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  • Since there are so many different notions of what is a multi verse, I think it it's Ah, it's quite usual.

  • Thio categorize them.

  • And that has been done.

  • Yeah, yeah, different.

  • There's a hierarchy of their multi verses on a guy called Max.

  • Take Mark.

  • Very well known physicist.

  • He's a classified into four levels, varying levels of controversy so well.

  • So we will begin with level one multi verse.

  • What techmark cause a level one multi vis on.

  • This is basically the idea that there is stuff beyond the observable universe.

  • So we think of that.

  • We talked about the observable universe, which is basically it's around 10 to the 26 meters distance.

  • Something like that, that order of magnitude.

  • So big distance scales.

  • That's a sw far as the eye can see.

  • Essentially, that's the fare this the way you can go that lights could have reached us from in principle since the beginning of the universe.

  • So pretty big distance scale.

  • But what's beyond that?

  • That's a legitimate question on dhe.

  • This is the notion of a Level one multi verse.

  • The universe is if you like the existence beyond that, well, I would agree But in a sense they're completely disconnected from us.

  • So in a sense that they are distinct.

  • I think that region beyond the observable horizon that we called the cosmological horizon, is not very big, but it could be enormous.

  • The universe could be absolutely John Enormous.

  • One of the reasons that you can make it biggest do something called its eternal inflation.

  • So we know that why our universe around is our corner of the universe.

  • We think it got big because at some point it just grew very quickly due to something called an inflow tonic.

  • Expanded out very, very fast, very, very big.

  • And what the influence on the funny thing about this inflict on field is what it does is is it generically it won't do this.

  • Okay, generically, it will appear, and it won't cause the universe to expand very quickly.

  • It won't have the right kind of conditions for that to happen, but every so often it will.

  • Okay, so what this influence on field does, it takes a little random walk in the beginning of the universe on Eventually it finds itself in just the right conditions to suddenly make that little pocket of the universe expand very quickly.

  • Okay, so if you like, I like to think about this.

  • Our friend Ed at Copeland.

  • He's a pretty chilled out guy, right?

  • I mean, you would agree with that.

  • Very chilled out.

  • But the thing about Eddie's is every soft I've known him for quite a few years now, and every so often something will rile him.

  • Doesn't happen very often, but it can happen on.

  • Then he'll explode and he'll explode the universe around race.

  • It doesn't very often, but it can't happen.

  • So you could imagine Ed going on little random walk through town.

  • Okay?

  • Most things keep him nice and serene, but every so often, something rousing and it blows up.

  • And that's like the influence on field.

  • So that's kind of how you explain how this is quite unlikely scenario suddenly made our pocket the universe big.

  • But the thing is, the IMF, Luton or Red doesn't stop doing its little random walks.

  • Okay?

  • Indifferent readers of space.

  • It can do its little random walks here and there.

  • And eventually, sometimes it can be it can jump up back into the right region where it's gonna cause this explosion again.

  • Or maybe it never stopped exploding.

  • Okay, So add, for example, might calm down, but some in some regions of space he's being maintained, annoyed because of these quantum fluctuations, they keeping him annoyed.

  • Okay, well, like the influence on is being kept to gain taming this expansion of space.

  • So you have this eternal process where regions of space and just continue to expand Our region has stopped expanding.

  • It's such a huge rate of expansion has slowed down somewhat, But the reasons it doesn't it just keeps it accelerated expansion, blowing up a crazy rate.

  • And so this is making those regions of space enormous.

  • On generally expect that some regions of space will be behaving like this on our eternally inflating.

  • This is what we call eternal inflation.

  • And so whilst our region of space of my have slowed down, that's not true.

  • Generically.

  • Now, of course, we now know that our region's places it's starting to speed off again.

  • But that's another story.

  • But what this means is that those regions of space outside of our observable units could be truly gargantuan.

  • And this is where you get all this sort of crazy sort of notions that we talked about in the in the Google Plex video arising.

  • There's also the idea of bolts for brains as well.

  • They can arise from this, but that's probably another video.

  • So one of the important things to realize about Level one universes is that the laws of physics, the values of the fundamental constants, they're the same here in our corner as they are in those far reaches level tier is certainly different to that.

  • A level two universe Where you've got to imagine now is, is that there are bubbles.

  • The universe is made up of.

  • Bubbles are corner is living inside of one bubble where the physical constants are what they are, and so on and so forth.

  • There are other bubbles where the physical constants might be different.

  • Where the laws of physics look different planks constant.

  • You know Houston's constant, you know, fine structure, constant.

  • All these concerts that underline physics.

  • Okay, these can change from from bubble to bubble.

  • Now, why might this happen?

  • What we believe this could happen because of string theory, string theory tells, is that there aren't just one possible universe.

  • One is possible vacuum for the universe that you can have.

  • There's an entire platter off them.

  • 10 to the 500 off them.

  • A huge number.

  • Okay, this is 33 When it was first introduced, it was hoped that it would come up with some sort of unique and vacuum solution, unique sort of second set of circumstances, which coincided with the universe we see.

  • That's not what happened.

  • It was the exact opposite.

  • It came out with a huge number of possibilities.

  • Now, of course, our corner of the universe is just one of those, but generically, there's many more on what will happen is you'll find these bubbles forming.

  • So within 11 bubble you can have through quantum tunneling, you can have a bubble of another unit, another type of universe form with different physical constants and different.

  • Yeah, this can happen within the bubble through quantum fluctuations.

  • And so, but then that expansion will a care with inside that bubble in the bubble outside, it can expand on the whole thing sort of, you know, proliferates.

  • You get percolation effects and you know the whole universe is gets filled with these with these bubbles of different, vacuous, different where the physical constants of different are they?

  • Because I'm thinking of bubbles in a bubble bath or, like, you know, stacked up against each other.

  • But what you're talking about their sounds more like Russian dolls.

  • Well, I mean, yeah, but boats.

  • Okay, It's both actually, so the bubbles can happen.

  • You know, you could have a bubble nuclear here.

  • There's a universe inside it where the laws are, what they are, everything is expanding.

  • So your bubble might get bigger, and so you don't really notice.

  • Or maybe that bubble will overtake Take the other one.

  • If this dynamics involved in the whole process, some bubbles might haven't even even experienced different dimensionality.

  • Different numbers of damages.

  • So if you're in one, but you may not experience a four dimensional universe, you might feel a five or six or seven dimensional.

  • What?

  • What's it?

  • The interface between bubbles.

  • Can I drive my car from one another?

  • Well, that would be quite destructive because, of course, the the laws that air the things that's holding your car together, the atomic forces that are holding the hardened electro magnetic forces that are holding a car together the physical constants that controlling that going to change as you traverse the edge of one of these bubbles and answer another bubble.

  • Okay, so what?

  • Your car looks like you get to the other side, Let me something extremely different.

  • And you know, who knows?

  • I don't want to speculate what it's gonna look like, right?

  • And so I mean, the the edges, these bubble walls are going to be sort of highly energetic regions.

  • Basically, you're gonna get shark radiance in the how things that things have changed it because, you know, planks constant, you know, is gonna perhaps change as you cross one of these walls that's going to create a strong gravitational effect on principle.

  • And so these are the thoughts, things that happening.

  • So but the walls themselves don't have to be sort of, you know, they compete wide.

  • Walls have to be sort of particularly thin.

  • They'll be thing compared to the public, the whole.

  • But you know, it said there's a there's a transition as you go from one to the other.

  • One of the things we're actually looking for in the cosmic microwave background is evidence of these bubbles.

  • Evidence of bubble collision.

  • So they're looking for circles in the sky.

  • That's what the court, because, you know, you might see two bubbles colliding.

  • The imprint it would have on the cosmic microwave background would be like a circle sort of circular shape within the temperature fluctuations.

  • So we're actually looking for these things, But this is a level two multi verse and I would say level ones not particularly controversial level to a little more controversial, but still, I mean, if you believe in string theory, you'd probably by that level three, on the other hand, is I would say controversial.

  • And this is the idea that you've already done a video about.

  • This is the idea that you know, in quantum mechanics, you know, you have this sort of idea of probabilities.

  • So on, whenever there's a chance that whenever there's any probability, ever there's any uncertainty, for example, in a coin, toss it, we're gonna try to picture it classic picture it sort of microscopically it doesn't really make sense to do that.

  • But if you were to try to do that, you might think that whenever you toss a coin that the universe splits into and, um, on one way that the coin came up has any on the other way, the coin came up tails and that both happened or physical space at two different universes occupy the same space.

  • Like like right now as we continually Faulcon.

  • Split, split, split, split, split, split.

  • Where these new university?

  • Well, I think the idea is that would be right on top of each other.

  • Essentially.

  • But you can't interfere from one to the other.

  • There's no way of communicating.

  • I don't think that's really necessary, though.

  • This is, this is me.

  • There's a few things I don't like about this fiercely, you know, your whenever you have a split.

  • Well, when does the split occurred?

  • At which point does the split occurred?

  • Precisely.

  • When is that?

  • I mean, it's already time uncertainty in this.

  • So in quantum mechanics.

  • So when does the splits a care that's not really clearly answered?

  • Secondly, and you can't just If you do the splits, then you're you're increasing to the split.

  • It see this late, you're increasing the number of states.

  • Suddenly you're increasing the size of this space that describes the quantum system on.

  • That doesn't seem satisfactory either, but the problem I have is is that this was originally introduced.

  • People like to think about it because they're uncomfortable with the idea of collapse of the way from any idea.

  • When you open the box, the car is out of dead or alive and not this mixture.

  • But we actually understand why there is no it's to do with a process called Deco here.

  • It's where the large environment outside of the box starts to mix up with the cat in such a way.

  • That's Ah, with the overlap region between cut dead and cut alive because of the large environmental system outside just gets made very small.

  • So if we're okay with wave function glass because we understand it through this d coherence, I don't think that we need what's wrong with the idea that nature's probabilistic?

  • Why do we have to have that?

  • Every possibility is realized.

  • It's a quantum.

  • Mechanics is probabilistic.

  • That's the way nature nature, what is on?

  • Just because, you know, our classical sort of intuition tells, is that and you know, we enjoy a deterministic existence.

  • That doesn't mean continent comics have to behave like doesn't mean NATO spirit, but I just means that day today that's how it seems to behave, but truly it's probabilistic and that's fine, Okay, doesn't mean every possibility has to be realized.

  • So that's level three.

  • So the last of these is the level for and this is the last one.

  • True, The last one.

  • There is no level five.

  • So the idea of a level four, this is very much tag marks thing and very much controversial.

  • So the idea of a Level four multi verses the fooling we look at our universe, we look off physical existence and we say, is described by a mathematical structure that seems reasonable, that I can build build up general relativity from the rules of math mags.

  • And they can They can describe the universe that I see, for example, or I can build the standard model for my mother.

  • And I have this mathematical structure which is the standard model plus general activity plus all these things that describes the universe.

  • I see.

  • And then you say OK, but there are other mathematical structures.

  • The fact there are many more mathematical structures that you can think off.

  • This master we use is just that we use to describe, you know, the standard model of General activity is just one corner of the possible huge round of mathematical structures that you could imagine you're saying if I just gave you a blank piece of paper and so gonna make up some new mathematics Exactly on this.

  • This is known as the subject of formal systems that this is actually exists.

  • You know, you just need certain operations, you know, Does your mathematical structure include multiplication?

  • For example?

  • It doesn't have to ours doors that we used to describe the universe that we see.

  • But in principle, you could imagine a mathematical structure that doesn't have multiplication in it.

  • That's just as good as any other.

  • Okay, so so there are other mathematical structures.

  • And then you have to ask the question Well, which ones corresponds of physical reality?

  • Is it just the one that we speak off or is it all of them?

  • Or is it?

  • None of them were clearly not none of them.

  • But, you know, it's like so what?

  • So Mark says, he says, Well, I can't really answer that question unless I have mathematical democracy.

  • So he says, I exploit this notion of mathematical democracy and says that all mathematical structures are physically riel.

  • So all of them.

  • So they have a physical reality too.

  • So this is the ultimate multi verse now, the all mathematical structures, not just general 70 not just string theory.

  • All these things, but anything any kind of operation that you can create in a formal system.

  • Anything that constitutes a mathematical study is physically riel.

  • Okay, this is quite a dramatic thing.

  • Are you saying each one has its own universe?

  • It is physically, really mean.

  • The notion of the universe may not even exist in some of them, but it's physically riel.

  • So you see what I mean?

  • So it's like it may not even contain gravity.

  • It may not even it's kind of hard for us to imagine what they are because we're so used to thinking about, you know, the one in which we're familiar with.

  • But you know, anything that is basically saying that maths is reality.

  • Hey, now, why is this widest time out like this?

  • Well, ah, one hand.

  • He likes it because it's kind of final in a sense that the set of all mathematical structures is itself a mathematical structure.

  • So it's the It's the end of the road.

  • It's sort of completes itself and in all senses of pierced once they're a question that we'll oppose.

  • Which is why why of all the possible mathematics that we could have to describe the universe?

  • Why this?

  • What's special about this one?

  • Is there anything special about it on?

  • Well, it seems to answer that well, no, there's nothing special about it that all structures air riel.

  • Is this like the bubbles?

  • Is this saying that next door to us is a bubble where multiplication?

  • I mean, I wouldn't say the bubbles, because the notion of a bubble mind I even exist for some mathematical stretches.

  • So I mean, it's so obstructed.

  • Kind of is bordering on the well, it's his bordering while they have said right.

  • But, I mean, let's think about it if it even makes sense, I mean, on one level, it's not clear that the set of my own mathematical structures is well defined, so that's a problem.

  • There's also this is challenged by girdles incompleteness there in which says that there are some things which, which are true, but it can't be proven to be true on uh, you know, is that if you have a universe where that's the case and we have a mathematical structure which is like that cannot properly describe the universe.

  • There seems to be some sort of weakness they're on.

  • In fact, 10 markers lessen his his statements and suggest that actually only girdle complete mathematical structures of physically riel.

  • But again, I think it comes back Thio.

  • For me, it comes back to the same debate that one would have with the with the level three multi verses just because they can exist on just because you can still have mathematical democracy.