The multiverse is a
hypothetical
A hypothesis (plural hypotheses) is a proposed explanation for a phenomenon. For a hypothesis to be a scientific hypothesis, the scientific method requires that one can test it. Scientists generally base scientific hypotheses on previous obser ...
group of multiple
universe
The universe is all of space and time and their contents, including planets, stars, galaxies, and all other forms of matter and energy. The Big Bang theory is the prevailing cosmological description of the development of the univers ...
s. Together, these universes comprise everything that exists: the entirety of
space
Space is the boundless three-dimensional extent in which objects and events have relative position and direction. In classical physics, physical space is often conceived in three linear dimensions, although modern physicists usually consi ...
,
time
Time is the continued sequence of existence and event (philosophy), events that occurs in an apparently irreversible process, irreversible succession from the past, through the present, into the future. It is a component quantity of various me ...
,
matter
In classical physics and general chemistry, matter is any substance that has mass and takes up space by having volume. All everyday objects that can be touched are ultimately composed of atoms, which are made up of interacting subatomic part ...
,
energy
In physics, energy (from Ancient Greek: ἐνέργεια, ''enérgeia'', “activity”) is the quantitative property that is transferred to a body or to a physical system, recognizable in the performance of work and in the form of ...
,
information
Information is an abstract concept that refers to that which has the power to inform. At the most fundamental level information pertains to the interpretation of that which may be sensed. Any natural process that is not completely random, ...
, and the
physical law
Scientific laws or laws of science are statements, based on repeated experiments or observations, that describe or predict a range of natural phenomena. The term ''law'' has diverse usage in many cases (approximate, accurate, broad, or narro ...
s 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 mo ...
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
Atomism (from Greek , ''atomon'', i.e. "uncuttable, indivisible") is a natural philosophy proposing that the physical universe is composed of fundamental indivisible components known as atoms.
References to the concept of atomism and its atoms a ...
, 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 C ...
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
In the history of Europe, the Middle Ages or medieval period lasted approximately from the late 5th to the late 15th centuries, similar to the post-classical period of global history. It began with the fall of the Western Roman Empire ...
.
The American philosopher and psychologist William James
William James (January 11, 1842 – August 26, 1910) was an American philosopher, historian, and psychologist, and the first educator to offer a psychology course in the United States.
James is considered to be a leading thinker of the lat ...
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
Dublin (; , or ) is the capital and largest city of Ireland. On a bay at the mouth of the River Liffey, it is in the province of Leinster, bordered on the south by the Dublin Mountains, a part of the Wicklow Mountains range. At the 2016 ...
in 1952, Erwin Schrödinger
Erwin Rudolf Josef Alexander Schrödinger (, ; ; 12 August 1887 – 4 January 1961), sometimes written as or , was a Nobel Prize-winning Austrian physicist with Irish citizenship who developed a number of fundamental results in quantum theo ...
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 during the Second World War, and again from ...
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 of Two Worlds!" is a landmark comic book story that was published in ''The Flash'' #123 (Sept. 1961). It introduces Earth-Two, and more generally the concept of the multiverse, to DC Comics. The story was written by Gardner Fox under th ...
(Flash Volume 1 #123) by Carmine Infantino
Carmine Michael Infantino (; May 24, 1925 – April 4, 2013) was an American comics artist and editor, primarily for DC Comics, during the late 1950s and early 1960s period known as the Silver Age of Comic Books. Among his character creations are ...
and Gardner Fox
Gardner Francis Cooper Fox (May 20, 1911 – December 24, 1986) was an American writer known best for creating numerous comic book characters for DC Comics. He is estimated to have written more than 4,000 comics stories, including 1,500 for DC ...
. In the story, Flash
Flash, flashes, or FLASH may refer to:
Arts, entertainment, and media
Fictional aliases
* Flash (DC Comics character), several DC Comics superheroes with super speed:
** Flash (Barry Allen)
** Flash (Jay Garrick)
** Wally West, the first Kid F ...
meets with his duplicate version of another Earth (Earth-2) and another Flash
Flash, flashes, or FLASH may refer to:
Arts, entertainment, and media
Fictional aliases
* Flash (DC Comics character), several DC Comics superheroes with super speed:
** Flash (Barry Allen)
** Flash (Jay Garrick)
** Wally West, the first Kid F ...
(Flash-2).
The term was first used in fiction in its current physics context by Michael Moorcock
Michael John Moorcock (born 18 December 1939) is an English writer, best-known for science fiction and fantasy, who has published a number of well-received literary novels as well as comic thrillers, graphic novels and non-fiction. He has worke ...
in his 1963 SF Adventures novella ''The Sundered Worlds'' (part of his Eternal Champion
The Eternal Champion is a fictional character created by British author Michael Moorcock and is a recurrent feature in many of his speculative fiction works.
General overview
Many of Moorcock's novels and short stories take place in a shared M ...
series). (see Multiverse (Michael Moorcock)
The multiverse is a series of parallel universes in many of the science fiction and fantasy novels and short stories written by Michael Moorcock (many other fictional settings also have the concept of a multiverse). Central to these works is the ...
)
Brief explanation
Multiple universes have been hypothesized in cosmology
Cosmology () is a branch of physics and metaphysics dealing with the nature of the universe. The term ''cosmology'' was first used in English in 1656 in Thomas Blount's ''Glossographia'', and in 1731 taken up in Latin by German philosopher ...
, physics
Physics is the natural science that studies matter, its fundamental constituents, its motion and behavior through space and time, and the related entities of energy and force. "Physical science is that department of knowledge which r ...
, astronomy
Astronomy () is a natural science that studies celestial objects and phenomena. It uses mathematics, physics, and chemistry in order to explain their origin and evolution. Objects of interest include planets, moons, stars, nebulae, g ...
, religion
Religion is usually defined as a social- cultural system of designated behaviors and practices, morals, beliefs, worldviews, texts, sanctified places, prophecies, ethics, or organizations, that generally relates humanity to supernatural, ...
, philosophy, transpersonal psychology
Transpersonal psychology, or spiritual psychology, is a sub-field or school of psychology that integrates the spiritual and transcendent aspects of the human experience with the framework of modern psychology. The '' transpersonal'' is defined ...
, music
Music is generally defined as the art of arranging sound to create some combination of form, harmony, melody, rhythm or otherwise expressive content. Exact definitions of music vary considerably around the world, though it is an aspe ...
, and all kinds of literature
Literature is any collection of written work, but it is also used more narrowly for writings specifically considered to be an art form, especially prose fiction, drama, and poetry. In recent centuries, the definition has expanded to include ...
, particularly in science fiction
Science fiction (sometimes shortened to Sci-Fi or SF) is a genre of speculative fiction which typically deals with imaginative and futuristic concepts such as advanced science and technology, space exploration, time travel, parallel uni ...
, comic books and fantasy
Fantasy is a genre of speculative fiction involving magical elements, typically set in a fictional universe and sometimes inspired by mythology and folklore. Its roots are in oral traditions, which then became fantasy literature and d ...
. 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 timeline
Alternate history (also alternative history, althist, AH) is a genre of speculative fiction of stories in which one or more historical events occur and are resolved differently than in real life. As conjecture based upon historical fact, altern ...
s", "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
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 ...
notion rather than a scientific hypothesis
A hypothesis (plural hypotheses) is a proposed explanation for a phenomenon. For a hypothesis to be a scientific hypothesis, the scientific method requires that one can test it. Scientists generally base scientific hypotheses on previous obser ...
because it cannot be empirically falsified
Falsifiability is a standard of evaluation of scientific theories and hypotheses that was introduced by the philosopher of science Karl Popper in his book ''The Logic of Scientific Discovery'' (1934). He proposed it as the cornerstone of a sol ...
. The ability to disprove a theory by means of scientific experiment is a critical criterion of the accepted scientific method
The scientific method is an empirical method for acquiring knowledge that has characterized the development of science since at least the 17th century (with notable practitioners in previous centuries; see the article history of scientific ...
. Paul Steinhardt
Paul Joseph Steinhardt (born December 25, 1952) is an American theoretical physicist whose principal research is in cosmology and condensed matter physics. He is currently the Albert Einstein Professor in Science at Princeton University, where ...
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
Steven Weinberg (; May 3, 1933 – July 23, 2021) was an American theoretical physicist and Nobel laureate in physics for his contributions with Abdus Salam and Sheldon Glashow to the unification of the weak force and electromagnetic interac ...
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 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 (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, which has a resolution three times higher than WMAP, did not reveal any statistically significant evidence of such a bubble universe 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
Brian Randolph Greene (born February 9, 1963) is a American theoretical physicist, mathematician, and string theorist. Greene was a physics professor at Cornell University from 19901995, and has been a professor at Columbia University since 1 ...
, Max Tegmark
Max Erik Tegmark (born 5 May 1967) is a Swedish-American physicist, cosmologist and machine learning researcher. He is a professor at the Massachusetts Institute of Technology and the president of the Future of Life Institute. He is also a scienti ...
, Alan Guth
Alan Harvey Guth (; born February 27, 1947) is an American theoretical physicist and cosmologist. Guth has researched elementary particle theory (and how particle theory is applicable to the early universe). He is Victor Weisskopf Professor of ...
, Andrei Linde, Michio Kaku
Michio Kaku (, ; born January 24, 1947) is an American theoretical physicist, futurist, and popularizer of science ( science communicator). He is a professor of theoretical physics in the City College of New York and CUNY Graduate Center. Kak ...
, David Deutsch
David Elieser Deutsch ( ; born 18 May 1953) is a British physicist at the University of Oxford. He is a Visiting Professor in the Department of Atomic and Laser Physics at the Centre for Quantum Computation (CQC) in the Clarendon Laboratory of ...
, Leonard Susskind
Leonard Susskind (; born June 16, 1940)his 60th birthday was celebrated with a special symposium at Stanford University.in Geoffrey West's introduction, he gives Suskind's current age as 74 and says his birthday was recent. is an American physicis ...
, Alexander Vilenkin
Alexander Vilenkin (russian: Алекса́ндр Виле́нкин; uk, Олександр Віленкін; born 13 May 1949) is the Leonard Jane Holmes Bernstein Professor of Evolutionary Science and Director of the Institute of Cosmology a ...
, Yasunori Nomura
Yasunori Nomura (born 1974) is a theoretical physicist working on particle physics, quantum gravity, and cosmology. He is a professor of physics at University of California, Berkeley, a senior faculty scientist at Lawrence Berkeley National Labor ...
, Raj Pathria
Raj Kumar Pathria (born March 30, 1933) is a theoretical physicist, a Distinguished Professor Emeritus at University of Waterloo, and an Urdu poet.Men of Sciences & Technology in India, Edited by Raj K Khosla, Premier Publishers (India), New Delh ...
, Laura Mersini-Houghton
Laura Mersini-Houghton (''née'' Mersini) is an Albanian-American cosmologist and theoretical physicist, and professor at the University of North Carolina at Chapel Hill. She is a proponent of the multiverse hypothesis and the author of a theor ...
, Neil deGrasse Tyson
Neil deGrasse Tyson ( or ; born October 5, 1958) is an American astrophysicist, author, and science communicator. Tyson studied at Harvard University, the University of Texas at Austin, and Columbia University. From 1991 to 1994, he was a p ...
, Sean Carroll and Stephen Hawking.
Scientists who are generally skeptical of the multiverse hypothesis include David Gross
David Jonathan Gross (; born February 19, 1941) is an American theoretical physicist and string theorist. Along with Frank Wilczek and David Politzer, he was awarded the 2004 Nobel Prize in Physics for their discovery of asymptotic freedom. ...
, Paul Steinhardt
Paul Joseph Steinhardt (born December 25, 1952) is an American theoretical physicist whose principal research is in cosmology and condensed matter physics. He is currently the Albert Einstein Professor in Science at Princeton University, where ...
, Anna Ijjas, Abraham Loeb
Abraham "Avi" Loeb ( he, אברהם (אבי) לייב; born February 26, 1962) is an Israeli-American theoretical physicist who works on astrophysics and cosmology. Loeb is the Frank B. Baird Jr. Professor of Science at Harvard University. He had ...
, David Spergel
David Nathaniel Spergel is an American theoretical astrophysicist and the Emeritus Charles A. Young Professor of Astronomy on the Class of 1897 Foundation at Princeton University. Since 2021, he has been the President of the Simons Foundation ...
, Neil Turok
Neil Geoffrey Turok (born 16 November 1958) is a South African physicist. He holds the Higgs Chair of Theoretical Physics at the University of Edinburgh since 2020, and has been director emeritus of the Perimeter Institute for Theoretical Physi ...
, 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 of th ...
, Michael S. Turner, Roger Penrose, George Ellis, Joe Silk,
Carlo Rovelli, Adam Frank, Marcelo Gleiser
Marcelo Gleiser (born March 19, 1959) is a Brazilian physicist and astronomer. He is currently Professor of Physics and Astronomy at Dartmouth College and was the 2019 recipient of the Templeton Prize.
Early life and education
Gleiser received ...
, Jim Baggott[
] and Paul Davies
Paul Charles William Davies (born 22 April 1946) is an English physicist, writer and broadcaster, a professor in Arizona State University and Director of BEYOND: Center for Fundamental Concepts in Science. He is affiliated with the Institute ...
.
Arguments against multiverse theories
In his 2003 ''New York Times'' opinion piece, "A Brief History of the Multiverse", author and cosmologist Paul Davies
Paul Charles William Davies (born 22 April 1946) is an English physicist, writer and broadcaster, a professor in Arizona State University and Director of BEYOND: Center for Fundamental Concepts in Science. He is affiliated with the Institute ...
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
A cosmological horizon is a measure of the distance from which one could possibly retrieve information. This observable constraint is due to various properties of general relativity, the expanding universe, and the physics of Big Bang cosmology. Co ...
. 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 testability
Testability is a primary aspect of Science and the Scientific Method and is a property applying to an empirical hypothesis, involves two components:
#Falsifiability or defeasibility, which means that counterexamples to the hypothesis are logicall ...
and falsifiability
Falsifiability is a standard of evaluation of scientific theories and hypotheses that was introduced by the philosopher of science Karl Popper in his book '' The Logic of Scientific Discovery'' (1934). He proposed it as the cornerstone of a s ...
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
Existence is the ability of an entity to interact with reality. In philosophy, it refers to the ontological property of being.
Etymology
The term ''existence'' comes from Old French ''existence'', from Medieval Latin ''existentia/exsistentia' ...
. He points out that it ultimately leaves those questions unresolved because it is a metaphysical
Metaphysics is the branch of philosophy that studies the fundamental nature of reality, the first principles of being, identity and change, space and time, causality, necessity, and possibility. It includes questions about the nature of conscio ...
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.
Types
Max Tegmark
Max Erik Tegmark (born 5 May 1967) is a Swedish-American physicist, cosmologist and machine learning researcher. He is a professor at the Massachusetts Institute of Technology and the president of the Future of Life Institute. He is also a scienti ...
and Brian Greene
Brian Randolph Greene (born February 9, 1963) is a American theoretical physicist, mathematician, and string theorist. Greene was a physics professor at Cornell University from 19901995, and has been a professor at Columbia University since 1 ...
have devised classification schemes for the various theoretical types of multiverses and universes that they might comprise.
Max Tegmark's four levels
Cosmologist
Cosmology () is a branch of physics and metaphysics dealing with the nature of the universe. The term ''cosmology'' was first used in English in 1656 in Thomas Blount's ''Glossographia'', and in 1731 taken up in Latin by German philosopher ...
Max Tegmark
Max Erik Tegmark (born 5 May 1967) is a Swedish-American physicist, cosmologist and machine learning researcher. He is a professor at the Massachusetts Institute of Technology and the president of the Future of Life Institute. He is also a scienti ...
has provided a taxonomy
Taxonomy is the practice and science of categorization or classification.
A taxonomy (or taxonomical classification) is a scheme of classification, especially a hierarchical classification, in which things are organized into groups or types. ...
of universes beyond the familiar observable universe
The observable universe is a ball-shaped region of the universe comprising all matter that can be observed from Earth or its space-based telescopes and exploratory probes at the present time, because the electromagnetic radiation from these ob ...
. 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 is the existence of an infinite ergodic universe, which, being infinite, must contain Hubble volumes realizing all initial conditions.
Accordingly, an infinite universe will contain an infinite number of Hubble volumes, all having the same physical law
Scientific laws or laws of science are statements, based on repeated experiments or observations, that describe or predict a range of natural phenomena. The term ''law'' has diverse usage in many cases (approximate, accurate, broad, or narro ...
s and physical constants. In regard to configurations such as the distribution of matter
In classical physics and general chemistry, matter is any substance that has mass and takes up space by having volume. All everyday objects that can be touched are ultimately composed of atoms, which are made up of interacting subatomic part ...
, almost all will differ from our Hubble volume. However, because there are infinitely many, far beyond the cosmological horizon
A cosmological horizon is a measure of the distance from which one could possibly retrieve information. This observable constraint is due to various properties of general relativity, the expanding universe, and the physics of Big Bang cosmology. Co ...
, there will eventually be Hubble volumes with similar, and even identical, configurations. Tegmark estimates that an identical volume to ours should be about 1010115 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
In modern physical cosmology, the cosmological principle is the notion that the spatial distribution of matter in the universe is homogeneous and isotropic when viewed on a large enough scale, since the forces are expected to act uniformly throu ...
, wherein it is assumed that our Hubble volume is not special or unique.
Level II: Universes with different physical constants
In the eternal inflation
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 ...
theory, which is a variant of the cosmic inflation 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
Spontaneous symmetry breaking is a spontaneous process of symmetry breaking, by which a physical system in a symmetric state spontaneously ends up in an asymmetric state. In particular, it can describe systems where the equations of motion or ...
, which results in different properties, such as different physical constants.
Level II also includes John Archibald Wheeler
John Archibald Wheeler (July 9, 1911April 13, 2008) was an American theoretical physicist. He was largely responsible for reviving interest in general relativity in the United States after World War II. Wheeler also worked with Niels Bohr in ...
's oscillatory universe theory and Lee Smolin
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 the ...
's fecund universes theory.
Level III: Many-worlds interpretation of quantum mechanics
Hugh Everett III
Hugh Everett III (; November 11, 1930 – July 19, 1982) was an American physicist who first proposed the many-worlds interpretation (MWI) of quantum physics, which he termed his "relative state" formulation. In contrast to the then-dominant Cop ...
's many-worlds interpretation
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 ...
(MWI) is one of several mainstream interpretations of quantum mechanics
An interpretation of quantum mechanics is an attempt to explain how the mathematical theory of quantum mechanics might correspond to experienced reality. Although quantum mechanics has held up to rigorous and extremely precise tests in an extraord ...
.
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
Probability is the branch of mathematics concerning numerical descriptions of how likely an event is to occur, or how likely it is that a proposition is true. The probability of an event is a number between 0 and 1, where, roughly speakin ...
. 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
In physics, an observable is a physical quantity that can be measured. Examples include position and momentum. In systems governed by classical mechanics, it is a real-valued "function" on the set of all possible system states. In quantum phy ...
. 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 doppelgängers 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. According to Yasunori Nomura
Yasunori Nomura (born 1974) is a theoretical physicist working on particle physics, quantum gravity, and cosmology. He is a professor of physics at University of California, Berkeley, a senior faculty scientist at Lawrence Berkeley National Labor ...
, Raphael Bousso, and Leonard Susskind
Leonard Susskind (; born June 16, 1940)his 60th birthday was celebrated with a special symposium at Stanford University.in Geoffrey West's introduction, he gives Suskind's current age as 74 and says his birthday was recent. is an American physicis ...
, 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
Yasunori Nomura (born 1974) is a theoretical physicist working on particle physics, quantum gravity, and cosmology. He is a professor of physics at University of California, Berkeley, a senior faculty scientist at Lawrence Berkeley National Labor ...
, this quantum multiverse is static, and time is a simple illusion.
Another version of the many-worlds idea is H. Dieter Zeh's ''many-minds'' interpretation.
Level IV: Ultimate ensemble
The ultimate mathematical universe hypothesis
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 ...
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
Jürgen Schmidhuber (born 17 January 1963) is a German computer scientist most noted for his work in the field of artificial intelligence, deep learning and artificial neural networks. He is a co-director of the Dalle Molle Institute for Artifi ...
, however, says that the set of mathematical structures is not even well-defined
In mathematics, a well-defined expression or unambiguous expression is an expression whose definition assigns it a unique interpretation or value. Otherwise, the expression is said to be ''not well defined'', ill defined or ''ambiguous''. A func ...
and that it admits only universe representations describable by constructive mathematics
In the philosophy of mathematics, constructivism asserts that it is necessary to find (or "construct") a specific example of a mathematical object in order to prove that an example exists. Contrastingly, in classical mathematics, one can prove th ...
—that is, computer programs
A computer program is a sequence or set of instructions in a programming language for a computer to execute. Computer programs are one component of software, which also includes documentation and other intangible components.
A computer program ...
.
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
In computability theory, the halting problem is the problem of determining, from a description of an arbitrary computer program and an input, whether the program will finish running, or continue to run forever. Alan Turing proved in 1936 that a ...
. He also explicitly discusses the more restricted ensemble of quickly computable universes.
Brian Greene's nine types
The American theoretical physicist
Theoretical physics is a branch of physics that employs mathematical models and abstractions of physical objects and systems to rationalize, explain and predict natural phenomena. This is in contrast to experimental physics, which uses experime ...
and string theorist
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 intera ...
Brian Greene
Brian Randolph Greene (born February 9, 1963) is a American theoretical physicist, mathematician, and string theorist. Greene was a physics professor at Cornell University from 19901995, and has been a professor at Columbia University since 1 ...
discussed nine types of multiverses:
;Quilted
:The quilted multiverse works only in an infinite
Infinite may refer to:
Mathematics
* Infinite set, a set that is not a finite set
*Infinity, an abstract concept describing something without any limit
Music
*Infinite (group), a South Korean boy band
*''Infinite'' (EP), debut EP of American m ...
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 is composed of various pockets in which inflation fields collapse and form new universes.
;Brane
:The brane multiverse version postulates that our entire universe exists on a membrane (brane
In string theory and related theories such as supergravity theories, a brane is a physical object that generalizes the notion of a point particle to higher dimensions. Branes are dynamical objects which can propagate through spacetime accordin ...
) 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 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 brane
In string theory and related theories such as supergravity theories, a brane is a physical object that generalizes the notion of a point particle to higher dimensions. Branes are dynamical objects which can propagate through spacetime accordin ...
s that have collided, causing Big Bangs. 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 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 creates a new universe when a diversion in events occurs, as in the real-worlds variant of the many-worlds interpretation
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 ...
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 exists on complex computer systems that simulate entire universes. A related hypothesis, as put forward as a possibility by astronomer Avi Loeb
Abraham "Avi" Loeb ( he, אברהם (אבי) לייב; born February 26, 1962) is an Israeli-American theoretical physicist who works on astrophysics and cosmology. Loeb is the Frank B. Baird Jr. Professor of Science at Harvard University. He h ...
, is that universes may be creatable in laboratories of advanced technological civilizations who have a theory of everything
A theory of everything (TOE or TOE/ToE), final theory, ultimate theory, unified field theory or master theory is a hypothetical, singular, all-encompassing, coherent theoretical framework of physics that fully explains and links together all asp ...
. Other related hypotheses include brain in a vat
In philosophy, the brain in a vat (BIV) is a scenario used in a variety of thought experiments intended to draw out certain features of human conceptions of knowledge, reality, truth, mind, consciousness, and meaning. It is a modern incarna ...
-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 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
In modern physics, antimatter is defined as matter composed of the antiparticles (or "partners") of the corresponding particles in "ordinary" matter. Antimatter occurs in natural processes like cosmic ray collisions and some types of radioac ...
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
In physical cosmology and astronomy, dark energy is an unknown form of energy that affects the universe on the largest scales. The first observational evidence for its existence came from measurements of supernovas, which showed that the univ ...
: 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
Infinite may refer to:
Mathematics
* Infinite set, a set that is not a finite set
*Infinity, an abstract concept describing something without any limit
Music
*Infinite (group), a South Korean boy band
*''Infinite'' (EP), debut EP of American m ...
, self-sustaining cycles – typically a series of Big Crunches (or Big Bounces). 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 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. This opens up the possibility that there are other brane
In string theory and related theories such as supergravity theories, a brane is a physical object that generalizes the notion of a point particle to higher dimensions. Branes are dynamical objects which can propagate through spacetime accordin ...
s 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
Cosmology () is a branch of physics and metaphysics dealing with the nature of the universe. The term ''cosmology'' was first used in English in 1656 in Thomas Blount's ''Glossographia'', and in 1731 taken up in Latin by German philosopher ...
is a cosmological model in which the observable universe
The observable universe is a ball-shaped region of the universe comprising all matter that can be observed from Earth or its space-based telescopes and exploratory probes at the present time, because the electromagnetic radiation from these ob ...
is the interior of a black hole existing as one of possibly many universes inside a larger universe. This includes the theory of white hole
In general relativity, a white hole is a hypothetical region of spacetime and singularity that cannot be entered from the outside, although energy-matter, light and information can escape from it. In this sense, it is the reverse of a black ho ...
s, which are on the opposite side of space-time.
Anthropic principle
The concept of other universes has been proposed to explain how our own universe appears to be fine-tuned
In theoretical physics, fine-tuning is the process in which parameters of a model must be adjusted very precisely in order to fit with certain observations. This had led to the discovery that the fundamental constants and quantities fall into suc ...
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), 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
In classical physics and general chemistry, matter is any substance that has mass and takes up space by having volume. All everyday objects that can be touched are ultimately composed of atoms, which are made up of interacting subatomic part ...
, astronomical structures, elemental diversity, stars, and planets that can exist long enough for life to emerge and evolve.
The weak anthropic principle
The anthropic principle, also known as the "observation selection effect", is the hypothesis, first proposed in 1957 by Robert Dicke, that there is a restrictive lower bound on how statistically probable our observations of the universe are, beca ...
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
Intelligent design (ID) is a pseudoscientific argument for the existence of God, presented by its proponents as "an evidence-based scientific theory about life's origins". Numbers 2006, p. 373; " Dcaptured headlines for its bold attempt to ...
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'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. 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
In algorithmic information theory (a subfield of computer science and mathematics), the Kolmogorov complexity of an object, such as a piece of text, is the length of a shortest computer program (in a predetermined programming language) that produ ...
the proposed multiverse is simpler than a single idiosyncratic universe.
For example, multiverse proponent Max Tegmark
Max Erik Tegmark (born 5 May 1967) is a Swedish-American physicist, cosmologist and machine learning researcher. He is a professor at the Massachusetts Institute of Technology and the president of the Future of Life Institute. He is also a scienti ...
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- ergodic, 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
Mechanics (from Ancient Greek: μηχανική, ''mēkhanikḗ'', "of machines") is the area of mathematics and physics concerned with the relationships between force, matter, and motion among physical objects. Forces applied to object ...
. In Quantum Darwinism
Quantum Darwinism is a theory meant to explain the emergence of the classical world from the quantum world as due to a process of Darwinian natural selection induced by the environment interacting with the quantum system; where the many possible q ...
one does not need to adopt a MWI in which all of the branches are equally real.
Modal realism
Possible world
A possible world is a complete and consistent way the world is or could have been. Possible worlds are widely used as a formal device in logic, philosophy, and linguistics in order to provide a semantics for intensional and modal logic. Their me ...
s 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).
See also
* Beyond black holes
* Cosmogony
* Impossible world
* Measure problem (cosmology)
The measure problem in cosmology concerns how to compute fractions of universes of different types within a multiverse. It typically arises in the context of eternal inflation. The problem arises because different approaches to calculating thes ...
* Modal realism
* Parallel universes in fiction
* Philosophy of physics
In philosophy, philosophy of physics deals with conceptual and interpretational issues in modern physics, many of which overlap with research done by certain kinds of theoretical physicists. Philosophy of physics can be broadly divided into thr ...
* Philosophy of space and time
Philosophy of space and time is the branch of philosophy concerned with the issues surrounding the ontology and epistemology of space and time. While such ideas have been central to philosophy from its inception, the philosophy of space and time wa ...
* Simulated reality
* Twin Earth thought experiment
Twin Earth is a thought experiment proposed by philosopher Hilary Putnam in his papers "Meaning and Reference" (1973) and "The Meaning of 'Meaning (1975). It is meant to serve as an illustration of his argument for semantic externalism, or the ...
* Ultimate fate of the universe
The ultimate fate of the universe is a topic in physical cosmology, whose theoretical restrictions allow possible scenarios for the evolution and ultimate fate of the universe to be described and evaluated. Based on available observational e ...
References
Footnotes
Citations
Further reading
*
*
*
* Andrei Linde
The Self-Reproducing Inflationary Universe
Scientific American
''Scientific American'', informally abbreviated ''SciAm'' or sometimes ''SA'', is an American popular science magazine. Many famous scientists, including Albert Einstein and Nikola Tesla, have contributed articles to it. In print since 1845, it ...
, November 1994 - Touches on multiverse concepts at the end of the article
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 with Melvyn Bragg, on BBC Radio 4
BBC Radio 4 is a British national radio station owned and operated by the BBC that replaced the BBC Home Service in 1967. It broadcasts a wide variety of spoken-word programmes, including news, drama, comedy, science and history from the BBC' ...
.
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