為什麼《星球大戰》絕地武士的光劍是假的
那是一個慵懶的星期天下午，我們看完了《星球大戰》傳奇的《最後的絕地武士》，走齣電影院眯著眼睛適應外面的強光。我們在一棵冷杉樹樹蔭下停下來，轉身觀望從電影院里魚貫而出的人們。
「我實在不能理解你怎麼會享受觀看兩個小時的打鬥，還有那些奇怪的臉孔，」Lu大聲說道，「雖然我覺得雷伊挺酷的！尤其是當她揮舞她那把激光劍……」
「那可不是什麼激光劍！」我忍不住打斷她。
「可他們用它來焊接斷裂的金屬，跟工業激光的用途一樣。它還可以切穿皮肉，就像醫用激光一樣。而且它的顏色鮮艷耀眼，就像在激光表演中看到的那些一樣。那還能是什麼呀？」
不是，不是激光。真正的激光是一束光，跟手電筒很像。我打開我手機上的手電筒應用，並用它指向冷衫樹的根部。我用另一隻手掏出帶有小激光指針的舊鎖匙扣，並指向同一個區域。一個小紅點出現在被手機照亮的圓形區域中間。快看呀，量子奇觀！
「誰都知道激光是光做的，」Lu顯然不為所動，「不過快告訴我：一個小紅點怎麼量子啦？」
光是由叫做光子的粒子組成的。一般的出光口，像手機背後的燈那樣的，光子一旦被釋放就會各走各的路。出於這個原因光束會鋪開並照亮比較寬的一個區域，比出光口要大得多。這個現象叫做擴散，屬於經典物理行為。為方便理解，想像滿滿一桶網球。當你把球往地上倒他們會四處亂跳。另一個例子是電影院的出口，把裡面的人比作光子。一場電影結束後大門打開，人們開始往電影院外走，每個人都朝著自己的方向走去。沒人會知道其他人都往哪裡走了。
激光中的光粒子表現得完全不一樣。回到電影院的那個例子，讓我們想像一個排長為他排里的士兵把一場電影全部包下來了。電影結束之後，排長會指揮士兵們有秩序地離開座位，他們一齣電影院就步伐一致地形成一條整齊的隊列，朝他們的營地踏步前進。激光中的光粒子，跟電影院的士兵一樣，也有秩序地流出，並朝同一個方向前進。出於這個原因激光光束並不會向四周擴散，而只有像出光口大小的一個小點會被照亮。光粒子的排長是量子物理，它告知所有的光粒子該走向哪個方向。這是通過把粒子調到同一個量子狀態來做到的，量子排長非常嚴格，所有的光粒子都會在很長一段距離里保持這個狀態。有多長？如果你拿一個強勁的激光指針指向月亮，你可以肯定只有一個小紅點會出現在月亮上。
「現在我開始明白了。這同你跟我講過的糾纏的骰子差不多。」Lu看來被說服了，「激光光束里的所有光粒子都會一直發生糾纏，因為量子物理的作用。但你還是沒告訴我為什麼電影里的劍不可能是激光做的。」
激光中的量子作用的確跟《如何用量子糾纏在賭博中出千》一文里的骰子的糾纏相似。它使得許多的粒子處於同一個量子狀態並保持如此，直到它們撞到一個不透光的物體，比如說月亮。激光也可以十分強大，用來焊接金屬、切割較軟的物體，因為光子釋放出來的能量持續聚集在較小的區域。那麼，為什麼不用激光光束造一把劍呢？原因之一是激光光束不會停留在半空中，而只會繼續前進直到遇到它不能擊穿的表面。如果我們能夠造出像《星球大戰》的光劍那樣強大的激光，可以切割金屬，它將會到處扎孔，直到用盡所有能量。太空船內的光劍很快會切開船體，所有的人都將飄在太空中。這並不是什麼方便的武器，對吧？在1970年出版的科幻小說《環形世界》出現過一把無限長的「真正的」激光劍。《星球大戰》傳奇裡面的光劍太短了，不可能是激光造的。
還有另外一個原因可以解釋為什麼這些光劍不可能是激光光束造的。其實，它們根本就不可能是光做成的。無論光束有多麼強勁，它都會無任何阻礙地穿過另一個激光束。激光是由糾纏中的光粒子組成的。根據量子力量，光粒子不會跟其它的光粒子發生反應，除非它們處於互相糾纏中。所以，用激光劍比武，看起來會像是一場激光秀表演，五顏六色的光線互相交織，不費吹灰之力。
「是啊，沒我們在電影里看的那麼酷！但是想想我們可以在口袋裡帶個量子裝置，還可以用肉眼看到量子物理的應用，這也挺酷的！」
激光光束並不是量子物理形成的唯一可見現象。你可知道燒著的木炭發出的乍明乍暗的紅光也是量子的？而且梵高把我們現實世界這個不為人所知的一面畫在他的畫里了？在下一篇文章我們將談到這個。
（插畫為作者原創，如需借用必須先得到作者同意。）
（照片來自網路。如涉侵權請馬上告知，我們會第一時間刪除照片。）
----------英文版本 English Version---------
Why Jedis』 Lightsabers Are Fake
On a lazy Sunday afternoon, we left the cinema after watching 「The Last Jedi」 of 「Star Wars Saga」, squinting our eyes in the bright light. We stopped in the shade of a large fir tree, then turned around to watch the people flowing out of the cinema.
「I really cannot see how you can enjoy two hours of battles and weird faces,」 exclaimed Lu, 「even though I find that Rey is very cool! The way she swings around her laser sword…」
「That』s definitely not a laser sword!」 I couldn』t help interrupting her.
「But they used it to wield chunks of broken metal, like industrial lasers are used. And it cuts through flesh like the lasers used in hospitals. And it is in bright shiny colours, like the ones used in laser shows. What else can it be?」
No, not a laser. A laser is in fact a beam of light, much like an electric torch. I turned on the torch app on my smartphone and lit with it the roots of the fir tree. With my other hand I pulled out my old keychain that has a small laser pointer attached to it, and pointed it to the same root. A small red dot appeared in the middle of the round area lit by the phone. Behold the wonders of quantum physics!
「Everybody knows that a laser is made of light,」 said Lu, apparently not impressed, 「but now tell me: what』s so quantum about a little red dot?」
Light is made of particles called photons. In an ordinary light emitter, like the one in the back of the smartphone, once the photons are released, each one will go its own way. For this reason the light beam spreads up and illuminates a wide area, much larger that the emitting device. This phenomenon is called diffusion, and it』s a classical behaviour. To better visualise it, think of a bucket full of tennis balls. If you pour the balls on the floor, they will bounce and scatter all over the place. Another example is the exit of a cinema, with the people inside being the photons. When the door opens at the end of a show, people start flowing out of the cinema, each person heading his/her own way. Nobody knows where all the other people are going.
The particles of light in a laser behave in a very different way. To go back to the image of the cinema, imagine a movie show fully booked by a sergeant for the soldiers of the platoon he commands. When the movie ends, the sergeant will order the soldiers leave the seats in order, and once outside to march at unison in a well ordered column towards their barrack. The particles of light in a laser, like the soldiers in the cinema, also flow out in an orderly way, heading to the same direction. For this reason the laser beam does not spread around and only a dot, as wide as the emitter, will be lit. The sergeant of the particles of light is quantum physics, which informs all the particles about which direction they should take. This is done by tuning the particles to be in the same quantum state, and the quantum sergeant is so strict that the particles of light will keep this state over very long distances. How long? If you point a powerful laser towards the Moon, you can be sure that only a small red dot will appear on the Moon.
「Now I start to understand. It is a similar situation to that of the entangled dice you told me about.」 Lu seemed convinced, 「All the particles of light in the laser beam will just stay entangled, thanks to quantum physics. But you still didn』t tell me why the swords in the movie cannot be made of lasers.」
The quantum effect in lasers is indeed similar to the entanglement of the dice in the article 「Entangled in quantum gambling」. It forces many particles to be in the same quantum state, and stay that way until they hit an opaque object, such as the Moon. Lasers can be very powerful and used to wield metal and cut soft matter because the energy released by the photons remains concentrated in a small surface. So, why not building a sword made of a laser beam? One reason is that the laser beam would not stop in mid-air, but continue until it hits a surface it cannot penetrate. If we were able to build a laser as powerful as a Star Wars lightsaber, capable to cut through metal, it would pierce holes in everything until it exhausts its power. A lightsaber battle inside a spaceship will quickly slice the hull and leave everybody floating in space. Not really a handy weapon, is it? A 「true」 laser sword of infinite length has been featured in a 1970 sci-fi novel called Ringworld. The lightsabers in the Star Wars saga are too short to be made of lasers.
There is another reason why lightsabers cannot be made of laser beams. In fact, they cannot be made of light. No matter how powerful the beam is, it will pass through another laser beam without any resistance. Laser beams are made of entangled particles of light. Light particles do not interact with other light particles, according to quantum mechanics, unless they are entangled. A laser sword fight, therefore, will look much like a laser show with colourful beams of light crossing each other effortlessly.
「Right, not as cool as what we just saw in the movie! But it』s cool to think that we can carry a quantum device in our pocket, and we can see quantum physics at work with our own eyes!」
A laser beam is not the only visible effect quantum physics creates. Did you know that the glow of a burning charcoal is also quantum? And that Van Gogh painted this secret side of our reality in his paintings? We will talk about this in the next article.
(The illustration was originally created by authors of this article. Please ask for permission before using it for any purpose.)
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