic level I multiverses. Different bubbles may experience different spontaneous symmetry breaking, which results in different properties, such as different

s. Level II also includes John Archibald Wheeler's

oscillatory universe A cyclic model (or oscillating model) is any of several cosmological models in which the universe follows infinite, or indefinite, self-sustaining cycles. For example, the oscillating universe theory briefly considered by Albert Einstein in 1930 ...

theory and Lee Smolin's

fecund universes theory Lee Smolin (; born June 6, 1955) is an American theoretical physicist, a faculty member at the Perimeter Institute for Theoretical Physics, an adjunct professor of physics at the University of Waterloo and a member of the graduate faculty of t ...

Level III: Many-worlds interpretation of quantum mechanics

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 universe, with some or many of the interpretation's proponents suggesting that these universes are as real as ours. Suppose a six-sided die is thrown and that the result of the throw corresponds to quantum mechanics observable. All six possible ways the dice can fall correspond to six different universes. In the case of the Schrödinger's cat thought experiment, 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 A doppelgänger (), a compound noun formed by combining the two nouns (double) and (walker or goer) (), doppelgaenger or doppelganger is a biologically unrelated look-alike, or a double, of a living person. In fiction and mythology, a doppelg ...

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 In mathematics, Hilbert spaces (named after David Hilbert) allow generalizing the methods of linear algebra and calculus from (finite-dimensional) Euclidean vector spaces to spaces that may be infinite-dimensional. Hilbert spaces arise natural ...

." 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. According to Yasunori Nomura,

Raphael Bousso Raphael Bousso () (born 1971) is a theoretical physicist and cosmologist. He is a professor at the Berkeley Center for Theoretical Physics in the Department of Physics, UC Berkeley. He is known for the Bousso bound on the information content of t ...

, and Leonard Susskind, 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. Another version of the many-worlds idea is

H. Dieter Zeh Heinz-Dieter Zeh (; 8 May 1932 – 15 April 2018), usually referred to as H. Dieter Zeh, was a professor (later professor emeritus) of the University of Heidelberg and theoretical physicist. Work Zeh was one of the developers of the many-minds ...

's ''many-minds'' interpretation.

Level IV: Ultimate ensemble

The ultimate mathematical universe hypothesis is Tegmark's own hypothesis. This level considers all universes to be equally real which can be described by different mathematical structures. Tegmark writes: He argues that this "implies that any conceivable parallel universe theory can be described at Level IV" and "subsumes all other ensembles, therefore brings closure to the hierarchy of multiverses, and there cannot be, say, a Level V." Jürgen Schmidhuber, however, says that the set of mathematical structures is not even well-defined and that it admits only universe representations describable by constructive mathematics—that is, computer programs. Schmidhuber explicitly includes universe representations describable by non-halting programs whose output bits converge after a finite time, although the convergence time itself may not be predictable by a halting program, due to the undecidability of the halting problem. He also explicitly discusses the more restricted ensemble of quickly computable universes.

Brian Greene's nine types

The American theoretical physicist and string theorist Brian Greene discussed nine types of multiverses: ;Quilted :The quilted multiverse works only in an infinite universe. With an infinite amount of space, every possible event will occur an infinite number of times. However, the speed of light prevents us from being aware of these other identical areas. ;Inflationary :The

inflationary multiverse Eternal inflation is a hypothetical inflationary universe model, which is itself an outgrowth or extension of the Big Bang theory. According to eternal inflation, the inflationary phase of the universe's expansion lasts forever throughout most of ...

is composed of various pockets in which inflation fields collapse and form new universes. ;Brane :The

brane multiverse Brane cosmology refers to several theories in particle physics and cosmology related to string theory, superstring theory and M-theory. Brane and bulk The central idea is that the visible, three-dimensional universe is restricted to a b ...

version postulates that our entire universe exists on a membrane ( brane) which floats in a higher dimension or "bulk". In this bulk, there are other membranes with their own universes. These universes can interact with one another, and when they collide, the violence and energy produced is more than enough to give rise to a

big bang The Big Bang event is a physical theory that describes how the universe expanded from an initial state of high density and temperature. Various cosmological models of the Big Bang explain the evolution of the observable universe from the ...

. The branes float or drift near each other in the bulk, and every few trillion years, attracted by gravity or some other force we do not understand, collide and bang into each other. This repeated contact gives rise to multiple or "cyclic" big bangs. This particular hypothesis falls under the string theory umbrella as it requires extra spatial dimensions. ;Cyclic :The cyclic multiverse has multiple branes that have collided, causing

s. The universes bounce back and pass through time until they are pulled back together and again collide, destroying the old contents and creating them anew. ;Landscape :The

landscape multiverse The string theory landscape or landscape of vacua refers to the collection of possible false vacua in string theory,The number of metastable vacua is not known exactly, but commonly quoted estimates are of the order 10500. See M. Douglas, "The ...

relies on string theory's Calabi–Yau spaces. Quantum fluctuations drop the shapes to a lower energy level, creating a pocket with a set of laws different from that of the surrounding space. ;Quantum :The

quantum multiverse The many-worlds interpretation (MWI) is an interpretation of quantum mechanics that asserts that the universal wavefunction is objectively real, and that there is no wave function collapse. This implies that all possible outcomes of quantum ...

creates a new universe when a diversion in events occurs, as in the real-worlds variant of the many-worlds interpretation of quantum mechanics. ;Holographic :The holographic multiverse is derived from the theory that the surface area of a space can encode the contents of the volume of the region. ;Simulated :The

simulated multiverse The simulation hypothesis proposes that all of our existence is a simulated reality, such as a computer simulation. The simulation hypothesis bears a close resemblance to various other skeptical scenarios from throughout the history of philosophy. ...

exists on complex computer systems that simulate entire universes. A related hypothesis, as put forward as a possibility by astronomer Avi Loeb, is that universes may be creatable in laboratories of advanced technological civilizations who have a theory of everything. Other related hypotheses include brain in a vat-type scenarios where the perceived universe is either simulated in a low-resource way or not perceived directly by the virtual/simulated inhabitant species. ;Ultimate :The

ultimate multiverse In physics and cosmology, the mathematical universe hypothesis (MUH), also known as the ultimate ensemble theory and struogony (from mathematical structure, Latin: struō), is a speculative "theory of everything" (TOE) proposed by cosmologist Max T ...

contains every mathematically possible universe under different laws of physics.

Twin-world models

There are models of two related universes that e.g. attempt to explain the baryon asymmetry – why there was more matter than antimatter at the beginning – with a mirror anti-universe. One two-universe cosmological model could explain the Hubble constant (H₀) tension via interactions between the two worlds. The "mirror world" would contain copies of all existing fundamental particles. Another twin/pair-world or "bi-world" cosmology is shown to theoretically be able to solve the cosmological constant (Λ) problem, closely related to dark energy: two interacting worlds with a large Λ each could result in a small shared effective Λ.

Cyclic theories

In several theories, there is a series of, in some cases infinite, self-sustaining cycles – typically a series of

Big Crunch The Big Crunch is a hypothetical scenario for the ultimate fate of the universe, in which the expansion of the universe eventually reverses and the universe recollapses, ultimately causing the cosmic scale factor to reach zero, an event potential ...

es (or

Big Bounce The Big Bounce is a hypothesized cosmological model for the origin of the known universe. It was originally suggested as a phase of the ''cyclic model'' or ''oscillatory universe'' interpretation of the Big Bang, where the first cosmological even ...

s). However, the respective universes do not exist at once but are sequential, with key natural constituents potentially varying between universes (see § Anthropic principle).

M-theory

A multiverse of a somewhat different kind has been envisaged within

string theory In physics, string theory is a theoretical framework in which the point-like particles of particle physics are replaced by one-dimensional objects called strings. String theory describes how these strings propagate through space and interac ...

and its higher-dimensional extension, M-theory. These theories require the presence of 10 or 11 spacetime dimensions respectively. The extra six or seven dimensions may either be compactified on a very small scale, or our universe may simply be localized on a dynamical (3+1)-dimensional object, a

D3-brane In string theory, D-branes, short for ''Dirichlet membrane'', are a class of extended objects upon which open strings can end with Dirichlet boundary conditions, after which they are named. D-branes were discovered by Jin Dai, Leigh, and Polchin ...

. This opens up the possibility that there are other branes which could support other universes.Richard J Szabo, ''An introduction to string theory and D-brane dynamics'' (2004)Maurizio Gasperini, ''Elements of String Cosmology'' (2007)

Black-hole cosmology

Black-hole cosmology is a cosmological model in which the observable universe is the interior of a

black hole A black hole is a region of spacetime where gravitation, gravity is so strong that nothing, including light or other Electromagnetic radiation, electromagnetic waves, has enough energy to escape it. The theory of general relativity predicts t ...

existing as one of possibly many universes inside a larger universe. This includes the theory of white holes, which are on the opposite side of

space-time In physics, spacetime is a mathematical model that combines the three-dimensional space, three dimensions of space and one dimension of time into a single four-dimensional manifold. Minkowski diagram, Spacetime diagrams can be used to visualize S ...

Anthropic principle

The concept of other universes has been proposed to explain how our own universe appears to be fine-tuned for conscious life as we experience it. If there were a large (possibly infinite) number of universes, each with possibly different physical laws (or different

fundamental physical constants In physics, a dimensionless physical constant is a physical constant that is dimensionless, i.e. a pure number having no units attached and having a numerical value that is independent of whatever system of units may be used. For example, if one c ...

), then some of these universes (even if very few) would have the combination of laws and fundamental parameters that are suitable for the development of matter, astronomical structures, elemental diversity, stars, and planets that can exist long enough for life to emerge and evolve. The weak anthropic principle could then be applied to conclude that we (as conscious beings) would only exist in one of those few universes that happened to be finely tuned, permitting the existence of life with developed consciousness. Thus, while the probability might be extremely small that any particular universe would have the requisite conditions for life ( as we understand life), those conditions do not require intelligent design as an explanation for the conditions in the Universe that promote our existence in it. An early form of this reasoning is evident in

Arthur Schopenhauer Arthur Schopenhauer ( , ; 22 February 1788 – 21 September 1860) was a German philosopher. He is best known for his 1818 work ''The World as Will and Representation'' (expanded in 1844), which characterizes the phenomenal world as the prod ...

's 1844 work "Von der Nichtigkeit und dem Leiden des Lebens", where he argues that our world must be the worst of all possible worlds, because if it were significantly worse in any respect it could not continue to exist.

Occam's razor

Proponents and critics disagree about how to apply

Occam's razor Occam's razor, Ockham's razor, or Ocham's razor ( la, novacula Occami), also known as the principle of parsimony or the law of parsimony ( la, lex parsimoniae), is the problem-solving principle that "entities should not be multiplied beyond neces ...

. Critics argue that to postulate an almost infinite number of unobservable universes, just to explain our own universe, is contrary to Occam's razor. However, proponents argue that in terms of Kolmogorov complexity the proposed multiverse is simpler than a single idiosyncratic universe. For example, multiverse proponent Max Tegmark argues:

Possible worlds and real worlds

In any given set of possible universes – e.g. in terms of histories or variables of nature – not all may be ever realized, and some may be realized many times. For example, over infinite time there could, in some potential theories, be infinite universes, but only a small or relatively small real number of universes where humanity could exist and only one where it ever does exist (with a unique history). It has been suggested that a universe that "contains life, in the form it has on Earth, is in a certain sense radically non-

, in that the vast majority of possible organisms will never be realized". On the other hand, some scientists, theories and popular works conceive of a multiverse in which the universes are so similar that humanity exists in many equally real separate universes but with varying histories. There is a debate about whether the other worlds are real in the many-worlds interpretation (MWI) of quantum mechanics. In Quantum Darwinism one does not need to adopt a MWI in which all of the branches are equally real.

Modal realism

Possible worlds are a way of explaining probability and hypothetical statements. Some philosophers, such as David Lewis, believe that all possible worlds exist and that they are just as real as the world we live in (a position known as

modal realism Modal realism is the view propounded by philosopher David Lewis that all possible worlds are real in the same way as is the actual world: they are "of a kind with this world of ours." It is based on the following tenets: possible worlds exist; p ...

References

Footnotes Citations

External links

Interview with Tufts cosmologist Alex Vilenkin
on his new book, "Many Worlds in One: The Search for Other Universes" on the podcast and public radio interview progra
ThoughtCast.

Multiverse
– an episode of the series

In Our Time In Our Time may refer to: * ''In Our Time'' (1944 film), a film starring Ida Lupino and Paul Henreid * ''In Our Time'' (1982 film), a Taiwanese anthology film featuring director Edward Yang; considered the beginning of the "New Taiwan Cinema" * ''In ...

with

Melvyn Bragg Melvyn Bragg, Baron Bragg, (born 6 October 1939), is an English broadcaster, author and parliamentarian. He is best known for his work with ITV as editor and presenter of ''The South Bank Show'' (1978–2010), and for the BBC Radio 4 documenta ...

, on BBC Radio 4.
Why There Might be Many More Universes Besides Our Own
by Phillip Ball, March 21, 2016, bbc.com. {{Authority control Physical cosmology Science fiction themes Quantum mechanics Astronomical hypotheses Anthropic principle Hypothetical astronomical objects