Do Lightsaber Blades Have Mass?

光剑的刃部有质量吗？

When you think of Star Wars, you think of lightsabers. Right? What could be better, from a moviemaking standpoint, than a futuristic sword that lets you create awesome fencing duels like in old-time Errol Flynn swashbucklers. (So much better than watching Stormtroopers fire their blasters into walls and ceilings and anything else except their targets.) 提到《星球大战》，你就会想到光剑，对吧？从电影制作的角度来看，还有什么比一把未来派的剑更棒的呢？它能让你创造出像埃罗尔·弗林（Errol Flynn）老式侠客电影中那样精彩的击剑决斗。（这可比看帝国冲锋队把爆能枪射向墙壁、天花板以及除了目标以外的任何地方要好得多。）

Lightsabers come in a cosmic rainbow of hues (color-coded blue or green for good guys, red for bad) and a variety of shapes. There’s even a double-bladed version in Phantom Menace. (I don’t want to start a nerd fight—yet—but the best lightsaber battle in the canon has to be the “Duel of the Fates” in that movie, thanks to the skills and scariness of Darth Maul actor Ray Park.) 光剑有着宇宙般绚丽的色彩（蓝色或绿色代表正派，红色代表反派），形状也各不相同。《幽灵的威胁》中甚至出现了双刃版本。（我不想引发一场极客争论——至少现在不想——但正史中最棒的光剑对决绝对是那部电影里的“命运之战”，这要归功于达斯·摩尔的扮演者雷·帕克所展现的技巧和压迫感。）

So … exactly what are lightsabers? Of course, they aren’t real, so nobody really knows how they work. Even the characters in the movies seem a little confused about it. In Phantom Menace, Anakin calls it a “laser sword.” Yeah, he was a kid, but both Din Djarin (the Mandalorian) and Luke Skywalker also refer to it as a laser sword—though I suspect Luke was being sarcastic. 那么……光剑到底是什么？当然，它们不是真实存在的，所以没人真正知道它们是如何运作的。甚至电影里的角色似乎也对此感到困惑。在《幽灵的威胁》中，安纳金称其为“激光剑”。没错，他当时还是个孩子，但丁·贾林（曼达洛人）和卢克·天行者也都称其为激光剑——尽管我怀疑卢克是在讽刺。

Anyway, that’s just wrong: It can’t be a laser. For starters, laser beams are invisible from the side, so you wouldn’t see a thing unless you staged the duels in a disco with fog machines to scatter the beams. Second, the beams go on forever; they don’t have an end. Third, laser beams can’t clank together like swords—they’d just pass through each other when you try to parry. 总之，这种说法是错误的：它不可能是激光。首先，激光束从侧面看是不可见的，除非你在迪斯科舞厅里用烟雾机散射光束，否则你什么也看不见。其次，激光束会无限延伸，没有尽头。第三，激光束不能像剑一样碰撞在一起——当你试图格挡时，它们只会相互穿过。

But what is it then? We can greatly narrow the possibilities by asking if the blade has mass. If it’s some kind of light (as you’d think from the name “lightsaber”), then the answer is no—light, or electromagnetic radiation, has no mass. If we can determine that it has mass, then it’s not light. 那它到底是什么呢？我们可以通过询问“刃部是否有质量”来极大地缩小可能性范围。如果它是一种光（正如你从“光剑”这个名字所想的那样），那么答案是否定的——光，即电磁辐射，是没有质量的。如果我们能确定它有质量，那么它就不是光。

This is a question we can answer, by analyzing how lightsabers move when you wave them around. In other words, it’s time for some physics! 这是一个我们可以通过分析光剑挥动时的运动方式来回答的问题。换句话说，是时候运用一些物理学知识了！

Mass and Motion

质量与运动

Don’t confuse mass and weight. Mass is a measure of how much “stuff” like protons, neutrons, and electrons are in an object, and weight is the amount of gravitational force acting on an object. Here we want to see what impact the mass of a lightsaber would have on its motion. But let’s start with something simpler. 不要混淆质量和重量。质量是衡量物体中含有多少“物质”（如质子、中子和电子）的量度，而重量是作用在物体上的引力大小。在这里，我们想看看光剑的质量会对它的运动产生什么影响。但让我们先从简单的开始。

Instead of a lightsaber, say we have a “lightball” made of the same buzzy substance. Since it’s symmetrical, we can describe its motion without worrying about rotation. If we want to move this ball back and forth, we call on Newton’s second law of motion. This says the acceleration (a) of an object depends on its mass (m) and the amount of force (F) applied to it. 假设我们有一个由同样嗡嗡作响的物质制成的“光球”，而不是光剑。由于它是对称的，我们可以描述它的运动而不必担心旋转。如果我们想让这个球来回移动，我们需要用到牛顿第二运动定律。该定律指出，物体的加速度 (a) 取决于其质量 (m) 和施加在它上面的力 (F)。

If the ball’s at rest and you kick it, it’ll accelerate—its speed will increase from 0 to some positive number. As you can see from the equation, the more massive the ball, the harder you have to kick it to achieve a certain acceleration. This is the simple case of linear motion. But what we want is … 如果球处于静止状态，你踢它一脚，它就会产生加速度——速度会从 0 增加到一个正数。正如你从公式中看到的，球的质量越大，要达到一定的加速度，你就必须踢得越用力。这是线性运动的简单情况。但我们想要的是……

Angular Motion

角运动

Newton’s second law is cool and everything, and it’s useful if you want to move a lightsaber back and forth. But if you’re up against a Sith lord, you’re going to want to swing that thing. Now you have rotational motion, also called angular motion. Think of it like the sweeping motion of a windshield wiper around a pivot point. And that requires a slightly different model. 牛顿第二定律很酷，如果你想让光剑来回移动，它很有用。但如果你面对的是西斯尊主，你肯定想挥舞它。现在你涉及到了旋转运动，也称为角运动。把它想象成雨刷器绕着支点扫动的过程。这需要一个稍微不同的模型。

Again, we’ll start simple: Imagine you’re holding a lightsaber horizontally and you want to turn it to a vertical orientation. In order to rotate the weapon, you need to give it an angular acceleration (α). Yes, that’s just like regular acceleration (a), except that it deals with rotational motion. 同样，我们从简单的开始：想象你水平拿着一把光剑，想把它转成垂直方向。为了旋转武器，你需要给它一个角加速度 (α)。是的，这就像普通的加速度 (a)，只是它处理的是旋转运动。

Well, there should be something like Newton’s second law for rotations, right? Yup, there is. We have a rotational force that we call torque (τ), and a “rotational mass” called the moment of inertia (I): 那么，应该有一个类似于旋转运动的牛顿第二定律，对吧？没错，确实有。我们有一种旋转力，称为力矩 (τ)，以及一种称为转动惯量 (I) 的“旋转质量”：

This moment of inertia (I) is the part we really need to understand. It’s the property of an object that resists changes in rotational motion, and it depends not just on the mass of the object but also where that mass is located. 这个转动惯量 (I) 是我们真正需要理解的部分。它是物体抵抗旋转运动变化的一种属性，它不仅取决于物体的质量，还取决于质量分布的位置。

Here’s a simple demo to get a feel for this. All you need is a plain stick. First, hold the stick at one end (yes, like a lightsaber) and use your wrist to rotate it back and forth as fast as you can. Now hold the stick in the center and again rotate it back and forth. 这里有一个简单的演示来感受一下。你只需要一根普通的棍子。首先，握住棍子的一端（是的，就像握光剑一样），用手腕尽可能快地来回旋转它。现在握住棍子的中心，再次来回旋转它。

It’s the same stick, so the length and the mass are unchanged. The only difference is the point of rotation. Holding it in the center makes it much easier to rotate. When you hold it at the end, some of the mass of the stick is much farther from the point of rotation. So, even though the mass didn’t change, the moment of inertia did change. 这是同一根棍子，所以长度和质量都没有改变。唯一的区别是旋转点。握住中心使旋转变得容易得多。当你握住末端时，棍子的一部分质量距离旋转点远得多。因此，即使质量没有改变，转动惯量也改变了。

What’s this got to do with lightsabers? Well, if the blade part of the lightsaber has a mass, it will also increase the moment of inertia. It would be like holding the stick on the end (or swinging an actual sword). It will swing slower and require more effort to wave it around. 这和光剑有什么关系呢？好吧，如果光剑的刃部有质量，它也会增加转动惯量。这就像握住棍子的末端（或者挥舞一把真正的剑）。它挥动起来会更慢，需要更多的力气来挥舞。

Now let’s think about lightsaber battles in Star Wars. Yes, sometimes things move fast—but it does indeed look like they are hacking with swords. You can see that they’re putting some oomph into it. That alone tells us these lightsaber blades have mass. Otherwise they’d move differently. 现在让我们思考一下《星球大战》中的光剑对决。是的，有时动作很快——但看起来确实像是在用剑劈砍。你可以看出他们确实在用力。仅凭这一点就告诉我们，这些光剑的刃部是有质量的。否则它们的运动方式会完全不同。

Center of Mass

质心

But wait! There’s more. Remember the climactic battle between Luke and Darth Vader in Return of the Jedi? At one point, Vader takes his lightsaber and throws it at Luke. Now, once the lightsaber leaves Darth Vader’s hand, it will rotate about its center of mass. 等等！还有呢。还记得《绝地归来》中卢克和达斯·维达之间那场高潮迭起的战斗吗？在某一时刻，维达拿起他的光剑扔向卢克。现在，一旦光剑离开达斯·维达的手，它就会绕着它的质心旋转。

If the blade has no mass, the lightsaber will rotate about the center of the hilt. If the blade has some mass, the rotation point will move upward into the blade part. The dot shows the center of mass in each case. 如果刃部没有质量，光剑将绕着剑柄的中心旋转。如果刃部有质量，旋转点就会向上移动到刃部。圆点显示了每种情况下的质心。

This will cause it to spin very differently. Look at that scene again: It’s quite clear that the center of mass is more like the case on the right, which means the blade has mass. QED. 这会导致它以非常不同的方式旋转。再看一遍那个场景：很明显，质心更接近右边的情况，这意味着刃部是有质量的。证毕。