When you think of Star Wars, what's the first image that comes to mind? Lightsabers, right? Everyone loves lightsabers. You can buy some nice replicas, and there are even classes on lightsaber training. But what about an actual working lightsaber? What it would take to make a real one?
Today, I'm going to focus on the power needed to run a lightsaber. How much power would it use, and what kind of battery would you need? (The issue of how you can make a light beam with a finite shape that slices through things … well, we'll just set that aside for now.)
For this estimation, I'm going to look at Episode I: The Phantom Menace. In an early scene, Qui-Gon Jinn (Liam Neeson) uses his lightsaber to cut through a thick metal door in a Trade Federation starship. There's a bunch of physics to consider, so let's get started.
Power and Energy
Remember in Episode IV when Darth Vader said, “The ability to destroy a planet is insignificant next to the power of the Force”? He was talking about military power, not physical power. Words are slippery—just like the Force is not the same as a force in physics. But to a physicist, power has a specific meaning: It's the rate of energy use or transmission.
We can write this as the change in energy (ΔE) with respect to a finite time interval (Δt). If the energy is measured in joules and time in seconds, then the power would be in units of watts.
Say you pick up a textbook off the floor and lift it to the level of a table. Since you're working against gravity, this would require about 10 joules of energy. You'd use the same amount of energy whether you did it fast or slow, but the power would differ. If it took you 10 seconds to lift the book, it would require a power of 1 watt. If you did it in one second, that would be 10 watts. Same energy, different power. Right?
This definition gives us a strategy to estimate the power of a lightsaber. We just need to see some event in which we can estimate a change in energy over a measurable time interval. This is exactly what we see when Qui-Gon thrusts his lightsaber into that metal door.
Thermal Energy and Change in Phase
To increase the temperature of a cup of coffee, you need energy. In this case, we call it thermal energy. The change in thermal energy of something like coffee depends on the mass of the object (m), the change in temperature (ΔT), and the “specific heat capacity” (c) of the material.
Just to see how this works, imagine you take 300 grams of coffee (which is basically water) and raise the temperature from 25 to 100 Celsius. This would require 94,000 joules—as much energy as you'd need to lift that textbook 9,400 times. You're appreciating that Mr. Coffee on your counter now, right?
But wait! If you melt a metal door, you aren't just raising the temperature but also changing the phase of the material from a solid to a liquid. That's what melting does. The amount of energy required to change phases depends on the mass of the object (m) and a property we call the latent heat of fusion (lf).
Melting an ice cube takes 3.34 x 105 joules per kilogram. So a single ice cube with a mass of 50 grams would take 16,700 joules. You can already see that melting stuff takes a bunch of energy. If you want to melt a metal door, you have to do two things—increase the temperature to the melting point and then actually liquefy it. The total amount of energy depends on the mass and type of the metal.
Estimating the Power
Now we have to make some guesses. The big question is what starship doors are made of. I mean, if I was making a giant spaceship to carry battle droids around, I think my metal of choice would be aluminum, which has a nice balance between weight and strength.
Luckily, we know all about aluminum. It has a density of 2,700 kilograms per cubic meter and a melting temperature of 660 degrees Celsius. The specific heat capacity is 900 joules per kilogram per degree Celsius, and it has a latent heat of fusion at 3.96 x 105 joules per kilogram. (I know that's a lot of numbers, but don't worry, we're just going to drop them into a computer.)
What is the mass of the metal that's melted? That's a tough one. On Qui-Gon's first cut, it looks like he cuts a path about 2 meters long. I'm not sure about the width of this path, but I'll go with 1 centimeter. Finally, the door is maybe 5 centimeters thick, and he's cutting all the way through in one pass. With those estimations, I get a total mass of 2.7 kilograms of melted metal.
Now I can calculate the energy required to melt this amount of aluminum. It's just the energy needed to increase the temperature of the metal to the melting point and then melt it. (Want to try the calculation with a different metal? Just plug your numbers into my Python code.) With my numbers, I get a required energy of 2.6 million joules. Melting metal isn't trivial.
Now, for the power, I just need to determine how long it takes to melt this metal. Using Tracker Video Analysis, I get a cut time of 11.5 seconds. This gives a power output of 2.28 x 105 watts. Yes, that's the actual, physical power of the Force. It's equivalent to 305 horsepower, so it's like a high-powered car right there in your hand.
A Lightsaber Battery
Maybe lightsabers don't have batteries. Maybe they just draw energy from an extra dimension or something. No one knows because lightsabers aren't real (and that's fine). But if they did have a battery, what would it look like? What kind of battery would work?
We already estimated the power output of the lightsaber. With this, we can calculate the total energy stored in the device. We just need to know how long it will run on a single charge. I've never seen a lightsaber fizzle out in Star Wars, and I've never seen them plugged in to the wall, so I assume they last a long time.
Let's just make a guess that a lightsaber can run for 10 hours. That's probably a conservative estimate. So if the lightsaber outputs 2.28 x 105 watts for 10 hours, that requires a total stored energy of 8.2 x 109 joules. Oof! That's the same as picking up a textbook 820 million times.
Could you store that much energy with a lithium-ion battery like the one in your phone? These cells can store 2.5 x 109 joules per cubic meter. Using our value for the stored energy, this means the battery would have a volume of 3.3 cubic meters. That is … daunting. A cylindrical handle to contain it might be, for instance, 4 meters long and 1 meter in diameter—14 feet long and 3 feet thick. It would be like dueling with tree trunks or telephone poles.
On the other hand, remember what Yoda tells Luke: “Size matters not.” So who's to say.
But maybe we were wrong about the run time. Let's just use the dimensions of the lightsabers we see in the movies. How about a handle 13 centimeters long and 6 centimeters in diameter? That would have a volume of 367 cubic centimeters, which is about 0.01 percent the size of the one above. If everything is proportional, that would give us a run time of 4 seconds. In this case, whoever's battery lasts longer wins.
Clearly, a lithium-ion battery isn't going to cut it. Using the reasonable handle size above, we would need an energy density of 2.23 x 1013 joules per cubic meter (or 2.23 x 104 megajoules per liter) to get the run time we want. Looking at a list of energy densities, the only thing that could work is some type of nuclear battery or an antimatter source. I'd be fine with an antimatter battery. That would be complicated but super cool.
By the way, do you have a favorite lightsaber duel from the Star Wars movies and shows? Let me know. A lot of people cite the “Duel of the Fates” in Phantom Menace, when Darth Maul takes on Qui-Gon Jinn and Obi-Wan Kenobi with his double lightsaber. Ray Park, who played Darth Maul with psycho ferocity, was a real-life black belt, and it shows.
I put together a list of my top 5 lightsaber duels a few years ago. See if you agree. May the 4th be with you!
