When you think of Star Wars, you think of lightsabers. Right? What could be better, from a movie-making 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.)

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.)

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.

Anyway, that’s just wrong: It can’t be a laser. For starters, lasers 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.

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.

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!

Mass and Motion

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.

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.