How Many Multiverses Are There?

[Music] in 1828 amateur cartographer edward quinn finished his magnum opus the historical atlas published two years after his death it depicts our knowledge of the world as a flickering light blowing away clouds of ignorance unveiling continents and civilizations shrouded beneath it starts in 2348 bc beginning with the bible the dispersal of the clouds mirror european expansion into the lands beyond his known world little more than the lands of the euphrates and tigris rivers time marches on as we step through the pages of the atlas through the exodus of the israelites and the coming of the

roman empire more of the world is revealed by the early 19th century the clouds are fully dispersed all the continents are visible there is no more land left to discover of course for the artist in china india australia or the americas the focus would be different each with their own origin story their own small portion of the world surrounded by their own darkness and indeed these clouds are not solely geographical all human endeavors begin in a fog of mystery a fog that only begins to clear when we learn something new like the sheathed surface of

the earth clouds of mystery have also shrouded the universe over the last few hundred years we have turned our attention to the heavens with our most powerful telescopes we have peered deeper and deeper into the cosmos we have spied stars and galaxies black holes and planets we have peered back over 13 billion years and seen the very afterglow of creation we can even deduce the future history of our universe looking to the days of the last star trillions of years hence and even further to the time when mata melts and black holes evaporate one by

one the universe has given up its secrets the cosmic clouds have steadily been lifted it might now appear that we have learned all there is to learn about the cosmos but just as the polymath simon newcombe found after claiming we had reached the limits of astronomy in 1888 and dying in 1909 as einstein was upending the world of physics it is always a bad idea to bet against further discoveries and so this is where scientists find themselves now standing tall on towers of complex equations elaborate theories and remarkable observational tools they try to peer further

into the fog but unlike quinn's atlas this is not only a fog of distance these are clouds that linger around us within us before us these are new universes born beyond our cosmic horizon in our distant cosmic past even inside us with every instance that passes as our knowledge of theoretical physics has grown what had once seemed radical has gradually forced its way into possibility and even probability multiverse theories have proliferated hoping to answer the deepest questions about what we and the entire cosmos is from parallel worlds of infinite use to barren deserts of endless

nothing scientists are blowing away the cosmic fog of unknowing revealing a warped russian doll of infinities nestled both within and right next to each other [Music] welcome to the multiverses [Music] according to the european space agency there are approximately 106 stillion stars in the observable universe and across the cosmos they form huge filaments and clusters but on a more personal level have you ever wondered what the night sky looked like when you were born under lucky stars have been kind enough to support this video and their beautiful framed star maps are a great gift option

to remember an important moment in your life a personalized chart with over 15 designs to choose from and printed on durable museum quality paper showing what the stars were like on a day and time you choose they are the original star map maker and their methods are verified by nasa astrophysicists and enjoyed by neil himself they also sent me one over and koko absolutely loves it really ties the room together the company are also committed to keeping the skies clean organizing fundraisers for the international dark sky association it's a great thoughtful gift for someone or

indeed yourself especially as they're now offering an exclusive 10 discount to history of the universe viewers head over to underluckystars.com forward slash universe and use your code universe to start your starmap journey in april 1920 a great debate was held at the smithsonian museum of natural history in washington dc on the stage were harlow shapley and heber curtis both renowned astronomers shapley was well known for mapping the locations of stars throughout the milky way curtis the first to observe a thin stream of material in the object m87 though he didn't know it at the time

he had discovered a relativistic jet of matter ejected from an immense black hole the topic of their debate just how big is the universe [Music] the two camps presented their arguments shapley argued that the cosmos was small the stars of the milky way he claimed were the entire universe that was it there was nothing beyond he knew that astronomers had spied several strange nebulae through their telescopes but these he told the audience were features buried within the stars of the milky way curtis however objected he was no fan of this compact cosmos instead he argued

the universe was a large much larger than most astronomers were comfortable with the spiral nebuli he declared were not within the milky way but were many millions of light years away each was an island universe an individual galaxy home to billions of stars and there could be uncountable galaxies beyond the edge of the milky way [Music] eventually the debate came to a close the audience had been taken on a wild ride across the universe but there was no clear winner shapley and curtis went their separate ways and the size of the universe remained unsolved but

things were changing quickly on the other side of the country edwin hubble had recently joined the mount wilson observatory in california he was on a quest to answer the question of the size of the universe once and for all the timing was fortuitous as the world's largest telescope the 100-inch hooker had just been installed it was the ideal instrument for surveying the sky and charting the distances to heavenly bodies and hubble's first target was the fuzzy nebula in the constellation of andromeda he knew that astronomers had seen individual stars within andromeda's nebula but just how

far away were they they could be intrinsically faint stars that were closer or they could be extremely luminous stars spied at a very large distance astronomical evidence had been building for several years in fact in 1917 it was heber curtis who had discovered an exploding star a nova within andromeda's nebula and in search of older photographic plates he spotted 11 more much fainter than any others that had been observed but hubble was searching for something different [Music] he was scouring andromeda for stars whose brightness varied lots of stars can change their brightness but hubble was

after a particular kind of variable star a cepheid variable these were stars whose brightness varies with a regular beating pattern in 1912 henrietta swan levitt had realized that how long the beat of a star takes depends on how luminous the star is fainter stars beat rapidly brighter stars beat more slowly this meant that if you observe a cepheid variable star and measure how long it takes to beat you can deduce how luminous it truly is and once you know its luminosity you can work out how far it is from how bright it appears in your

telescope it was these standard candles that hubble was searching for and he found them buried within the fuzzy light of andromeda once he measured the regular beating of these stars the picture became crystal clear hubble was observing bright giant stars many hundreds of thousands of light years away far outside the milky way in 1924 hubble sent a letter to shapley telling him that andromeda is a galaxy in its own right and that the other nebula spied through telescopes must also be galaxies at immense distances the great debate was settled curtis was correct and shapley's response

here is the letter that destroyed my universe the clouds parted and our cosmos suddenly became much much bigger and throughout the 20th century we have continued to peer deeper and deeper into our universe in measuring distances millions of light years became billions of light years but was there a limit to how far we could see in 1964 a chance discovery of a hiss of radio emission from all over the sky marked the limit of our observations the cosmic microwave background radiation whose light has been traveling for 13.7 billion years marked the edge of the map

a boundary between the observable universe we can see and what we cannot this of course leads us to ask just what is beyond the observable universe through the 20th century telescopes got bigger and astronomers saw further but with their paper and pens the theorists were also hard at work building on the insights of einstein and friedman astronomers now had a mathematical description of the cosmos belgian cleric georges lemaitre realized that the universe was not infinitely old but had been born at a finite time in the past he called this initial event the cosmic egg the

ultimate origin of the cosmos and from this fiery birth and expansion in the big bang the universe had been cooling and changing for billions of years but what did their equations say about the size of the universe they realized that the universe we see around us the observable universe must only be part of a much larger whole given the finite age of the universe we can only see the bits from the universe from which we have received light and beyond this observable space there must be more more universe more galaxies more stars or planets but

how much can there be beyond the horizon and it is here that we meet the first of our infinities the cosmological equations suggested that there might be an infinite amount of space in all directions as the universe most likely had no spatial curvature then it had to go on forever this might seem strange given that the universe was born a finite time in the past expanding from a single point but if the cosmic equations are correct and the universe is infinite today it always has been even at its birth from an infinitely dense cosmic egg

it has simply gotten less and less dense and all we can observe is the small patch around us our observable universe embedded in the infinite but as the cosmos ages more and more of it should come into view as the observable universe grows eventually we should be touched by the light of countless distant stars and if we wait long enough maybe an infinite amount of time eventually we will see it all or so we thought but by the end of the 20th century everything had changed and this change shook our understanding of the universe to

its very core at the close of the 1990s astronomers announced the unexpected discovery of the accelerating expansion of the universe the culprit was dark energy a mysterious repulsive something that filled the cosmos it was realized that galaxies were and are moving away from each other faster and faster but what did this mean for our observable universe as time marches on our map will once again be plunged into darkness we will begin to lose sight of galaxies outside our own in about 100 billion years from now all other galaxies will have vanished completely from view beyond

our local stars there will be nothing but the black is black at every moment there is less and less of the universe we will ever be able to explore astronomers estimate that there are about two trillion galaxies within our observable universe and that due to the accelerated expansion almost 99 of those galaxies are already beyond our cosmic horizon if we could set off in a spaceship skimming close to the speed of light we would never arrive but though we will never visit these distant worlds or feel the warmth of their sons we can use a

logic to deduce their properties let's start with the simplest picture of all that beyond is an infinity of space in all directions and that the universe over the horizon is just more of the universe we have here not exactly the same just the same in character born 14 billion years ago and containing more stars and galaxies if we pick one of these galaxies at random it will sit at the center of its own observable universe surrounded by trillions of other galaxies it will be at the center of its own bubble of observability in the infinity

of the universe just as a native american would have produced a different historical atlas to the englishman edward quinn so would their outlook be their own but with a key difference the inhabitants of this universe always have been and always will be completely separate from us being causally disconnected means that no light rays no signals of any kind can be exchanged between the regions they are beyond each other's horizons and are destined to be forever so universe upon universe out there in the void and it is now things begin to get complicated so let's start

with new definitions let's call something like our observable universe simply a universe so beyond our horizons there are countless other universes but what do we call the universe that contains this myriad of individual universes welcome to the first level of the multiverse it is important to note that this is just a multiverse one of many ideas that carry that name the physicist max tegmark calling this a level one multiverse let's explore as we know in this particular multiverse beyond our horizon there is possibly an infinite number of other universes rich in stars and galaxies just

like our own with the potential for life just like our own however if we follow this logic and think a little harder a disturbing picture emerges in our level 1 multiverse each individual universe is roughly the same size each contains similar numbers of atoms arranged as planets stars and galaxies but with an infinity of space to play with strange things can occur out there in the void some of these universes will be identical to ours absolutely and precisely the same in this other universe there will be a milky way galaxy containing a star identical to

the sun and orbiting this star will be a planet identical to the earth with continents oceans plants and animals and of course there are people everyone you know and don't know are there playing out their copies of their lives and finally there will be another version of you watching this exact video such a situation would be vanishingly rare with most universes being only roughly similar to our own but a vanishingly rare percentage of infinity is still infinity how far away is this doppelganger max tegmark estimates something like 10 to the power of 118 meters this

is much much larger than the roughly 10 to the 27 meter diameter of our universe and of course they are causally disconnected so you will never interact with them but they would be out there in the infinite along with every possible variation anything that has a chance of happening must happen somewhere in the infinite somewhere in the level one multiverse of course we are forever locked within our own observational universe forever bound to see nothing but our tiny patch the infinite parallel worlds of the level 1 multiverse lost behind our cosmic horizon and yet this

is just the beginning this is just level one just one mind-melting deduction from our laws of physics there are three more levels in max tegmark's multiverse of multiverses and with each step our picture of the cosmos becomes even stranger why does our universe have its particular laws of physics this might seem like a strange question how could it have been otherwise but as our knowledge of the workings of space advanced across the 20th century it became harder and harder to ignore in the 1960s cosmologist robert dickey suggested that the forces of gravity and electromagnetism were

finally balanced if they were not stars simply would not shine and life in the universe would be impossible and as physicists stared deeply into their equations more examples of this fine tuning were revealed astronomer fred hoyle found that tiny changes in the nuclear forces would result in a universe without carbon and steven weinberg noted that a universe with more dark energy would expand too rapidly for stars to form serendipity piled on top of serendipity and it became apparent that if our universe were even very slightly different we simply would not exist our cosmos appeared to

have been just the right physics to allow for the complexity of life and it is in that key adjective that the answer lies our cosmos it is through this problem we can take our first steps into the level two multiverse although there is more than one way to build a level 2 multiverse we will start with the idea of inflation a burst of rapid universal expansion in its earliest moments when originally proposed by alan guth inflation turned on and off in a trillionth of a trillionth of a trillionth of a second smoothing the universe and

seeding the structure that led to the formation of galaxies it seemed an essential ingredient for explaining the universe around us but there was immediately a problem that threatened the very idea how could inflation be synchronized across the entire cosmos within this tiny window of time calculations suggested that the end of inflation would be incredibly messy whilst in some places inflation would come to an end after a short burst in others inflation could continue this lack of a graceful exit appeared to spell doom for the theory but inflation was too attractive to simply abandon and by

the 1980s a flurry of activity gave birth to the new theory of inflation its key architects were andre linda andreas albrecht and paul steinhardt and it was this second wave of thought that introduced a now essential part of the idea the inflaton the inflaton field permeated the early universe and provided the impetus for inflation this new inflation embraced the messiness of the original inflation as the cosmos inflates instead of ending everywhere individual universes continually crystallize out of the inflation in this picture inflation never ceases it continues for eternity and as it does more and more

universes crystallize within this eternal inflation our level 1 infinite multiverse is just one of countless many separated by ever inflating space and time so here we sit at the center of our observable universe across the horizon there is more universe that we can never explore and beyond that inflation continues carrying away the other infinite level 1 multiverses that we will never know of course many questions remain none less than just how a universe would emerge from the eternal inflation some have suggested the process is similar to the formation of bubbles when opening a soda and

it is this bubble nucleation that cements the properties of the universe it forms not just the overall properties but the intricate details this includes the very laws of physics themselves meaning that in some universes the forces of nature would be different in others the masses of the fundamental particles would be different and in others again there might be more or fewer dimensions of space and time physicists have wondered what these other multiverses would be like and they have arrived at a stark conclusion virtually all of these other level 1 multiverses would be dead and sterile

lacking the complexity essential for life though within this endless sea of dead universes there would be tiny pockets of habitability and it is no surprise that this is where we find our universe in this level 2 multiverse our level 1 multiverse is one of the lucky few that can host complexity and life the apparent fine-tuning of our physical laws little more than a statistical fluke a roll of an infinite dice across an even higher infinity but this is not the only level 2 multiverse that has been proposed there are other theories that would require a

multiverse of which our level one existence would only be a part and in fact paul steinhardt who developed the new theory of inflation in the 1980s plays a key role in a completely different idea [Music] in hindu cosmology the universe is without beginning or end instead it is cyclical with each cycle lasting for four million three hundred and twenty thousand years each universe starts with rebirth and perfection a time of enlightenment as it ages the universe degenerates and decays before ultimate destruction and eventual rebirth but what of the physical universe could it too be truly

cyclical since the dawn of modern cosmology many minds have pondered this question in the 1920s physicists including einstein explored this very possibility within the cosmological equations derived from general relativity the prospect for a cycling universe existed indeed the idea that our universe was just one of an infinitely repeating cycle was far more comforting than a universe born from nothing but there was a lingering problem and that problem was entropy [Music] entropy is a measure of disorder the inexorable running down of the cosmos at every moment through every process useful energy is being degraded into useless

energy life on earth is completely governed by such processing of energy from a fire turning wood to ash to the running of a cheetah turning food into movement energy is used and whilst the total flow of energy balances energy in equals energy out the entropy the movement from potential to disorder continues to increase across the universe this is unavoidable at the end of time the disorder will be greater than at the start there is no way to escape and so if a universe is just part of an endless cycle this entropy is passed from parent

to child every cosmos beginning encumbered with the disorder from all the universes that existed before and if there is an endless maybe infinite cycle of universes how can there be order at all in our current universe and it was this inexorable increase of entropy that doomed the concept of a cyclical universe to the wastebin of forgotten ideas although sometimes it is hard to completely kill a scientific theory and the concept of a cyclical universe has itself been recycled [Music] recent nobel prize winner roger penrose proposed the conformal cyclic cosmology he suggested that in the extreme

distant future of our universe all matter must decay away into nothing and that the resulting empty universe will continue expanding into eternity when it is large enough and empty enough it will be a very simple featureless place featureless like our own universe at the start of the big bang penrose then hypothesized that the past and the future are effectively one a constant cycle between cosmic birth and cosmic death and that is just one example in the 1980s physicist lee smolin suggested that a baby cosmos could be created when black holes formed tearing space and time

this tear giving birth to an entire new universe not a birth in the existing cosmos but spawning a completely new universe of space and time if this idea has even the spark of truth then every single second a new cosmos is born from a stellar death in our universe smolin however went further borrowing from darwinian evolution he wondered if the baby universe could mutate and be different to its parent the difference is lying in the laws of physics underlying the cosmos if the mutation is too severe gravity may be too weak or there may be

not enough atoms to form stars such a baby universe is an evolutionary dead end but those that can form stars and form black holes beget more baby universes eventually these star-forming universes must come to dominate in the evolutionary tree of the multiverse as stars are essential for the habitability of our universe smallins multiverse could be teeming with life of course there is a lot of uncertainty in smolin's cosmic natural selection and whilst it is intriguing to think about not everyone is convinced finally the cyclic universe was recently reborn again but this time in fire [Music]

think of the phoenix the immortal bird of greek mythology it lives its life in cycles dying an old age and a burst of flames but from the ashes the phoenix is reborn to live its life again and this is where paul steinhardt reappears in conjunction with neil turock he has since come to reject his own inflationary theory and proposed a new level two multiverse the ecpirotic universe developing out of string theory for them our entire universe floats in higher dimensions like a sheet blowing in the wind here cosmic rebirth occurs when multiple cosmic sheets collide

huge amounts of energy released forming the mata in the cosmos and the newborn universe has low entropy the phoenix universe rising from the ashes but today it has few true supporters many physicists have trouble accepting the notion of sheet universes floating in higher dimensions however recently this concept has gained widespread attention and the answer may lie in one of the most complex and mind-blowing theories of all ed whitton followed a rather unconventional path on his way to becoming a professor of physics in 1971 he completed an arts degree in history and linguistics he tried journalism

and politics eventually enrolling in and dropping out of a graduate program in economics by 1976 he had finally found his niche completing a phd in fundamental physics and exploring how the universe works witten's mathematical insights about the topology of space and time set him apart from other physicists in 1990 he was awarded the fields medal for his work often called the nobel prize of mathematics it is usually awarded to mathematicians but by this time witton had set his sights on bigger things indeed one of the biggest dreams in physics a single theory that encompassed everything

we knew about the universe [Music] the theory of everything but he was just the latest in a long line who had hunted and failed to find this physics el dorado just why was this search so hard [Music] of the four fundamental forces of nature three are written in the language of quantum mechanics these are the strong and weak nuclear forces and electromagnetism our best description of gravity however is einstein's general theory of relativity a distinctly non-quantum theory gravity is described by the curving and bending of space and time why the forces of nature are written

in two different mathematical languages has long bothered physicists there is a feeling at least a desire that the true mathematics of the universe should be one one neat single theory encompassing all the physics of the universe and so physicists have searched for this theory for a very very long time early attempts to unite gravity and electromagnetism were made by theodore kaluza and oscar klein they added more dimensions to einstein's relativity but ultimately failed in their quest other great names also litter the path of failed theories of everything this includes einstein himself as well as hermann

vile arthur eddington and david hilbert by the 1970s a brave new contender supergravity had been proposed this was based upon a mathematical formalism to iron out problems in our understanding of fundamental particles known as supersymmetry it's suggested that particles should have strange alter egos for electrons there should be selectrons for quarks there should be squawks and most counter-intuitively the mathematics of supergravity seem to work best in 11 dimensions but as scientists peered more deeply into these equations things began to unravel the universe predicted by the ideas of supergravity just didn't match the one we inhabit

and eventually supergravity quietly fell by the scientific wayside but again it can be difficult for scientific theories to completely die and disappear especially those with the prefix super and so in 1984 super string theory was born in the minds of michael green and john schwartz [Music] the key idea of super strings is that all fundamental particles are really the same thing each is really just a vibration on a loop of stringy stuff different notes on the strings correspond to different particles one note for an electron another for a quark and just like supergravity super strings

need far more dimensions than we experience day to day in its earliest mathematical form superstring theory was written in 26 dimensions though eventually things settled down to a more manageable eleven echoing its super ancestor a way was found to hide the extra dimensions a process known as compactification unlike our everyday dimensions of up down and across these supposed extra dimensions were curled up on a microscopic scale potentially as bizarre geometrical shapes called calabi yao manifolds and just as we cannot experience the individual atoms in the air we cannot experience the existence of these microscopic dimensions

again physicists were excited again they thought they were about to arrive at a theory of everything a complete description of the universe and again their hopes were dashed [Music] they had hoped that a single mathematical framework for the theory of everything would emerge but the mathematical complexity grew and ideas splintered in total there were five different versions of string theory following five different paths towards enlightenment by the mid-1990s frustrations had reached boiling point and that was when in 1995 at the annual string theory conference in the university of california ed whitton put a new idea

on the table perhaps these five differing theories weren't so distinct after all perhaps they were all the same theory intrinsically linked it was from this revelation m theory was born and the second superstring revolution was begun no one seems to remember what the m in m theory stands for some say mystery or perhaps magic but to many it stands for membrane reflecting how the one-dimensional strings of string theory are drawn out into sheets or brains within the higher dimensions this eases some of the mathematical issues facing string theory but it does not solve them and

in truth m theory isn't yet a theory it is more of an idea a dream a goal to which physicists are working but it has brought one thing however into sharp relief the original hope was that the theory of everything would be pure and unique it would explain why the electron has the mass it does and why gravity has the strength it currently does that hope died with m theory m theory predicts that due to the intense heat in the first moment the early universe would be featureless and simple as the universe cools the fundamental

features of physics crystallize out but this crystallization of the laws of physics would not be unique there would be uncountable ways that this crystallization could occur each resulting in different fundamental physics different particle masses and different forces physicists refer to this myriad of possible combinations of the laws of physics as the string theory landscape how many solutions are there out in the landscape nobody really knows it might be 10 to the power of 500 or it might be infinite once again our supposedly fine-tuned universe finds another dice to roll and indeed some physicists have gone

much further and suggested that these possible universes are not just possible they really do exist [Music] in this picture the multi-dimensions of m theory an entire universe can exist on a membrane or brain and in this space of multi-dimensions known as the bulk there are an uncountable number of brains for each universe floating in the bulk the crystallizing features of physics will be different differing universes will find themselves at differing locations in the string theory landscape there are clear parallels to the bubble multiverses of eternal inflation as we've already seen a random mix of physical

laws is most likely to end up with a universe that is simply dead and sterile and as the vast multiverse of eternal inflation brings about a desolate graveyard so too would the m-theory bulk be a grim sight but every so often somewhere in the mix there would be a universe that won the multiverse lottery and unsurprisingly we find ourselves in such a lucky universe of course given the undeveloped form of m theory much of this is pure speculation we don't know if the string theory landscape is real or if universes do exist as brains in

the bulk but to some physicists m theory simply feels right and a brain multiverse feels inevitable only time and intense mathematical effort will tell and so the fog of unknowing has retreated and revealed a plethora of possible level 2 multiverses some contrasting some able to work in harmony some currently in scientific favor and some out our finite observable universe sits within its larger infinite multiverse perhaps crystallizing from eternally inflating bubbles or reborn in fire from a previous multiverse or resting on a vast brain floating in the bulk of hyperspace [Music] the clouds are parting is

this the end is this as weird as it gets in a word no [Music] what if i told you that you are immortal that you will live forever what if i could guarantee that no matter what challenges confronted you you would emerge victorious what if i told you that you will with all certainty conquer illness and even death with a gun pointed at your head i could ensure your survival you might think that i am mad but we have now arrived at the level 3 multiverse and here immortality is guaranteed [Music] we begin with a

seemingly simple insight with earth-shattering consequences the question of the fundamental nature of light by the year 1800 whether light was a wave or a collection of particles had been argued over for more than a century but a new experiment was about to seemingly provide a definitive answer to this question the experimenter was thomas young a polymath known by some as the last man to know everything over his life young made important contributions to medicine music and language indeed he was instrumental in deciphering egyptian hieroglyphics and the rosetta stone but we are interested here in young's

many contributions to physics and so it was in 1801 that he turned his impressive intellect to the nature of light his experimental setup was deceptively simple it consisted of a source of light and a screen in between the source and screen young placed another screen containing two narrow slits he reasoned that if light was composed of particles some would fly through one slit and some would fly through the other this would result in two bright spots of light on the distant screen however if light was a wave this wave would pass through both slits radiating

outwards towards the screen instead of just two spots of light the result would be a complex mix of brightness and darkness an interference pattern young set up his experiment shrouding it to ensure complete darkness he directed sunlight through a small hole to act as a source and then he inspected the screen and it was there he found the beautiful pattern of interference indisputable evidence that light was a wave this viewpoint lasted a century but as it has since tended to do in the world of physics whenever it has raised its head quantum mechanics came along

and seriously muddied the water [Music] the key message of quantum mechanics was that the superficially continuous universe of classical physics was actually quantized chopped into discrete chunks and this included light it was einstein who first proposed the discrete nature of light tiny particles we now know as photons but this left scientists with a simple but fundamental problem if quantum mechanics says that light is truly particles how could they explain young's observation of an interference pattern this strange duality light as both a particle and a wave has been argued about for more than a century and

to understand it and its far-reaching implications we need to explore the language of quantum mechanics to galileo and newton objects could be described definitively that was there this was here and things move at the speed that is measured in classical terms an electron has a definite position and a definite velocity in the mathematics of quantum mechanics however the definitive language has to be abandoned in quantum terms an electron is now defined by a wave function this wave function encodes all the properties of the electron but it has no unique position or unique velocity it is

smeared over space in 1926 erwin schrodinger derived an equation that allows us to determine how a wave function changes with time and it is with this equation we can calculate how light or any quantum particle flows through young slits as the wave function is a wave and waves can interfere we get interference at the screen problem solved but not quite let's run young's experiment again in 1801 he used photographic film to detect his light waves but today we use electronic detectors and with the newfound accuracy they provide it is clear that single particles are detected

on the screen single particles that are not smeared out like their wave function the light travels as a wave thus creating the interference pattern but the photons actually interact with the screen as particles single points this is true for all quantum particles and this is where the real problems begin how does this smeared out wave function turn into a discrete electron or photon it was two of the giants of quantum mechanics niels bohr and werner heisenberg who first put a solution on the table named the copenhagen interpretation of quantum mechanics at the time it was

a radical development for bohr and heisenberg put the observer squarely in the experiment it is the observer that demanded an outcome to understand this the wave function has to be interpreted as a wave of probability this idea was first proposed by max born in 1926. he said that you should expect to find your photon or electron where the probability amplitude is high and to be rarer in places where your probability amplitude is low but an extra ingredient is needed to turn your wave function into the detection of an individual photon or electron you need the

wave function to collapse so when you fire an individual particle through young slits its wave function fans out the wave function goes through both slits fanning out again and interfering on its journey towards the screen but at the screen the wave function collapses to a single detection we cannot predict exactly where on the screen the particle will be detected but its location will be dictated by the probabilistic amplitude of the wave function many experiments over the last century with photons electrons and other particles have borne this out but there is a problem with the copenhagen

interpretation and it is this problem that will eventually lead us to our only level 3 multiverse nowhere in the successful mathematics is the mechanism for the wave function to collapse the wave function collapse has to be bolted on an ad hoc piece of mathematical machinery that seems to have no real justification this is one of the most furiously disputed questions in physics what quantum physicists call the measurement problem some have claimed it is the act of detection that collapses the wave function with john van neumann even suggesting that consciousness is needed for collapse the presence

of the observer becoming critical to the outcome of the experiment indeed this leads to some bizarre questions does that mean no wave functions collapsed until the first consciousness in whatever primeval creature arose it does however remain a popular interpretation often taught to students who are new to quantum mechanics but to many physicists and philosophers collapsing wave functions are more than discomforting cosmologist sean carroll went as far as to say the need for the wave function collapse should be very embarrassing to physicists and so other interpretations of quantum physics have been proposed and the most controversial

of these is something known as many worlds [Music] in 1942 a twelve-year-old hugh everett iii sent a letter to albert einstein asking whether the universe was held together by chance or unified by some kind of force dear hugh there is no such thing as an irresistible force and immovable body but there seems to be a very stubborn boy who was forced his way victoriously through strange difficulties created by himself for this purpose sincerely yours a einstein and it was 12 years later in 1954 that this stubborn boy came up with what would be his singular

contribution to theoretical physics during a late night drinking session ruminating on the ridiculous things about the nature of quantum mechanics his aim was to follow the mathematics of the theory real world consequences be damned he wanted to tackle the measurement problem head on the copenhagen interpretation is hopelessly incomplete because of its prior reliance on classical physics as well as a philosophic monstrosity with a reality concept for the macroscopic world and denial of the same for the microcosm indeed as we've seen the copenhagen interpretation relied on this so-called heisenberg cut the ad-hoc line between the micro

and macro which held the more bizarre features of the quantum world at bay everett rejected this he through the notion of wave function collapse out the window let's begin by thinking about an experiment in a laboratory once set up traditionally the experimenter is distinct from their apparatus they can influence the experiment by adjusting the temperature or magnetic field but they are just an observer not part of what is being experimented on everett realized however that this cannot be correct the experiment is made of electrons and nuclei each described by individual wave functions in terms of

quantum mechanics this experiment is a superposition of all its wave functions and so is the experimenter to fully describe the laboratory we need to consider the superposition of experiment and experimenter the entire laboratory is the superposition of all the particles within but if the experimenter is now coupled to the experiment through superposition where is the collapse the problems only get worse from here everett realized that the experimental laboratory is not isolated but is part of the universe the total wave function the universal wave function should encode the properties of every particle in the universe within

this universal wave function is all of the particles that make up u and as in all of quantum mechanics the schrodinger equation churns the universal wave function into the future but if this is the case where has the probabilistic aspect of quantum mechanics gone everett proposed that the universal wave function encodes more than just our universe it also encodes all of the possible universes as a superposition of universal wave functions within these universes an electron would be in a supposition of two states but the act of observing the electron severs this supposition the coherence between

nearby universes is broken by the observation and all the universes decohere in some universes the electron is found to be definitively in state a in others it is definitively in state b and remember you are in the universal wave function so you in some universes measure the electron in a and you in other universes measure the electron in b effectively two separate universes have become distinct within the universal wave function and this would happen at every quantum measurement at every quantum interaction every probabilistic outcome in quantum mechanics is played out within the superposition of universal

wave functions individual universes with distinct futures crystallize out of the ultimate superposition in many worlds theory the other universes are just as real as our own and in some of them there are other versions of you in nearby universes which hold coherence these other versions of you are almost identical to the one you are experiencing but in another universe somewhat more distant in the mix and distinctly separated from our own you are very different in others you don't exist at all this feels very much like the level one multiverse we encountered earlier with parallel worlds

playing out but in reality in the level 3 multiverse every branching universe can be its own level 1 multiverse or even level 2 infinities upon infinities and more worlds beyond worlds from young's seemingly simple double slit experiment we have arrived at what max tegmark calls the level 3 multiverse in removing the need for collapse of the wave function we have ended up with an endless sea of parallel universes some physicists dismiss many worlds out of hand calling it exceedingly wasteful nobel prize winner gerard went as far as to say it would be a stupendous number

of parallel worlds which are only there because physicists couldn't decide which of them is real and indeed everett's insights were largely ignored during his lifetime after receiving the cold shoulder from much of the physics community and niels bohr himself he left physics after completing his phd going on to work as a computer programmer and dying of a heart attack at the age of just 51 but his ideas resonated with a new generation of physicists with a resurgence of interest in recent years to some many worlds represents the only realistic solution to the various interpretations of

quantum mechanics the only way to truly follow the science and explain wave function collapse but there are still questions not least of all consciousness what happens to the you that makes you as individual universes diverge are there two distinct versions of you each now imprinted with the memories and experiences before the split it is in such exciting playgrounds that philosophers love to play [Music] imagine you have a coin a perfectly fair coin with equal chances of coming up heads or tails the stakes on flipping this coin are high tails you are executed heads you survive

as the coin spins in the air you know that the outcome is 50 50. the coin lands and settles on heads you sigh with relief but you are instructed to toss the coin again and again and again you realize this is a game you are bound to lose you would have to be extremely lucky to keep getting heads on every throw and so your execution awaits this is what you ultimately face with the copenhagen interpretation of quantum mechanics with many worlds however on the first toss the universe splits in two in one universe the coin

came up heads and you survived in the other universe the coin came up tails and you died and on the second toss the universe split again into one universe in which you live another in which you're executed the lucky path of a continuous run of heads will exist through the superposition of possible universes one future history will guarantee eternal survival what would the coin tosser experience will they find themselves on the lucky immortal path given the mysteries of consciousness nobody knows it may seem that we have come to the end of our exploration of multiverse

ideas what possibly could contain more than the ultimate universal superposition of all possible universes to get to this point we have already rid ourselves of the usual understanding of reality we've had to consider parallel worlds beyond our own vast nested infinities stretching out beyond eternity but to arrive at the next step we will have to completely and utterly let go four in the level four multiverse we may be little more than a mathematical dream [Music] what is reality as you look around it seems obvious you are surrounded by stuff some solid some liquid and some

as ethereal as the air but you should know by now that this reality is an illusion if you appear closely enough it is all made of atoms combined into various molecules and we can strip back reality even further atoms can be dismantled into smaller pieces particles such as electrons and quarks to these fundamental particles represent the purest representation of reality not quite as we also need to include the ways these particles interact through the fundamental forces of course all this is written in the language of mathematics numbers and equations that describe reality but where do

the rules of reality exist this might seem like a very strange question where could these rules written in the language of mathematics exist and it is now that we arrive at max tegmark's level 4 multiverse this multiverse is filled with nothing but mathematical structures encompassing the entire realm of possibility these mathematical structures can encode every kind of universe within this level 4 multiverse every other multiverse can be found it is the ultimate ensemble of every kind of reality and much much more indeed this theory bears a lot of similarities to the simulation hypothesis that our

reality is nothing more than a super powerful program on a computer a computer in a universe above ours it's programs written by beings to whom we are but play things but the level 4 multiverse max tegmark proposes would be more than just a physical computer it would be in existence of just math and so we now rest at the edge of speculation after speculation theoretical reach after theoretical reach the ultimate view of the multiverse we started our journey by peering over the horizon and wondering what was beyond we now find ourselves gazing at the meaning

of reality and questioning our very existence what is clear is that the multiverse is not a single concept it is more a grab bag of ideas some built on solid science some almost science fiction and so before we end our exploration a word of caution discussions of the multiverse in the public arena often end in shouting matches for some physicists the multiverse seems inevitable ideas like eternal inflation are simply too alluring to be wrong other physicists are certain they are on the right path with m theory and the existence of the string theory landscape even

the seemingly simple idea of wandering about dark energy and our cosmic horizon implies some kind of multiverse but some still vehemently oppose the notion to them talking about multiverses that are forever beyond our grasp is unscientific ideas they tell us are only scientific if they are amenable to scientific scrutiny and therefore parallel universes that will never be visible to telescopes or microscopes should be struck from science but in truth neither side holds the scientific high ground in reality our multiverse ideas are simply too early in development to truly question their scientific validity some of the

foundational mathematics might be in place but so many mysteries still abound questions like how universes crystallized out of eternal inflation remain unanswered string theory is hugely popular yet has no definitive proof behind it the many worlds of quantum mechanics has some passionate backers but remains just an interpretation and tegmark's mathematical structures of the level 4 multiverse a little more than a dream none of this means that the multiverse will be forever off limits with enough mathematical effort we might find a crack into parallel worlds we might find complex space-time structures akin to the idea of

wormholes that will bring the seemingly unseeable into sight some have suggested that patterns may be written into the sky in the cosmic microwave background these they claim might be the scars of universal collisions in the many dimensions of m theory the cosmic microwave background could also give us suggestions as to whether our universe is flat and therefore infinite or simply too large for us to see the curvature the search continues but for now contact or evidence of these alternate worlds is little more than wishful thinking we have no idea how theories will be developed into

the future and we simply have no idea if the multiverse will ever be spied through our telescopes or experimented on in our laboratories in reality the multiverse may be forever hidden within the fog you've been watching the entire history of the universe don't forget to like and subscribe and leave a comment to tell us what you think thanks for watching and we'll see you next time