Despite its almost instant recognition, very little seems to be known regarding the physics behind the lightsaber blade or the exact energy requirements of the weapon. The weapon of a Jedi Knight is seen to cut through both organic flesh as well a number of Star Wars alloys. However, quantification of a lightsaber's energy or thermal characteristics is difficult as very little is known regarding the thermal properties of materials cut by lightsabers. This post will be a first pass attempt to quantify the energy characteristics of a lightsaber by examining its destructive effects on biological tissue. It is a rather crude, somewhat questionable, and probably flammable approximation based largely on a laser ablation heating approach.
Laser ablation is a medical technique in which a high intensity, focused laser beam is applied to a selected region. The energy transfer at the tissue interface raises the tissue temperature to 100 degrees Celsuis. As biological tissues are mostly water, this results in tissue vaporization as the intracelluluar and extracellular fluid is transformed into a gaseous state.
However, lasers and lightsabers share very little other than being associated with the word "light." It is well known from ESB and ROTJ that lightsabers do not appear to cast EM radiation in a darkened room. Lightsabers are of finite length, cast shadows, and appear to physically deflect other lightsabers. Despite the lack of shared characteristics, I have decided to still model a lightsaber's tissue heating behavior on a laser ablation mainly because both appear to be able to thermally destruct tissue in a localized fashion.
In ESB, a lightsaber is employed by Han Solo to cut and expose the abdominal cavity of a Tauntaun Lizard. Han Solo takes approximately eight frames to slice open an approximately 78cm section of tissue implying a cutting speed of 290 cm/s. In the ESB novelization, it is stated that Han exposes enough of the cavity to fully insulate Luke. At the above cutting speed, it would only take Han approximately 1-2 seconds to cut open the necessary area of tissue to insulate Luke. This is consistent with the movie visuals as the camera cuts away to Luke for several seconds while Han is still presumably cutting.
For this example, it will be assumed that the Tauntaun is composed of biological tissue and that the abdominal region of the Tauntaun cut does not contain bone. The heat required to raise water from 37 degrees to 100 degrees Celsius is 4.18 J/g/C *(100-37)=263 J/g = 263 J/g. The latent heat of vaporization for water is 2260 J/g and thus the total Q required to vaporize water is 2260+263=2523 J/g=2523 J/cm^3.
The approximate formula for the ablation speed of a CO laser is given by v= f I / Q, where v is the ablation speed, I is irradiance, f is an efficiency term and Q is the heat required to vaporize water from basal temperature. For this example, I will assume %100 efficiency even thought typical laser ablations efficiencies vary from 30%-60%. [1]
The calculated irradiance at Han Solo's cutting speed of 290cm/s is thus 731,670 W/cm^2. For comparison, typical irradiances from lasers are between 100-1000 W/cm^2.
Power can be derived from this number by determining the area of lightsaber contact. As this is potentially a complicated contact problem involving a semicircle and an infinite plate, I am punting on it for now. However, for a quick and dirty approach, lets look at a straight lightsaber thrust in which the area of contact is roughly circular. The power is thus P = I * 4 pi r^2. Estimating the diameter of a lightsaber to be approximately 4cm, the calculated power output is 36.7 MW. Your typical medical ablation laser may vary between 20W-100W.
The above approach could also be applied to the Darth Maul, Anakin, and Luke amputations. Care should be taken as these examples will involve cutting through bone. Either the heat required to melt bone should be included or the heat required to transform water in bone into superheated steam, and pressurize the steam past bone's failure stress should be included. Or another approach that does involve a laser ablation model should be used.
It should be noted that these estimates, at best, are extremely crude lower indicators of a lightsabers power and cutting ability. The cutting speed for the laser ablator is a physical limit whereas the speed used by Han may simply be limited by how hard he chose to swing (or limited by his clothes) rather than a physical limitation of the lightsaber.
Again we have little reason to model the lightsaber as a laser ablatar other than the false nomenclature implied by the term light as well as the localized heating characteristics of both devices. Considering we have seen lightsabers cut through bone, unknown Star Wars alloys, AT-AT armor, the true power characteristics and thermal heating ability are presumably many, many orders of magnitude higher.
Back to takehome.
[1] Jacques, Steven, "Continuous Laser Ablation of Carbonized Tissues: Simple Rules." News Etc, May 1998.
