ESOTERICA: On Laser Weaponry

October 25, 2017

 

 

I have mentioned in numerous places my interest in creating science fiction that relies on realistic physics wherever possible. Occasionally, this requires me to do some research on some unusual topics, and I thought I might share the results with you.

Today I am going to talk about lasers as weapons – specifically, how they work in fiction versus reality, and how effective a laser might be at punching a hole through a person. Most of my information comes from a variety of online sources, some less reputable than others (and therefore not strictly relied upon for hard facts), referenced at the bottom.

Laser weapons have been featured in sci-fi for close to a century now, and perhaps partly due to the fact that such weaponry was imagined before a real laser ever even came into being, most of the depictions of these weapons are wildly inaccurate. Of course, many games, books, and movies have avoided this inaccuracy by renaming the weaponry: blasters, rayguns, plasma rifles, disruptor charges, etc. But others have maintained the use, including such notable international sensations as the Starcraft franchise, which describes the Terran Battlecruiser unit as being equipped with “ATA/ATS laser batteries.”

The chief problem with most of these fictional laser weaponry depictions is that they aren’t – read, literally could not be - lasers. A laser is, by definition, a beam of focused light (or other EM radiation), and, therefore, travels at the speed of light. In other words, almost instantly. Most depictions of laser weaponry, including those in Starcraft, feature the weapons discharging ammunition in the form of discrete, brightly colored bolts that can be seen leaving the muzzle of the weapon and traveling to the target. In reality, a discharging laser would appear to be a solid line of light instantly materializing between the weapon’s muzzle and its target, connecting them like a rigid wire - if it appeared at all. In the absence of a diffusing medium (smoke, dust, and other airborne particulates) the laser beam would actually be invisible, and if the laser was using infrared or ultraviolet radiation, it would be invisible regardless of whether such a medium were present.

 

But this, as it happens, is just the tip of the iceberg. Once we push past the fundamentally inaccurate, we still have to consider the question of how a laser strike would actually affect a variety of targets. For a detailed description of how such a strike could be used against mechanical targets, the Popular Mechanics article referenced below will serve you best; the remainder of this post will be focused on laser weaponry being directed towards human targets.

 

Caution – this is going to get gory.

 

The chief inquiry guiding my research here was, “If you point a powerful enough laser at a person, would it pierce clean through their body and leave a cauterized hole in its wake?”

 

The short answer is, “Oh sweet gods, no, it’s so much worse than that.”

 

You see, despite the elegance the notion of a laser implies, it turns out that laser weaponry is extraordinarily messy against an active human combatant. Lasers can be supremely precise, of course; this is why they are used by surgeons. However, it should be noted that such lasers work not by cutting through tissue, but by burning it apart. This is the key point to remember about lasers as weapons: they aren’t just lines of “energy” that sever anything they cross like a lightsaber, but rather, can be best thought of as heat beams. A weaponized laser would cause the target to absorb thermal energy until it melted or combusted. As a result, the precision of the weapon is only part of the problem; the rest is a matter of how heat actually effects the human body.

 

Much of the “burning” in the use of surgical laser is actually the result of a process called “protein denaturation,” a heat-induced process that causes proteins in human tissue to cease their normal function. One of the chief ways this happens in softer tissues, like skin, fat, and organs, is through a tightening or coagulation of the proteins, causing the material to shrink and curl away from the heat source even though it hasn’t actually been destroyed. Muscle, on the other hand, is a much tougher customer. As you may know from having cooked the flesh of other animals, muscle only condenses slightly when it is heated – mostly, it just changes firmness. It may start to blacken at cooking temperatures, but will not actually be incinerated.

 

In other words, a laser beam that would do an excellent job of “burning” through soft tissue would only cook, and possibly char (with prolonged, sustained exposure) muscle. It would not pierce through it at those temperatures; to achieve deeper penetration, we’d need to go much, much hotter.

 

According to several crematorium resources, under relatively optimal conditions, human flesh (i.e. muscle as well as softer tissue) will not actually combust until temperatures near 1400 degrees Fahrenheit are reached, and even then, it takes close to an hour to burn all the way through a human body (ignoring the skeleton, which takes several hours more). Since a combatant is only likely to leave one part of their body in the sight of a laser for a few seconds or less, the user of the laser weapon definitely will not have an hour to shoot through them. But even assuming that the time to achieve total incineration wasn’t an issue, the time to achieve that temperature is another.

 

The specific heat of a human body (averaging all components) is usually around 3500 J/kg*K. Supposing a human body starts at 98 degrees F, that means a 1302 degree F increase is called for, or about 560 degrees K. Supposing a patch of a human’s torso weighed several kilograms (let’s say 3), that would mean that 3500 * 560 * 3 = 5,880,000 joules, or 5.88 megajoules of energy would be required in about 2 seconds of time, which would require a laser with a wattage of 2.94 megawatts. By way of comparison, a typical surgical laser is only 40 watts. A “burn through a human” laser would have to be over 70,000 times as powerful as a contemporary surgical one.

 

Well, that’s no great deterrent – in sci-fi we often imagine fantastic sources of energy and efficiency for established technology, so let’s suppose such a laser could, in fact, be made and directed at a human and that it would create sufficient heat to incinerate the flesh in its path. Here is where things get really messy.

 

The human body is made up primarily of water, and it uses that water to naturally dissipate heat to avoid injury; accordingly, laser heat would spread from the point of impact. In a surgical laser, the heat spread is reportedly limited to 50 microns, but with the wattage on the doomsday laser we’re considering, the spread would likely be much further. At first, this would actually work in the human target’s favor; dissipating heat reduces its intensity. However, heated water will eventually boil and evaporate, turning to steam, and at the relatively low temperature of 212 degrees Fahrenheit. Compare this to the sustained 1400 degrees needed to incinerate flesh, and you will begin to see the problem.

 

Well before enough heat has been absorbed to burn through muscle, the water in the target area will evaporate. With the wattage of our hypothetical doomsday laser, the evaporation would occur so quickly and forcefully as to actually be explosive, and instead of punching a nice clean hole through the target, the laser will most likely cause a massive, bloody crater to erupt on the target’s body in a cloud of steam and shredded muscle. To illustrate, let’s see some specifics:

 

A typical human is about 80% water by weight, so of those 3 kilograms in the torso we were considering, let’s call it 2.4 kg of water. The typical density of steam at the instant of vaporization (i.e. at 1 atm of pressure) is .6 kg per cubic meter. If all of the water in the targeted chunk of our unfortunate human were to evaporate, then, it would want to expand to a size of 4 cubic meters. But it would still be contained within its normal cellular spaces, which, by water’s typical density of 1000 kg in a cubic meter, would be only .0024 cubic meters. Using Boyle’s law of P1V1 = P2V2, we can determine the expected pressure within the cellular spaces: 1atm * 4 m^3 = x atm * .0024 m^3. Solving for x, the pressure within the cellular spaces would be nearly 1700 atm of overpressure (pressure above standard 1 atm). By way of comparison, 20 atm of overpressure was found by one study to be enough to mostly demolish heavy concrete buildings. Another source quotes 50 atm of overpressure resulting from the detonation of a 1-megaton nuclear warhead.

 

In other words, the vaporization of this water would generate nearly 85 times the requisite pressure to destroy buildings, and 34 times the pressure generated by a nuke. The body surrounding this expanding ball of steam would be lucky to even still be a body by the time the explosion was over, and not just a sticky, steaming pile of pulped human juice – and that’s to say nothing of what might happen to the surrounding bodies, including the person firing the laser!

 

Of course, we don’t know for sure if a laser would cause this sort of result, even at a greatly diminished magnitude, because we’ve never done any experiments that involve firing a high yield laser at living tissue. Moreover, thanks to the Protocol on Blinding Laser Weapons (an accord most countries have reached with the U.N.), there’s already precedent banning at least one form of laser weaponry (specifically targeting the eyes) against humans. Since most laser weaponry research now seems to be geared more towards destroying machinery, it may be a while before there is an opportunity to field test this particular case of physics.

 

In the meantime, we writers now know that lasers will most probably NOT ever be able to pierce clean through a target. Instead, they might be used to raise hellish, unspeakable boils within the target’s body that, in a fraction of a second, will explode like nuclear bombs beneath the skin.

 

As a side note, my current draft of The Abadon Relic features a character being shot through with a laser and just carrying on with the pain. Looks like I’ll need to do some rewrites!

Thank you so much for reading, friends. Please feel free to reblog with questions or corrections!

-Lyra A. Schneider

Sources:

http://www.atomicarchive.com/Effects/effects4.shtml
https://en.wikipedia.org/wiki/Overpressure
https://www.cdc.gov/niosh/docket/archive/pdfs/niosh-125/125-explosionsandrefugechambers.pdf
https://hypertextbook.com/facts/2007/DmitriyGekhman.shtml
https://en.wikipedia.org/wiki/Laser_scalpel
http://www.chegg.com/homework-help/questions-and-answers/1-average-specific-heat-capacity-human-body-approximately-3500-j-kgk-normal-conditions-spe-q2108015
http://themedicinejournal.com/articles/does-cauterizing-a-wound-really-work/
https://en.wikipedia.org/wiki/Protocol_on_Blinding_Laser_Weapons#Protocol_text
https://www.fda.gov/Radiation-EmittingProducts/RadiationEmittingProductsandProcedures/SurgicalandTherapeutic/ucm115910.htm
http://starcraft.wikia.com/wiki/Battlecruiser
http://boards.straightdope.com/sdmb/archive/index.php/t-430279.html
http://www.cremationresource.org/cremation/how-is-a-body-cremated.html

Popular Mechanics article:
http://www.popularmechanics.com/military/weapons/news/a19877/how-the-military-will-be-revolutionized-by-laser-weaponry/

 

 

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