时间是什么

时间是什么？物理学家探究其终极理论 时间是什么？物理学家探究其终极理论 (2010-03-01 11:14:08) 时间是什么？物理学家探究其终极理论   艾琳·彼芭（Erin Biba）2010-2-26 译者：kiwi       艺术家的多元宇宙想像图/贾森·托尔钦斯基（JasonTorchinsky）   美国圣地亚哥（SAN DIEGO）——作为科学家使自己被别人所知的一个方法就是去解决一个真正困难的问题。物理学家肖恩·卡罗尔（Sean Carroll）就是其中之一。他因为尝试去回答一个目前还没有科学家能够真正解释的古老问题，已经有了一点极客圈摇滚明星的感觉；这个问题就是：时间是什么？   这里，在美国科学促进会（American Association forthe Advancement of Science）的年会上，他作了关于时间之箭（the arrow of time）的报告；走廊中的科学家们拦下他，告诉肖恩他们对他的工作有多么感兴趣。   2月19日，在美国科学促进会（AAAS）的会场，卡罗尔坐下来向《连线》杂志记者解释他的理论，以及为什么马蒂麦福莱（Marty McFly，经典时光旅行电影《回到未来》中的角色；驾驶时间机器从1985年回到三十年前的1955年）的冒险在现实世界中永远不会存在，因为时间只能向前走不可能回到过去。     肖恩·卡罗尔（Sean Carroll）是加州理工大学的一位理论物理学家，他的研究兴趣是宇宙学的理论、场论（field theory）和地心引力（gravitation），他主要研究宇宙的进化。卡罗尔的最新著作《从永恒到现在：探求时间的终极理论》（From Eternity to Here: The Quest for theUltimate Theory of Time），试图将他的时间和宇宙理论带给物理学家，同样也带给普通大众。       连线：你能从一个外行人的角度解释一下你的时间理论吗？   卡罗尔：我正在试着去理解时间是如何工作的。同时这也是一个有很多不同面向的庞大问题，其中有许多方面可追溯到爱因斯坦及其时空理论，以及我们如何使用钟来测量时间。但是关于时间我感兴趣的一个方面是时间之箭：即过去与未来不同这一事实。我们能记住过去但是我们无法记住未来。这是一个不可逆的过程。就像有些事情的发生，比如你能把一个鸡蛋做成煎蛋，但是你不可能将煎蛋做成最初的鸡蛋。   同时可以说是我们只是理解了一小部分。时间之箭是基于奥地利物理学家路德维格·波尔兹曼（Ludwig Boltzmann）在1870年代提出的理论。他将他的理论称为熵（entropy）理论。熵是对无序程度的一个度量，而且熵往往是增加的。这是根据热力学第二定律（second law of thermodynamics）得到的：即熵随着时间增长，世间万物会变得更加无序。所以，如果你桌上的文件整整齐齐地放着，你离开之后返回，发现桌子上乱成一团你也会不觉得奇怪。如果回来时发现之前本就乱成一团的桌子变得整整齐齐，你将会觉得非常奇怪。这就是熵和时间之箭。熵因为无序程度的增加而增加。  所以，波尔兹曼了解这一点并且他解释了熵是如何与时间之箭相关的。不过他的解释还缺少了一块，即为什么熵最初总是处在一个开始向上增长的低水平起点？为什么宇宙中的文件最初总是整整齐齐的？基本上，我们可观测的宇宙是从大约137亿年前的一种精致秩序（异常低的熵）状态中开始的。比方说，宇宙就像一个发条玩具，在137亿年前扭动发条上足了劲，那么最终将逐渐平息下来。但是为什么它一开始总是上足发条的呢？为什么它处于这么奇怪的、低熵的、不寻常的状态？ 这就是我现在正试图解决的问题。我试着去理解宇宙学（cosmology），为什么大爆炸（the Big Bang）理论能够解释宇宙的性质？同时这样思考也是很有趣的：即直接回到我们的厨房，我们如何能够做煎蛋？我们如何能够记住时间的方向？为什么会产生“先前效应”（precede effects）？为什么我们出生时年轻并慢慢变老？所有这一切都是因为熵的增加，都是因为有大爆炸。 连线：所以大爆炸开启了这一切。但是你推导出在大爆炸之前还存在某种东西。某种东西制造了大爆炸。它们又是什么呢？   卡罗尔：如果你在冰箱里发现一个鸡蛋，你不会觉得奇怪。你不会说，“哇，一个低熵形态。它真罕见。”因为你知道宇宙中并不是只有这一个鸡蛋。它是母鸡生下的，它是饲养场的一部分，它是生物圈的一部分，等等。但是对于宇宙，我们无法做这些联想思考。我们无法说出宇宙是其他什么东西的一部分。但是这正是我要表达的。我与现代宇宙学的一系列观点相一致，现代宇宙学说我们观察到的宇宙并非现存的这些，它只是更大的多元宇宙（multiverse）的一部分。大爆炸并不是宇宙的开端。 如果这种观点正确的话，那将改变你想提的问题。如果不正确，你的问题将变成“为什么宇宙以低熵状态为开端？”如果它正确，那你的问题将变成“为什么多元宇宙的一部分宇宙要经历一个低熵的阶段？”而且如果真正确的话，你的问题将更容易回答。   多元宇宙示意图/肖恩·卡罗尔 连线：在这个多元宇宙理论里，你假设在中间有一个静止的宇宙。从那里开始，一些小宇宙从中脱离，并且向不同的方向（或说时间之箭）延伸。所以这是否意味着处于中间的多元宇宙中不存在时间？ 卡罗尔：所以这是一个值得刻画的区别。在宇宙的历史中有许多不同的时刻，时间能告诉你哪一个时刻是你正在谈论的。然后就有了时间之箭，它给了我们过程的感觉，向时间流动或移动的感觉。所以这个处于中间的静止的多元宇宙具有作为坐标的时间，但是没有时间之箭。没有相对于过去的未来，每一个时刻都是彼此平等的。 连线：所以那是一个我们无法理解无法感知的时间？  卡罗尔：我们能够测量它，但是你无法感觉到它。你将不可能体验到它。因为像我们一样的客体将无法在那个环境中存在。因为我们依赖时间之箭正是为了我们的存在。   连线：那么，在那个宇宙中的时间又是什么呢？   卡罗尔：即便是在真空的宇宙里，时间和空间仍然存在。物理学家回答这样的问题是完全不成问题的：即“如果一棵树倒在树林里并且那里没有人听见声音，那么树倒下发出声音了吗？”他们回答说，“有！当然发出声音了！”同样地，如果时间在没有熵的情况下流逝并且没有人在那个宇宙里体验到它，那里仍然还存在时间吗？是的，仍然存在时间。这依然是自然基本定律的一部分，甚至是关于宇宙的基本定律的一部分。这正是没有因果性，没有记忆，没有进程，没有年龄老化或新陈代谢或像这样的任何事物的空宇宙里发生的事情。那是一个任意的波动。   连线：如果这个处于中间的宇宙只是在那里，而什么也没有发生；那么具有时间之箭的这些小宇宙究竟如何从中间的多元宇宙中脱离呢？因为这似乎是一个可衡量的事件。   卡罗尔：完全正确。这是一个很好的观点。而就是，那里几乎什么也没发生。所以我试图建立的观点的重点就是“为什么我们看见我们身边的宇宙时刻在变化？”这个问题的答案就是宇宙不可能真正永远保持静止。宇宙不可能存在这样一个状态，即永远停留在那里的状态。如果存在，我们就应当居住在那个状态里并且永远呆在那里。   具有时间之箭的小宇宙从中间的多元宇宙上脱离就像一个球从山上滚下来，但是山下没有尽头。这个球始终在过去与未来之间滚动。于是，处于中间部分的多元宇宙在原地保持静止——那个中间的小区域似乎什么也没发生。但是，根据量子力学（quantum mechanics），事件能够偶然地发生。中间的宇宙也可能从任意的波动转变成存在。这种改变发生的可能性还是存在的。   所以，我认为宇宙类似于原子核（atomic nucleus）。它不是完全稳定的，而是有一个半衰期（half-life），它将按半衰期进行衰变。如果你看着它，它看起来绝对稳定，没有任何事情发生；看着看着，然后嘣地一声发生衰变。突然有一个阿尔法粒子（alpha particle）从原子核里出来，而从中间的多元宇宙里出来的是具有时间之箭的小宇宙。   连线：那些新产生的、沿着时间之箭前进的不同小宇宙里，存在物理定律不同的地方，比如时空异常。那么时间之箭在那些地方还仍然存在吗？   卡罗尔：这可能。关于时间之箭奇怪的事情是在物理学的基本定律中并没有发现它，它不在那里。于是时间之箭只是我们能看见的宇宙的属性，而不是某个不同粒子的定律的属性。所以说，时间之箭建立在任何适用当地物理定律的地方。   连线：如果时间之箭是基于我们的意识和感知它的能力，那么像你这样更充分地理解它的人与我们其他人相比是否有不同的时间体验？   卡罗尔：事实上不是。它工作的方式是知觉先产生，然后才是对它的理解。所以理解并不会改变先前的知觉，理解只是帮助你将知觉放进更广泛的背景中。我的里有一句圣奥古斯汀（St. Augustine）的名言，他说的大意是这样的，“直到你问我时间的定义，我才知道时间是什么，不过我无法给你答案。”所以我认为我们所有人知觉时间推移的方式非常类似。然后试图去理解它并不会改变我们的知觉。   连线：那么在像黑洞（blackhole）或者高速运动以致我们对它们的知觉已经改变的地方，那里的时间之箭发生了什么呢？   卡罗尔：这就回到了爱因斯坦的相对论。对任何一个在时空中移动的人而言，他们自身以及他们带在身上的时钟——包括他们的生物钟（biological clock），比如心脏和心理认知——没有任何一个会感觉时间走得快一点或者慢一点。或者，无论如何，如果你身上带着精确的时钟，你的时钟将始终每秒钟走动一下（不快不慢）。无论你是在黑洞里，现在在地球上，还是在那个处于中间的什么也没发生的多元宇宙，都没关系，都会是这样。但是爱因斯坦告诉我们的是你通过空间和时间的途径能够戏剧性地影响你感觉在流逝的时间。   时间之箭是关于方向的，而不是关于速度的。一个重要的事实是有一个一致的方向（速度则不一致）。在时空中的任何地方，都是过去和未来这一方向。   连线：所以你将告诉迈克尔·福克斯（Michael J. Fox，加拿大男演员，主演《回到未来》电影三部曲），回到过去去拯救他的家人是不可能的？   卡罗尔：走出时光旅行的迷雾最简单的方式是宣布那不可能做到。这很可能是正确答案。然而，我们仍然不能确信。我们无法绝对地证明这不可能发生。   连线：至少，你不可能回到过去。   卡罗尔：嗯，回不去。你可以很容易进入未来，这是没有问题的。   连线：我们现在就正在朝未来走去！   卡罗尔：昨天，我向未来（现在这个时刻）走去。   我在我的书中指出的一点是如果我们想象回到过去是可能的，假设，回到了过去，将会引起的所有矛盾都最终追溯到一个事实，即如果你能够回到过去你将无法定义一个方向一致的时间之箭。因为那时你认为是未来（你已经经历过）的时刻在宇宙的“过去”里。所以不可能有一个时刻在任何地方都相同。同时这也与物理定律不相符，而且更与我们的日常经验不相符；日常生活中我们能够作出抉择来影响未来，但是我们无法作出抉择来影响过去。   连线：所以，多元宇宙理论的一部分是我们自己身处的宇宙最终将变成空的、静止的宇宙。那么是否意味着我们自己身处的宇宙最终将会脱离出另一个宇宙？   卡罗尔：时间之箭不会永远前进。在宇宙的历史中存在一个阶段，那时你从低熵走入高熵。但然后一旦你到达这个宇宙的最大熵，你将不可能再前进，再没有时间之箭了。这就像房间。如果你抽走房间里的所有空气，并把这些空气放在一个角落里，那么这个角落就处于低熵状态。然后你释放角落里的空气，最终空气将充满整个房间；最后将停止下来，房间里的空气再不会有什么变化。当还在变化的时候，存在时间之箭。但一旦你达到均衡点时，那么时间之箭将停止存在。最后，理论上，新的宇宙脱离。   连线：所以在我们的后面有无数的宇宙，在前面也有无数的宇宙。这是否意味着我们能够前进到在我们前面的那些宇宙里去拜访他们？   卡罗尔：我认为不可能，但是我也不知道。事实上，我在加州理工大学（Caltech）有一个博士后，他对宇宙间的互相碰撞很感兴趣。现在我们把它们都称为宇宙。但事实上，说实话，它们是拥有不同当地情况的空间的区域，而并非形而上学地彼此不同。它们很遥远。你可能可以想象不同的宇宙相互碰撞并留下痕迹和可观察的效应。也可能这一切不会发生。或者如果它们在某个地方，我们也不可能得到任何它们在那里的信号。如果这是真的话，要是这些描述变得有意义，唯一的方式是你假设多元宇宙不是一种理论，而是某种理论的预测结果。       如果你认为你真正理解重力法则和量子力学，真正很好地理解，那么你可以说，“根据这些定律，宇宙是经脱离而存在的。即使我无法观察到它们，这也是我的理论的一个预测结果，同时我已经使用其他方法测试了这些理论。”我们现在还无法到达那里。我们还不知道如何建立一个好的理论，以及如何测试理论。但是我们展望的课题将提出一个关于量子引力的良好理论，在我们的宇宙中进行测试，然后严谨地预测我们无法观测到的事物。 时间是什么？物理学家探究其终极理论   英文原文：   What Is Time? One Physicist Hunts for the Ultimate Theory Read More http://www.wired.com/wiredscience/2010/02/what-is-time/#ixzz0gtJMrqnn SAN DIEGO — One way to get noticed as a scientist is to tackle a really difficult problem. Physicist Sean Carroll has become a bit of a rock star in geek circles by attempting to answer an age-old question no scientist has been able to fully explain: What is time? Sean Carroll is a theoretical physicist at Caltech where he focuses on theories of cosmology, field theory and gravitation by studying the evolution of the universe. Carroll’s latest book, From Eternity to Here: The Quest for the Ultimate Theory of Time, is an attempt to bring his theory of time and the universe to physicists and nonphysicists alike. Here at the annual meeting of the American Association for the Advancement of Science, where he gave a presentation on the arrow of time, scientists stopped him in the hallway to tell him what big fans they were of his work. Carroll sat down with Wired.com on Feb. 19 at AAAS to explain his theories and why Marty McFly’s adventure could never exist in the real world, where time only goes forward and never back. Wired.com: Can you explain your theory of time in layman’s terms? Sean Carroll: I’m trying to understand how time works. And that’s a huge question that has lots of different aspects to it. A lot of them go back to Einstein and spacetime and how we measure time using clocks. But the particular aspect of time that I’m interested in is the arrow of time: the fact that the past is different from the future. We remember the past but we don’t remember the future. There are irreversible processes. There are things that happen, like you turn an egg into an omelet, but you can’t turn an omelet into an egg. And we sort of understand that halfway. The arrow of time is based on ideas that go back to Ludwig Boltzmann, an Austrian physicist in the 1870s. He figured out this thing called entropy. Entropy is just a measure of how disorderly things are. And it tends to grow. That’s the second law of thermodynamics: Entropy goes up with time, things become more disorderly. So, if you neatly stack papers on your desk, and you walk away, you’re not surprised they turn into a mess. You’d be very surprised if a mess turned into neatly stacked papers. That’s entropy and the arrow of time. Entropy goes up as it becomes messier. So, Boltzmann understood that and he explained how entropy is related to the arrow of time. But there’s a missing piece to his explanation, which is, why was the entropy ever low to begin with? Why were the papers neatly stacked in the universe? Basically, our observable universe begins around 13.7 billion years ago in a state of exquisite order, exquisitely low entropy. It’s like the universe is a wind-up toy that has been sort of puttering along for the last 13.7 billion years and will eventually wind down to nothing. But why was it ever wound up in the first place? Why was it in such a weird low-entropy unusual state? That is what I’m trying to tackle. I’m trying to understand cosmology, why the Big Bang had the properties it did. And it’s interesting to think that connects directly to our kitchens and how we can make eggs, how we can remember one direction of time, why causes precede effects, why we are born young and grow older. It’s all because of entropy increasing. It’s all because of conditions of the Big Bang. Wired.com: So the Big Bang starts it all. But you theorize that there’s something before the Big Bang. Something that makes it happen. What’s that? Carroll: If you find an egg in your refrigerator, you’re not surprised. You don’t say, “Wow, that’s a low-entropy configuration. That’s unusual,” because you know that the egg is not alone in the universe. It came out of a chicken, which is part of a farm, which is part of the biosphere, etc., etc. But with the universe, we don’t have that appeal to make. We can’t say that the universe is part of something else. But that’s exactly what I’m saying. I’m fitting in with a line of thought in modern cosmology that says that the observable universe is not all there is. It’s part of a bigger multiverse. The Big Bang was not the beginning. And if that’s true, it changes the question you’re trying to ask. It’s not, “Why did the universe begin with low entropy?” It’s, “Why did part of the universe go through a phase with low entropy?” And that might be easier to answer. Wired.com: In this multiverse theory, you have a static universe in the middle. From that, smaller universes pop off and travel in different directions, or arrows of time. So does that mean that the universe at the center has no time? Carroll: So that’s a distinction that is worth drawing. There’s different moments in the history of the universe and time tells you which moment you’re talking about. And then there’s the arrow of time, which give us the feeling of progress, the feeling of flowing or moving through time. So that static universe in the middle has time as a coordinate but there’s no arrow of time. There’s no future versus past, everything is equal to each other. Wired.com: So it’s a time that we don’t understand and can’t perceive? Carroll: We can measure it, but you wouldn’t feel it. You wouldn’t experience it. Because objects like us wouldn’t exist in that environment. Because we depend on the arrow of time just for our existence. Wired.com: So then, what is time in that universe? Carroll: Even in empty space, time and space still exist. Physicists have no problem answering the question of “If a tree falls in the woods and no one’s there to hear it, does it make a sound?” They say, “Yes! Of course it makes a sound!” Likewise, if time flows without entropy and there’s no one there to experience it, is there still time? Yes. There’s still time. It’s still part of the fundamental laws of nature even in that part of the universe. It’s just that events that happen in that empty universe don’t have causality, don’t have memory, don’t have progress and don’t have aging or metabolism or anything like that. It’s just random fluctuations. Wired.com: So if this universe in the middle is just sitting and nothing’s happening there, then how exactly are these universes with arrows of time popping off of it? Because that seems like a measurable event. Carroll: Right. That’s an excellent point. And the answer is, almost nothing happens there. So the whole point of this idea that I’m trying to develop is that the answer to the question, “Why do we see the universe around us changing?” is that there is no way for the universe to truly be static once and for all. There is no state the universe could be in that would just stay put for ever and ever and ever. If there were, we should settle into that state and sit there forever. It’s like a ball rolling down the hill, but there’s no bottom to the hill. The ball will always be rolling both in the future and in the past. So, that center part is locally static — that little region there where there seems to be nothing happening. But, according to quantum mechanics, things can happen occasionally. Things can fluctuate into existence. There’s a probability of change occurring. So, what I’m thinking of is the universe is kind of like an atomic nucleus. It’s not completely stable. It has a half-life. It will decay. If you look at it, it looks perfectly stable, there’s nothing happening … there’s nothing happening … and then, boom! Suddenly there’s an alpha particle coming out of it, except the alpha particle is another universe. Wired.com: So inside those new universes, which move forward with the arrow of time, there are places where the laws of physics are different — anomalies in spacetime. Does the arrow of time still exist there? Carroll: It could. The weird thing about the arrow of time is that it’s not to be found in the underlying laws of physics. It’s not there. So it’s a feature of the universe we see, but not a feature of the laws of the individual particles. So the arrow of time is built on top of whatever local laws of physics apply. Wired.com: So if the arrow of time is based on our consciousness and our ability to perceive it, then do people like you who understand it more fully experience time differently then the rest of us? Carroll: Not really. The way it works is that the perception comes first and then the understanding comes later. So the understanding doesn’t change the perception, it just helps you put that perception into a wider context. It’s a famous quote that’s in my book from St. Augustine, where he says something along the lines of, “I know what time is until you ask me for a definition about it, and then I can’t give it to you.” So I think we all perceive the passage of time in very similar ways. But then trying to understand it doesn’t change our perceptions. Wired.com: So what happens to the arrow in places like a black hole or at high speeds where our perception of it changes? Carroll: This goes back to relativity and Einstein. For anyone moving through spacetime, them and the clocks they bring along with them – including their biological clocks like their heart and their mental perceptions – no one ever feels time to be passing more quickly or more slowly. Or, at least, if you have accurate clocks with you, your clock always ticks one second per second. That’s true if you’re inside a black hole, here on Earth, in the middle of nowhere, it doesn’t matter. But what Einstein tells us is that path you take through space and time can dramatically affect the time that you feel elapsing. The arrow of time is about a direction, but it’s not about a speed. The important thing is that there’s a consistent direction. That everywhere through space and time, this is the past and this is the future. Wired.com: So you would tell Michael J. Fox that it’s impossible for him to go back to the past and save his family? Carroll: The simplest way out of the puzzle of time travel is to say that it can’t be done. That’s very likely the right answer. However, we don’t know for sure. We’re not absolutely proving that it can’t be done. Wired.com: At the very least, you can’t go back. Carroll: Yeah, no. You can easily go to the future, that’s not a problem. Wired.com: We’re going there right now! Carroll: Yesterday, I went to the future and here I am! One of things I point out in the book is that if we do imagine that it was possible, hypothetically, to go into the past, all the paradoxes that tend to arise are ultimately traced to the fact that you can’t define a consistent arrow of time if you can go into the past. Because what you think of as your future is in the universe’s past. So it can’t be one in the same everywhere. And that’s not incompatible with the laws of physics, but it’s very incompatible with our everyday experience, where we can make choices that affect the future, but we cannot make choices that affect the past. Wired.com: So, one part of the multiverse theory is that eventually our own universe will become empty and static. Does that mean we’ll eventually pop out another universe of our own? Carroll: The arrow of time doesn’t move forward forever. There’s a phase in the history of the universe where you go from low entropy to high entropy. But then once you reach the locally maximum entropy you can get to, there’s no more arrow of time. It’s just like this room. If you take all the air in this room and put it in the corner, that’s low entropy. And then you let it go and it eventually fills the room and then it stops. And then the air’s not doing anything. In that time when it’s changing, there’s an arrow of time, but once you reach equilibrium, then the arrow ceases to exist. And then, in theory, new universes pop off. Wired.com: So there’s an infinite number of universes behind us and an infinite number of universes coming ahead of us. Does that mean we can go forward to visit those universes ahead of us? Carroll: I suspect not, but I don’t know. In fact, I have a postdoc at Caltech who’s very interested in the possibility of universes bumping into each other. Now, we call them universes. But really, to be honest, they are regions of space with different local conditions. It’s not like they’re metaphysically distinct from each other. They’re just far away. It’s possible that you could imagine universes bumping into each other and leaving traces, observable effects. It’s also possible that that’s not going to happen. That if they’re there, there’s not going to be any sign of them there. If that’s true, the only way this picture makes sense is if you think of the multiverse not as a theory, but as a prediction of a theory. If you think you understand the rules of gravity and quantum mechanics really, really well, you can say, “According to the rules, universes pop into existence. Even if I can’t observe them, that’s a prediction of my theory, and I’ve tested that theory using other methods.” We’re not even there yet. We don’t know how to have a good theory, and we don’t know how to test it. But the project that one envisions is coming up with a good theory in quantum gravity, testing it here in our universe, and then taking the predictions seriously for things we don’t observe elsewhere. Images: 1) Artist’s rendition of the multiverse./Jason Torchinsky. 2) Diagram of the multiverse./Sean Carroll. 3) Ken Weingart. Read More http://www.wired.com/wiredscience/2010/02/what-is-time/#ixzz0gtKXKX3L