In the latest poll of our Science fiction, science fact project you told us that you wanted to know what time is. Here is an answer, based on an interview with Paul Davies, a theoretical physicist and cosmologist at Arizona State University and Director of BEYOND: Centre for Fundamental Concepts in Science. Click here to see other articles on time and here to listen to our interview with Davies as a podcast.
在我們最近有關(guān)科學(xué)小說的民意調(diào)查中，科學(xué)事實項目告我我們大家最想知道的是“什么是時間”。經(jīng)過采訪亞利桑那州立大學(xué)的理論物理學(xué)家、宇宙學(xué)家，《超越》雜志主編Paul Davies，我們得到了想要的答案——科學(xué)概念的中心。
Everyone knows what time is. We can practically feel it ticking away, marching on in the same direction with horrifying regularity. Time has enslaved the Western world and become our most precious commodity. Turn it over to the physicists however, and it begins to morph, twist and even crumble away. So what is time exactly?
每一個人都了解什么時間。我們可以實際地感到時間的流逝。時間嚴格的按照它既定的規(guī)律運行，即持續(xù)性與不可逆性。時間一直被西方世界所控制，它是我們最珍貴的財富。但是在物理學(xué)家眼中，時間是可以變形的，它可以扭曲甚至崩潰。時間到底是什么呢？
To many people throughout history time would have been synonymous with the rhythms of nature; the passing of the seasons and the cycles of the celestial bodies. If this idea seems naive today, it's not only because modern clocks are infinitely more accurate time keepers than the celestial bodies ever were. It's also because we've come to think of time as something universal, something that would keep marching on even if all clocks, celestial or man-made, were to stop. The notion of an absolute time, one that's measurable and the same for all observers, was expressed most succinctly by Newton: "absolute, true and mathematical time, of itself, and from its own nature, flows equably without relation to anything external."
在歷史上的，人類認為時間就像自然的旋律，季節(jié)的更替，天體的循環(huán)。這樣的觀點也許在今日看起來相當幼稚，那是因為現(xiàn)代的時鐘可以對時間進行精確的記錄，而天體卻不能。而且我們現(xiàn)在也認為時間是普遍存在的東西，即使所有的時鐘、所有天體，所有人造產(chǎn)品都停止運行，它依然一刻不停地流逝。絕對時間是可以被測量的，而且他對所有的觀察者都是一樣的。這一切被牛頓用簡單的概念詮釋為：“絕對、真實及數(shù)學(xué)的時間本身，從其性質(zhì)來說，均勻流逝與此外的任何事物無關(guān)。”
Einstein's time
愛因斯坦的時間
Newton's absolute time may feel like an accurate description of the beast that rules our daily lives, but in science the notion was shattered in 1905 by Einstein's special theory of relativity. "Einstein showed that there isn't a universal time," explains Davies. "Your time and my time get out of step with each other if we move differently." In other words, the duration of time between two events can vary depending on how fast you are moving in the period between the events.
牛頓的絕對時間概念野獸般精確地定義著我們每天的生活，但是在愛因斯坦1905年發(fā)表的《狹義相對論》中，這個概念被否定了。“愛因斯坦認為這里并不存在標準時間”，Davies解釋說，“你的時間和我的時間決定于我們的移動速度。”換句話說，兩件事情間隔的時間根據(jù)我們在這兩件事情間的移動速度來決定。
At the root of this strange time warping effect lies Einstein's postulate that the speed of light should be the same for all observers, no matter how fast they are moving. Imagine two observers, one travelling on a train and the other stationary by the side of the tracks. As the two pass each other the traveller emits a pulse of light from a torch shining vertically up. The two observers will disagree on the distance the pulse has travelled when it hits the ceiling of the train, because the stationary observer perceives not just the vertical motion of the pulse, but also the horizontal motion of the train.
實際上，根據(jù)愛因斯坦假設(shè)的時間隧道效應(yīng)，光速在所有觀察者眼中是不變的，無論他們移動的多快。試想兩個觀察者，一個正在火車上旅行，而另一個在火車旁邊靜止。當兩個觀察者垂直發(fā)射一束激光，兩個個觀察者會對激光照射在天花板上移動過的距離產(chǎn)生異議，因為靜止的觀察者認為激光不僅僅只有垂直速度也應(yīng)該有與火車相同水平速度。
Since both observers measure the same speed of light, and since speed is distance per time, this implies that they must also disagree on the time it took the pulse to travel that distance. Time is relative to the observer, or as the physicist Kip Thorne prefers to put it, time is "personal". (For a more detailed description, read the Plus article What's so special about special relativity?.)
因為兩個觀察者測量的光速都是一樣的，因為速度是單位時間內(nèi)前進的距離，這就意味著他們對光束傳播用時定義不同。時間與測量者有關(guān)就像物理學(xué)家Kip Thorne喜歡說的，時間是個人的。（如果你想知道得更多，請閱讀什么是相對論。）
在日常生活中，我們并沒有感覺到這種時間膨脹，但是它并不至于小到無法測量。“如果我從鳳凰城飛到倫敦再返回，然后將我的表與放在辦公室的比較，他們會有十億分之一秒的誤差”戴維斯說。對人類來說這只是很小的一個數(shù)字，但是對于現(xiàn)代的計時技術(shù)，測量如此小的數(shù)字是易如反掌的。
In fact, time dilation has a real impact on the global positioning system (GPS), which many of us have come to rely on for navigating around the world. "The system works with orbiting satellites that are moving very fast," explains Davies. "If you didn't factor in this time distorting effect of motion, then your GPS would very quickly begin to accumulate errors so that in an hour or two you'd be lost. So this is a real effect, not just some sort of mad mathematician's nightmare."
事實上，時間膨脹對于我們大多數(shù)人都依賴的全球定位系統(tǒng)有著真實的影響。這套系統(tǒng)所使用的衛(wèi)星運行速度非常高，Davies解釋說，如果你沒有把高速移動對時間的影響考慮進去，你們的GPS系統(tǒng)將很快地堆積錯誤，并且在一到兩個小時中你就會迷路。所以時間膨脹是真實存在的，而不是某些瘋狂的數(shù)學(xué)家的臆想。
But motion isn't the only thing that can distort time. In his general theory of relativity, published in 1916, Einstein showed that gravity too can slow time. Rather than thinking of gravity as an invisible force that wafts across the ether, Einstein thought of it as the effect of massive bodies distorting the very fabric of space. A famous analogy is that of a bowling ball sitting on a trampoline, which creates a dip that a nearby marble will roll into. According to general relativity, massive objects like stars and planets warp space in a similar way, and thus "attract" other bodies that pass nearby. However, Einstein realised that time and space are inextricably linked in what he called spacetime, so the warping effect of gravity does not just effect space, but also time.
其實，運動不是唯一可以是時間膨脹的東西。在他1916年發(fā)表的廣義相對論中，愛因斯坦證明了重力可以減慢時間。愛因斯坦認為，重力不僅僅是一種在宇宙中不可見的力，具有巨大質(zhì)量的物體可以很大程度上扭曲時空。這里有一個著名的例子，一個保齡球放在彈簧床上，如果在床上扔一塊大理石，大理石會是床產(chǎn)生下陷，于是球會滾進去。根據(jù)廣義相對論，高質(zhì)量的物體例如行星，它們可以以相同的方式扭曲宇宙，同時吸引周圍的其他星體。但是，愛因斯坦意識到，時間和空間是無法分割的一體，所以他稱它們?yōu)闀r空，重力的翹曲效應(yīng)不僅作用于空間，也作用于時間。
"Gravity slows time, so that it runs a little bit slower in the basement of your house than it does on the roof," says Davies. "It's a tiny effect, but it can be measured, even on distances that are that small. But if you want a seriously big time warp from gravity, you have to go where there's a very big gravitational field. If you had a clock on the surface of a neutron star, for example, it would tick at about 70% of the rate of a clock on Earth. The ultimate time warp is at the surface of a black hole, where in a sense time stands still relative to our time. If you went there, you wouldn't notice anything peculiar about time, but if you compared clocks between the two locations, they'd be enormously out of step."
“重力影響了時間，所以時間在你家地下室里要比在房頂上慢一點。” Davies說，“這種影響很小，但是仍然可以測量，即使在距離上來說它們很微小。但假如你希望重力對時間有巨大的影響，那么你就不得不去一個有著極大的重力場的地方。舉個例子，假如你在中子星的表面有一個時鐘，那么它的運行速度只有在地球的70%。最終極的時間扭曲發(fā)生在黑洞的表面，在那里從某種意義上說時間仍是我們的時間。如果你到了那里，你不會感覺到時間有什么特殊，但是如果你比較一下兩地的時鐘，它們會有相當巨大的不同。”
Einstein drew an interesting conclusion from his results about the nature of time. In a letter to the family of a recently deceased friend, Michele Besso, Einstein wrote, "... for us physicists believe the separation between past, present, and future is only an illusion, although a convincing one." Since time is relative to the observer, it is impossible to divide it up into past, present and future in a way that is universally meaningful. In some sense, past, present and future are all there at once.
愛因斯坦根據(jù)他的結(jié)果得出了一個有關(guān)自然時間的有意思的結(jié)論。在一封給最近去世的朋友Michele Besso家人的信中愛因斯坦寫到：“對于我們物理學(xué)家來說，過現(xiàn)未來的區(qū)分只是一個幻象，盡管看起來很真實。”因為時間與觀察者相關(guān)，所以我們就不能用某種即使普遍認為是有意義的方法將時間分成過去，現(xiàn)在，未來。從某種程度上說，過去，現(xiàn)在，未來都在同一時刻同一地點。
"This notion is sometimes called block time, but I like to call it the timescape because it's a bit like a landscape," says Davies. "If you look at a map, the whole of the landscape is there before you, all at once. If you add time as the fourth dimension on this map, then all of time is there at once too. The fact that nothing in physics singles out a particular 'now' is a mystery."
“這種觀點有時被叫做塊時間（block time），但是我喜歡叫它時間柱（timescape），因為這有一點像風(fēng)景，”Davies 說，“如果你在看一個地圖，那么整個風(fēng)景就是擺在你面前的地圖此刻的全貌，假如你在地圖上定義了第四維度——時間，那么全部時間都在這里。實際上，在物理學(xué)中，‘此刻’仍然是一個迷。”
Incidentally, there is nothing in Einstein's theory that prohibits time travel, be it into the future or into the past. But this is a can of worms we won't open here, as you can read about it in Kip Thorne's Plus article Is time travel allowed? (or read Davies book How to build a time machine).
順便一說，在愛因斯坦的理論中沒有什么能阻止時間旅行，通過它可以走向未來也能回到過去。但是最麻煩的卻是我們沒法開啟時間旅行，你可以讀一讀Kip Thorne 的文章Is time travel allowed。
The arrow of time
時間箭頭
Thinking of past and future brings us to another problem that has foxed scientists and philosophers: why time should have a direction at all. In every day life it's pretty apparent that it does. If you look at a movie that's being played backwards, you know it immediately because most things have a distinct time direction attached to them: an arrow of time. For example, eggs can easily turn into omelettes but not the other way around, and milk and coffee mix in your cup but never separate out again.
過去和未來帶給我們的另一個問題已經(jīng)被科學(xué)家和哲學(xué)家解決了：為什么時間總是朝著一個方向流逝。在生活的每一天，時間箭頭總是很明顯地指向他的方向。假如你讓電影倒帶播放，你會發(fā)現(xiàn)任何事情都帶有明顯的時間方向性質(zhì)，就像有東西拉著他們：時間箭頭。例如，雞蛋很容易變成雞蛋餅，但是卻不能變回雞蛋?？Х群团Ｄ淘诒又谢旌?，但是卻不能再一次把他們分開。
The most dramatic example is the history of the entire Universe, which, as scientists believe, started with the Big Bang around thirteen billion years ago and has been continually expanding ever since. When we look at that history, which includes our own, it's pretty clear which way the arrow of time is pointing.
最明顯的例子就是宇宙的進化史，正如科學(xué)家所相信的，宇宙開始與130億年前的大爆炸，從那時起宇宙開始不斷膨脹。當我們審視這段包括我們自己歷史的時候，我們可以很清楚的看到時間箭頭所指的路。
"But the mystery is that the laws of physics show no preference for forward time or backward time," says Davies. For example, if you can make an object move one way by applying a force, then, as Newton's second law of motion tells you, you can make it retrace its path by applying the same force in the opposite direction. So when you watch a movie of this process you wouldn't be able to tell if it's being played forwards or backwards, as both are equally possible.
“但是這個迷作為物理定律置于未來和過去中間。”Davies說。例如，如果你通過一定的力是的某個物體向前移動，同時就如牛頓第二定律所描述的，你可以是把這個物體放回原位置通過在相反的方向上用相同的力量。所以當你看在電影中看到這個過程的時候，你無法分辨這是在正常播放還是在倒序播放。因為這兩種情況是等可能性的。
"So the problem is how to account for the asymmetry of time in daily life when the laws that govern all the atoms that make up everything around us are symmetric in time," says Davies. Much has been made of this problem, which affects Einstein's physics just as it did Newton's classical description of the world.
Davies說：“現(xiàn)在的問題是如何在每天的生活中解釋時間的非對稱性的，尤其是這定律管理著所有原子，并將它們組成我們身邊的一切，但我們身邊的一切在時間中都是對稱的。”這個問題很大程度上影響了愛因斯坦的物理學(xué)說就像他影響牛頓傳統(tǒng)物理學(xué)對世界的描述一樣。
But the answer isn't all that difficult to find. Most processes we feel are irreversible in time are those that (for whatever reason) start out in some very special, highly ordered state — Davies uses a pack of cards as an example. When you first open up a new pack the cards will be ordered according to suit and numerical value. When you shuffle them for a while they will become disordered, so it seems that, as time passes, things will always move from order to disorder. "We might think that this is very strange because there is nothing in the act of shuffling that chooses a direction in time, yet we see a distinct arrow," says Davies.
但是，問題的答案并不難以尋找。絕大多數(shù)我們感覺在時間上不可逆的過程不論什么原因，他們都開始于非常特別的，高度有序的狀態(tài)。Davies 用了一盒撲克給我們舉例子。當你剛剛打開一盒新的撲克牌是，他們按照花色和數(shù)量排好了順序。當你洗牌的時候他們就會被打亂，所以看起來就是時間流逝，某些東西從有序變成無序。“我們可能會想，這種現(xiàn)象非常奇怪，因為在洗牌的時候我們沒有任何動作去選擇方向，但是我們可以看到時間箭頭。”Davies如是說。
However, there is nothing in the laws of physics that prevents the act of shuffling from producing a perfectly ordered set of cards. It's just that the ordered state is only one of a total of around 8 × 1067 possible states, so the chance that we come across it while shuffling the cards is vanishingly small. So small that it would never happen even within several lifetimes of shuffling.
但是，這里也沒有某種物理定律使得洗牌不能使撲克完美有序。完美的排序只是8 × 1067種可能性中的一種狀態(tài)，所以當我們洗牌的時候，我們也是有機會把撲克完美排序的，雖然這樣的機會近乎為零。所以這么小的幾率也許我們洗上幾輩子的牌也不一定能碰得到。
So the apparent asymmetry of time is really just an asymmetry of chance. Systems of many components — like a cup full of milk and coffee particles or a bowl full of egg particles — evolve from order to disorder not because the reverse is impossible, but because it's highly unlikely. This, in a nutshell, is the second law of thermodynamics, which states that the entropy (a measure of the disorder) in a closed physical system never decreases. It's a statistical principle, rather than a fundamental law describing the behaviour of individual atoms. The apparent arrow of time emerges as a property of the macroscopic system, but it's not there in the laws that govern the individual particle interactions. As the physicist John Wheeler put it, "If you ask an atom about the arrow of time, it will laugh in your face."
所以表面上對稱的時間實際上卻是不對稱的幾率。一個由許多部分組成的系統(tǒng)，例如一杯混合了牛奶的咖啡，盛滿蛋清的杯子，他們從有序變成無序不是因為相反的過程不可能，而是因為概率很小。簡單來說，熱力學(xué)第二定律表明在一個封閉的系統(tǒng)中，熵是不會降低的。這是統(tǒng)計學(xué)原理，而不是描述單個原子行為的基本物理法則。很顯然時間箭頭成為了宏觀系統(tǒng)的一種性質(zhì)，但沒有定律來控制單個原子間的相互作用。物理學(xué)家John Wheeler說：“如果你問一個原子時間之箭在哪里，它就會當面笑話你。”
This also applies to the whole Universe. "The Universe started out very smooth and expanding uniformly ," says Davies. From a gravitational view point the Big Bang was a low entropy state and the Universe has been increasing its entropy ever since, hence the arrow of time. The question now is why the Universe started in the way it did. "Why our Universe went bang in such an ordered state is still a mystery," says Davies. "There is no agreed answer to that, partly because there is no agreed model of cosmology. We all think the Universe began with a Big Bang and we know it's expanding. What we don't know is if the Big Bang is the ultimate origin of time or whether there was a time before that." (Read the Plus article What happened before the Big Bang? for more on this subject.)
這個道理同樣應(yīng)用于宇宙。Davies說：“宇宙的開始與膨脹都很平穩(wěn)。”從重力學(xué)的觀點來看，宇宙大爆炸時其處于低熵的狀態(tài)，從那時開始因為時間箭頭，宇宙的熵一直在增大。問題是為什么宇宙因大爆炸開始。“為什么我們的宇宙從一個有序的狀態(tài)爆炸至今仍然是一個謎，”Davies說，“對于這個一直沒有讓大家共同認同的答案，一定程度上說，至今仍然沒有一個可以令人信服的宇宙模型。我們都知道宇宙從大爆炸開始，也知道他在不斷膨脹。我們不知道的是大爆炸是不是時間的開始或者時間在大爆炸之前就有了。”
Time disappears
時間消失
One thing we have neglected to say so far is that Einstein's theory, which describes the macroscopic world so admirably well, doesn't work for the microscopic world. To describe the world at atomic and subatomic scales, we need to turn to quantum mechanics, a theory that's fundamentally different from Einstein's. Reconciling the two, creating a theory of quantum gravity, is the holy grail of modern physics.
目前為止，有一件事被我們一直忽略，那就是愛因斯坦的理論對于宏觀世界的描述非常完美，但是他的理論在微觀世界就行不通了。為了描述在原子以及原子內(nèi)部量級的世界，我們需要把視線引向量子力學(xué)。量子力學(xué)與愛因斯坦的理論有著本質(zhì)上的不同。融合兩者，我們創(chuàng)造出了量子引力理論，它是當代物理學(xué)的核心。
When Schrodinger and Heisenberg formulated quantum mechanics in the 1920s, they ignored Einstein's work and treated time in Newton's spirit, as an absolute that is ticking away in the background. This already gives us a clue as to why the two theories might be so hard to reconcile. The status of time in quantum mechanics has also created profound problems within the theory itself and has lead to "decades of muddle and subtlety," as Davies puts it.
當薛定諤和海森堡在二十世紀二十年代將量子力學(xué)公式化，他們無視了愛因斯坦的工作，用牛頓的思想對待時間，即時間是在不知不覺中恒定流逝的。這已經(jīng)給我們稍稍解釋了為什么這兩種學(xué)說難以融合的原因。在量子力學(xué)中，時間概念也產(chǎn)生了麻煩的問題，而且導(dǎo)致了“幾十年的混亂和微妙”，就像Davies所說。
We won't go into this muddle here, but we'll note the conundrum that unfolds when you try to apply quantum mechanics to the Universe as a whole (a rather controversial approach not all physicists agree with). "If you try to write down a quantum mechanical description of the whole Universe, you find that the time parameter actually drops out [of the equations], it's not there at all," says Davies. Time is replaced by correlations. "For example, you might have a correlation between the size of the Universe and the value of some [physical] field. We would describe this by saying 'as the Universe evolves over time and gets bigger, so this field changes in value'. We use that language, but actually all you're talking about is a correlation [between physical quantities] and time can be removed completely."
這里，我們不會陷入混亂，但是當你把宇宙看為一個整體（一種極具爭議的方式，并沒有的到所有物理家的贊同）并將量子力學(xué)應(yīng)用于它的時候，我們將提起那些未解之謎。Davies說：“如果你想用量子力學(xué)來描述整個宇宙，你會發(fā)現(xiàn)時間參數(shù)在方程中消失了，它根本不存在。”時間被相關(guān)性代替了。“例如，你可能理解在某些場中宇宙的尺度與重要性的相關(guān)性。我們可以這樣描述‘隨著宇宙的膨脹，這個場的數(shù)量改變了’。我們用這樣的語言描述它，但是事實上你們大多數(shù)在討論物理量的相關(guān)性，所以時間可以被完全移除。
Some people have interpreted this to say that time doesn't exist at all, but Davies disagrees. "I think time exists just as telephones do. It's a real thing and we can measure it. But it does suggest that the way it enters into our description of the world is different from other quantities we're used to."
有些人如此認為時間從未存在過，但是Davies并不同意。“我認為時間是存在的就像手機是存在的。這是真實的東西，我們可以測量他。但是我們現(xiàn)在討論的描述世界的方式與之前我們曾經(jīng)討論過的不同。”
One possibility is that time, and also space, are emergent properties of the Universe, which are not part of the bottom level of reality. "It may be that for the extreme conditions at the Big Bang a description in terms of other variables is more appropriate. When we see the world with a well-defined space and time [or spacetime as Einstein put it] this may just be some particular state of the Universe that has emerged out of the Big Bang." Davies uses a block of rubber as an example: it's got its very own physical properties, its elasticity for example, but these properties aren't there at the atomic level. They are a result of the atoms and the laws that govern them combining in one particular way. Similarly, the Universe, as it cooled down from the Big Bang, may have just happened to give rise to spacetime. Perhaps, if it had cooled down in another way, spacetime wouldn't have come up.
時間或者空間都可能是宇宙的一種自然性質(zhì)，但二者都不是現(xiàn)實的組成。“這可能是對于在宇宙大爆炸時的極端條件下，依靠其他變量進行描述更加合理恰當。當我們發(fā)現(xiàn)世界擁有了完善的時間與空間的定義（或者如愛因斯坦所說稱之為時空），那么它可能處于宇宙特別的狀態(tài)，發(fā)生了大爆炸。”Davies用橡膠塊舉例子：它擁有自身獨特的物理特性，用它的彈性舉例，但是這里并沒有原子尺度的道具。他們是原子結(jié)合體，根據(jù)一定的法則組合在一起。類似的，比如宇宙，當他從大爆炸中冷卻下來，時空產(chǎn)生了?；蛟S，如果他用另一種方式冷卻下來，時空就不會存在了。
But if space and time aren't fundamental, what are the fundamental properties of the Universe? There is no theory that people agree on. "We can invent words to describe them and people have, but these things are not anything we are going to encounter in daily life. So we're just resorting to [mathematical descriptions]. But even if one day we manage to explain time and space in terms of something else, that only pushes the question to another level, because you then have to explain [the something else]."
但假如時間和空間都不是基本性質(zhì)，那么什么是宇宙的基本性質(zhì)呢？關(guān)于此，還沒有能讓大眾信服的學(xué)說。“我們可以創(chuàng)造詞匯來形容他們，但是這些并不會出現(xiàn)在我們的日常生活中。所以我們?nèi)匀灰蕾囉跀?shù)學(xué)描述。但是即使有一天我們希望通過某種東西去解釋時間或者空間，那只不過是把問題推向了另一個層次，因為你還需要解釋你解釋時間用的這些東西是什么。”
So it seems that we're no closer to understanding what time is than Newton was — perhaps we understand it even less. But then, perhaps the job of the scientist isn't to fully explain the Universe, but merely to describe it. "You postulate a theory, usually in the form of mathematical equations, and then you test it against reality," says Davies. "If it does work, you don't argue where those equations come from. It's just your best attempt to describe the world."
現(xiàn)在，我們似乎還沒有想牛頓那樣理解時間，甚至可能我們理解的還要少。不過，科學(xué)家的工作并不都在解釋宇宙是什么，也只有極少數(shù)可以解釋的了它。“你常常是用數(shù)學(xué)方程組來假設(shè)一個理論，然后你檢測他是否與現(xiàn)實相違背，”Davies說，“如果方程組有效，你就不必為方程組解釋方程組來自哪里。這就是你解釋世界最好的嘗試。”
Whether it's fundamental, emergent, or just a set of correlations in disguise, the fact is that something we call time manifests itself undeniably and we all know about it. As a friend of mine put it, "If you want to know what time is, just look at my face."
無論什么是基礎(chǔ)，意外，或者一系列的相關(guān)性，事實上，這些是我們稱作時間的那個東西表現(xiàn)自己的方式，而且我們了解它。像一個朋友對我說的，“如果你想知道時間是什么，那就看看你自己的臉吧。”
