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  • [music] [airplane engine]

  • Brian Greene: We have found, over the last hundred years,

  • that a very fruitful way

  • of thinking about the universe,

  • is to allow for the possibility

  • that left-right, back-forth, and up-down,

  • the three dimensions that we all are immersed within,

  • we move through freely in day-to-day life...

  • We have found it useful to imagine

  • that there may be more than those three dimensions

  • of space.

  • And, we have found that, at least theoretically,

  • using mathematics, if you engage

  • that possibility of more than

  • three dimensions of space,

  • certain problems of physics, all of a sudden,

  • admit to a solution.

  • And this is an idea that goes

  • all the way back to the early part

  • of the twentieth century, around roughly 1920,

  • when a German mathematician Theodor Kaluza

  • was studying Einstein's theory of gravity,

  • it's called the General Theory of Relativity,

  • and he found, rather surprisingly,

  • that if he imagined applying

  • Einstein's insights not to a universe

  • with three dimensions of space,

  • which is what Einstein had in mind,

  • but into one with four dimensions

  • of space with time included

  • in both approaches. He found that

  • he could find a framework

  • that put together the laws of gravity,

  • and the laws of electromagnetism,

  • in one theoretical package. So, he found a way of unifying

  • the then known laws of physics.

  • You move through the world, and you want to give some information

  • about your location.

  • You have to give a street,

  • a cross street, and a height, a floor number.

  • If you want to meet somebody, that would be the data

  • that you need to give.

  • The ideas of these higher dimensional theories

  • would be that that's not enough information.

  • You have to give those three pieces of information

  • plus more to really nail down where you are.

  • [piano music playing]

  • A very natural question, when confronted with this strange idea,

  • of more than left-right back-forth and up-down, is

  • well we immediately see those three dimensions,

  • why don't we see the others?

  • And this is a real puzzle. It's a real problem

  • that people have come up with very interesting

  • and creative solutions for.

  • If the math of your theory is suggesting more dimensions

  • but you don't see them, where are they hiding?

  • And one of the places that they may be hiding

  • is in plain sight.

  • It may be that those other dimensions are all

  • around us just like the three that we know about,

  • but they're tightly curled up, to a very very small size.

  • So tiny, that we can't see it with the naked eye

  • or even with our most powerful equipment to date.

  • And, often people get confused 'what do you mean by a small,

  • curled up dimension?'

  • And all I mean by that is, if you're a little tiny ant,

  • and you're walking along one of these tiny dimensions

  • you circle around back to your starting point very quickly

  • you can't go very far in that dimension

  • because it curls back in on itself.

  • Like, a sphere, or like a little tiny circle

  • and you go around it you come back to your

  • starting point, and if that circle or that sphere

  • is so tiny that we don't have the visual acuity to pick it out,

  • those dimensions can be all around us, and yet be invisible.

  • So according to these ideas if I, you know, if I take my hand

  • and I sweep it in front of my face, I wouldn't just be moving in the

  • obvious dimensions that we can all see with the naked eye,

  • I'd also be in some sense moving around those other

  • tiny dimensions, I'd just be returning to my starting point

  • over and over again so quickly that on average, I wouldn't even

  • have recognized that I took the journey at all.

  • To me the most exciting thing that one can do with one's life, really

  • is try to, get some sense of how we got here.

  • Where did the universe come from? Where did life come from?

  • Where does consciousness come from?

  • These are the big questions, that frame existence.

  • And, what we are doing in physics and in string theory in particular,

  • is trying to answer one of those.

  • The real motivation, the driving force behind what we do

  • behind these ideas of extra dimensions and strings and quantum mechanics

  • and general relativity we are all trying to figure out

  • where the universe came from.

  • And the current laws of physics, remarkably, take us back to a

  • fraction of a second after the beginning.

  • But the current laws breakdown. So we are really working hard

  • to find laws that don't breakdown under any circumstance

  • no matter how extreme.

  • The early universe was very hot, it was very dense

  • it was a very violent arena.

  • We want laws that can cope with that arena,

  • allowing us to peer back using mathematics

  • and understand what happened at time zero.

  • That, to me, would be a remarkable achievement if our species can get to the

  • point where we can say:

  • Here's how the universe began, here's why there's a universe at all.

  • The notion of before the big bang, could be a meaningless question.

  • It could be that time comes into existence with the bang itself,

  • and, asking questions 'what happened before?'

  • doesn't make sense because the notion of before doesn't apply

  • that is certainly a possibility, but I am certainly open to other possibilities.

  • For instance, some of the theories that we're working on today suggest that

  • the big bang was not the actual beginning

  • of the universe in the grandest sense of the term,

  • but rather was an interesting event that gave rise to our part of the big cosmos

  • but there could be other big bangs happening here, there, other far flung places

  • throughout space,

  • so the big bang may simply be an interesting event but not the starting

  • event.

  • So, the notion of what happened before it may be perfectly well-defined,

  • and, using our mathematics, we may even be able to describe what it was

  • like

  • in those "pre" big bang eras, those pre big bang arenas.

  • Will we ever observationally be able to probe those realms if they exist?

  • Maybe not. Probably not.

  • But I don't think that's any cause to say that it's not in the realm of science

  • or it's not worth studying or thinking about.

  • Because one of the key things that's often lost is, when you're testing a theory,

  • you don't need to test every single prediction of the theory,

  • you need to test enough of the predictions that you gain confidence that the theory is

  • a good guide to the nature of reality and then if that theory tells you things about

  • realms that you can't see, like a realm before the big bang, you're actually learning

  • about that realm,

  • even though you can't test it or see it.

  • These kinds of ideas are really helping us gain the deepest insight into the

  • very nature of reality.

  • And I don't know how long it takes.

  • A hundred years, a thousand years, whatever. When our picture of reality shifts,

  • when we thought there were three dimensions and there are more

  • or when we thought that time was universal and we learned that it isn't.

  • Or when we learned that space is flexible as opposed to the previous Newtonian version

  • of it being this unchanging stage,

  • these shifts in our basic understanding of reality,

  • do begin to infiltrate the way we think of ourselves

  • and the way we picture how we fit in, to the grand scheme of the cosmos.

  • [serene outro music]

[music] [airplane engine]

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ブライアン・グリーン:隠された次元の探索 (Brian Greene: The Search For Hidden Dimensions)

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    michaelfun に公開 2021 年 01 月 14 日
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