How a Light Saber Works

A popular explanation

[Metanote: This explanation arose out of a lunch-time discussion with a colleague who claimed a light saber could not exist since the light beam it emits could not just stop in mid-air after 75 cm. I thank this colleague for the challenge.]

[Metanote 2: I am looking for a person whose physics is less rusty than mine and who can help me with putting some quantitative estimates to the phenomena described here.]

Unlike what is generally thought, and even sometimes taught, the construction and workings of a light saber are not secret. So, you might wonder, what is keeping a galaxy-wide firm like Jax-Bethal from mass-producing them, so we can all merrily and effortlessly dismember our enemies? The answer is simple: the power supply in the handle. Traditional technology could easily make the blade part, but without the multi-TeraWatt Force-operated hand-controlled power supply in the handle it would just be a coil of high-grade glass in a metal-and-ceramic casing.

Which is exactly what it is. The prime component of a light saber blade is a filament of ultra-low-loss glass, about 3 meters long, properly doped, with a perfect mirror at one end and a slightly transparent mirror at the other. The transparent end (called the input end) is connected to an energy source in the handle, and if the filament is perfectly straight, it is easy to see that this set-up is a laser, except that you don't notice so because no power emanates from the free end (usually called the noput end) - the mirror there is perfect.

Now imagine that the laser is on and you try to bend it (DON'T try this at home; you'll burn your fingers or worse). Then three things happen, each important in its own right.

First, the filament will try to straighten itself again. Why does it do that? Believe it or not, light falling on a surface exerts pressure on it. An awfully minuscule bit, it is true, but it's there. And a lot of light exerts a tiny bit of pressure, and a huge amount of light exerts a noticeable amount of pressure. And we are talking TeraWatts here. So the filament is just like a garden hose with one end closed off and the other end connected to an open water tap with enough pressure. The force by which it fights back is not at all large, and you could easily win, but it's there.

Second, you'd expect the laser to stop because the light would leak away, but it doesn't. The photons inside the filament bounce off the wall when they hit it, back into the laser, just like light in classical fiber optics. This effect is important to keep the laser going.

The third effect is a little weirder, and has to do with exactly what happens when a photon hits the inner wall of the filament. Above we said it bounces off the wall, but why does it do so? Why doesn't it traverse the wall and fly off into the air, where it can propagate much faster than inside the glass? Now that is exactly the point: the photon is a wave packet, and as soon as part of the wave packet moves outside the glass and into the air, it speeds up with respect to the rest of the wave packet which is still in the glass. So the part outside gets ahead of the part inside, but since they form together one single wave packet they cannot go their separate ways. And depending on the angle at which the photon hits the wall, either the part outside pulls the part inside out into the air (for sharp angles), or the part inside pulls the part outside safely back into the glass (for oblique angles). Exactly what is a sharp angle and what an oblique one, depends on the coefficients of refraction of the glass and the air, but inside our bent filament all angles at which photons can hit walls are guaranteed to be oblique, so all photons are shepherded back into the filament.

This phenomenon is called total reflection and has in fact been known since the middle of the 17-th century CE. It can easily be observed in many places in more or less daily life: when you look into a glass filled with water standing on a lit table, the inside of the glass reflects like a very good silvery mirror rather than allowing you to look through it; and when you are at the bottom of a swimming pool and look up, the surface of the water straight above you is transparent, but where you look at it at an oblique angle it shimmers like mercury.

So we see that total reflection prevents the curved laser from quitting by keeping the photons inside - but not quite. All the time there are some photons that are just treading outside the filament and being bent back in, so part of them is outside the filament, as shown in the picture above. So the outer rim of the curved laser is surrounded by a microscopically thin layer of light. This effect is very important in making the light saber a formidable weapon.

Now we are getting somewhere. The blade of the light saber at rest consists of several thousands of such filament lasers, all coiled up in loops of about 2.5 cm diameter. Half of them are wound clockwise, half of them counterclockwise, and they are interwoven like the threads in a wick. All input ends are connected to energy sources in the handle, all noput ends are just tied in a circle to a ceramic plate, and the whole assembly resides in the top of the handle, covered by the plate, looking innocent.

But as soon as the Jedi activates the power source in the handle, energy (lots of it) is pumped into the filaments, and laser action starts in all of them. Since very little energy is lost inside the glass and no energy is lost at the noput end, very high levels of luminosity can quickly be built up. You'll have to think in terms of between ??? and ??? Teralumens here. As the light intensity increases, the light pressure inside each filament rises, trying to unwind the coil it is lying in. But the mesh they are woven in prevents the coils from just getting straight, so the filaments stretch in the only direction they can go, sideways. This makes the coils emerge like a wick out of the handle, and as we all know the blade of the saber extends to its full length of about 80 cm in roughly 0.3 seconds.

So what we see as a column of light is actually a hollow cylinder of thousands of glass filaments, each standing in a spiral, each supported by its internal light pressure and by all the other filaments. But why then do we see it as a shaft of light, if none of the photons leave the filaments due to total reflection? As explained above, although the photons do not permanently leave the lasers, they do step a little bit outside before being reflected back in. Each such photon sticking its nose out creates a small electro-magnetic field, and together (we are talking about roughly 10??? photons here!) they are strong enough to ionize the air around; and the ionized air sends out the light we see. This composite field is called the aura field. Ionizing the air constitutes a small energy loss, but it is negligible compared to the energy stored in the entire blade.

It is the aura field that does the cutting and makes the light saber blade so extremely dangerous. Anything it touches is heated to 4000 to 5000 °C within milliseconds, depending on the properties of the material. And we all know what that can do!

We can now also understand some other properties of the light saber. When it is waved through air, the air passes through the mazes in the filament mesh (the filaments are so thin it's mostly holes), gets ionized when it enters, and de-ionizes when it leaves. This causes the whoosh you hear during fights. Also, the filaments are so thin and so far apart (relatively speaking), that beams from energy weapons hardly affect them and just pass through.

Since the tremendous energy of the blade is not expended until something comes inside the aura field and diverts the photons out of the curved lasers, the blade is not hot in the normal sense of the word. It just emanates the heat of a normal plasma, as in a household variety fluorescence tube. But unlike the latter there is no tube of fluorescent material around it, so it does not count for much as a light source.

It is less known, and indeed sometimes denied, that the "blade" of a light saber is not rigid but fairly flexible and handles like a stiff rubber stick; after all it is mechanically just a wick of glass fibers held up by light pressure. Normally you don't see this because the blade is so light that it follows the movements of the wielder immediately, and, when it hits something, it is so destructive that it has already cut though before it bends. But if you hit for example a stone or a steel wall with it, it will give somewhat and fold to the contours of the object, thus prolonging the time of contact to several hundreds of milliseconds, enough to blow any known substance into oblivion. An additional advantage is that it does not "hit back" in your hands, like a baseball bat may do when you hit something very hard with it.

It is even less known, and has for a long time been kept semi-secret, that the blade of a light saber can relatively easily be compressed length-wise, axially. When that happens, disaster is near: the humongous energy in the blade is compressed into a smaller volume, raising the energy-per-volume ratio. More in particular, if an active saber drops on its tip from a sufficient height, or is otherwise pushed back completely into its hilt, the weapon explodes, taking a considerable chuck of its surrounding with it and creating one of those situations in which absence of body is better than presence of mind. Some safe-guards are in place: 1. under normal operation, the saber switches off as soon as the owner lets go; 2. some sabers have an internal titanium cross-bar system that locks when the blade is fully extended. My advice: don't count on them.

Little study has been made of the behavior of the blade under water. There is some boiling and splutter, but nothing indicative of the real destructive capacity of the weapon. The prevailing theory is that a thin layer of steam builds up around it immediately, preventing the rest of the water to come into contact, much as the thin layer of aluminum oxide prevents further oxidation of aluminum exposed to air. But further study may prove this theory wrong.

Another property is that a light saber in vacuum is almost invisible, but still works. There is no air to be ionized by the aura field and emit light, but the aura field is there nonetheless and will do its devastating work on whatever comes near enough to it. Needless to say, a light saber in vacuum does not whoosh.

When the light saber is switched off, the lasers stop working, the light pressure disappears and all the filaments coil up again in the handle. All the energy stored in the lasers is reabsorbed in the power cell in the handle, another feat that can be performed by Jedi technology only.

Notes of the Katcon 2012 Lecture

Aantekeningen bij de lezing op Katcon 2012

How a Light Saber Works / Dick Grune / dick@dickgrune.com