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00:00Welcome to the vast and infinite realm of the multiverse.
00:04There's nothing quite like it in all of science, physics, space, or theoretical thinking.
00:10It's a concept that essentially laughs in the face of most mainstream models on how the world works,
00:16and provides us instead with an endless, glittering, constantly regenerating expanse of potential.
00:22By now, thanks in large part to various works of science fiction,
00:26we of course know that the multiverse is multiple universes.
00:30A web of realities that's in some way entangled to form the true, wider, total physical structure that we are but a tiny part of.
00:39It's parallel dimensions and alternate realities.
00:42It's bridges, wormholes, quantum phenomena, and time travel.
00:47It's thrilling, and it's disconcerting.
00:50Thankfully then, some have at least tried to make even a little bit of sense out of it all.
00:55And today we're moving through the many levels of one explanation of the multiverse in particular.
01:01This is Unveiled, and today we're taking an in-depth look at the nine types of multiverse, according to the theoretical physicist Brian Greene.
01:10Do you need the big questions answered? Are you constantly curious?
01:13Then why not subscribe to Unveiled for more clips like this one? And ring the bell for more thought-provoking content!
01:20Brian Greene is a US physicist and mathematician.
01:23He was born in New York City, graduated Harvard and Oxford, he's a professor at Columbia University,
01:29and he founded and chairs the World Science Festival.
01:32He's also deeply integrated in the modern history of the multiverse.
01:36In 2012, Greene released his book, The Hidden Reality – Parallel Universes and the Deep Laws of the Cosmos.
01:43In it, he outlines one of the most comprehensive attempts to explain the multiverse in full, dividing it all into nine more specific categories.
01:51While variations of the categories had been seen and debated before by others, Greene pulls them all together in a bid to finally nail down the true nature of reality.
02:01Number 1. The Quilted Multiverse
02:04In general, this is the simplest of all of Greene's multiverse types.
02:08The quilted multiverse only really requires this universe to be infinite, which many believe that it is.
02:14From there, all manner of arguably quote-unquote parallel worlds are possible.
02:19It certainly works in theory, because in an infinite structure, everything should be and happen an infinite number of times, including all possible variations of physical matter.
02:30Greene suggests that it's only due to the universal speed limit, the speed of light, that we're wholly unaware of the multiverse we're in.
02:37If true, one of the implications is that not only do we share this universe with other lifeforms, but we share it with other versions of ourselves.
02:46Naturally, we know that life, the likes of which we find on Earth, is possible, because we're the proof of it.
02:53So, in the quilted multiverse, somewhere beyond the catchment of light speed, life as we know it should happen again, and again, and again, an infinite number of times.
03:04On a larger scale, the same should be true for planets and stars.
03:08Meaning that, in this quilted reality, there should be other instances of Earth and the Sun, of our entire solar system, exactly as we know them.
03:18And, in fact, there should be an infinite number of other solar systems just like this one.
03:23The other implication is that if we could only find a way to travel beyond light speed, then we would eventually discover this multiverse with our own eyes.
03:31In simple terms, it should be only a matter of travelling in any direction for long enough, before we inevitably encounter ourselves again.
03:40As to all of the other variations of life, matter and energy that we might find along the way, it's a patchwork of yet more infinite possibilities.
03:49Number two, the inflationary multiverse.
03:52Sometimes referred to as the bubble multiverse, this rendering asks us to take our first major step away from the conventional cosmos as we typically know it.
04:00The inflationary multiverse is born out of eternal inflation theory, which says that, at the highest level, our reality is always expanding…
04:09and that an event such as the Big Bang could actually be caused by that expansion, rather than being the cause of it.
04:16Along our own timeline, we know that there was a period of extremely rapid expansion that occurred fractions of a second after our universe began.
04:24During this fleeting moment, spacetime grew faster than light speed.
04:28A subatomic speck was transformed into a sprawling expanse, teeming with energy and potential.
04:34And this initial burst set everything in motion.
04:37But in the higher, wider, inflationary multiverse, it didn't happen just once.
04:42Such inflation wasn't an isolated event, but was and is an ongoing process, occurring in different regions at different times.
04:50Like popcorn kernels, popping endlessly in a vast, cosmic popper, each kernel represents potential universes within bubbles that are expanding within higher-dimensional space.
05:01But no one piece of popcorn pops at exactly the same time, nor in exactly the same way.
05:06Scaled up to multiverse level, this means that every bubble universe that's created isn't just separate, but should also have its own unique laws and parameters.
05:15The laws of physics that we know are universal, yes, but perhaps they aren't multiversal.
05:20Every bubble stands alone.
05:22One might have stronger gravity, for example, or no electromagnetism.
05:27Another could be teeming with forms of matter that are fundamentally alien to us.
05:32Number 3. The Brain Multiverse
05:34With its roots in M-theory, an extension of string theory that requires eleven dimensions to work,
05:40the Brain Multiverse model also suggests that everything we know exists as just one of many on another, higher plane.
05:47This time, however, the universe is a three-dimensional membrane floating in a higher-dimensional space, sometimes called the bulk or hyperspace.
05:57Brain cosmologists also suggest that the infinite nature of reality should mean that there are endless other membranes out there, all also suspended in the same, broader structure.
06:07One key implication, however, is that it might be possible for separate membranes to interact, and for those interactions to then have a profound effect.
06:15From our point of view, perhaps our membrane, our universe, has before collided with another.
06:21Or maybe we've spent our entire 13.8 billion years overlapping with a nearby membrane, another reality, which has then in some way shaped this reality in a fundamental sense.
06:32The Brain Multiverse also triggers all new considerations when it comes to potentially moving through the multiverse.
06:38Whereas in the quilted and, to some degree, inflationary models, travelling the multiverse may only ever boil down to reaching and breaking light speed,
06:47in the brain, a would-be traveller would need to physically cross over from this world to another.
06:52Number 4. The Cyclic Multiverse
06:55This grade of multiverse is closely related to the previous brain model, but it rests on one idea in particular.
07:02In the cyclic multiverse, membrane universes certainly do interact with each other, and in the most profound way possible,
07:09because collisions between brains are here what double up as Big Bang events.
07:14Now the multiverse assumes a more dynamic and even more eternal, regenerative trait,
07:19as the membranes in the bulk also become the birthers of other universes, just by being there.
07:25The precise mechanics of how a membrane collision could cause something as dramatic as a singularity cascading into life are unknown,
07:33but if true, it's a model that repaints hyperspace as being something like an easel, upon which a chain reaction of creation is constantly unfolding.
07:42Of course, this is hardly the only cyclic model in cosmology in general.
07:46The Big Bounce Theory has fallen in and out of favour over the years, suggesting that universal expansion will one day halt,
07:53and everything will contract back inwards from that point, until such time as it's all compacted back down into a singularity,
08:01and the Big Bang happens again. At that moment, it could be said that the universe will have been bounced back out into space.
08:09The cyclic multiverse, by comparison, employs a similar logic, but on an even grander scale.
08:15Number 5. The Landscape Multiverse
08:17Next, and we're heading full tilt into String Theory, probably the most well-known attempt at a quote-unquote theory of everything there is.
08:25It posits that the fundamental constituents of reality are one-dimensional, vibrating strings, rather than point-like particles,
08:33and one of many things that this would affect is the make-up of the multiverse.
08:38In general, String Theory allows for many different ways in which its strings can vibrate,
08:43and each mode of vibration corresponds to a different elementary particle.
08:47Meanwhile, and as a result of that, reality is really divided into many more dimensions than just the three space plus time that we're familiar with.
08:56There are, then, many possible shapes that all of those extra dimensions can take when they're compacted down.
09:03In fact, there could be an effectively endless number of shapes possible, each leading to an all-new universe with its own distinct set of physical laws.
09:12The landscape of the Landscape Multiverse, then, pitches these different universes as though they exist along a never-ending series of hills and valleys.
09:20Every universe of the multiverse appears somewhere along this structure.
09:25Some are more stable than others, but none are identical. All are unique.
09:29In this view, our universe just so happens to reside in one valley within this broader landscape created by the fundamentals of String Theory.
09:39And our particular set of physical laws corresponds to just one way in which strings can vibrate.
09:45But all the while, there are infinite other valleys and environments out there,
09:49to represent all other universes with different laws derived from their own unique vibrational patterns,
09:55and their own unique shape taken as a result.
09:58Number 6. The Quantum Multiverse
10:01Along with the Quilted Multiverse, this is probably the most widely known and discussed of all Green's nine types.
10:08The Quantum Multiverse suggests that at every moment when a choice or diversion is made, a new universe is created along with the one we inhabit.
10:17It works on both the micro and macro levels, and is best laid out by the famed Many Worlds Interpretation of Quantum Mechanics.
10:24On the quantum level, due to various phenomena including the observer effect and wave function collapse,
10:30we know that particles can be one thing or another until they eventually settle down into what they ultimately become.
10:38But the Quantum Multiverse imagines that nothing is lost as reality takes shape,
10:43that all possible outcomes take place just along an endless mesh of different timelines.
10:49These timelines, or what some refer to as parallel worlds, are constantly branching off from the one you're in right now,
10:57which in itself has branched off of uncountable others in the past.
11:01It's perhaps more satisfying to visualise at a larger scale, though.
11:05Consider all of the choices you've already made today.
11:09What you had for breakfast, what you said to a colleague, how many YouTube videos you've watched,
11:14whether or not you scratched behind your left ear, switched your alarm to snooze, or tripped on your own doorstep.
11:21You might think of some of your actions as being more consequential than others,
11:25but in the Quantum Multiverse they're all equally valuable, because they all spawn parallel worlds of what could have been.
11:32Apply the same thinking to every single quantum fluctuation that's happening all around you,
11:37countless times, during every single second of your existence.
11:41And the sheer density of this particular multiverse becomes clear.
11:45Some strands of it will almost be indistinguishable from the one along which you're currently living.
11:50But still, take just a short journey through it, and you could very quickly end up in an entirely alien place.
12:02This wouldn't be a video about the true underlying physics of reality without at least a passing mention of black holes.
12:08And this is that mention.
12:10The holographic multiverse builds on the holographic principle,
12:14which says that our three-dimensional reality could be encoded on a two-dimensional plane.
12:19That everything we know, from stars and planets to cars and sandwiches,
12:24is actually projected from that 2D structure to generate our universe.
12:29It ties back to black holes as a possible solution for the black hole information paradox.
12:34This is the realisation that information is seemingly lost in black holes that eventually evaporate away,
12:40despite that apparently being impossible as per the conventional laws of physics.
12:44Grand cosmic holography isn't the only or even the most well-supported solution,
12:49but for advocates, it could be that the seemingly lost information within a black hole
12:54is actually encoded in 2D, along the event horizon.
12:58Apply a similar model to the entirety of the universe,
13:02and it could well be a process that happens time and time and time again.
13:06And suddenly, the multiverse begins to feel like a multiplex cinema,
13:10with multiple movies, or realities, playing across multiple screens, or spaces.
13:16Perhaps more than any other model up until this point, it could quickly make you feel quite inconsequential,
13:22suggesting as it does that at your root you're not even a three-dimensional being.
13:27However, and as with all proposed multiverse types,
13:30it can also be easily argued that really, it doesn't matter.
13:34That even if life, the universe, and everything is a hologram,
13:38nothing actually changes from our point of view.
13:41Number 8. The Simulated Multiverse
13:44This is another of the more widely known multiverse possibilities,
13:48given how it's permeated pop culture in recent times.
13:51Born out of the simulation hypothesis, as proposed by Nick Bostrom,
13:55it says that our universe is first and foremost a product of complex digital code.
14:01That everything we know, see, and feel is only ever the result of a program
14:06that's running on some far-off supercomputer in a higher dimension.
14:10For those in favour, it might even be argued that a simulated multiverse is inevitable
14:14so long as technology progresses.
14:17In our own world, we've seen how computer-simulated games and virtual reality experiences
14:22have become more and more lifelike over a relatively short period of time,
14:26to the point that some have predicted it may be impossible to tell the difference
14:30between the real world and a simulated environment in the future.
14:34Ideas on the simulated multiverse simply approach that line of thinking from a different perspective.
14:39Because what if, for some other, higher, more advanced civilization or entity,
14:44that line has already been passed, and we're the ones being convinced or tricked by it?
14:49What if our world is just a very good sim?
14:52Traditionally, so to speak, it was an idea that led some to envisage one single controller, or player,
14:58a supreme god-like being who's watching over our every move,
15:02because they've designed every single thing about us.
15:05But combine all the potential for a simulated reality with the endless repetition of a multiverse,
15:10and you quickly land on something even bigger.
15:13If simulated universes are possible, and if multiple universes are possible,
15:17then all that we know could, and even should, be just one variation of cosmic code
15:22in amongst an endless stream.
15:24Consider that we ourselves have constructed simulated realities,
15:28albeit quite primitive and superficial at present.
15:31This would mean that our controller created us to create and control others.
15:36So, why not a creator for our controller to control them while they control us
15:41while we control our sims, and so on?
15:44Then consider that comparable chains must surely be erupting an infinite number of times
15:49along an infinite number of other chains,
15:51and we again have one of the deepest and densest multiverses imaginable.
16:00Finally, to an effectively unknowable multiverse
16:03that represents the most expansive and all-encompassing of all concepts possible.
16:08In this view, every conceivable universe with every possible set of physical laws
16:13and constraints exists within a grand cosmic collection.
16:17Imagine that you could lay all of everything we've covered in this video out on the table
16:21and bag it up.
16:22The ultimate multiverse is that bag.
16:25Sometimes referred to as the mathematical multiverse,
16:28and directly comparable to Max Tegmark's Level Four multiverse,
16:32it transcends all possible constraints that might be placed on any other multiverse version.
16:37In this staggering vision, not only are there universes like ours with slight variations,
16:42or even like ours but with radical differences,
16:45there are also universes that are governed by completely different mathematics.
16:50This means that any form of logical or mathematical structure that can exist does exist
16:56as a separate reality somewhere within this overriding, ultimate landscape.
17:00In this way, most of it is impossible for us to even picture,
17:04given the limited view that we inescapably have.
17:07It's a fine structure that pushes infinitely far beyond our understanding of physics,
17:12because it suggests that our own universe's laws
17:15– everything from quantum mechanics to general relativity –
17:18are not uniquely fundamental in any way whatsoever.
17:22Instead, we're merely cast adrift in an endless sea of theoretical frameworks.
17:27While highly speculative and abstract,
17:30contemplating such a multiverse challenges us to reconsider
17:33what we deem as necessary truths about existence in general.
17:37It opens up profound questions about reality's true nature and our place within it.
17:41It may be, then, that our universe isn't just one page in a much longer book,
17:46but that it's one page in an endless library.
17:49For some, the ultimate multiverse could even lead to a multiverse of multiverses,
17:55contained only by the one truth that it cannot be contained.
18:00So, what do you think?
18:02Which of Brian Greene's nine types of multiverses do you think hits closest to the truth?
18:07Or are you unconvinced by the idea of a multiverse in any form?
18:11Let us know your take in the comments.
18:13For now, while the prospect of a multiverse is increasingly covered in contemporary science,
18:18it remains true that, at its heart, there are questions that humankind has always wrestled with
18:24and will always wonder about.
18:26Why are we here?
18:27What is our purpose?
18:28How real is real?
18:30And what does tomorrow hold?
18:32Importantly, the multiverse has yet to be proven, but it's an idea that's very much alive
18:38and challenging us all to take a higher, wider view.
18:42What do you think?
18:43Is there anything we missed?
18:44Let us know in the comments, check out these other clips from Unveiled,
18:48and make sure you subscribe and ring the bell for our latest content.