為什麼沒有東西能夠超過光速？

We are told that nothing can travel faster than light. This is how we know it is true.

It was September 2011 and physicist Antonio Ereditato had just shocked the world. The announcement he had made promised to overturn our understanding of the Universe. If the data gathered by 160 scientists working on the OPERA project were correct, the unthinkable had been observed.

2011年9月，物理學家安東尼奧伊·雷迪塔托震驚了世界。他宣布的消息將徹底改變我們對宇宙的理解方式。如果參與OPERA項目的160名科學家收集的數據正確的話，說明我們已經成功觀測到了不可能發生的事情。

Particles _ in this case, neutrinos _ had travelled faster than light.

According to Einstein"s theories of relativity, this should not have been possible. And the implications for showing it had happened were vast. Many bits of physics might have to be reconsidered.

這件事就是：粒子(這裡指的是中子)的運動速度超過了光速。

根據愛因斯坦的相對論，這應該是不可能發生的。假如這件事成真，它的影響也十分巨大，許多物理學知識都必須予以重新考慮。

Although Ereditato said that he and his team had "high confidence" in their result, they did not claim that they knew it was completely accurate. In fact, they were asking for other scientists to help them understand what had happened.

雖然伊雷迪塔托和他的團隊稱，他們對自己的研究結果抱有「高度自信」，但他們從未說過自己的結果是完全精確的。事實上，他們還邀請了其他科學家來幫助他們弄清究竟發生了什麼事情。

In the end, it turned out the OPERA result was wrong. A timing problem had been caused by a poorly connected cable that should have been transmitting accurate signals from GPS satellites.

There was an unexpected delay in the signal. As a consequence, the measurements of how long the neutrinos took to travel the given distance were off by about 73 nanoseconds, making it look as though they had whizzed along more quickly than light could have done.

最後他們發現，OPERA項目的結果是錯誤的。由於一處電纜接觸不良，從GPS衛星傳來的信號出現了延遲。結果中子的運動時間縮短了73秒，看上去就好像比光速還快一樣。

Despite months of careful checks prior to the experiment, and plentiful double-checking of the data afterwards, this time the scientists got it wrong. Ereditato resigned, though many pointed out that mistakes like these happen all the time in the hugely complex machinery of particle accelerators.

雖然科學家們在實驗之前進行了好幾個月的細緻檢查，在實驗之後也進行了反覆核查，但這一次，科學家們還是犯了錯誤。雖然很多人指出，在粒子加速器這麼複雜的機器中，這樣的錯誤總會發生，但伊雷迪塔托還是引咎辭職了。

Why was it such a big deal to suggest _ even as a possibility _ that something had travelled faster than light? And are we really sure that nothing can?

為什麼人們都將「某種東西比光速還快」這件事看得這麼嚴重呢？我們真就那麼確定沒有東西能超過光速嗎？

We cannot go as fast as light ↑

Let"s take the second of those questions first. The speed of light in a vacuum is 299,792.458 km per second _ just shy of a nice round 300,000km/s figure. That is pretty nippy. The Sun is 150 million km away from Earth and light takes just eight minutes and 20 seconds to travel that far.

讓我們先來看看第二個問題。真空中的光速是每秒.458公里，約等於每秒30萬公里，速度非常之快。太陽距地球約1.5億公里，光只需要8分20秒就能跑過這段距離。

Can any of our own creations compete in a race with light? One of the fastest human-made objects ever built, the New Horizons space probe, passed by Pluto and Charon in July 2015. It has reached a speed relative to the Earth of just over 16km/s, well below 300,000km/s.

我們造出來的東西能與光速相提並論嗎？新視野號空間探測器是人類造出的速度最快的東西之一，相對地球的運行速度只有每秒鐘16公里，比每秒鐘30萬公里差了一大截。

However, we have made tiny particles travel much faster than that. In the early 1960s, William Bertozzi at the Massachusetts Institute of Technology experimented with accelerating electrons at greater and greater velocities.

Because electrons have a charge that is negative, it is possible to propel _ or rather, repel _ them by applying the same negative charge to a material. The more energy applied, the faster the electrons will be accelerated.

但粒子的速度可以比這快得多。上世紀60年代初，麻省理工學院的威廉·貝托齊開展了一項實驗，不斷給電子加速，使電子的速度越來越快。由於電子帶負電荷，只要使一塊材料帶上同樣的負電荷，就能把電子向前推出去。施加的能量越高，電子的速度也就越快。

You might imagine that you just need to increase the energy applied in order to reach the required speed of 300,000km/s, but it turns out that it just is not possible for electrons to move that fast. Bertozzi"s experiments found that using more energy did not simply cause a directly proportional increase in electron speed.

Instead, he needed to use ever-larger amounts of additional energy to make ever-smaller differences to the speed the electrons moved. They got closer and closer to the speed of light but never quite reached it.

你可能會以為，要想達到每秒鐘30萬公里的速度，只要增加所施加的能量就可以了。但我們發現，電子是不可能達到那麼高的運行速度的。貝托齊的實驗顯示，增加能量之後，電子的運行速度並不會簡單地成比例增加。到了後來，就算施加了大量能量，電子的速度也只能加快一點點。這一速度會不斷接近光速，但永遠無法真正追上光速。

Imagine travelling towards a door in a series of moves, in each of which you travel exactly half the distance between your current position and the door. Strictly speaking, you will never reach the door, because after every move you make you still have some distance still to travel. That is the kind of problem Bertozzi encountered with his electrons.

But light is made up of particles called photons. Why can these particles travel at the speed of light when particles like electrons cannot?

想像一下，你正在朝一扇門走過去，每次走的長度都是你現在和門之間距離的一半。嚴格來說，你永遠也走不到門跟前，因為每走一步之後，你和門之間仍然存在一定距離。貝托齊的電子加速實驗遇到的也是類似的問題。但光也是由一種叫做光子的粒子構成的。為什麼這些粒子就能達到光速，電子之類的粒子就不行呢？

New Horizons visited Pluto in 2015

"As objects travel faster and faster, they get heavier and heavier _ the heavier they get, the harder it is to achieve acceleration, so you never get to the speed of light," says Roger Rassool, a physicist at the University of Melbourne, Australia.

"A photon actually has no mass," he says."If it had mass, it couldn"t travel at the speed of light."

「物體的運動速度越快，它就會變得越重；而物體變得越重，要想加速也就越難，因此你永遠不可能達到光速。」墨爾本大學的一名物理學家羅傑·拉索爾說道，「光子實際上是沒有質量的。如果它有質量，也就不可能以光速運行了。」

Photons are pretty special. Not only do they have no mass, which gives them free reign when it comes to zipping about in vacuums like space, they do not have to speed up. The natural energy they possess, travelling as they do in waves, means that the moment they are created, they are already at top speed.

光子是一種非常特殊的粒子。不僅因為它們沒有質量，讓它們在宇宙這樣的真空中可以無拘無束地自由穿梭，還因為它們根本不需要加速。光的能量藉助波的形式傳播，這意味著從光子誕生的那一刻起，它就已經達到了最高速度。

In fact, in some ways it makes more sense to think of light as energy rather than as a flow of particles, though truthfully it is _ a little confusingly _ both.

事實上，在一些方面把光想像成能量而不是粒子的流動更能被理解，雖然實際上_有點困惑_它兩者都是。

Still, light sometimes appears to travel more slowly than we might expect. Although internet technicians like to talk about communications travelling at "the speed of light" through optical fibres, light actually travels around 40% slower through the glass of those fibres than it would through a vacuum.

不過，光有時似乎傳播得比我們認為的要慢一些。雖然互聯網技術人員喜歡說信息「以光速」在光纖中傳播，但光在光纖的玻璃中傳播的速度其實比在真空中慢40%。

In reality, the photons are still travelling at 300,000km/s, but they are encountering a kind of interference caused by other photons being released from the glass atoms as the main light wave travels past. It is a tricky concept to get your head around, but it is worth noting.

Similarly, special experiments with individual photons have managed to slow them down by altering their shape.

事實上，這些光子的運行速度仍然是每秒鐘30萬公里，但在光波穿過玻璃時，會從玻璃原子中釋放出其它的光子，對之前的光子造成一定干擾。這一點可能很難理解，但值得我們去注意一下。與之類似，科學家在實驗中通過改變光子的形狀，成功減慢了單個光子的速度。

Optical fibers carry information ↑

Still, for the most part it is fair to say that light travels at 300,000km/s. We really have not observed or created anything that can go quite that quickly, or indeed more quickly. There are a few special cases, mentioned below, but before those, let"s tackle that other question. Why is it so important that this speed of light rule be so strict?

不過，在絕大多數情況下，我們還是可以說光速就是每秒30萬公里。我們還未觀察到過、或者造出過能與光速媲美、甚至超過光速的東西。下文中提到了一些特殊的案例，但在此之前，讓我們先來解決另一個問題：為什麼光速這麼重要呢？

The answer lies, as so often in physics, with a man named Albert Einstein. His theory of special relativity explores many of the consequences of these universal speed limits.

One of the important elements in the theory is the idea that the speed of light is a constant. No matter where you are or how fast you are travelling, light always travels at the same speed.

But that creates some conceptual problems.

答案與一位叫做阿爾伯特愛因斯坦的男人有關。他的狹義相對論對這一速度上限引發的許多後果進行了探討。該理論最重要的觀點之一是，光速是一個常量。無論你身在何處，無論你速度多快，光傳播的速度始終保持不變。但這也帶來了一些概念上的問題。

Imagine shining light from a torch up to a mirror on the ceiling of a stationary spacecraft. The light will shine upwards, reflect off the mirror, and come down to hit the floor of the spacecraft. Let"s say the distance travelled is 10m.

想像一下這樣的場景：手電筒的光柱投射到一艘靜止的宇宙飛船的天花板上。光線先是朝上，被鏡子反射回來，然後投射到地板上。假設光線經過的距離為10米。

Now let"s imagine that the spacecraft begins travelling at a hair-raising speed, many thousands of kilometres per second.

When you shine the torch again, the light will still seem to behave as before: it will shine upwards, hit the mirror, and bounce back to hit the floor. But in order to do so the light will have to travel diagonally rather than just vertically. After all, the mirror is now moving quickly along with the spacecraft.

然後再想像一下，宇宙飛船開始以超高速運行，速度為每秒數千、甚至數萬公里。你打開手電筒之後，光線的運動方式看上去和之前一樣：先是往上走，然後被鏡子反射回來，投射到地板上。但由於鏡子此時正和宇宙飛船一起高速運行，要實現這樣的效果，光線的運動軌跡必須傾斜於地面，而不是垂直於地面。

The distance the light travels therefore increases. Let"s imagine it has increased overall by 5m. That is 15m in total, instead of 10m.

因此光線經過的距離比之前增加了。假設這段距離增加了5米，光線經過的總距離就變成了15米，而不是之前的10米。

And yet, even though the distance has increased, Einstein"s theories insist that the light is still travelling at the same speed. Since speed is distance divided by time, for the speed to be the same but the distance to have increased, time must also have increased.

不過，雖然這段距離增加了，根據愛因斯坦的理論，光速仍然是不變的。速度等於距離除以時間，既然速度不變，距離增加，時間應該也增加了才對。

Yes, time itself must have got stretched. That sounds wacky, but it has been proved experimentally.

不錯，時間本身也被拉長了。這聽上去很異想天開，但實驗已經證實了這一點。

Time can slow down or speed up ↑

It is a phenomenon known as time dilation. It means time travels slower for people travelling in fast-moving vehicles, relative to those who are stationary.

這種現象名叫時間膨脹效應。這意味著對於在高速運行的汽車中的人來說，時間過得比靜止時要慢一些。

For example, time runs 0.007 seconds slower for astronauts on the International Space Station, which is moving at 7.66 km/s relative to Earth, compared to people on the planet.

例如，國際空間站相對地球的運動速度是每秒7.66公里，對於宇航員來說，時間比地球上慢了0.007秒。

Things get interesting for particles, like the electrons mentioned above, that can travel close to the speed of light. For these particles, the degree of time dilation can be great.

而套用到粒子身上，事情就更有趣了。比如上文提到的電子，它們可以以接近光速的速度運行。對於這些粒子來說，時間膨脹效應就更明顯了。

Steven Kolthammer, an experimental physicist at the University of Oxford in the UK, points to an example involving particles called muons.

牛津大學的一名實驗物理學家史蒂文科爾斯海默用渺子舉例說明了這一點。

Muons are unstable: they quickly fall apart into simpler particles. So quickly, in fact, that most muons leaving the Sun should have decayed away by the time they reach the Earth. But in reality muons arrive at Earth from the Sun in great numbers. This was something scientists long found difficult to understand.

渺子十分不穩定，很快就會分裂成其它更簡單的粒子。按照它們的衰變速度，大部分渺子在離開太陽之後，等到抵達地球時，就應該已經衰變了才對。但事實上，仍有大批渺子能成功抵達地球。長時間以來，科學家一直對這一點感到大惑不解。

"The answer to this puzzle is that the muons are generated with so much energy that they"re moving at velocities very near the speed of light," says Kolthammer."So their sense of time, if you will, their internal clock, actually runs slow."

「原因是渺子在誕生時的能量極其巨大，因此渺子能夠以接近光速的速度運行，」科爾斯海默說道，「所以對於它們而言，時間其實放慢了不少。」

The muons were "kept alive" longer than expected, relative to us, thanks to a real, natural bending of time.

渺子之所以能「存活」得比我們以為的更久，靠的就是實際存在的、天然的時間彎曲效應。

Light travels from the Sun to Earth ↑

When objects move quickly relative to other objects, their length contracts as well. These consequences, time dilation and length contraction, are both examples of how space-time changes based on the motion of things _ like you, me or a spacecraft _ that have mass.

當物體相對於其它物體的運動速度更快時，它們的長度也會收縮。時間膨脹效應和尺縮效應都是時空根據物體的運動狀態發生改變的例子。比如你，比如我，比如宇宙飛船，物體只要有質量，就會出現這些現象。

Crucially, as Einstein said, light does not get affected in the same way _ because it has no mass. That is why it is so important that all of these principles go hand-in-hand. If things could travel faster than light, they would disobey these fundamental laws that describe how the Universe works.

但愛因斯坦指出，最關鍵的是，光不會受到這些效應的影響，因為光沒有質量。正是因為這一點，這些定律之間的統一才那麼重要。如果有什麼東西的運動速度超過了光速，它們就會與宇宙運作的基本法則相違背。

That sums up the key principles. At this point, we can consider a few exceptions and caveats.

For one thing, while nothing has ever been observed travelling faster than light, that does not mean it is not theoretically possible to break this speed limit in very special circumstances.

但也有一些例外的現象。首先，雖然我們還沒觀察到有什麼東西能超過光速，但這並不意味著，在非常特殊的情況下，理論上是無法打破光速的限制的。

Take, for instance, the expansion of the Universe itself. There are galaxies in the Universe moving away from one another at a velocity greater than the speed of light.

Another interesting situation concerns particles that seem to be expressing the same properties at the same time, no matter how far apart they are.

This is called "quantum entanglement". In essence, a photon will flip back and forth between two possible states at random _ but the flips will exactly mirror the flipping of another photon somewhere else, if the two are entangled.

宇宙膨脹就是一個例子。宇宙中有一些星系，它們從彼此身邊逃離的速度就超過了光速。另一個有趣的例子則與粒子有關。這些粒子無論相隔多遠，似乎都能同時表達出相同的特性。這一現象叫做「量子糾纏」。從本質上來說，光子可以在兩種狀態間隨機轉換，但如果兩個光子之間存在量子糾纏的話，其中一個光子的狀態將恰好與另一處的光子完全相同。

Two scientists each studying their own photon will therefore get the same results at the same time, faster than the speed of light.

However, in both these examples it is crucial to note that no information is travelling faster than the speed of light between two entities. We can calculate the Universe"s expansion, but we cannot observe any faster-than-light objects in it: they have disappeared from view.

As for the two scientists with their photons, while they might achieve the same result simultaneously, they could not confirm the fact with each other any more quickly than light could travel between them.

因此，如果兩名科學家各負責觀察一個光子，他們就能同時得到相同的結果，而這一速度是超過了光速的。不過，在上述兩個例子中，我們必須注意到，信息在兩個實體之間傳播的速度是無法超過光速的。我們可以計算宇宙的膨脹速度，但我們無法在其中觀察到任何超過光速運行的物體，就好像它們從我們的視線中消失了一樣。至於那兩名研究光子的科學家，雖然他們能同時得到相同的結果，但他們向對方確認這一事實的速度也不可能超過光速。

Galaxies are flying away from us ↑

"This gets us out of any problems, because if you are able to send signals faster than light you can construct bizarre paradoxes, under which information can somehow go backwards in time," says Kolthammer.

There is yet another possible way in which faster-than-light travel is technically possible: rifts in space-time itself that allow a voyager to escape the rules of normal travel.

「這讓我們避免了各種棘手的問題，因為如果你發射信號的速度超過光速的話，就可能引發一些詭異的悖論，讓信息在時間上出現了倒退。」科爾斯海默說道。不過，從技術層面來講，還有另一種方法能實現超光速運動：利用時空中本身存在的縫隙，從而避免受到普通運動法則的牽制。

Gerald Cleaver at Baylor University in Texas has considered the possibility that we might one day build a faster-than-light spacecraft. One of the ways to do this might be to travel through a wormhole. These are loops in space-time, perfectly consistent with Einstein"s theories, which could allow an astronaut to hop from one bit of the Universe to another via an anomaly in space-time, a sort of cosmic shortcut.

德州貝勒大學的傑拉德克利佛對製造超光速宇宙飛船的可行性進行了研究。一種方法是穿越蟲洞。時空中存在一些環狀迴路，這與愛因斯坦的理論是完全一致的。宇航員可以利用這些捷徑，從宇宙中的某一處地方直接跳到另一處去。

The object travelling through the wormhole would not exceed the speed of light, but it could theoretically reach a certain destination faster than light could if it took a "normal" route.

But wormholes might not be available for space travel. What if instead you actively distorted space-time in a controlled way, to travel faster than 300,000km/s relative to someone else?

物體在蟲洞中運行的速度不會超過光速，但從理論上來說，它到達目的地的時間的確比光走正常路線所需的時間要短。但我們也許無法利用蟲洞進行空間旅行。那麼，我們能否以某種可控的方式主動使時空發生彎曲，從而使相對的運動速度超過光速呢？

Wormholes would be handy, if they exist ↑

Cleaver has investigated an idea known as an "Alcubierre drive", proposed by theoretical physicist Miguel Alcubierre in 1994. Essentially, it describes a situation in which space-time is squashed in front of a spacecraft, pulling it forward, while space-time behind the craft is expanded, creating a pushing effect.

克利佛對一種名為「曲速引擎」的概念進行了研究，這一概念是理論物理學家米格爾·阿庫別瑞於1994年提出的。從根本上來說，它描述的是這樣一種情境：宇宙飛船前方的時空會收縮，將宇宙飛船向前拉去，而與此同時，飛船後方的時空則會膨脹，產生推動效應。

"But then," says Cleaver,"there"s the issues of how to do that, and how much energy it"s going to take."

In 2008, he and graduate student Richard Obousy calculated some of the energies involved.

"We worked out that, if you assume a ship that"s about 10m x 10m x 10m _ you"re talking 1,000 cubic metres _ that the amount of energy it would take to start the process would need to be on the order of the entire mass of Jupiter."

「但問題是，我們怎樣才能實現這一點呢？實現它又需要多大的能量呢？」克利佛說道。2008年，克利佛和他手下的研究生理查德·奧伯塞對所需的能量進行了計算。「我們發現，假設飛船大小為10米*10米*10米、即總體積為1000立方米的話，光是啟動這一過程所需的能量數量級就與木星的質量相當。」

After that, the energy would have to continue being provided constantly in order to ensure the process did not fail. No-one knows how that would ever be possible, or what the technology to do it would look like.

"I don"t want to be misquoted centuries from now for predicting it would never come about," says Cleaver,"but right now I don"t see solutions."

Faster-than-light travel, then, remains a fantasy at the moment.

而在啟動之後，我們還需要不斷供應能量，保證這一過程不會中斷。沒人知道我們要怎樣才能做到這一點，也沒人知道這需要什麼樣的技術。「我可不想預言說這永遠不可能成真，結果被後人詬病數百年，」克利佛說道，「但就目前而言，我真不知道怎樣才能做到這一點。」因此就現在來說，超光速旅行依然如神話般遙不可及。

But while that may sound disappointing, light is anything but. In fact, for most of this article we have been thinking in terms of visible light. But really light is much, much more than that.

不過先別失望。在本文中，我們考慮的主要是可見光。但事實上，真正的光比這要寬泛得多。

Visible light is only part of the electromagnetic

Everything from radio waves to microwaves to visible light, ultraviolet radiation, X-rays and the gamma rays emitted by decaying atoms _ all of these fantastic rays are made of the same stuff: photons.

從無線電波到微波，再到可見光、紫外線、X射線和原子衰變時釋放的伽馬射線，這些神奇的射線都是由同一種物質組成的——光子。

The difference is the energy, and therefore their wavelength. Collectively these rays make up the electromagnetic spectrum. The fact that radio waves, for instance, travel at the speed of light is enormously useful for communications.

In his research, Kolthammer builds circuitry that uses photons to send signals from one part of the circuit to another, so he is well placed to comment on the usefulness of light"s awesome speed.

它們之間的區別在於能量和波長的不同。這些射線加起來，就構成了完整的電磁光譜。無線電波能以光速傳播，這對於通訊的用處非常巨大。科爾斯海默在他的研究中搭建了一個電路系統，用光子從電路的一部分向另一部分發射信號。因此他在光速的用途上很有發言權。

"The idea that we"ve built the infrastructure of the internet for example and even before that, radio, based on light, certainly has to do with the ease with which we can transmit it," he points out.

He adds that light acts as a communicating force for the Universe. When electrons in a mobile phone mast jiggle, photons fly out and make other electrons in your mobile phone jiggle too. It is this process that lets you make a phone call.

「現在的互聯網和以前的無線電都是這樣的例子，光速為我們提供了巨大的便利。」他指出。科爾斯海默還補充說，光在宇宙中還起到了溝通的作用。當一部手機中的電子振動時，便會釋放出光子，讓另一部手機中的電子也開始振動。你打電話的時候，就會經歷這樣的過程。

The jiggling of electrons in the Sun also emits photons _ at fantastic rates _ which, of course, produces the light that nourishes life on Earth.

Light is the Universe"s broadcast. That speed _299,792.458 km/s _ remains reassuringly constant. Meanwhile, space-time is malleable and that allows for everyone to experience the same laws of physics no matter their position or motion.

太陽中的電子振動時也會釋放出光子，正是它們產生的光線孕育了地球萬物。光就像宇宙中的廣播節目。光速為每秒鐘.458公里，這一速度始終保持不變。並且，時空還具有延展性，無論人們身在何方，無論他們正處於怎樣的運動狀態，每個人都遵循著相同的物理法則。

Who would want to travel faster than light, anyway? The show it puts on is just too good to miss.

不過，誰會願意運動得比光速還快呢？那場景一定太美，讓人不容錯過。

By Chris Baraniuk