星球大战的死忠粉都梦想拥有绝地武士手中的激光剑，铮亮的手柄发出固定长度的激光，嗡嗡作响手起刀落干净利落。不过科幻归科幻，今天我们从物理理论上分析一下为什么永远也做不出来这样的激光剑。
在展开讨论之前，我们先思考以下几个简单问题：
- 手电筒发出的光会是固定长度还是无限长度？你能控制发射到多远么？
- 一个红光手电筒和一个蓝光手电筒发出的光交叉会让你感觉有实体物之间那样的相斥力吗？
- 升级上述光束为红蓝激光后，你认为情况会有改观吗？
如果你不太确定，我们不得不深入理解什么是光，以及什么是激光。
光
光作为一种纯能量大致分为:
- 不可见高能射线（波长小于400纳米，能量由高至低依次为：伽马射线，X射线，紫外线。能量高而带有放射性）；
- 可见光波段的光（波长400-700纳米，能量由高到低依次为：紫蓝青绿黄橙红。从带辐射的紫外线波段继续延伸到无辐射的可见紫色光）；
- 不可见低能射线（波长大于700纳米，能量由高到低依次为：红外线，微波-是的就是你厨房微波炉里的微波，无线电波FM，无线电波AM，短波，长波。）
激光
我们从可见光中分离出激光。所以和可见光一样，紫色激光的能量比其他颜色高，红色激光最低。由于激光是由波形完全步调一致的单一波形射线组成的，所以激光是笔直的，发散率极低。这使得激光照射在任何物体表面上都会形成一个极小的能量焦点，从而烧穿该物质（具体怎样烧穿，微观上发生了什么以后再讲）。工业中经常利用激光来切割材料。
比较
和普通光的比较，以交通灯的红光来举例，红绿灯的红光蔟是发散性的。红光蔟里面包含了各式各样无数不同波长的红光，波长不一样和波段的不统一性使得该光蔟传播后散乱的波形大面积接触在物质表面上，并不能像单一波段的激光那样形成能量的聚合焦点，所以用红光蔟是不能像激光那样有效烧穿一张白纸的。这非常类似在太阳下用放大镜把阳光中各种颜色波长的光线都聚焦在一个非常小的点上，不久就可以轻易烧穿白纸。
从波形分析
蓝光具有比红光更短的波长且携带比红光要高的能量，这也可以理解为相同长度内蓝色激光的波峰比红色激光的波峰更多。1米等于10亿纳米（1,000,000,000纳米）。那么1米的蓝色激光剑包含大约2,127,660个蓝光波包（波长470纳米），1米的红色激光剑包含大约1,503,760个红光波包（波长665纳米）。蓝光的能级容量大约相当于红光的1.414倍。也就是说如果绝地武士之间比武，手拿蓝色激光剑的单在武器能量上就已经占了上风，红剑绝地武士处于绝对劣势。其实这也是为什么蓝光DVD比早期的红光CD容量更大，因为相同面积内，蓝光的短波比红光占据更小的单位空间。但这是另外一个话题，我们以后再展开。
从量子力学分析
深度分析，我们再来看看量子力学是怎么说的。从标准模型图表中我们知道光子γ（质量为零，自旋为整数1，γ是希腊字母伽马）是规范波色子中的一种。光子不遵守泡利不相容原理，也就是说任意多个全同光子可以同时占有完全相同的量子态。这句拗口的简单解释是：再多的光子叠加在一起也不可能形成物理实体物质。这是激光剑发出的光子将会穿过彼此之间永远不会有相斥力的深层原因。换个角度说，相斥力是由不同物质原子的核外电子的负电荷间负负相排斥产生的（属于四大原力的电磁力范畴）。由于光子是玻色子且不带有电荷，所以光子之间也是不可能产生任何电磁斥力的。
固定光束的长度
如果我们把光束缚在物质里面不就可以造出有固定长度的激光剑了么？多少人都这样尝试过但在Lene Hau之前从来没人如此接近成功过，1998年她用激光射向一团波色-爱因斯坦超冷凝聚态物质（原子云）中，并成功将光束缚了1毫妙（实现超冷凝的设备占满整个实验室，即便可以暂时束缚光子也不可能把设备缩小到激光剑柄里）。请注意，束缚和停止的含义是不一样的，光进入超冷凝物质被减速并被束缚了1毫秒或更长时间，但这不意味着光的波形脉冲完全停止，光子也不可能处于绝对的静止状态。在麦克斯韦Maxwell的解释中，光是永远持续前进的电磁波，电磁波（就是光）必须永远前进才能维持自身是光，一个电场（波形）产生一个磁场（波形），一个磁场（波形）产生一个电场（波形），以此类推。如果波形停止前进，则光停止存在。所以光不能被绝对停止，且永远按照原来的轨迹射向远方，永远不会有自然光或高能激光是只有1米长的。
物质的其他状态（气态，液态，固态）要么反射光子要么作为能量吸收掉光子，但是从来不能把光子俘获为其物质自身分子或原子系统的一部分。被吸收光子会立即引起原子核外电子向更高能级轨道跃迁从而在亚原子尺度增加整个系统的能级，光子在这个吸收过程中消失，所以不存在光子与物质的联合结构态。光子也不能被磁场所束缚，因为光子本身不包含电荷。
但是
如果你把要求降低到典型星战光剑的标准（有斥力，激光有固定长度）以下，则做出一把玩具光剑还是可以的，在网上就可以轻易购买都很多玩具光剑，但永远不会有真正的绝代武士光剑。
最后，还是忘掉激光剑吧，烧脑结束。
The Star Wars’ diehard fans all want a lightsaber of a Jedi warrior, glossy hilt with a fixed length laser beam, buzzy shinny with clean cut. But back from sci-fi movie to real, today we will analyse from physical theory why a star war lightsaber is impossible to make.
Before start, now let’s think the below question:
- Does beam of flash light go limitedly or unlimitedly in distance, can you control how far it goes?
- Does crossing 2 flash beams (red & blue) make you feel any repulsive force like between 2 real substances do?
- Does upgrading above flash light to laser beams make any improvement for question 1 & 2?
If you are not sure, we have to understand what is light and laser.
Light
Light being pure energy is roughly divided to:
- Invisible high energy rays (wavelength shorter than 400 nano metre, energy from high to low: Gamma ray, X-ray, Ultra Violet. High energy with radiation);
- Visible light rays (wavelength is between 400-700 nm, energy from high to low: Violet, Blue, Aqua, Green, Yellow, Orange, Red. Colour extends from radioactive invisible ultra violet to non-radioactive visible violet);
- Invisible low energy rays (wavelength greater than 700 nm, energy from high to low: Infrared, Microwave-yes the one used in your kitchen, Radio wave FM, Radio wave AM, long wave)
Laser
We managed to extract laser from visible spectrum. Similarly, Violet laser carries higher energy than others, while red laser has the lowest energy among. Because laser is composed by photons that have completely the same wavelength, so laser goes perfectly straight, with extremely low divergent rate. This nature also creates a extremely concentrated high energy point to any surface it lights up to, and then burn up the substance (will expand details on what and how it happens in microscopic level in other articles another time). This nature is used in various industries for cutting materials.
Comparison
Comparatively from laser, take the red beam from a traffic light for example, the light cone is emitted to a broad direction. Unlike the concentrated energy point by the laser, the normal red light cone includes all different kinds of red wavelengths, photons splashing on large area to the surface of matter, this makes it never efficiently penetrate a paper sheet like laser. It is very much like you put a magnifier under the sun, focusing all energy into a small point and burn the sheet.
Analysis from wavelength
Blue light has shorter wavelength and carries higher energy than the red light, understandably that given the same length, the blue light has more wave packs than the red light. One metre equals to 1 billion nano metres (1,000,000,000 nm). There are 2,127,660 blue wave packs (470 nm wavelength) in 1 metre long blue lightsaber, while there are 1,503,760 red wave packs (665 nm wavelength) in 1 metre long red lightsaber. The energy level contained in blue lightsaber is 1.414 times greater than in the red. So, in Jedi warriors battle, the one holding the blue lightsaber wins from the energy level and the red lightsaber holder is surely doomed. Besides, this also explains why blue ray DVD is has more capacity and informative than the red laser disc, as the blue wavelength occupies smaller space than the red laser in a given disc volume space. But this is another topic, we will explain this in other articles another time.
Analysis from Quantum theory
To dig further, let’s see what Quantum Theory says. We know the photon γ (zero mass, with integer spin of 1, γ is geek letter gamma) is one of the gauge bosons in the Standard Model. Photons are not subject to the Pauli Exclusion Principle, that any number of identical bosons can occupy the same quantum state. The simple meaning of this tongue twister is: photons can not add up to real substance. This is the deeper reason why lightsaber beams will go through with each other, never repulse. From another perspective, repulsive force is generated between the negative charge carried by the outer electrons of nuclear between different matters (Electromagnetic force is 1 of 4 basic forces). Photon has no electric charge, so there is no repulsive force between photons.
Fixed length of light beam
What if we combine the laser beam to matter so that light can be constrained into a fixed length? Well, many have thought this but never been so succeeded until Prof. Lene Hau, who successfully shot a laser beam through a Bose-Einstein Condensate matter (atom cloud) and constrained light for 1 millisecond in 1998 (the equipment for making the condensate matter takes the whole lab room, so even if we can constrain light, it will not fit for a lightsaber hilt). Be careful between constraint and stop in here, light is slowed and constrained (trapped) in condensate matter for a millisecond or maybe even longer time, but the light pulse still exists and light will never be completely stopped to still state. In Maxwell’s explanation, light is continuing electromagnetic wave, this means the wave will continue for maintaining itself as light, an electric field (wave) created a magnetic field (wave), an magnetic field (wave) created a electric field (wave) and so on. if the wave cease to pulse, the light won’t exist. So light can never be completely stopped and always goes from its original path to far distance, there will never be a natural light or laser with great power in a fixed length of 1 metre.
Matters (either in gas, liquid or solid state) reflect light or absorb light as energy but can never combine the photon into their molecule or atomic level. The absorption of photon will immediately quantum lead the electron to higher orbit and increase the whole energy level in subatomic scale, the photon will disappear during the process, so there is really no light-matter combined state. Light also can not be constrained by magnetic field because light carries no electric charge.
However
If you lower requirements to not that Star War identical lightsaber standard (repulsive, fixed length), then we can make a toy one, in fact you can easily get it from online store, but never as real as a for a Jedi’s.
So this’s it, no shinny lightsaber, end of story.
