Parallel universes are no longer just a feature of a good sci-fi story. There are now some scientific theories that support the idea of parallel universes beyond our own. However, the multiverse theory remains one of the most controversial theories in science.
Parallel Theory
But is it all that's out there? Science fiction loves the idea of a parallel universe, and the thought that we might be living just one of an infinite number of possible lives. Multiverses aren't reserved for "Star Trek," "Spiderman" and "Doctor Who," though. Real scientific theory explores, and in some cases supports, the case for universes outside, parallel to, or distant from but mirroring our own.
Multiverses and parallel worlds are often argued in the context of other major scientific concepts like the Big Bang (opens in new tab), string theory (opens in new tab) and quantum mechanics (opens in new tab).
Around 13.7 billion years ago, everything we know of was an infinitesimal singularity. Then, according to the Big Bang theory, it burst into action, inflating faster than the speed of light in all directions for a tiny fraction of a second. Before 10^-32 seconds had passed, the universe had exploded outward to 10^26 times its original size in a process called cosmic inflation (opens in new tab). And that's all before the actual expansion of matter that we usually think of as the Big Bang itself, which was a consequence of all this inflation: As the inflation slowed, a flood of matter and radiation appeared, creating the classic Big Bang fireball, and began to form the atoms, molecules, stars and galaxies that populate the vastness of space that surrounds us.
That mysterious process of inflation and the Big Bang have convinced some researchers that multiple universes are possible, or even very likely. According to theoretical physicist Alexander Vilenkin of Tufts University in Massachusetts, inflation didn't end everywhere at the same time. While it ended for everything that we can detect from Earth 13.8 billion years ago, cosmic inflation in fact continues in other places. This is called the theory of eternal inflation. And as inflation ends in a particular place, a new bubble universe forms, Vilenkin wrote for Scientific American (opens in new tab) in 2011.
But even if we could reach the next bubble, according to eternal inflation (combined with string theory), our familiar universe with its physical constants and habitable conditions could be totally different from the hypothetical bubble universe next to our own.
The many-worlds theory is the most "courageous" take on the quandary of quantum mechanics, physicist Sean Carroll wrote in his book, "Something Deeply Hidden: Quantum Worlds and the Emergence of Spacetime (opens in new tab)" (Dutton, 2019). He also argued that it is the most straightforward theory, although not without wrinkles.
One of those wrinkles is that the many-worlds idea is not really falsifiable. This is an important component of scientific thought and is the way the scientific community develops ideas that can be explored with observation and experimentation. If there's no opportunity to find evidence against a theory, that's bad for science as a whole, science journalist John Horgan argued in a blog post for Scientific American (opens in new tab).
Countless works of myth and fiction draw from ideas of parallel universes and the multiverse. Overlapping worlds make appearances in Norse mythology as well as Buddhist and Hindu cosmology. The idea of multiple universes coming into contact showed up in print as early as Edwin A. Abbott's novella "Flatland: A Romance of Many Dimensions" (Seeley & Co., 1884), and can still be seen in recent movies such as the 2016 Marvel film "Doctor Strange." An entire genre of Japanese graphic novels, called isekai, deals with characters transported to parallel worlds, as described by the New York Public Library (opens in new tab).
And comics, as well as their corresponding movies, delve deeply into the idea of parallel worlds. Recent Marvel Comics' storylines (both film and in print), DC's Flashpoint arc and 2018's "Into the Spider-Verse" all explore multiple universes and the intersections between them.
The multiverse is a hypothetical group of multiple universes.[a] Together, these universes comprise everything that exists: the entirety of space, time, matter, energy, information, and the physical laws and constants that describe them. The different universes within the multiverse are called "parallel universes", "other universes", "alternate universes", or "many worlds".
In his 1930 autobiography My Early Life, Winston Churchill cited the theory when explaining his preference for "believing whatever I want to believe":[original research?] .mw-parser-output .templatequoteoverflow:hidden;margin:1em 0;padding:0 40px.mw-parser-output .templatequote .templatequoteciteline-height:1.5em;text-align:left;padding-left:1.6em;margin-top:0
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".
Some physicists say the multiverse is not a legitimate topic of scientific inquiry.[14] 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.[15] 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.[16] Paul Steinhardt has famously argued that no experiment can rule out a theory if the theory provides for all possible outcomes.[17]
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.[19][20][21] 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.[22][23] In addition, there was no evidence of any gravitational pull of other universes on ours.[24][25]
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.
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.
In the eternal inflation 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,[63] 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.
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."[29]
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.[84] This includes the theory of white holes, which are on the opposite side of space-time.
[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.[62][87] 2ff7e9595c
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