StarDestroyer.Net BBS

I am taking a well deserved break from my take-home final and decided to post this.

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.

Impressive. Your premise seems about right, since armor and other metals do tend to show more resistance to lightsabers. It could be due to better thermal properties, higher density, or a combination of the two.

I should note that the body temperature of a tauntaun may be colder that 37 degrees due to their environement.

Does this allow for quantification of the thermal properties of Star Wars alloys?

Hardy wrote:Impressive. Your premise seems about right, since armor and other metals do tend to show more resistance to lightsabers. It could be due to better thermal properties, higher density, or a combination of the two.

I should note that the body temperature of a tauntaun may be colder that 37 degrees due to their environement.

Does this allow for quantification of the thermal properties of Star Wars alloys?

It might allow for quantification IF:

A) The lightsaber does infact cut in a manner similar to a laser
B) The lightsaber could only cut the tauntaun as quickly as Han did so, or within a certain, small, range.

I think A is a fair assumption but AFAIK that cannot be proven, and we have no way to know if Han could have cut faster or not, so B kills us anyway. But nevertheless I am impressed by the analysis!

The Silence and I wrote: I think A is a fair assumption but AFAIK that cannot be proven,

Well, of course that assumption is the basis of the analysis if I'm interpreting it correctly. It does appear to heat up the taunatun's flesh in a similar manner, so it seems reasonable to assume that the process is similar. The mechanisms are very likely different, though.
I agee with B, though.

Hardy wrote: Well, of course that assumption is the basis of the analysis if I'm interpreting it correctly.

You are.

To be honest, I am not entirely happy with that assumption. I do like the fact the properties of the coherent beam (focusing, power, duty cycle etc) appear to affect the irradiance term only, and that this equation describes another class of laser, with a slight modification. It appears to be mostly driven by the irradiance term and the heating term Q, which is object dependent.

Still, it would be best to look at the derivation of the formula to see what assumptions were made. Perhaps after finals.

While both A and B are questionable, I think B is really the kicker. Han's jacket seems to be very confining so I suspect he was limited more by range of motion than anything else. In the films, we do not really see an example of someone struggling to cut through tissue or even bone (indicating that organic resistance is not a significant factor) so I doubt the speed constraint imposed here is valid.

Thanks Hardy and The Silence and I for the feedback.

My pet theory was always that the lightsaber "blade" is hot plasma. The lightsaber itself has some sort of retractable magnetic wire in it, and when you turn it on the plasma (probably superheated air, like a blow dryer but way hotter, based on the lack of any apparent fuel tank) is magnetically attracted to the wire and coils around it to form the blade. The thing I like about it is that it explains why lightsabers bounce off each other like regular swords. If lightsabers were lasers or some other form of energy the blades should just pass through each other.
The real thorny problem is how they deflect blaster bolts. So far I can't think of any explanation whatsoever for that.

Junghalli wrote:My pet theory was always that the lightsaber "blade" is hot plasma. The lightsaber itself has some sort of retractable magnetic wire in it, and when you turn it on the plasma (probably superheated air, like a blow dryer but way hotter, based on the lack of any apparent fuel tank) is magnetically attracted to the wire and coils around it to form the blade. The thing I like about it is that it explains why lightsabers bounce off each other like regular swords. If lightsabers were lasers or some other form of energy the blades should just pass through each other.
The real thorny problem is how they deflect blaster bolts. So far I can't think of any explanation whatsoever for that.

I like this. It also helps to get around the finite length of the blade. I thought about the plasma route but decided to punt on it due to my lack of knowledge of plasma physics and plasma thermodynamics.

Junghalli wrote:My pet theory was always that the lightsaber "blade" is hot plasma. The lightsaber itself has some sort of retractable magnetic wire in it, and when you turn it on the plasma (probably superheated air, like a blow dryer but way hotter, based on the lack of any apparent fuel tank) is magnetically attracted to the wire and coils around it to form the blade. The thing I like about it is that it explains why lightsabers bounce off each other like regular swords. If lightsabers were lasers or some other form of energy the blades should just pass through each other.
The real thorny problem is how they deflect blaster bolts. So far I can't think of any explanation whatsoever for that.

Plasma will not glow green at any temperature. This is the chief problem with that solution. The second is that if the color of the laightsabre was determined by its released heat, then why does the ANH novel say the blade emitted no heat?

The ROTS ICS makes mention of lightsabres in the wheelbike page. It's basically the same as a blaster bolt or TL blast, but contained in a kind of shield.

DM: Low end quantification of qui gon vs the blast door put lightsabres at 200-400 MW. Good work though.