by ROBERT LEA<\/p>\n\n Y<\/span>ou might have noticed, if you\u2019ve set foot in a cinema this year, that Hollywood has fallen in love with the multiverse. From Marvel to DC to Disney, alternate universes, realities and timelines are being written into scripts to wow audiences and make life a bit easier when A-list celebrities tire of yanking on the latex. <\/p>\n\n It\u2019s not just the big studios that are at it. The sublimely joyful indie film Everything <\/em>Everywhere <\/em>All <\/em>At <\/em>Once <\/em>asks and answers, \u2018why, if everything is <\/em>happening everywhere and all at once, should any of it matter?\u2019 Likewise, Rick <\/em>And <\/em>Morty, <\/em>Dark <\/em>and Man <\/em>In <\/em>The <\/em>High <\/em>Castle <\/em>use the idea of alternate universes as a kind of funhouse mirror to ponder (sometimes) serious questions about our own Universe. And it\u2019s fair to point out that the idea is nothing new. Who could forget Spock\u2019s evil doppelg\u00e4nger with his suitably sinister goatee? Clearly, the idea of the multiverse has permeated the fabric of our culture.<\/span><\/p>\n\n But what do the scientists think about multiverses? Is there science to back them up?<\/span><\/p>\n\n Many physicists believe that multiverses could exist, ranging from universes lurking behind the event horizons of black holes, to growing universes expanding like bubbles in soap foam. <\/p>\n\n \u201cA multiverse is something which is really not that strange if you think of it historically, from the point of view of science,\u201d says Prof Ulf Danielsson, a theoretical physicist at Uppsala University, Sweden. \u201cOur horizons have continuously been expanding.<\/span> At some time, we thought that Earth was the only planet and that this was the whole world. We now know there\u2019s a Universe full of other planets. It\u2019s also quite natural to speculate that there is another step and that our Universe is not the only one.\u201d<\/p>\n\n So what are some of the leading multiverse theories, and which of them could harbour an evil, possibly moustachioed, you.<\/p>\n\n This is a theory that has grown out of cosmology, particularly from the discovery that our own Universe is expanding. This concept of a multiverse asks if the initial rapid inflation that our Universe underwent some 13.8 billion years ago, could be happening in distant regions of space-time disconnected from our Universe.<\/p>\n\n \u201cThe basic idea is that our Universe is one particular patch of space-time that is evolving as a well-defined entity,\u201d explains astrophysicist Prof Fred Adams, from the University of Michigan. \u201cThis region is homogeneous, isotropic [the same in all directions] and expanding in a well-defined manner. If you trace the evolution backward in time, then you find an age for the Universe of about 13.8 billion years from this initial expansion.\u201d<\/p>\n\n Adams, who wrote the book Our Living Multiverse <\/em>and authored a Physics Report <\/em>paper on the topic, also believes that other regions of the multiverse could be experiencing their own Big Bangs, and therefore their own expansions. This means that they are not able to affect our Universe. \u201cThey are thus other universes and the collection of all such universes is the multiverse,\u201d Adams says.<\/p>\n\n This multiverse idea caught on in fiction because it is an excellent storytelling device. It became popular in cosmology because it could address lingering mysteries, while still fitting with existing physics.<\/p>\n\n \u201cOne reason that the concept of the multiverse became popular is that it can naturally arise from the theory of inflation,\u201d explains Heling Deng, a postdoctoral researcher in cosmology, particle physics and astrophysics at Arizona University.<\/p>\n\n \u201cIt was shown by [physicists] Andrei Linde and Alex Vilenkin, in separate works, that if inflation did occur, it could create infinite disconnected regions.\u201d<\/p>\n\n \u201cONE REASON THAT THE CONCEPT OF THE MULTIVERSE BECAME POPULAR IS THAT IT CAN ARISE FROM THE THEORY OF INFLATION\u201d<\/em><\/strong><\/span><\/p><\/blockquote>\n\n Although inflation ended 13.8 billion years ago in the Universe we are living in, Deng says that quantum effects can always bring inflation back in another region of space-time. This results in bouts of inflation never ending \u2013 referred to as \u2018eternal inflation\u2019\u2013 and the possibility of an infinite number of \u2018different universes\u2019.<\/p>\n\n Russian-American theoretical physicist Andre Linde puts forward one suggestion for the arrangement of this multiverse. He sees the universes as \u2018bubbles\u2019 expanding on something resembling a cosmic canvas, squeezing away from each other in bouts of eternal and chaotic inflation.<\/p>\n\n How these universes within a multiverse would differ is also currently the topic of speculation, but Adams suggests there\u2019s no reason to believe that the laws of physics would be the same in these separate regions.<\/p>\n\n \u201cOne reason that these other universes are of interest is that they could have other versions of the laws of physics,\u201d he says. That variation could apply across a range of physical parameters, including gravity and the rate at which that universe expands.<\/p>\n\n That means some of these universes could have laws of physics that aren\u2019t fit for the formation of large-scale structures like galaxies or stars. They may not even have the same fundamental particles.<\/p>\n\n Consequently, these universes aren\u2019t variations of our Universe and thus could not host any life at all, never mind some version of you or I.<\/p>\n\n String theory is a suggestion put forward by physicists to connect quantum mechanics and General Relativity, which are the best descriptions we have of the infinitesimally small and incomprehensibly large. The underlying idea of string theory is that fundamental particles like quarks and electrons are actually a single point in one-dimensional strings, vibrating at different frequencies. <\/span><\/p>\n\n\n\n This \u2018string-landscape\u2019 provides a popular setting for the multiverse, thanks to one of the key elements upon which string theory depends. In order to be mathematically sound, string theory needs \u2018extra dimensions\u2019 to exist. <\/p>\n\n\n\n These aren\u2019t parallel dimensions like we see in science fiction. Instead, string theorists believe these extra dimensions are curled up within the three traditional dimensions of space. They remain invisible to us, as we evolved only to see in three dimensions. These extra dimensions could offer a \u2018way in\u2019 to the string theory multiverse.<\/p>\n\n\n\n \u201cYou need to have these extra dimensions, and the number of dimensions needed in total is 10 or 11,\u201d Danielsson says. \u201cIt could also be that you would need to go into some extra dimension in order to get to these other universes.\u201d <\/p>\n\n\n\n Even if this was the case and a connection via these dimensions of space to other universes existed, they may still remain permanently out of reach and view, thanks to the fact that the inflation of the Universe means that there is a cosmic horizon beyond which we can\u2019t see. If there is no \u2018connectivity\u2019 between universes in a multiverse, it makes the cosmological concept of a multiverse almost impossible to test experimentally. <\/p>\n\n\n\n \u201cThe \u2018evidence\u2019 to date is theoretical, not experimental. And, unfortunately, we just cannot do any direct experiments to verify or falsify what goes on in other universes,\u201d Adams explains. <\/p>\n\n\n\n Our inability to test these ideas is a double-edged sword. While the lack of ways to test a multiverse means we can\u2019t prove its existence, it also means we can\u2019t disprove it either. <\/p>\n<\/div><\/section>\n\n At the end of a massive star\u2019s life, when it has run out of fuel for nuclear fusion, it\u2019ll collapse into a black hole \u2013 a region of space-time bounded by a surface called an event horizon from which nothing, not even light, can escape. <\/p>\n\n Einstein\u2019s General Theory of Relativity tells us that a large mass can curve space-time. The theory also says that the heart of a black hole has a singularity where the mass is so great that the space-time curvature becomes infinite and, consequently, the laws of physics break down. This is a concept that troubles physicists, but one hypothesis could do away with the singularity and replace it with an entire universe and in turn, a multiverse. <\/p>\n\n \u201cSingularities are unphysical because they cannot be measured. That means their existence indicates that a theory is incomplete,\u201d says theoretical physicist Dr Nikodem Poplawski, from the University of New Haven, Connecticut. \u201cIn my hypothesis, every black hole produces a new, baby universe inside \u2013 on the other side of the event horizon \u2013 and becomes an Einstein-Rosen bridge, also known as a wormhole, that connects this infant universe to the parent universe in which the black hole exists.\u201d <\/p>\n\n \u201cA UNIVERSE MAY PRODUCE BILLIONS OF BLACK HOLES AND EACH OF THEM COULD PRODUCE A BABY UNIVERSE\u201d <\/em><\/strong><\/span><\/p><\/blockquote>\n\n In this theory, when viewed from the new universe, the parent universe appears as the other side of a white hole, a region of space that cannot be entered from the outside and which can be thought of as the reverse of a black hole. <\/p>\n\n \u201cAn analogy of the matter going to a black hole and ending up in a new universe could be blowing a soap bubble through a circular wand,\u201d Poplawski says. \u201cThe wand is the event horizon \u2013 albeit in one dimension less \u2013 the soap liquid is the matter crossing the event horizon, and the surface of the bubble is the new universe.\u201d<\/span><\/p>\n\n In the hypothesis suggested by Poplawski, a universe may produce billions of black holes and each of them could produce a baby universe. In January of this year, researchers at the International School of Advanced Studies (SISSA) in Italy estimated that there could be as many as 40 trillion \u2013 that\u2019s a four followed by 13 zeros \u2013 black holes in our Universe alone. That\u2019s a lot of baby universes!<\/span><\/p>\n\n These infant universes would be hidden from the occupants of their parent universe by the light-trapping surface of the event horizon, and once that event horizon is crossed there\u2019s no going back. That, and the fact nothing can enter a white hole (which is still purely theoretical but allowed by General Relativity), means no interaction between parent and infant. <\/p>\n\n However, if two black holes existed in the same universe, and each of these black holes created a new universe, then there is a possibility that these two sibling universes could merge, \u201cjust as two black holes merge to create one black hole,\u201d says Poplawski.<\/span><\/p>\n\n He adds that this would manifest in a baby universe as a large-scale asymmetry in space. This means that if we ever discover some preferred direction in our Universe \u2013 a direction with increasing matter and energy, for example \u2013 it could be attributed to our Universe interacting with a sibling. <\/p>\n\n As for the possibility of an alternate version of you existing beyond the event horizon of a black hole, Poplawski concludes that chances are not good. \u201cThere would be no \u2018alternate you.\u2019 At any time, an object can only exist in one universe,\u201d he says. <\/p>\n\n But one pop culture mainstay reflects his concept: \u201cI think the closest thing could be the TARDIS in Doctor Who. <\/em>You enter the police box and you realise that you are in something bigger than the box.\u201d <\/p>\n\n In quantum physics, which deals with the physical laws of the subatomic, the term multiverse doesn\u2019t exist. Alternate universes are instead referred to as \u2018many worlds\u2019 and are part of a radically different concept, as these aren\u2019t geographic in nature like the multiverses explored previously. <\/p>\n\n\n\n The many-worlds hypothesis was first suggested by the US physicist Hugh Everett III to explain how a quantum system can exist in seemingly contradictory states at the same time \u2013 called a \u2018superposition\u2019 \u2013 and how these paradoxical states seem to vanish. <\/p>\n\n\n\n The effect of many worlds on the existence of a superposition of states can be imagined by considering Erwin Schr\u00f6dinger\u2019s infamous thought experiment, Schr\u00f6dinger\u2019s cat. <\/p>\n\n\n\n In the thought experiment, a hapless moggy is placed in a sealed box with a device containing a vial of lethal poison, released only if an atomic nucleus in the box decays. Treating the box, the cat and the device as a single quantum system, each state \u2013 in this case, \u2018dead\u2019 or \u2018alive\u2019 \u2013 is described by a wave. As waves can overlap to form a single wave function, the cat can exist in a superposition of states. This means that in quantum mechanics the cat is both simultaneously dead or alive.<\/p>\n\n\n\n This seemingly contradictory state persists only until the box is opened \u2013 analogous to making a measurement on the system \u2013 and the wave function collapses meaning the superposition is gone and the state is resolved. The cat is either dead or alive. Yet why <\/em>measurement causes this collapse of superposition, also known as \u2018decoherence\u2019, is still a mystery. <\/p>\n\n\n\n The many-worlds hypothesis does away with decoherence altogether. <\/p>\n\n\n\n Instead, it suggests that rather than the opening of the box collapsing the wave function, measurement causes it to grow exponentially and \u2018swallow\u2019 the experimenter and eventually the entire Universe. <\/p>\n\n\n\n \u201cIn the many-worlds formulation of quantum mechanics, each state of a system is a physically distinct world,\u201d says Prof Jeffrey Barrett, a philosopher of science at the University of California Irvine. <\/p>\n\n\n\n This means each flick of a light switch would create a near-infinity of worlds. One for each possible path of each photon as the light fills your living room, not just a world in which you didn\u2019t flick the switch at all. <\/p>\n\n\n\n That means that in terms of the Schr\u00f6dinger\u2019s cat thought experiment, the experimenter isn\u2019t opening the box to discover if the cat is dead or alive. Rather, they are opening the box to discover if they are in a world in which the cat is dead, or one in which it lives. <\/p>\n\n\n\n At first, the worlds that comprise this quantum multiverse are similar, with infinitesimally small differences. But these changes grow from universe to universe, meaning those that diverged earlier could be strikingly different from each other. <\/p>\n\n\n\n \u201cThe objects, events and physical records of observers are different in different worlds. There is a world where the Eiffel Tower is in Los Angeles,\u201d Barrett says. \u201cAll of the worlds \u2013 universes \u2013 are part of a single global universe. It looks just like this universe from the perceptive of our branch world.\u201d<\/p>\n\n\n\n Barrett addresses the question of how likely it is that one of these \u2018many worlds\u2019 would contain an alternate \u2018you\u2019. He reveals that it isn\u2019t just possible, it\u2019s demanded.<\/p>\n\n\n\n \u201cIt certainly would contain many alternate copies of me,\u201d he says. \u201cThat is fundamental to how the theory addresses the quantum measurement problem.\u201d <\/p>\n\n\n\n All of this makes the quantum version of the multiverse the one that most closely resembles pop culture, at least in principle. This is because it doesn\u2019t just probably <\/em>contain infinite versions of you, it definitely <\/em>does. <\/p>\n<\/div><\/section>\n\n Every tale that exploits a multiverse needs a MacGuffin, a device that allows access to these weird worlds. Here are some of our favourites\u2026 <\/strong><\/span><\/em><\/p>\n\n\n\n In a 1960s comic book, superhero the Flash visits an alternative world. He does so by performing his own version of the Indian rope trick, which sees a person climb a length of rope and disappear. To perform the trick, he vibrates his hands at super-speed, which causes the rope to rise into the air. He then climbs up the rope and phases into a different universe. Maybe string theory is founded on vibrating ropes in the DC Universe?<\/span><\/p>\n\n\n\n The X-Men\u2019s mentor Professor X has a contentious relationship with his son David Haller, known as Legion. In a 1995 story, Legion travels back in time to kill his father\u2019s nemesis Magneto, but accidentally murders Professor X. This leads to a twisted world in which the X-Men enemy Apocalypse rules, and Magneto forms a very different X-Men team. The many-worlds theory would suggest that Legion travelling back in time created a new \u2018world\u2019. This is something that has been suggested as an \u2018out\u2019 for the grandfather paradox, in which a time traveller kills his own grandfather before his parent is conceived. Rather than causing the time traveller to no longer exist, in the many-worlds theory the event would cause worlds to split, ensuring the time traveller\u2019s existence in the primary world he left.<\/span><\/p>\n\n\n\n Doctor Who\u2019s time-travelling police box isn\u2019t supposed to travel to parallel worlds, but this is exactly what it does in David Tennant\u2019s run in Doctor Who. <\/em>This allows the Doctor to watch the rise of his enemies the Cybermen all over again, as they dominate a new world. Doctor Who <\/em>lore posited a void between universes, perhaps similar to the voids that separate bubble universes in a cosmological multiverse. <\/span><\/p>\n\n\n\n![\"\"](\"https:\/\/dj9jqhxgw9833.cloudfront.net\/uploads\/sites\/42\/2022\/07\/f70857d6-327c-4134-8568-1b2f0424a597.jpg\")
<\/figure>\n\nThe cosmological inflating multiverse<\/h3>\n\n
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The string theory multiverse<\/span><\/h3>\n\n\n\n
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The black hole multiverse<\/h3>\n\n
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The many-worlds multiverse of quantum mechanics<\/span><\/h3>\n\n\n\n
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PLOT TWIST<\/span><\/strong><\/h4>\n\n\n\n
1. <\/strong>THE FLASH\u2019S ROPE TRICK <\/span><\/h5>\n\n\n\n
\n\n\n\n2. <\/strong>LEGION KILLING HIS DAD <\/span><\/h5>\n\n\n\n
\n\n\n\n3. <\/strong>THE TARDIS <\/span><\/h5>\n\n\n\n
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