伦纳德·萨斯坎德：量子力学、弦理论与黑洞

我之前提到过超导体，这是一个很明显的例子。

I said superconductors before, it's an obvious one.

超导体看起来极其复杂，有着各种奇怪的电磁性质等等。

A superconductor seems like a monstrously complicated thing with all sorts of crazy electrical properties, magnetic properties, and so forth.

但归根结底，它其实是一个非常简单的量子力学现象，叫做自发对称性破缺，我们在其他场合也学过并且很熟悉。

And when it finally is boiled down to its simplest elements, it's a very simple quantum mechanical phenomenon called spontaneous symmetry breaking, and which we, in other contexts, we learned about and we're very familiar with.

所以，是的，我们确实会把复杂的东西简化，但我们不想做的是，把本质上就很复杂的东西，还自欺欺人地以为能把它变简单。

So, yeah, I mean, yes, we do take complicated things, make them simple, but what we don't wanna do is take things which are intrinsically complicated and fool ourselves into thinking that we can make them simple.

我们不想让它们变得比实际更简单，我不知道这话是谁说的，但就是这个意思。

We don't wanna make I don't know who said this, but we don't wanna make them simpler than they really are.

对。

Right.

明白吗？

Okay?

大脑最终是靠一些简单的规则运作，还是它本身就是复杂的？

Is the brain a thing which ultimately functions by some simple rules, or is it just complicated?

就人工智能而言，没人真正知道我们目前的方法有什么局限。

In terms of artificial intelligence, nobody really knows what are the limits of our current approaches.

你提到了机器学习。

You mentioned machine learning.

我们怎么才能创造出人类级别的智能呢？

How do we create human level intelligence?

似乎有很多非常聪明的物理学家可能过于简化了智能的本质，把它看作信息处理，因此似乎没有理论上的理由说明我们不能人工创造出人类级别或超人类级别的智能。

It seems that there's a lot of very smart physicists who perhaps oversimplify the nature of intelligence and think of it as information processing, and therefore there doesn't seem to be any theoretical reason why we can't artificially create human level or superhuman level intelligence.

实际上，他们的推理是，如果你创造了人类级别的智能，你用来创造人类级别智能的同样方法应该能让你很容易地、指数级地创造出超人类级别的智能。

In fact, the reasoning goes, if you create human level intelligence, the same approach you just used to create human level intelligence should allow you to create superhuman level intelligence very easily, exponentially.

那么，你觉得物理学家那种思维方式是怎么回事？

So what do you think that way of thinking that comes from physicists is all about?

我希望我知道，但我特别想知道是有原因的。

I wish I knew, but there's a particular reason why I wish I knew.

我还有第二份工作。

I have a second job.

我给谷歌做顾问。

I consult for Google.

哦。

Ah.

不是给谷歌，是给谷歌的X项目。

Not for Google, for Google X.

我是那群机器学习物理学家的资深学术顾问。听起来挺疯狂的，因为我对此一无所知。

I am the senior academic adviser to to a group of machine learning physicists at now that sounds crazy because I know nothing about the subject.

我对这个领域了解不多。

I know very little about the subject.

不过，我挺擅长给建议的，所以我就给他们提点意见。

On the other hand, I'm good at giving advice, so I give them advice on things.

总之，我看到这些年轻物理学家正在研究机器学习的问题。

Anyway, I see these young physicists who are approaching the machine learning problem.

机器学习确实有个核心问题，就是：为什么它效果这么好？

There is a there is a real machine learning problem, namely why does it work as well as it does?

对。

Right.

似乎没有人真正理解它为什么能够进行那样的泛化，等等。

It nobody really seems to understand why it is capable of doing the kind of generalizations that it does and so forth.

有三组人思考过这个问题。

And there are three groups of people who have thought about this.

有工程师。

There are the engineers.

工程师们非常聪明，但他们往往不太深入思考为什么这个东西有效，而更多考虑如何使用它，显然。

The engineers are incredibly smart, but they tend not to think as hard about why the thing is working as much as they do how to use it, obviously.

他们提供了大量数据，正是他们证明了机器学习的效果可以远超你的预期。

They provided a lot of data, and it is they who demonstrated that machine learning can work much better than you had any right to expect.

机器学习系统就是系统。

The machine learning systems are systems.

这个系统与物理学家研究的那些系统并没有太大不同。

The system's not too different than the kind of systems that physicists study.

在数学结构上，量子计算和这个差别并不大。

There's not all that much difference between quantum com in in the structure of the mathematics.

物理上确实有差别，但在数学结构上，用于描述量子系统的张量网络和机器学习中使用的那些网络，差别不大。

Physically, yes, but in the structure of the mathematics between a tensor network designed to describe a quantum system on the one hand, and the kind of networks that are used in machine learning.

所以我觉得越来越多的年轻物理学家被吸引到机器学习这个领域。

So there are more and more, I think, young physicists are being drawn to this field of machine learning.

有些非常非常优秀的。

Some very, very good ones.

我和一些非常优秀的物理学家合作，但不是搞机器学习，而是一起吃午饭。

I work with a number of very good ones, not on machine learning, but on having lunch.

一起吃午饭？

On having lunch?

对。

Right.

是的。

Yeah.

然后呢？

And

我可以告诉你，他们超级聪明。

I can tell you, they are super smart.

是的。

Yes.

他们似乎不会因为自己的物理背景而傲慢到觉得自己能做一些别人做不到的事情。

They don't seem to be so arrogant about their physics backgrounds that they think they can do things that nobody else can do.

但物理的思维方式，我觉得，会给机器学习带来巨大的价值。

But the physics way of thinking, I think, will add will add great value to will bring value to the machine learning.

我相信会的。

I I believe it will.

而且我觉得已经带来了。

And I think it already has.

以你长期的经验和对新发现的惊讶，你觉得预测未来在什么时间尺度上会变得无用？

At what time scale do you think predicting the future becomes useless in your long experience in being surprised at new discoveries?

有时候是一天，有时候是二十年。

Sometimes a day, sometimes twenty years.

有些事情我原以为我们离理解还差得很远，结果几乎在一瞬间就突然被理解了，这让我非常惊讶。

There are things which I thought we were very far from understanding, which practically in a snap of the fingers or a blink of the eye suddenly became understood, completely surprising to me.

还有其他一些事情，我看着它们，心想，这些我们五百年都理解不了，特别是量子引力。

There are other things which I looked at, and I said, we're not gonna understand these things for five hundred years, in particular, quantum gravity.

那个时间尺度是二十年，二十五年。

The scale for that was twenty years, twenty five years.

我们现在理解了很多，但远非完全理解，但我原以为要五百年才能取得任何进展。

And we understand a lot, and we don't understand it completely now by any means, but we I thought it was five hundred years to make any progress.

结果远非如此。

It turned out to be very, very far from that.

结果更像是从我以为要五百年的时候起，过了二十或二十五年。

It turned out to be more like twenty or twenty five years from the time when I thought it was five hundred years.

如果可以的话，我们能聊聊量子引力和物理学的一些基本概念吗？

So if we may, can we jump around quantum gravity, some basic ideas in physics?

从数学上讲，弦理论的梦想是什么？

What is the dream of string theory mathematically?

希望是什么？

What is the hope?

它从何而来？

Where does it come from?

它试图解决什么问题？

What problem is it trying to solve?

我认为弦理论的梦想与整个基础理论物理学的梦想并无不同。

I don't think the dream of string theory is any different than the dream of fundamental theoretical physics altogether.

理解一个统一万物的理论。

Understanding a unified theory of everything.

对。

Yeah.

我不喜欢把弦理论看作一个独立的学科，有所谓的弦理论家，他们是这个弦理论的实践者。

I I don't like thinking of string theory as a subject unto itself with people called string theorists who are the practitioners of this thing called string theory.

我更愿意把他们看作是在试图回答关于自然的深层基本问题的理论物理学家，特别是引力，特别是引力与量子力学的联系，而目前他们发现弦理论是一个有用的工具。

I much prefer to think of them as theoretical physicists trying to answer deep fundamental questions about nature, in particular gravity, in particular gravity and its connection with quantum mechanics, and who, at the present time, find string theory a useful tool.

而不是说有一个叫弦理论的学科，我不喜欢被称为弦理论家。

Rather than saying there's this subject called string theorist, I don't like being referred to as a string theorist.

是的。

Yes.

但作为一种工具，用多维空间和振动的弦来思考自然是否有用？

But as a tool, is it useful to think about our nature in multiple dimensions, the strings vibrating?

我相信它是有用的。

I believe it is useful.

我来告诉你到目前为止它的主要用途。

I'll tell you what the main use of it has been up till now.

嗯。

Mhmm.

嗯，它有几个主要用途。

Well, it has had a number of main uses.

最初，弦理论被发明出来，而且我知道我当时就在场。

Originally, string theory was invented, and I know that I was there.

我正好就在它被发明出来的地方，当时是为了理解强子。

I was right at the spot where it was being invented literally, and it was being invented to understand hadrons.

强子是亚核粒子，比如质子、中子、介子。

Hadrons are sub nuclear particles, protons, neutrons, mesons.

那时候，也就是六十年代末、七十年代初，实验清楚地表明，这些叫做强子的粒子能够振动、旋转，能做出一根闭合弦能做的所有事情。

And at that time, the late sixties, early seventies, it was clear from experiment that these particles called hadrons had could vibrate, could rotate, could do all the things that a little closed string can do.

对。

Mhmm.

它过去是、现在也是描述强子的有效且正确的理论。

And it was and is a valid and correct theory of this hadrons.

它已经经过实验验证，这已经是板上钉钉的事了。

It's been experimentally tested, and that is a done deal.

后来它又作为引力理论获得了第二次生命。

It had a second life as a theory of gravity.

同样的基础数学，只不过是在非常非常小的距离尺度上。

The same basic mathematics, except on a very, very much smaller distance scale.

引力研究的对象比质子小19个数量级。

The objects of gravitation are 19 orders of magnitude smaller than a proton.

但同样的数学又出现了。

But the same mathematics turned up.

同样的数学又出现了。

The same mathematics turned up.

它的价值是什么？

What has been its value?

它的价值在于，它在很多方面数学上很严谨，使我们能够找到既包含量子力学又包含引力的严谨数学结构。

Its value is that it's mathematically rigorous in many ways and enabled us to to find to find mathematical structures which have both quantum mechanics and gravity with rigor.

我们可以检验一些想法。

We can test out ideas.

我们可以检验一些想法。

We can test out ideas.

我们没法在实验室里测试它们。

We can't test them in the laboratory.

我们感兴趣的东西太小了，小了19个数量级，但我们可以用数学来测试它们，并分析它们的内在一致性。

They're they're 19 orders of magnitude too small, the things that we're interested in, but we can test them out mathematically and and analyze their internal consistency.

到现在，四十年前，三十五年前，人们非常质疑引力和量子力学之间的一致性。

By now, forty years ago, thirty five years ago, and so forth, people very very much questioned the consistency between gravity and quantum mechanics.

史蒂芬·霍金就以此闻名。

Stephen Hawking was very famous for it.

确实如此。

Rightly so.

现在没人再质疑这种一致性了。

Now nobody questions that consistency anymore.

因为我们现在有数学上精确的弦理论，它同时包含了引力和量子力学，并且是一致的。

They don't because we have mathematically precise string theories which contain both gravity and quantum mechanics in a consistent way.

所以它证明了量子力学和引力可以共存。

So it's provided that certainty that quantum mechanics and gravity can coexist.

这可不是小事。

That's not a small thing.

这是件非常大的事。

A very huge thing.

这是件大事。

It's a huge thing.

爱因斯坦会感到骄傲的。

Einstein would be proud.

爱因斯坦可能会感到震惊，我不知道，他不太喜欢量子力学，但他肯定会被震撼到。

Einstein, he might be appalled, I don't know, he didn't like quantum mechanics very much, but he would certainly be struck by it.

对。

Yeah.

我认为这可能是目前弦理论对物理学最大的贡献，它几乎明确地证明了量子力学和引力是紧密相关的，而且彼此并不矛盾。

I think that may be, at this time, its biggest contribution to physics in illustrating almost definitively that quantum mechanics and gravity are very closely related and not inconsistent with each other.

有没有可能还有更深层、更深刻的东西，仍然与弦理论一致，但更深刻，还有待发现？

Is there a possibility of something deeper, more profound, that still is consistent with string theory but is deeper, that is to be found.

嗯，你可以对量子力学问同样的问题。

Well, you could ask the same thing about quantum mechanics.

这是不是某种东西？正是。

Is this something Exactly.

对。

Yeah.

对。

Yeah.

想想弦理论，它只是量子力学系统的一个例子，这个系统同时包含了引力和量子力学。

Think string theory is just an example of a quantum mechanical system that contains both gravitation and quantum mechanics.

那么，量子力学背后是否还有更深层的东西？

So is there something underlying quantum mechanics?

也许是某种确定性的东西。

Perhaps something deterministic.

也许是某种确定性的东西。

Perhaps something deterministic.

我的朋友杰拉德·特胡夫特，你可能知道他的名字，他是一位非常著名的物理学家。

My friend Gerard Itwuft, whose name you may know, he's a very famous physicist.

荷兰人，名气不如他应得的那么大，但是但是

Dutch, not as famous as he should be, but but

他的名字很难拼写，所以

Hard to spell his name, so

那是他的名字很难说。

that's It's hard to say his name.

不，他的名字很容易拼写。

No, it's easy to spell his name.

有个撇号，他是唯一一个名字以撇号开头的人。

An apostrophe, he's the only person I whose name begins with an apostrophe.

他是我物理学界的英雄之一。

And he's one of my heroes in physics.

虽然比我年轻一点，但他仍然是我崇拜的偶像之一。

Mean, he's a little younger than me, he's nevertheless one of my heroes.

卢托夫特认为，世界背后存在一些具有经典特征、决定论特征的底层结构，只是他很难说清楚具体是通过什么机制，才涌现出了量子力学。

Lutovt believes that there is some substructure to the world which is classical in character, deterministic in character, which somehow, by some mechanism that he has a hard time spelling out, emerges as quantum mechanics.

也就是说，波函数是某种程度上的涌现现象。

I The wave function is somehow emergent.

不只是波函数，而是整个量子力学体系——包括不确定性原理、纠缠态等等这些概念——都是涌现出来的。

The wave function and the whole not just the wave function, but the whole mech the whole thing that goes with quantum mechanics, uncertainty, entanglement, all these things are emergent.

所以你认为量子力学就是最底层的理论？

So you think quantum mechanics is the bottom of the well?

是不是

Is is the

我觉得，这时候你得学会谦虚。

Well, here I think is here I think is where you have to be humble.

这就是谦逊的意义所在。

Here's where humility comes.

我认为现在谁都不该断言说已经触及了问题的底线。

I don't think anybody should say anything is the bottom of the well at this time.

对。

Yeah.

我觉得我们可以合理地说，或者我可以说，当我望向那口井时，我看不到量子力学之外的东西。

As I think we I think we can reasonably say or I can reasonably say, when I look into the well, I can't see past quantum mechanics.

我看不出有任何理由认为量子力学之外还有什么。

I don't see any reason for there to be anything beyond quantum mechanics.

我认为Etof确实提出了非常有趣且深刻的问题。

I think Etof does ask very interesting and deep questions.

但我不喜欢他的答案。

I don't like his answers.

好吧，再让我问一次，如果我们看现实最深层的本质，无论是决定论的，还是被观察时表现为概率性的，这对我们人类层面的自由意志观念意味着什么？

Well, again, let me ask, if we look at the deepest nature of reality, whether it's deterministic or when observed as probabilistic, what does that mean for our human level of ideas of free will?

我们拥有的这种自由意志的感知，或许是一种幻觉，它与现实的基本本质之间有任何联系吗？

Is there any connection whatsoever from this perception, perhaps illusion of free will that we have, and the fundamental nature of reality?

我只能说，我和你一样对此感到困惑。

The only thing I can say is I am puzzled by that as much as you are.

它的幻觉。

The illusion of it.

意识的幻觉，自由意志的幻觉，还有……

Illusion of consciousness, the illusion of free will, the illusion of

自我的幻觉。

self.

这是否与……

Does that connect to

一个物理系统怎么能做到这一点，我和其他人一样困惑。

How can a physical system do that, and and I am as puzzled as anybody.

这在观察者效应中有体现。

There's echoes of it in the observer effect.

是的。

Yep.

那你明白作为观察者意味着什么吗？

So do you understand what it means to be an observer?

我在技术层面理解它。

I understand it at a technical level.

观察者是一个具有足够自由度的系统，能够记录信息，并且能与它所测量的对象发生纠缠。

An observer is system with enough degrees of freedom that it can record information and which can become entangled with the thing that it's measuring.

纠缠是关键。

Entanglement is the key.

当一个系统，我们称之为仪器或观察者，本质上是一样的，与它所观察的系统相互作用时，它不仅仅是看着它，而是物理上变得纠缠在一起。正是这种纠缠，我们称之为观察或测量。

When a system, which we call an apparatus or an observer, same thing, interacts with the system that it's observing, it doesn't just look at it, it becomes physically entangled And with it's that entanglement which we call an observation or a measurement.

那么，这作为我个人作为观察者，是否让我满意呢？

Now does that satisfy me personally as an observer?

既满意又不满意。

Yes and no.

我觉得非常满意的是，我们有了一个数学表示来描述观察一个系统的含义。

I find it very satisfying that we have a mathematical representation of what it means to observe a system.

你现在在意识层面正在观察事物。

You are observing stuff right now at the conscious level.

对。

Right.

你觉得这种纠缠在宏观尺度上会有回响吗？

Do you think there's echoes of that kind of entanglement in our macro scale?

是的，绝对有。

Yes, absolutely.

当然。

For sure.

我们跟这个房间里的一切都在量子力学层面上纠缠在一起。

We're we're entangled with quantum mechanically entangled with everything in this room.

如果我们没有纠缠，那就意味着我们并没有在观察它。

If we weren't, then it would just well, we wouldn't be observing it.

但另一方面，你也可以问自己：我真的对此感到安心吗？

But on the other hand, you can ask, do I really am I really comfortable with it?

我对这一点感到不安，就像我永远无法适应五个维度一样。

And I'm uncomfortable with it in the same way that I can never get comfortable with five dimensions.

我的大脑就是没法理解它。

My my brain isn't wired for it.

你对四维空间感到自在吗？

Are you comfortable with four dimensions?

稍微自在一点，因为我总能想象第四维是时间。

Little bit more, because I can always imagine the fourth dimension is time.

那时间之箭呢？你对那个箭头感到自在吗？

So the arrow of time, are you comfortable with that arrow?

你觉得时间是涌现的现象，还是自然界的基本属性？

Do you think time is an emergent phenomena, or is it fundamental to nature?

这是目前物理学中的一个重大问题。

That is a big question in physics right now.

我们做的所有物理学，至少是我能自在交谈的那些人，我的朋友们。

All the physics that we do, or at least at the people that I am comfortable with talking to, my friends.

是的。

Yeah.

我的朋友们。

My friends.

不。

No.

我们都问过你刚才问的那个问题。

We all ask the same question that you just asked.

对于空间，我们相当确定它是涌现的，它从纠缠和其他东西中涌现出来。

Space, we have a pretty good idea, is emergent, and it emerges out of entanglement and other and other things.

时间似乎总是内置于我们的方程中，就像牛顿基本上会认为的那样。

Time always seems to be built into our equations as just what Newton pretty much would have thought.

牛顿，经过爱因斯坦稍加修改，会称之为时间。

Newton, modified a little bit by Einstein, would have called time.

而且在我们的方程中，时间大多不是涌现的。

And in mostly in our equations, is not emergent.

物理学中的时间是完全对称的，向前，对吧。

Time in physics is completely symmetric, forward Right.

而且是对称的。

And Symmetric.

所以，你

So so you

在大多数物理现象中，其实不需要考虑时间箭头的问题？

don't really need to think about the arrow of time for most physical phenomena?

对于大多数微观现象来说，确实不需要。

For most microscopic phenomena, no.

对。

Right.

只有当现象涉及的系统足够大，使得热力学变得重要时，情况才会如此。

It's only when the phenomena involve systems which are big enough for thermodynamics to become important.

没错。

Right.

熵开始发挥重要作用的时候。

For entropy to become important.

对于一小部分、小型系统来说，熵并不是一个适用的概念。

For a small subset a small system, entropy is not a good concept.

熵是从大量元素中涌现出来的现象。

Entropy is something which which emerges out of large numbers.

它是一种概率性的概念，也是统计学的概念，同时还是热力学的概念。

It's a probabilistic idea, it's a statistical idea, and it's a thermodynamic idea.

热力学需要非常多、非常多的微观结构支撑。

Thermodynamics requires lots and lots and lots of little substructures.

明白吗？

Okay?

所以，只有到了热力学层面，时间箭头才会出现。

So it's not until you emerge at the thermodynamic level that there's an arrow of time.

我们理解它了吗？

Do we understand it?

是的。

Yeah.

我觉得我们比大多数人以为的要更了解它，大多数人都说他们认为我们了解它。

I think I think we understand better than most people think they most people say they think we understand it.

对。

Yeah.

我觉得我们确实理解了。

I think we understand it.

这是一个统计学的概念。

It's a statistical idea.

你是指热力学第二定律、熵之类的概念吗？

The you mean, like, second law of thermodynamics, entropy, and so on?

对。

Yeah.

是的。

Yeah.

应该是

Would be

比如把一副扑克牌抛向空中，然后看看它们会怎样散开。对。

If pack of cards, you fling it in the air, and you and you look what happens to it Yeah.

但结果会变得随机。

But gets random.

我们明白。

We understand

它不会从随机变成有序。

It doesn't go from random to simple.

它从有序走向无序。

It goes from simple to random.

但你觉得它会失效吗？

But do you think it ever breaks down?

我觉得在实验室环境下，你可以选取一个介于微观和宏观之间的系统，对，然后让它逆向运行。

What I think you can do is in a laboratory setting, you can take a system which is somewhere intermediate between being small and being large Mhmm.

并让它倒回去。

And make it go backward.

有些东西看起来因为统计力学的原因，或者说因为热力学第二定律，只能单向进行。但你可以通过非常精细的操作，让它倒着运行。

A thing which looks like it only wants to go forward because of statistical mechanical reasons, because of the second law, you can very, very carefully manipulate it to make it run backward.

我觉得你不能把一只掉在地上摔碎的鸡蛋——就像那个蛋头先生一样——

I don't think you can take an egg humpty dumpty who fell on the floor

对。

Yeah.

再把它复原回去。

And reverse that.

但在非常受控的环境下，你可以让那些看似正在向随机状态演化的系统停下来，逆转它们，让它们倒回去。

But you can in a in a very controlled situation, you can take systems which appear to be evolving statistically toward randomness, stop them, reverse them, and make them go back.

这背后的直觉是什么？

What's the intuition behind that?

我们具体该怎么做呢？

How do how do we do that?

我们要怎么实现逆转呢？

How do we reverse it?

你是说一个封闭系统吗？

A you're saying a closed system?

对。

Yeah.

基本上是封闭系统。

Pretty much closed system.

是的。

Yes.

你刚才是不是说时间旅行是可能的？

Did you just say that time travel is possible?

不。

No.

我没说时间旅行是可能的。

I didn't say time travel is possible.

我说的是你可以让一个系统倒退。

I said you can make a system go backward.

在时间上。

In time.

你可以让它倒退，可以逆转它的步骤，也可以改变它的轨迹。

And you can make it go back, you can make it reverse its steps, you can make it reverse its trajectory.

对。

Yeah.

我们该怎么做呢？

How do we do it?

这背后的直觉是什么？

What's the intuition there?

这是不是只是我们在实验室小规模下能做到的一个巧合现象，而没有更深层的意义呢？

Does it have, is it just a fluke thing that we can do at a small scale in the lab that doesn't have Well,

我的意思是，你不仅能做到小规模，还能做得比小规模稍微大一点。

what I'm saying is you can do it on a little bit better than a small scale.

用简单的微小系统肯定是可以实现的。

You can certainly do it with a simple small system.

小系统对时间箭头没有任何概念。

Small systems don't have any sense of the arrow of time.

确实。

Mhmm.

原子。

Atoms.

原子也没有时间箭头的概念。

Atoms are no sense of an arrow of time.

它们是完全可逆的。

They're completely reversible.

只有当你面对大量粒子时，也就是热力学第二定律所描述的大数定律情况，才会出现不可逆性。

It's only when you have, you know, the second law of thermodynamics is the law of large numbers.

所以你可以打破这个定律，因为它并不适用于微观层面。你可以

So you can break the law because it's not You can

打破它。

break it.

可以打破，但很难做到。

Can break it, but it's hard.

这需要非常小心。

It requires great care.

系统越大，就越需要小心，难度也越高。

The bigger the system is, more care, the more the harder it is.

你得克服所谓的混沌。

You have to overcome what's called chaos.

是的。

Yeah.

这很难，而且需要越来越高的精度。

And that's hard, and it requires more and more precision.

对于10个粒子，你可能费点劲还能做到。

For 10 particles, you might be able to do it with a with some effort.

但对于100个粒子，那就真的很难了。

For a 100 particles, it's really hard.

对于一千或一百万个粒子，就别想了，但这并不是因为什么根本性的原因，只是因为技术上太难了，就是太难了。

For a thousand or a million particles, forget it, but not for any fundamental reason just because it's technologically too hard It's just hard.

要让系统倒回去。

To make the system go backward.

所以，所以，没有时间旅行，因为工程上的原因。

So so no no time travel for for engineering reasons.

哦，不。

Oh, no.

不。

No.

不。

No.

不。

No.

什么是时间旅行？

What is time travel?

时间旅行，是去未来的那种吗？

Time travel time travel to the future?

那太简单了。

That's easy.

对。

Yes.

你只要闭上眼睛，睡一觉，醒来就到未来了。

You just close your eyes, go to sleep, and you wake up in the future.

对。

Yeah.

是的。

Yeah.

睡个好觉就能到那儿了。

A good nap gets you there.

没错。

Yeah.

睡个好觉就能到未来。

A good nap gets you there.

对。

Right.

那逆转热力学第二定律呢？让任何人类尺度的事物回到过去，是非常困难的工程。

What happened in reversing the second law of thermodynamics, the going backward in time for anything that's human scale is a very difficult engineering effort.

我不会把那称为时间旅行，因为那会和科幻小说里的时间旅行混淆。

I I wouldn't call that time travel, because it gets too too mixed up with what the science fiction calls time travel.

对。

Right.

这仅仅是逆转一个系统的能力。

This is just the ability to reverse a system.

你拿一个系统，然后逆转其中每个分子的运动方向。

You you take the system, and you reverse the direction of motion of every molecule in it.

这个你可以对一个分子做到。

That in you can do it with one molecule.

如果你发现一个粒子朝某个方向运动，咱们先不说粒子，说个棒球吧，你把它完全停住，然后简单地让它的运动方向反转。

If you find a particle moving in a certain direction, let's not say a particle, a baseball, you stop it dead, and then you simply reverse its motion.

原则上来说，这并不难。

In principle, that's not too hard.

对。

Mhmm.

它就会沿着原来的轨迹往回走。

And it'll go back along its trajectory in the backward direction.

只是把程序倒着运行。

Just running the program backwards.

把程序反向运行。

Running the program backward.

对。

Yeah.

好的。

Okay.

如果有两个棒球碰撞，嗯，你可以做到，但你必须非常、非常小心，才能做得恰到好处。

If you have two baseballs colliding, well, you can do it, but you have to be very, very careful to get it just right.

对。

Right.

如果有十个棒球，真的，或者更好的是，在一张理想化的无摩擦台球桌上放十颗台球。

If you have 10 baseballs, really, really or better yet, ten ten billiard balls on an idealized frictionless billiard table.

是的。

Mhmm.

好。

K.

所以你把球都摆成三角形。

So you start the balls all on a triangle.

对吧？

Right?

对。

Yep.

然后你用力击打它们。

And you whack them.

对。

Yep.

根据你在玩的游戏不同，你是用力击打，或者非常小心地操作，但总之是击打它们。

Depending on the game you're playing, you either whack them or you're really careful, but but you whack them.

然后它们就会向各个可能的方向飞散出去。

And they go flying off in all possible directions.

对。

Yeah.

好。

Okay.

试着把它倒过来。

Try to reverse that.

试着把它倒过来。

Try to reverse that.

想象一下，试着把每个台球都停下来，在某个点让它完全静止，然后反转它的运动，让它朝相反的方向去。

Imagine trying to take every billiard ball, stopping it dead at some at some point, and reversing its motion so that it was going in the opposite direction.

如果你极其小心地这么做，它就能重新拼回那个三角形。

If you did that with tremendous care, it would reassemble itself back into the triangle.

好。

Okay.

这是事实，而且你大概能用两个台球做到，运气好的话，三个台球也行。

That is a fact, and you can probably do it with two billiard balls, maybe with three billiard balls if you're really lucky.

但问题是，随着系统变得越来越复杂，你就必须越来越精确，不能出一点差错，因为最小的错误都会被放大。对。

But what happens is as the system gets more and more complicated, you have to be more and more precise not to make the tiniest error, because the tiniest errors will get magnified Yeah.

然后你就根本没法完成反转了。

And you'll simply not be able to do the reversal.

所以，是的，你可以那样做，但我不会称之为时间旅行。

So yeah, you could, that, but I wouldn't call that time travel.

对，那是另一回事。

Yeah, that's something else.

但如果你想想，这让我想到，如果我们把状态的展开看作一个程序，如果我们看这个世界，那么把世界看作一个模拟、一台计算机的想法。

But if you think of it, it just made me think, if we think the unrolling of state that's happening as a program, if we look at the world, so the idea of looking at the world as a simulation, as a computer.

对。

Right.

但它不是计算机，它只是一个单一的程序。

But it's not a computer, it's just a single program.

有一个问题可能很有用。

A question arises that might be useful.

运行宇宙的计算机有多难？

How hard is it to have a computer that runs the universe?

好。

Okay.

我们知道一些数学宇宙。

So there are mathematical universes that we know about.

其中一个叫做反德西特空间，在那里我们，而且它是量子力学。

One of them is called anti de Sitter space, where we and it's quantum mechanics.

我觉得我们可以在量子计算机上模拟它，在量子计算机上模拟。

Well, I think we could simulate it in a computer, in a quantum computer.

用经典计算机的话，你只能求解它的方程。

Classical computer, all you can do is solve its equations.

你没法让它像真实系统那样运作。

You can't make it work like the real system.

如果我们能造出一台足够大、足够稳定的量子计算机，也许就能模拟一个宇宙，一个反德西特宇宙的缩小版。

If we could build a quantum computer, a big enough one, like a robust enough one, we could probably simulate a universe, a small version of an anti Sitter universe.

反德西特是一种宇宙学模型。

Anti de Sitter is a kind of cosmology.

对。

Mhmm.

好。

Alright.

所以我觉得我们知道该怎么做。

So I think we know how to do that.

问题在于，我们生活的宇宙并不是反德西特空间的几何结构。

The trouble is the universe that we live in is not the anti de Sitter geometry.

它是德西特空间的几何结构，而我们对它的量子力学完全没搞懂。

It's the de Sitter geometry, and we don't really understand its quantum mechanics at all.

所以目前来讲，我们恐怕一点头绪都没有，怎么去模拟一个和我们自己的宇宙类似的宇宙。

So at the present time, I would say we wouldn't have the vaguest idea how to simulate a universe similar to our own.

没有。

No.

我本来想问，我们能不能在实验室里建一个小型版本，一个量子力学版本，把一堆量子计算机纠缠在一起、耦合起来，重现宇宙中发生的那些现象，哪怕是在很小的尺度上。

Was gonna ask, could we could we build in the laboratory a small version, a quantum mechanical version, the collection of quantum computers entangled and and coupled together, which would reproduce the the phenomena that go on in the universe, even on a small scale.

可以，如果那是在反德西特空间里的话。

Yes, if it were anti De Sitter space.

但如果是德西特空间，那就没办法。

No, if it's De Sitter space.

你能不能稍微描述一下德西特空间和反德西特空间？

Can you slightly describe De Sitter space and anti De Sitter space?

对。

Yeah.

它们的几何性质是什么？

What are the geometric properties of

它们的不同之处在于一个叫做宇宙学常数的常数的符号。

They differ they differ by the sign of a single constant called the cosmological constant.

一个是负曲率的，另一个是正曲率的。

One of them is negatively curved, the other is positively curved.

反德西特空间，也就是负曲率的那一个，你可以把它想象成一个放在有反射壁的盒子里的孤立系统。

Anti de Sitter space, which is the negatively curved one, you can think of as an isolated system in a box with reflecting walls.

嗯。

Mhmm.

你可以把它想象成一个在孤立环境中的量子力学系统。

You could think of it as a system of quantum mechanical system isolated in an isolated environment.

德西特空间才是我们真正生活的那个，它是呈指数膨胀的。

The Sitter space is the one we really live in, and that's the one that's exponentially expanding.

指数膨胀、暗能量，不管我们怎么称呼它，我们在数学上都不理解。

Exponential expansion, dark energy, whatever we wanna call it, and we don't understand that mathematically.

我们理解吗？

Do we understand

不是所有人都同意我的观点，但我确实不理解。

Not everybody would agree with me, but I don't understand.

我是说，他们肯定会同意我这一点：我不理解。

I mean, they would agree with me they definitely would agree with me that I don't understand it.

那关于大爆炸的起源，也就是它的诞生，我们有没有什么理解呢？

What about, is there an understanding of the the birth, the origin No.

大爆炸？

The Big bang?

没有。

So No.

不，没有。

One No.

有一些理论。

There's theories.

理论倒是有一些。

There are theories.

我最喜欢的是一个叫“永恒暴胀”的理论。

My favorite is the one called eternal inflation.

无限可以两边都有，也可以只有一边有，或者两边都没有。

The infinity can be on both sides, on one of the sides, and none of the sides.

那什么是永恒的无限？

So what's what's eternal infinity?

好的。

Okay.

两边都有无限。

Infinity on both sides.

哦，天哪。

Oh, boy.

对，对。

Yeah, yeah.

那为什么这是你最喜欢的？

That's Why is that your favorite?

因为它最让人震撼吗？

Because it's the most just mind blowing?

不是。

No.

因为我们想要一个起点。

Because we want a beginning.

不是的。

No.

为什么我们非要有个起点不可呢？

Why do we want a beginning?

实际上，确实是有个开端的。

I In practice, there was a beginning, of course.

实际上，确实有一个开端。

In practice, there was a beginning.

但有没有可能，在无限的时间中，它只是一个随机的涨落？

But could it have been a random fluctuation in an otherwise infinite time?

也许吧。

Maybe.

无论如何，永恒暴涨理论，如果理解正确的话，我认为它在两个方向上都是无限的。

In any case, the the eternal inflation theory, I think if correctly understood, would be infinite in both directions.

你怎么看待无限？

How do you think about infinity?

天哪。

Oh god.

老天爷。

Lord.

好吧。

Okay.

当然，你可以从数学的角度来思考这个问题。

Of course, you can think about it mathematically.

我刚刚跟谢尔盖·布林聊完这个话题。

I just finished this I just finished this discussion with my friend Sergey Brin.

对。

Yes.

你是怎么看待无限这个概念的？

How do you think about infinity?

我说，好吧，谢尔盖·布林富得没边儿。

I say, well, Sergey Brin is infinitely rich.

你怎么验证这个假设呢？

How do you test that hypothesis?

行吧。

Okay.

这话真妙啊。

That's such a good line.

对。

Right.

是的。

Yeah.

所以有些东西真的没法想象出来吗？

So there's no there's really no way to visualize some of these things?

比如

Like

对，这是个很好的问题。

Yeah, no, this is a very good question.

物理学里有没有无穷的位置？

Does physics have any, does infinity have any place in physics?

对。

Right.

对，我只能说这是个很好的问题。

Right, and all I can say is very good question.

那你对最近事件视界望远镜拍到的第一张黑洞图像有什么看法？

So what do you think of the recent first image of a black hole visualized from the Event Horizon Telescope?

这是科学的巨大胜利。

It's it's it's an incredible triumph of science.

本身来说，黑洞碰撞并不令人惊讶，而且它们的行为完全符合预期。

In itself, the fact that there are black holes which collide is not a surprise, and they seem to work exactly the way they're supposed to work.

我们能从中获得很多知识吗？

Will we learn a great deal from it?

我不知道。

I don't know.

也许吧，我们可能会学到一些东西。

I can't I I I we might.

但真正学到的可能不是关于黑洞本身的。

But the kind of things we'll learn won't really be about black holes.

为什么自然界会有这种质量尺度的黑洞，为什么它们如此普遍，这可能告诉我们关于宇宙结构及其演化的信息。

Why there are black holes in nature of that particular mass scale and why they're so common may tell us something about the structure, evolution of structure in the universe.

对。

Mhmm.

但我觉得它不会让我们对黑洞有新的认识。

But I don't think it's gonna tell us anything new about black holes.

不过从某种意义上说，这确实是个巨大的成就。回想一百年前，广义相对论不断发展，黑洞被发现，LIGO问世，还有为LIGO投入的惊人技术……这一切都是水到渠成的结果。

But it's a triumph in the sense that you go back a hundred years, and it was a continuous development, general relativity, the discovery of black holes, LIGO, the incredible technology that went into LIGO.

说实话，要是放在三四十年前，我根本不敢相信这些真会发生。

It is something that I never would have believed was gonna happen, you know, thirty, forty years ago.

我觉得广义相对论的演化是一个宏伟的结构，非常了不起。对。

And I I think it's a magnificent structure, magnificent thing, this evolution of general relativity Mhmm.

LIGO具有极高的精度，能够测量到10的负21次方尺度的变化。

LIGO, high precision, ability to measure things on a scale of 10 to the minus 21.

所以

So

画面和你预想的不同吗？没有。

and is the picture different than you realized No.

它只是证实了。

It simply confirmed.

你知道，能够证实这一点是一个巨大的胜利。

You know, it's a magnificent triumph to have confirmed Confirmed.

直接观测。对。

A direct observation Yeah.

爱因斯坦的引力理论在黑洞碰撞层面确实有效，这太了不起了。

That Einstein's theory of gravity at the level of black hole collisions actually works is awesome.

真的非常了不起。

It is really awesome.

你知道，我认识一些参与其中的人。

You know, I know some of the people who are involved in that.

他们只是普通人。

They're just ordinary people.

是的。

Yeah.

而且他们真的能把这件事做成了，真是厉害。

And the idea that they could carry this out Mhmm.

我……我真的被震撼到了。

I I just I'm I'm shocked.

对。

Yeah.

这些小小的智人

These little homo sapiens

是的。

Yeah.

就这些

Just these

小猴子。

little monkeys.

对。

Yeah.

聚在一起了。

Got together.

对。

Right.

然后拍了张照片，我觉得是稍微先进一点的灵长类。

And took a picture of Slightly advanced limers, I think.

什么

What

科学目前无法回答，但你希望很快能回答的问题有哪些？

kind of questions can science not currently answer, but you hope might be able to soon?

嗯，你已经提到了。

Well, you've addressed them.

比如，意识是什么？

What is consciousness, for example?

你觉得科学能解答这个问题吗？

You think that's within the reach of science?

我觉得科学在一定程度上能解答这个问题，但我觉得现在这个问题掌握在计算机科学家和神经科学家手中。

I think it's somewhat within the reach of science, but I think now I think it's in the hands of the computer scientists and the neuroscientists.

不是物理学家。

Not a physicist.

物理学家会帮忙吗？

Would the help.

也许在某个时候会，但我觉得物理学家会试图把它简化到他们能用自己方法处理的程度，但也许那些方法并不合适。

Perhaps at some point, but I think physicists will try to simplify it down to something that they can use their methods, and maybe they're not appropriate.

也许我们只需要在更大规模上做机器学习，让机器进化，不仅仅是学习的机器，还要在过程中进化自己的架构，这个架构不完全受我们控制，只部分受我们控制，而是受机器学习控制。

Maybe we we maybe we simply need to do more machine learning on bigger scales, evolve machines, machines not only that learn, but evolve their own architecture, as a process of learning, evolve an architecture, not under our control, only partially under our control, but under the control of machine learning.

对。

Yeah.

我再告诉你一件我觉得很酷的事。

I'll tell you another thing that I find awesome.

你知道谷歌教计算机下棋那件事吗？

You know this Google thing that they taught computers how to play chess?

对，对。

Yeah, yeah.

好的。

Okay.

他们教计算机下棋，不是通过教它们怎么下棋，而是让它们互相下棋。

They taught computers how to play chess, not by teaching them how to play chess, but just having them play against each other.

互相下，自我对弈。

Against each other, self play.

互相下。

Against each other.

这是一种进化形式。

This is a form of evolution.

这些机器进化了。

These machines evolved.

它们进化出了智能。

They evolved an intelligence.

嗯。

Mhmm.

它们在没有任何人告诉它们怎么做的情况下进化出了智能。

They evolved an intelligence without anybody telling them how to do it.

它们不是被设计出来的。

They were not engineered.

它们只是互相下棋，然后变得越来越好。

They just played against each other and got better and better and better.

这让我觉得机器可以进化出智能。

That makes me think that machines can evolve intelligence.

具体是什么样的智能呢？

What exact kind of intelligence?

我不知道。

I don't know.

但通过更好地理解这一点，也许我们能更好地了解我们自身智能的运作方式。

But in understanding that better and better, maybe we'll get better clues as to what goes on in our own intelligence.

生命和智能，是的。

Will life and intelligence is Yeah.

最后一个问题。

Last question.

有哪些问题是科学目前无法回答，甚至可能永远无法回答的？

What kind of questions can science not currently answer and may never be able to answer?

对。

Yeah.

是否存在某种智能，是这一切的基础？

Is there intelligence out there that underlies the whole thing?

如果你想的话，可以用那个以G开头的词来称呼它。

You can call them with a g word if you want.

我可以说，我们是不是一个有目的的计算机模拟？

I can say, are we a computer simulation with a purpose?

是否存在一个智能体，一个智能的代理，是这一切的基础或负责这一切的？

Is there an agent, an intelligent agent that underlies or is responsible for the whole thing?

那个智能体符合物理定律吗？

Does that intelligent agent satisfy the laws of physics?

它符合量子力学定律吗？

Does it satisfy the laws of quantum mechanics?

它是由原子和分子构成的吗？

Is it made of atoms and molecules?

对。

Yeah.

问题真的很多。

There's a lot of questions.

我觉得这似乎

And I don't see this it seems to me

是个真正的问题。

a real question.

是可以回答的问题。

It's an answerable question.

嗯，不知道它是否可回答。

Well, don't know it's answerable.

问题必须可回答才算真实。

The questions have to be answerable to be real.

有些哲学家会说，一个问题如果不能回答，就不算问题。

Some philosophers would say that a question is not a question unless it's answerable.

这个问题在我看来，没有任何已知方法可以回答，但它似乎是

This question doesn't seem to me answerable by any known method, but it seems to me

真实的。

real.

没有比这更好的结尾了。

There's no better place to end.

伦纳德，非常感谢你今天来交谈。

Leonard, thank you so much for talking today.

好的。

Okay.

好的。

Good.