Another piece I wrote for the campus science magazine. I really enjoyed writing this one!

Ask a hundred people to name a sci-fi film, and there’s a good chance that your most popular answer will be Star Wars. Ask those same people again to name the most memorable thing from that film, and the answer would always be the same: lightsabers. It’s easy to see why lightsabers enjoy such a reputation, even with people who otherwise have no interest in science fiction, or indeed science. The lightsaber is an impressive sight; a blade of coloured energy wielded expertly by the mystical and powerful Jedi, able to deflect blaster shots and cut through anything except another sabre. Millions of children (myself included) spent their childhoods playing with light-up, plastic versions, yet the question always remained: would science ever be able to create a real, working lightsaber?

The name lightsaber is itself misleading – a beam of light would never be able to be contained in such a way. However, it is certainly theoretically possible that such a device could be made, using plasma to create the blade. A plasma is essentially an ionized gas, consisting of an equal number of gas ions and free electrons. Plasmas behave very differently to standard fluids; due to their composition, they are governed by electromagnetic interactions between the ions and electrons, rather than by physical collisions. Plasmas can also be highly energetic – obviously a necessary quality if we’re planning on using them to cut through walls. Plasmas used in magnetic confinement fusion, such as in the JET experiment at Culham, can reach temperatures of around 100 million Kelvin, whilst temperatures a short distance away remain unchanged due to the electromagnetic qualities of the plasma. To put that into perspective, the surface of the sun is around 6,000 Kelvin, and the temperature at the centre has been estimated to be about 16 million Kelvin.

The power, then, is evidently not a problem. Neither is the creation of a plasma, which itself is very easy to accomplish using a pair of electrodes. What does prove difficult is the containment of the plasma into a blade-like shape, which is quite important if we’re to battle the forces of evil in epic displays of swordsmanship. Unfortunately, simply projecting a beam of plasma a certain length from the handle seems impossible. Magnetic fields are necessary to contain plasmas, as they restrict the movement of the charged particles to specific field lines, and would prevent the beam from exploding into an impressive-looking yet likely catastrophic cloud. Tokamaks, such as JET, use toroidal (donut shaped) and poloidal (circular around the donut) magnetic fields to contain the plasma into a toroidal shape, around a central control rod. Theoretically, it would be possible to extend the height of such a device, allowing the ring of plasma to be shaped as a long cylinder. If the plasma were to be emitted along the central rod, then a lone toroidal field, which could be produced by passing a current around the rod, should be sufficient to contain the plasma into a cylindrical shape around the rod. We have our lightsaber’s shape!

However, it’s not quite that simple, or I can assure you that thousands of physicists would have done it already. Plasma Physics as a field is still in its infancy; there are many unexplained phenomena that we really would like to understand, and counteract. For example, ELMs (edge localised modes), which are violent offshoots of toroidal plasma, can destabilise the magnetic fields and cause the plasma to collapse. Or the loss of energy when a field comes into contact with matter or simply flows out of the volume, as described by the Poynting vector. These problems, the latter in particular, will likely not remain unsolved forever, yet with our current level of knowledge, factors such as these prevent a lightsaber from being built.

But it’s not all doom and gloom! A cylindrical “plasma sword”, albeit attached to a power pack rather than being self-contained in the hilt, is not an unrealistic aim. In Michio Kaku’s excellent book Physics of the Impossible, he classes lightsabers as a Class I impossibility: technologies which are impossible today, but do not violate the known laws of Physics, and might be possible within a century. And we already know how to create an array of different coloured plasmas for use in such a weapon, by using different gases: silicon tetrafluoride (SiF 4 ) or tetrachlorosilane (SiCl 4 ) for blue, chlorine for green, and for those tempted by the dark side, bromine or neon for red. It seems that the dream of a real-world lightsaber isn’t that childish after all.

Sources:

Michio Kaku, Physics of the Impossible, published by Penguin Books (2009).

Diener Electronic, Glossary of Terms – Plasma colour