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  • Everyone loves the idea of parallel universes - maybe it's the appeal of an ideal world

  • where you have second chances and things turn out differently - an alternate reality where

  • you do get into Hogwarts and the Star Wars prequels aren't made and you finally plug

  • in your asymmetric computer cord correctly on the first try... but is there really a

  • place in science for such wistful speculation?

  • I mean, if "the universe" is everything that there is, you can't have two versions of it,

  • right? Otherwise the pair would really be everything and what you started off calling

  • the universe, wasn't.

  • The problem here is terminology: physicists speaking informally often say "universe" when

  • they really mean "observable universe" - that is, the part of the whole universe that we've

  • so far been able to see. And it's perfectly fine to talk about multiple different observable

  • universes - for example, an alien near the edge of OUR observable universe will see parts

  • of the Whole Universe that we cannot yet see, but that's a well-understood question and

  • not what physicists normally talk about when they discuss multiple observable universes,

  • or "multi-verses."

  • So let's cut to the chase: in physics, the word "Multiverse" normally refers to one of

  • three distinct and largely unrelated proposed physical models for the universe - none of

  • which has been tested or confirmed by experiment, by the way. The three "multiverse" models

  • are:

  • Type 1) Bubble universes or baby black hole universes. This is the most straightforward

  • kind of multiverse: the basic idea is that perhaps there are other parts of the universe

  • which are so far away that we will never see them (or are inside black holes so similarly

  • we will never see them). This kind of model was created as an attempt

  • to explain why our universe is so good at making stars and galaxies and black holes

  • and life - as the argument goes, if each of these separate mutually un-seeable "bubbles"

  • in the universe had slightly different laws of physics, then by definition we could only

  • exist in one that had the right physical laws to allow us to exist. If you're not convinced

  • by this logic, don't worry too much: there's not yet any experimental evidence for this

  • kind of multiverse. Multiverse type 2) Membranes and extra dimensions.

  • Inspired in part by the inability of the mathematics of string theory to predict the right number

  • of dimensions for the universe in which we live, string theorists proposed the idea that

  • perhaps what we think of as our universe is actually just a three-dimensional surface

  • embedded within a larger super-universe with 9 spatial dimensions. Kind of like how each

  • page of a newspaper is its own two-dimensional surface embedded within our three-dimensional

  • world. And of course, if space had 9 dimensions rather

  • than three, there'd be plenty of space for other three-dimensional surfaces that appeared,

  • like ours, to be universes in their own right, but, like the pages of a newspaper, were actually

  • part of a bigger whole. These kinds of surfaces are called "membranes" or "branes" for short.

  • And as a reminder, there is not yet any experimental evidence for this kind of multiverse.

  • Multiverse type 3) The many-worlds picture of quantum mechanics. Surprisingly, physicists

  • still don't fully understand how the collapse of the wavefunction in quantum mechanics happens,

  • and the many-worlds hypothesis makes an attempt at explanation by proposing that every possible

  • alternate timeline for the universe is real and they all happen in an ever-larger, ever-branching

  • way. Like, a universal choose-your-own-adventure where every possible story happens!

  • If this were the case, we might not realize it because we'd be stuck living out just one

  • of the infinitely many possible lives available to us. In some ways, many-worlds is similar

  • to the bubble multiverse model by proposing "maybe anything that can happen, does. And

  • we just happen to exist in the series of happenings that were necessary for us to exist." If you're

  • still not convinced by this logic, don't worry: there is not yet any experimental evidence

  • for this kind of multiverse.

  • Of course if you want to get imaginative, you could also combine several of these models

  • together into a multi-multiverse... a new super-speculative model based, itself, on

  • speculative and experimentally unconfirmed models.

  • But that's not to say we couldn't test these multiverse hypotheses. For example, if our

  • observable universe were really just one of many disconnected bubbles or membranes and

  • if it happened to collide with another bubble or membrane some time in the past, then that

  • collision would certainly have had some sort of effect on what we see when we look up at

  • the night sky. On the other hand, the many-worlds interpretation

  • might be tested fairly soon since experimentalists are becoming increasingly able to manipulate

  • and control ever-larger quantum mechanical systems in their labs - systems that approach

  • the line between the quantum realm and our everyday experience.

  • So as always, we must remember that physics is science, not philosophy; and in our attempts

  • to explain the universe that we observe, we have to make claims that can in principle

  • be tested - and then test them!

Everyone loves the idea of parallel universes - maybe it's the appeal of an ideal world


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B2 中上級

平行宇宙の真の科学 (The True Science of Parallel Universes)

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    何平平 に公開 2021 年 01 月 14 日