이번 강연은, 물리학자 애런 오코넬의 양자 물리학에 관한 이야기 입니다. 양자역학 (Quantum Mechanics)는 정말 말만 들어도 어려워 보이죠. 하지만 강연에서 그는 양자역학의 개념을 누구나 알아들을 수 있도록 아주 간단히 소개를 하며 또 양자 현상의 한 단면을 시각화 하였던 자신의 시도를 소개합니다.


강연자는 자신의 뇌는 논리적인 동시에 직관적인 면을 고루 가지고 있다고 말합니다. 아시다시피 흔히 좌뇌는 논리적이고, 우뇌는 직관을 담당한다고 알려져 있죠. 보통 사람들은 자신이 논리적이다 혹은 직관적이다 라고 한 쪽으로 치우친 성향으로 표현하죠. 하지만 강연자는 어려운 양자물리학의 개념들을 연결짓기 위해서 논리적이여야 하고, 또 한편으로는 실험이 제대로 이루어지도록 직관적인 면도 있어야 한다고 합니다.

다시 이런 직관이 왜 필요할까요? 직관이라는 것은 경험으로 말미암아 얻게 됩니다. 직관이라는 것은 우리가 태어날 때부터 얻어지는 것이 아닙니다. 아기는 물건을 보고 만지고 경험하면서, '하나의 물체는 한 곳에 있구나' 라는 직관을 배웁니다. 양자물리학이 어려운 이유가 여기 있습니다. 양자물리학은 쉽게 말해 하나의 물체가 두 곳의 다른 장소에 있을 수 있다고 하는 것입니다. 하지만 우리는 이런 현상을 경험한 적이 없고 우리의 일반적인 직관과 완전히 반대되는 것입니다.

그러나 어느날부터 입자물리학자들은 원자보다도 작은 입자들을 연구하면서 얻은 자료들이 기존의 거시세계에서 사용되던 직관과 위배되는 경우를 많이 목격하게 되었습니다. 벽을 통과하거나, 두 장소에서 하나의 입자가 존재하거나 하는 것들이 확인되었습니다. 그래서 새로운 역학이 필요하게 되었고, 그 것은 지금 양자역학으로 불리고 폭넓게 연구되고 있습니다.

여기서 강연자는 하나의 의문을 갖습니다. "입자 세계에서 양자역학이 성립하면, 그 입자들로 이루어진 거시세계에서는 양자역학이 성립하지 않을까?" 라는 것이었지요. 그는 이 의문을 해결하기 위해 거시세계의 물체가 양자역학을 따르는 모습을 관찰하는 실험을 했습니다. 그가 만들어야 했던 장치는 그림과 같습니다. 

아래 사진은 위 사진을 확대 한 것이다. 아래 사진에 수영장 널뛰기 판처럼 생긴 것이 보인다. 이 작은 금속조각의 움직임이 이 실험의 관찰 목표이다.


비록 실험하는 대상이 현미경의 도움을 받아야 하는 마이크로 칩의 한 부분에 있는 금속 조각이지만, 여전히 거시세계의 역학법칙이 적용이 됩니다. 강연자는 이 금속으로부터 양자역학을 따르는 모습을 관찰하려면 특수한 환경을 갖추어 주어야 합니다. 마치 엘레베이터에 혼자 있을 때는 온갖 모습을 다 보이지만, 한사람이라도 들어온다면 혼자 있을 때와 다른 모습을 보이는 것처럼, 거시세계의 물체는 양자역학을 따르는 모습을 보여주려면 완전히 혼자 있어야 합니다. 그래서 저 마이크로 칩은 진공, 절대온도 0도, 그리고 암실과 같은 특수한 환경에서 관찰을 해야합니다. 

그렇다면 어떤 실험결과가 나왔을까요? 특수한 환경에 있는 금속조각은 매우 특이한 움직임을 보입니다. 먼저 가만히 정지하는 대신에, 확장과 수축을 반복하는 진동의 움직임을 보입니다. 게다가 살짝 건드려주면, '진동하는 상태' 와 '진동하지 않는 상태' 를 동시에 보여줍니다. 진동하는 상태의 금속의 원자는 찰나의 시간동안만 정지한 상태의 원자와 일치됩니다. 그리고 대부분의 시간을 분리된 공간에서 존재하게 됩니다. 그리고 이런 모습은 특수한 환경이 제거되자마자 사라지고 맙니다.


그럼 이 관찰 결과가 우리에겐 어떤 의미가 있는 것일까요? 연사는 양자역학에 지배당하는 물체가 아닌 바로 자신을 상상해보라고 제안해 봅니다. 우리 주위의 모든 것은 실제로는 양자 역학에 의해 지배받는 존재들입니다. 하지만 주위의 공기 온도 빛 이 모든 것에 의해, 마치 꽉꽉 메워진 엘레베이터 속의 사람처럼, 우리는 양자역학에 따르는 실제 모습을 드러내고 있지 않는 것입니다. 거꾸로 말하면, 우리의 지금 이 모습은, 주변 모든 것에 연결되고 상호작용하면서 영향받은 모습이라고 할 수 있겠네요.

역시 이 세상 모든 것은 하나도 고립되어 있는 것이 없고, 함께 영향을 주고받으며 살 수 밖에 없어보입니다!! 하나의 물리 현상으로 삶의 모습을 돌아보면 너무 유치한 패턴이겠지만 자연스럽게 생각이 그렇게 흘러버리네요.ㅎㅎ 오늘의 테드 리뷰 역시 시간 정말 오래걸리지만 재밌었습니다.


원문 dictation.

This is representation of your brain. And your brain can be broken into two parts. There is left half, which is logical side. and the other is right half which is intuitive. And so if we had a scale to measure the aptitude of each hemisphere. then we can plot our brain. And for example, this would be somebody who's completely logical. This would be someone who is entirely intuitive. So where would you put your brain on this scale? Some of us may have opted for one of these extremes, but i think for most of people in the audience, your brain is something like this. High aptitude in both hemispheres at the same time. It's not like they are mutually exclusive or anything. You can be logical or be intuitive. And so i consider myself one of these people along with most of the other experimental quantum physicists who need a good deal of logic to string together these complex ideas but at the same time, we need a good deal of intuition to make the experiment actually work. How do we develop this intuition? Well we like to play with those stuff. So we go out and play with it, and then we see how it acts and then we develop our intuition from there. And really you do the same thing. So some intuition that you may have developed over the years, is one thing is only in one place at a time. I mean, it can sound weird to think about one thing being in the two different places at the same time. 
But you weren't born with this notion, you developed it. Now I remember watching kids playing on a car stop. He was just a toddler and he wasn't very good at it, and he kept falling over. But I bet playing with this car stop taught him a really valuable lesson and that's that large things don't let you get right past them, and that they stay in one place. 
And so this is a great conceptual model to have of the world unless you are particle physicist. It would be a terrible model for a particle physicist. because they don't play with the car stop. They play with these little weird particles. And when they play with their particles they find they do all sorts of really weird things like they can fly right through walls or they can be in two different places at the same time. And so they wrote down all these observations. and they called it the theory of quantum mechanics. That is where physics was at a few years ago. you needed quantum mechanics to describe little tiny particles. But you didn't need it to describe the large everyday objects around us. This didn't really sit well with my intuition. and maybe it's just because i don't play with particles very often. I've never seen them, and nobody's ever seen a particle. But it didn't sit well with my logical side either. Because if everything is made up of little particles and all the little particles follow quantum mechanics then shouldn't everything just follow quantum mechanics? I don't see any reason why it shouldn't. So I feel a lot better about the whole thing if we could somehow show that an everyday object also follows quantum mechanics. So few years ago I set off to do just that. So I made one. This is first object that you can see that has been mechanical quantum superposition. So what we are looking at here is a tiny computer chip. You can sort of see this green dot right in the middle. And that is this piece of metal I'm going to be talking about in a minute. This is photograph of the object and here I'll zoom-in a little bit. We are looking right there in the center. And then here's really big close-up of the little piece of metal. So what we are looking at is little chunk of metal, and it's shaped like a driving board, and it's sticking out over a ledge. I made this thing in nearly the same way as you make a computer chip. I went to clean room with a fresh silicon wafer, and then I just cranked away at all the big machines for about 100 hours. For the last stuff I had to build my own machine- make this swimming pool-shaped hole underneath the device. This device has ability to be in a quantum superposition. But it needs a little help to do it. Let me give you an analogy. You know how uncomfortable it is to be in a crowded elevator? When I'm in an elevator all alone I do all sorts of weird things. But then other people came in and i stop doing those things because i don't want to bother them or frankly scare them. So quantum mechanics say that intimate object feel the same way. The fellow passengers for intimate objects are not just people, but it's also the light shining on it and the wind blowing past it and the heat of the room. So when we wanted to see this piece of metal behave quantum mechanically we are going to have to kick out all the other passengers. So that is what we did. We turned off the lights, then we put it in a vacuum and sucked out all the air, and then we cooled it down to just a fraction of degree above absolute zero. Now all alone in the elevator, the chunk of metal is free to act however it wanted. 

We found it was moving in really weird ways. Instead of just sitting perfectly still, it was vibrating. In the way it was vibrating was breathing something like this-like expanding and contracting bellow. And by giving it gentle nudge we were able to make it both vibrate and not vibrate at the same time. something that is only allowed with quantum mechanics. So what I'm telling you here is something truly fantastic. What does it mean for one thing to be both vibrating and not vibrating at the same time? So let's think about the atoms. So in one case all the trillions of atoms that make up that chunk of metal are sitting still and at the same time those same atoms are moving up and down. Now it's only at precise times when they align.The rest of time they are delocalized. That means that every atom is in the two different places at the same time which in turn means the entire chunk of metal is in two different places. I think this is really cool. I was worth locking myself in a clean room to do this for all those years. Because check this out, the difference in scale between single atoms and that chunk of metal is about the same as the difference between that chunk of metal and you. So if the single atom can be in two different places at the same time, that chunk of metal can be in two different places then why not you? This is my logical side talking. Imagine you are at multiple places at the same time, what would that be like. How would your consciousness handle your body being delocalized in space? There is one more part of story. It's when we warmed it up, and we turned on the lights and looked inside the box, we saw that the piece metal was still there in one piece. And so i had to develop this new intuition that it seems like all the objects in the elevator are really just quantum objects just crammed into a tiny space. You hear a lot of talk about how quantum mechanics says that everything is all interconnected. But that is not quite right. It's more than that,  it's deeper. It's that those connections all the things around you, literally define who you are. And that is the profound weirdness of quantum mechanics. 

 Thank you.


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