Multiverse

The idea of infinite worlds was first suggested by the pre-Socratic Greek philosopher Anaximander in the sixth century BCE.[1] The evidence is not clear whether the worlds, as Anaximander viewed them, were co-existent or successive[2] but he believed they were all perishable and in a cycle of creation and destruction.

Other early recorded examples of the idea of infinite worlds existed in the philosophy of Ancient Greek Atomism, which proposed that infinite parallel worlds arose from the collision of atoms. In the third century BCE, the philosopher Chrysippus suggested that the world eternally expired and regenerated, effectively suggesting the existence of multiple universes across time.[3] 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.[4]

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".[5] This sort of duality is called "superposition".

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 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))

Multiple universes have been hypothesized in cosmology, physics, astronomy, religion, philosophy, transpersonal psychology, music, and all kinds of literature, particularly in science fiction, comic books 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.[6] 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.[7] 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.[8] Paul Steinhardt has famously argued that no experiment can rule out a theory if the theory provides for all possible outcomes.[9]

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 is doomed, for their values would be an accident of the particular part of the multiverse in which we live."[10]

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.[11][12][13] 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.[14][15] In addition, there was no evidence of any gravitational pull of other universes on ours.[16][17]

Modern proponents of one or more of the multiverse hypotheses include Don Page,[18] Brian Greene,[19][20] Max Tegmark,[21] Alan Guth,[22] Andrei Linde,[23] Michio Kaku,[24] David Deutsch,[25] Leonard Susskind,[26] Alexander Vilenkin,[27] Yasunori Nomura,[28] Raj Pathria,[29] Laura Mersini-Houghton,[30][31][32][33] Neil deGrasse Tyson,[34] Sean Carroll[35] and Stephen Hawking.[36]

Scientists who are generally skeptical of the multiverse hypothesis include David Gross,[37] Paul Steinhardt,[38][39] Anna Ijjas,[39] Abraham Loeb,[39] David Spergel,[40] Neil Turok,[41] Viatcheslav Mukhanov,[42] Michael S. Turner,[43] Roger Penrose,[44] George Ellis,[45][46] Joe Silk,[47] Carlo Rovelli,[48] Adam Frank,[49] Marcelo Gleiser,[49] Jim Baggott[50] and Paul Davies.[51]

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:[52]

For a start, how is the existence of the other universes to be tested? To be sure, all cosmologists accept that there are some regions of the universe that lie beyond the reach of our telescopes, but somewhere on the slippery slope between that and the idea that there is an infinite number of universes, credibility reaches a limit. As one slips down that slope, more and more must be accepted on faith, and less and less is open to scientific verification. Extreme multiverse explanations are therefore reminiscent of theological discussions. Indeed, invoking an infinity of unseen universes to explain the unusual features of the one we do see is just as ad hoc as invoking an unseen Creator. The multiverse theory may be dressed up in scientific language, but in essence, it requires the same leap of faith.

— Paul Davies, The New York Times, "A Brief History of the Multiverse"

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 testability and falsifiability is a major concern, but he is opposed to that line of thinking:

Many physicists who talk about the multiverse, especially advocates of the string landscape, do not care much about parallel universes per se. For them, objections to the multiverse as a concept are unimportant. Their theories live or die based on internal consistency and, one hopes, eventual laboratory testing.

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:[53]

As skeptical as I am, I think the contemplation of the multiverse is an excellent opportunity to reflect on the nature of science and on the ultimate nature of existence: why we are here.... In looking at this concept, we need an open mind, though not too open. It is a delicate path to tread. Parallel universes may or may not exist; the case is unproved. We are going to have to live with that uncertainty. Nothing is wrong with scientifically based philosophical speculation, which is what multiverse proposals are. But we should name it for what it is.

— George Ellis, "Does the Multiverse Really Exist?", Scientific American

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.[54]

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

Twin-world models

Concept of a twin universe, with the beginning (of time) in the middle.

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.[68][69][70] 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.[71][72] 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.[73][74][75]

A multiverse of a somewhat different kind has been envisaged within string theory and its higher-dimensional extension, M-theory.[76]

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 branes which could support other universes.[77][78]

Black-hole cosmology is a cosmological model in which the observable universe is the interior of a black hole existing as one of possibly many universes inside a larger universe.[79] This includes the theory of white holes, which are on the opposite side of space-time.

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), 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'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.[80]

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.[81] However, proponents argue that in terms of Kolmogorov complexity the proposed multiverse is simpler than a single idiosyncratic universe.[57]

For example, multiverse proponent Max Tegmark argues:

[A]n entire ensemble is often much simpler than one of its members. This principle can be stated more formally using the notion of algorithmic information content. The algorithmic information content in a number is, roughly speaking, the length of the shortest computer program that will produce that number as output. For example, consider the set of all integers. Which is simpler, the whole set or just one number? Naively, you might think that a single number is simpler, but the entire set can be generated by quite a trivial computer program, whereas a single number can be hugely long. Therefore, the whole set is actually simpler... (Similarly), the higher-level multiverses are simpler. Going from our universe to the Level I multiverse eliminates the need to specify initial conditions, upgrading to Level II eliminates the need to specify physical constants, and the Level IV multiverse eliminates the need to specify anything at all... 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. Perhaps we will gradually get used to the weird ways of our cosmos and find its strangeness to be part of its charm.[57][82]

— Max Tegmark

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.[83] 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".[84] 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.[85]

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.[86]

Footnotes

Citations

Look up multiverse in Wiktionary, the free dictionary.

Wikiquote has quotations related to Multiverse.

Wikimedia Commons has media related to Multiverse.

• 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 program ThoughtCast.
• Multiverse – an episode of the series In Our Time with Melvyn Bragg, on BBC Radio 4.
• Why There Might be Many More Universes Besides Our Own, by Phillip Ball, March 21, 2016, bbc.com.