When you think of Star Wars, you think of lightsabers. RIGHT? What’s better, from a cinematic point of view, than a futuristic sword that allows you to create stunning fencing duels like in Errol Flynn’s old swashbucklers. (So much better than watching Stormtroopers fire their blasters at walls and ceilings and anything other than their targets.)
Lightsabers come in a cosmic rainbow of hues (blue or green for the good guys, red for the bad guys) and in a variety of shapes. There is even a double blade version 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 this film, thanks to the skills and fear of the Darth Maul actor Ray Park.)
So…exactly what are lightsabers? Of course, they’re not real, so no one really knows how they work. Even the characters in the films seem a little confused about this. In Phantom MenaceAnakin calls it a “laser sword”. Yes, it was a child, but Din Djarin (the Mandalorian) and Luke Skywalker also call it a laser sword, although I suspect Luke was being sarcastic.
Either way, it’s simply not true: it can’t be a laser. For starters, the laser beams are invisible from the side, so you won’t see anything unless you stage the duels in a nightclub with fog machines to disperse the beams. Second, the beams last indefinitely; they have no end. Third, laser beams can’t clash like swords: they would simply cross each other when you try to parry.
But what is it then? We can narrow down the possibilities considerably by asking if the blade has mass. If it is a few type of light (as one might 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.
This is a question that we can answer by analyzing how the lightsabers move when you wave them. In other words, it’s time to do some physics!
Mass and movement
Do not confuse mass and weight. Mass is a measure of the quantity of “stuff” like protons, neutrons and electrons. In an object, and weight is the amount of gravitational force acting on an object. Here we want to see what impact a lightsaber’s mass would have on its movement. But let’s start with something simpler.
Instead of a lightsaber, let’s say we have a “luminous ball” made of the same buzzing substance. As it is symmetrical, we can describe its movement without worrying about rotation. If we want to move this ball back and forth, we appeal to Newton’s second law of motion. That says the acceleration (A) of an object depends on its mass (m) and the amount of force (F) applies to it.
If the ball is at rest and you hit it, it will accelerate: its speed will go from 0 to a positive number. As the equation shows, the more massive the ball, the harder you have to hit it to get some acceleration. This is the simple case of linear motion. But what we want is…
Angular movement
Newton’s second law is cool and all, and it’s useful if you want to move a lightsaber back and forth. But if you’re facing a Sith Lord, you’ll want to swing this thing. You now 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.
Again, we’ll start simple: imagine you’re holding a lightsaber horizontally and want to orient it vertically. To rotate the weapon, you must give it angular acceleration (A). Yes, it’s like regular acceleration (A), except that it deals with rotational motion.
Well, there should be something like Newton’s second law for rotations, right? Yeah, there is. We have a “rotational force” which we call torque (t), and a “rotational mass” called moment of inertia (I):
This moment of inertia (I) is the part we really need to understand. It is the property of an object that “resists” changes in rotational motion, and it depends not only on the mass of the object but also on where that mass is located.
Here’s a simple demo to get a feel for this. All you need is a simple stick. First, hold the stick at one end (yes, like a lightsaber) and use your wrist to swing it back and forth as quickly as possible. Now hold the stick in the center and swing it back and forth again. I use PVC pipe below:
It’s the same stick, so the length and mass remain unchanged. The only difference is the rotation point. Holding it in the center makes it much easier to rotate. When you hold it by the end, some of the mass of the stick is much further away from the point of rotation. So, even though the mass has not changed, the moment of inertia did change.
What does this have to do with lightsabers? Well, if the lightsaber blade has mass, that will also increase the moment of inertia. It would be like holding the stick at the end (or swinging a real sword). It will swing more slowly and require more effort to swing.
Now let’s think about lightsaber battles in Star Wars. Yes, sometimes things move quickly, but it does indeed look like they are hacking with swords. You can see they’re packing some punch into it. This alone tells us that these lightsaber blades have mass. Otherwise, they would move differently.
Center of mass
But wait! There is more evidence. Remember the climactic battle between Luke and Darth Vader in Return of the Jedi? At some point (2:37 in this excerpt), Vader takes his lightsaber and throws it at Luke. Now, once the lightsaber leaves Darth Vader’s hand, it will rotate around its center of mass.
If the blade has no mass, the lightsaber will rotate around the center of the hilt. If the blade has a certain mass, the rotation point will move upward in the blade part, as shown on the right below. The dot indicates the center of mass in each case.
This will make it run very differently. Look at this scene again: it’s quite clear that the center of mass looks more like the one on the right, which means the blade has a mass. CQED, debate over. The lightsaber does not use light for commercial purposes.
Yeah, I know, it moves like that because it’s a movie prop with a real stick for a blade, onto which they then superimposed the special lighting effects. I’m not crazy. But it all fits together, don’t you see? If the lightsaber blade had no mace, a duel would look like two people waving laser pointers. The blades would move so fast it wouldn’t even be entertaining.
So what exactly creates the glowing blade part? Perhaps there is an internal mechanism that extends from the hilt and emits some sort of plasma field. I’m not really sure. But not knowing all the answers is what makes science fiction so great.
