A Blog by Jonathan Low

 

Aug 14, 2021

Are Light Sabers Theoretically Possible?

Theoretically? Maybe. And scientists agree it would be fun to try to build one. JL

Daniel Kolitz reports in Gizmodo:

Of course what every child thinks as he or she swings that lighted plastic rod around is: I wish this thing was real! Trapping photons in a crystal inside a lab (been done) and making a hand-held lightsaber are two different things, but the basic physics are, while “not there yet,” support the idea of a trapped beam of light. There could be a configuration of the magnetic fields of two different light sabers to repel each other. Many of the practical features are orders of magnitude away from reality or even possibility. But it would still be fun to try.

I’ve never been a Star Wars fan. I’ve seen only one of the movies, The Phantom Menace, and I was eight years old at the time. And yet even I, as a child, owned and played with one of those little lightsaber toys, as have millions of children before and after me (not to mention the millions of children yet-unborn, who will not be spared induction into the Disney Industrial Complex). And of course what every child thinks as he or she swings that lighted plastic rod around is: I wish this fucking thing was real! No doubt Disney would rake in even more money if it sold actual lightsabers, but, ignoring the impact this might have on lightsaber-related crime, the question then becomes: are actual lightsabers even possible, scientifically? For this week’s Giz Asks, we reached out to a number of laser and plasma scientists to find out.


Dennis K. Killinger

Professor Emeritus, Physics, University of South Florida

The Star Wars universe lightsaber is usually associated with a laser or laser beam that is able to burn, cut, or damage an object/target or enemy. Technically, since their invention in 1960, we have had many different kinds of lasers, and these have found a wide variety of uses, from 0.001 Watt red lasers used to scan the UPC code at grocery checkouts, eye-safe 1 Watt IR lasers used to map buildings and roadways using lidar (laser radar), to remote sensing using laser probes to measure the earth’s ozone hole and CO2 levels. As far as burning or cutting, we already have industrial lasers that are used to weld car bodies and cut metal plates. However, the power supplies that these lasers use are usually the size of a large suitcase and weigh about 50 lbs (not exactly conducive to saber-wielding). In addition, laser beams come in different colors or wavelengths, so this aspect is already there.

The one aspect of lightsabers that seems infeasible is the concept of its acting as a solid physical rod or saber that can “hit” or “strike” an opponent. In the movies the mechanical hitting of the dueling lightsabers is enforced by sound effects—the lightsabers have a “hum,” and you can hear them hitting each other. But if you take two flashlight beams and cross one beam with the other beam, there is no sound or force experienced by one light beam on the other. This is because photons have no mass, which means that a laser or optical beam has no mass. To get the point across: I like to say that “You can’t use a light beam to hammer a nail.” So in this sense, it is not feasible that two laser beams can “hit” each other in the mechanical sense. However, there is a scientific exception to this: as discovered by recent physics Nobel Prize winner A. Ashkin, a laser beam under the right conditions can be used as an optical trap or as an optical tweezer to trap and move very small objects, such as a bacteria. While one could stretch the truth and call this a Star Wars Tractor Beam, there is a 1,000 billion billion times difference between moving a bacteria and the mass of a starship (ie. SpaceX second stage starship.)

All of that said, it can be argued that the lightsaber is not a laser beam but is instead made up of a gaseous high-temperature plasma or a plasma-like tube. Plasmas are high-temperature, gaseous discharges consisting of electrons and ions at temperatures of around 5,000 to 10,000 C or higher, and are represented by the gas discharge inside a fluorescent tube, a lightning bolt in the atmosphere, and the solar wind of plasma that causes the northern lights. But how to create a stable rod of plasma in the atmosphere? One way is to use a high-power laser and focus it to a spot in the air such that you have Laser Induced Breakdown Spectroscopy (LIBS), which creates a plasma spot in the air, which then emits the fluorescent light from the plasma ball; then, by proper adjustment of laser power and optical alignment, an elongated filament or plasma in the air can be created. Such a technique has been demonstrated under laboratory conditions using a femtosecond laser, and could yield a glowing plasma saber, albeit one with a limited lifetime. However, several of the issues noted above would still limit the usefulness of this plasma saber.

So in summary, some of the physics aspects of the lightsaber can be shown to be theoretically possible, but many of the practical features are orders of magnitude away from reality or even possibility. But it would still be fun to try.

Mark Csele

Professor, Photonics, Niagara College, Canada

You’d be surprised how many times I have been asked this one.

First, let’s consider using a laser (my favorite). Consider a beam of light like that from a powerful laser. One of the interesting qualities about laser light is that it is collimated—it travels in a straight beam that has little divergence. A regular flashlight, for example, will always have a beam which spreads out as it travels, no matter what you do with the optics, but the coherent beam from a laser can have surprisingly little spread. It will maintain its “power” to cut/destroy/etc. for a long distance from the laser (be it 10 cm, 1 m, or perhaps even 100 m). From that standpoint, it would be ideal for our lightsaber.

Problem is, light doesn’t just stop in free space. To build a lightsaber we’d need to devise a way to have those photons of radiation go about 1.5 m and then just magically stop—and that is well beyond our understanding of physics. I’m not saying we could never find a way to do it (heck, a hundred years ago splitting of the atom seemed pretty unrealistic), but our current understanding of physics says this just isn’t possible.

Could we use a particle other than a photon? Say, something like a pion, which can travel some fixed distance and then decay (and in doing so “stop”)? Perhaps, but we don’t know of any particles that would maintain lethality until some point where they just disappear. Maybe one day we can “design” such a particle, but today, this is strictly the stuff of science fiction. I might add that particle accelerators are huge beasts in terms of kilometers in length—the laser is a better bet for miniaturization.

Perhaps the best approach, using current technology, would be a plasma: a hot stream of ionized gas molecules confined by a large magnetic field. This would require a gas supply. The confining magnetic field is the hard part, though—it would be huge, and require an immense amount of power—so, hardly small enough to fit in the palm of the hand, but at least “doable” in theory with current technology.

Back to the laser, the problem is having the beam just stop in free space. Well, it is kind of possible to do this right now. Our current understanding of physics does indeed allow us to stop or “freeze” photons within a crystal. If this could be applied to the “shaft” of the lightsaber, and done in free space as opposed to a photonics crystal, it might well be possible to create a 1 m long beam of light in which radiation stays trapped inside that area (and if the power is high enough, anything touching that beam would be destroyed).

Now trapping a few photons in a crystal inside a lab (been done) and making a hand-held lightsaber are two different things, but at least the basic physics are, while “not there yet,” at least supporting of the idea of a trapped beam of light.

Notwithstanding that, don’t go looking for it at your local gun shop just yet. (I’m thinking of the Terminator scene with the plasma rifle here).

Lorin Matthews

Professor, Physics, Baylor University

A lightsaber is, according to Wikipedia at least, a magnetically confined plasma. That makes sense, as one thing that lightsabers are good for is cutting through various materials, and we already use plasma torches to cut through dense materials like steel. However, the flames used in a plasma torch are only a few inches long, as the torch is ionizing gas that flows through a nozzle. A short distance from the electrodes inside the torch, the electrons and ions in the gas have collided with the neutral gas in the atmosphere and lost energy.

So the problem is to sustain the plasma, keep the plasma particles from colliding with the neutral air, and extend the distance over which the plasma has energy.

One actually can confine charged particles (plasma) in a magnetic bottle, but the problem is that the “bottle” leaks at both ends, so the plasma quickly escapes. Current state-of-the-art physics does make use of a magnetic field to confine plasmas, especially for the use of fusion reactors. To get around the fact that a linear bottle leaks at both ends, the magnetic field lines are curved to create a doughnut—no ends! However, the plasma still leaks out in different directions, among other problems, and the magnetic fields needed are quite complex. (This is why we still don’t have fusion energy, but we are working on it.)

Back to the light saber—the magnetic field will contain the charged particles, but it has no effect on the neutral gas particles in the air. Perhaps this is what determines the length of the light saber, and the plasma would have to be very dense towards the hilt and fall off towards the end. Note that the magnetic field has no effect on neutral particles, so it can’t keep the atmospheric gas out of the magnetic bottle.

The color of the plasma glow is determined by the atomic energy levels in the gas which is ionized. Thus neon plasmas are red and argon plasmas are pinky-purple and oxygen plasmas tend to be green. The Wikipedia article says that the color of the lightsaber is controlled by a “kyber crystal.” This is true for lasers, where the color of the laser light is controlled by the transition levels for electrons in the crystalline material, but not true for a plasma. To get different colors, the lightsaber would need to use different working gases or excite a different energy level for the electron transitions. A strong Jedi could use the force to change the energy and excite a different transition level—but in an earth-like atmosphere, the primary gas is nitrogen, so the glow will be purply-blue. That said, check out the colors of the aurora (northern lights)—the solar wind plasma is directed to the earth’s atmosphere by the earth’s magnetic field, and at different altitudes (and energies) many different gases are excited, producing an array of colors.

One of the important characteristics of the lightsaber is that it can be deflected by another lightsaber. In this case, the plasma would have to be really dense—at least as dense as steel. This goes back to how well the magnetic bottle is designed to trap the plasma. It would not only have to trap the plasma, but collect extra gas from the surrounding atmosphere to concentrate it into a high enough density. (Question: does a lightsaber work in outer space?) I suppose it is possible that there could be a configuration of the magnetic fields such that the magnetic fields of two different light sabers repel each other.

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