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Let's say in a magical world, there exists a basic cooling spell which slightly cools down an area by reducing the heat energy of everything inside it.

In said world there also exists a particularly creative and capable mage that has managed to take this spell and massively compress it, reducing the effective area to a small cylinder but massively amplifying the effect in the process.

A few high-level enchantments later and now this spell is attached to a handle, creating some sort of incorporeal lightsaber which cools down anything it phases through. The incorporeal part means that the conventional protection against cold won't work, this weapon is capable of directly affecting someone's internal organs. The compressed spell can't be activated for too long at a time, so it can only last for very few seconds per activation.

Question is, just how effective of a weapon would this be? What would happen when this blade passes through a human being, rapidly cooling down a section of their body in their process?

To summarize:

  • I have a magical lightsaber-like weapon
  • When activated, creates an incorporeal cylinder blade ~5cm in diameter and lightsaber-like in length.
  • Activation is limited to about 3 seconds at a time, followed by a significant recharge period of 10-15 seconds.
  • Anything touched by the blade's area is rapidly cooled down, say a 15C temperature drop.
  • How effective is this weapon against fleshy beings like humans?

I'm trying to balance this so that it is an effective side-weapon when used right, but not so devastating that it's basically an instant win button.

Edit log: Freezing rate changed from 40-50 C/s -> 15 C. It no longer freezes over time but a flat temperature reduction, to promote swinging instead of just sticking it at the same place for the whole duration.

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  • $\begingroup$ does it also cut the limb like sword do? or just make the part that contact get froze bite? $\endgroup$
    – Li Jun
    Aug 12 '20 at 10:34
  • $\begingroup$ @LiJun no, it's incorporeal other than the handle so it's not very effective in dealing physical damage, and it also doesn't protect you from incoming attacks. $\endgroup$
    – votbear
    Aug 12 '20 at 10:41
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    $\begingroup$ sounds like shardblade then, even if they survive, the dead limbs part that still attached to their body will be a great hindrance and a dead weight to them. $\endgroup$
    – Li Jun
    Aug 12 '20 at 10:51
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    $\begingroup$ How is it different from an icicle? $\endgroup$
    – user6760
    Aug 12 '20 at 11:39
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    $\begingroup$ How do you define "instant win button"? Even in the scope of non-magical medieval warfare, a weapon that can instantly kill something within melee range once every 10 seconds is hardly a guarantee of victory. It would be more useful for intimidation/morale than it would be practically speaking if/when the mage charges onto the field. $\endgroup$ Aug 12 '20 at 18:39

12 Answers 12

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For an assassin, it's indispensable

The Hypothermia datasheet on Wikipedia will be quite useful.

40-50 degrees per second means one degree in 0.02-0.025 seconds.

At 35 degrees Celsius, or about 0.05 seconds in, mild hypothermia sets in. Normally the person would be shivering at that point, but things are so local and so quick, this won't have a chance to happen.

At 32 degrees, or 0.1s, moderate hypothermia sets in. If you managed to hit the whole brain, it would start to not function. If your mage is an informed biologist, they might aim at a specific part of the brain. But frankly, it doesn't matter.

At 28 degrees, or 0.2s, the brain straight up doesn't work. The heart also has trouble. That is, if they are being targeted.

At 20 degrees, or 0.4s, there are no vital signs in the affected area. If the targeted area is in the brain, it stops working. If the targeted area is the heart, it stops working.

Less than a second in, the blood in whatever area has been targeted has turned into ice. If that ice lies anywhere between the heart and the brain, the brain starts to quickly run out of oxygen. At that point, it would likely really hurt, but at that point they're already dead. If the victim is fast asleep, they might wake up at that point. Briefly. Even if you depart at that point, they're dead.

And frankly, even if the blade was much slower, as long as it has enough juice to create ice in the bloodstream, it's a great way to kill somebody asleep. And a pretty decent way to kill someone who's awake but not paying attention. VIPs might know better than to stand still when their chest is mysteriously feeling cold, but a regular low-budget security guy might not.

And the best part? No fingerprints on the victim, no chemicals that could be traced to the one place that sells it and you happened to be the only buyer this week, just a pair of small really frostbitten areas on the skin. On the other hand, if you happen to be the only criminal with a frostblade, that one piece of evidence might be enough to put you in jail, but your criminal syndicate should have plenty of good lawyers that will cast enough doubt to, perhaps with a little bit of evidence fuzzing, get you out scot-free.

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    $\begingroup$ This is mighty helpful, thank you so much! These benchmark numbers are precisely what I needed to know in order to better balance the numbers to my liking. $\endgroup$
    – votbear
    Aug 12 '20 at 12:45
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    $\begingroup$ Be careful. This is for the whole body. Not for a tiny slice. $\endgroup$
    – Trioxidane
    Aug 12 '20 at 16:30
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    $\begingroup$ It might be even worse. That data is for natural cooling from the outside, giving a chance for normal bodily processes to react. Instant cooling could introduce new problems. $\endgroup$ Aug 13 '20 at 3:12
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    $\begingroup$ "Localized hypothermia" isn't a thing. Unless you manage to cool the entire body, bloodflow will rapidly re-warm the affected body part. $\endgroup$
    – Phil Frost
    Aug 13 '20 at 14:49
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    $\begingroup$ @phil not sure about the reheating part before freezing, but after that the blood isn't flowing, which reduces the reheating from -123c ability significantly. It will melt for sure, but not nearly enough to stop the brain from suffocating. Even if you bring the 5cm of blood to just below freezing, it'll be enough to cause death. Even if the deep frozen veins don't shatter outright. $\endgroup$ Aug 13 '20 at 18:02
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Simply deadly

There's a lot about hypothermia I could say, but in the end it doesn't really matter. At 40-50 degrees per second it is very deadly. The body and blood consists mostly of bound water. Even thought he consistency might make it more difficult, a full second would be deadly. Blood would freeze, as well as most tissue touched. This would block the blood, produce high pressures in the veins and cause internal bleeding. Together with the burn from freezing, it's already going to feel like a knife or sword into your flesh where hit even for a moment. Imagine sticking your tongue to freezing metal, then imagine this everywhere you get hit as well as internally. It's likely to kill, even though for some it might take a moment before death sets in from internal bleeding and damage.

As a side weapon it is more dangerous than a normal knife. As it goes through armour and skin, you don't require force to kill. But if it's a side weapon I guess it's short ranged, making it too dangerous to use it as a main weapon. So yes, I would personally think it's a good side weapon you would only depend on as your secondary option.

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    $\begingroup$ sounds like its good as arrow blade or crossbow bolt to me. $\endgroup$
    – Li Jun
    Aug 12 '20 at 10:37
  • $\begingroup$ @LiJun a normal arrow or crossbow bolt is plenty deadly. I wouldn't use a magic weapon like that as it seems to be a huge cost to make something that is very deadly even more deadly. In addition, you might miss, making the cost a great waste. So I would use it only if it's incredibly abundant. $\endgroup$
    – Trioxidane
    Aug 12 '20 at 10:41
  • $\begingroup$ Thank you for the answer. It is indeed far too deadly for my liking - I'd much prefer it if this was a weapon that would slowly sap the strength of the enemy or cause internal damage, and will be devastating over multiple activations instead of a one shot kill. How much temperature drop do you think is the limit to make it have an impact on the human body, but not outright cause instant devastation? what about ~15/20C per activation, with ~10s cooldown per activation? $\endgroup$
    – votbear
    Aug 12 '20 at 10:46
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    $\begingroup$ unless you wear plate armor and shield to deflect it, i was thinking in a volley shot in warfare its unlikely you miss in big formation with plenty ppl in it. but yeah it all depend on the cost. $\endgroup$
    – Li Jun
    Aug 12 '20 at 10:48
  • $\begingroup$ @LiJun that's a good point, this would be awful as a mass produced weapon. I'll be sure to make this specialized weapon of one particular character instead of a mass producable one - maybe adding a requirement of for it to be held by a capable/compatible user. $\endgroup$
    – votbear
    Aug 12 '20 at 10:52
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I think you're dead.

You cannot parry your opponent's weapon with your incorporeal frostsaber; while you are cooling part of their body with questionable effect, they are sticking a sharpened piece of steel through a vulnerable part of your anatomy which is guaranteed 100% to be damaging.

I would suggest you work on icicle arrows, freezer darts, shiver spears or other ranged weapons that don't put you in such peril.

Also the problem is with warm blood circulating everything heats up again pretty quickly from a 15C cooling. A weapon powerful enough to actually freeze a portion of flesh -- ideally heart or brain -- would be a more effective killer.

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  • $\begingroup$ if the frostsaber a one hand weapon, it can block or parry by wielding shield to compensate, or dual wield it with normal weapon (risky against range weapon though), beside i heard steel can get weaker or shatter/crack upon impact in extreme cold (i dont know the temperature though) especially getting warm again due to expansion and contraction effect. $\endgroup$
    – Li Jun
    Aug 13 '20 at 1:37
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    $\begingroup$ @LiJun Steel requires temperatures significantly below -20°C to become brittle, so that's not a factor here. $\endgroup$
    – toolforger
    Aug 13 '20 at 16:05
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I do not think it would be very effective overall in a warfare situation. It depends of the situation you want to use it.

At minus 40 to minus 50 F (minus 40 to minus 45 C), hypothermia can set in 5 to 7 minutes, he said. A drop in body temperature prevents critical organs from working properly — including the brain and heart, according to the Mayo Clinic.

This is a problem of heat transfer between the beam/laser and the body, more specific the area that is being affected. In a classic combat situation where a swing of the sword would not last more than a second and would not have time to actually cool it off. You could have an impact if your sword would have a clean hit on the chest, around the heart area but even then if you cant keep it for all the duration and the victim is healthy and have a good metabolism than it will not do much, at max it will give him a good shock, which might be effective if you only want to incapacitate someone. And then there are other factors as the humidity in the air which would play a role or the environment.

However if you can make the best out of those 3 seconds, you could pull some really nice assassination, as it will all seem as a heart attack, especially if the person in question has a fragile body.

Overall I do not think it would be very effective combat wise, but I might be wrong so I encourage you to do more research on the body-heat transfer.

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    $\begingroup$ Thank you for the answer! I took the feedback from you and the others and modified it a bit - I want this weapon to be swung around (because its more exciting that way), so now it's a flat reduction of temperature per activation instead of cooling per second. The thing about body heat transfer is that a lot of it is about cold seeping in from the outside, not instantly applied internally, so my initial research attempts were not very applicable. $\endgroup$
    – votbear
    Aug 12 '20 at 10:54
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Do you want to make this a deadly weapon, or a secondary tool that can disable an opponent or give you an advantage?

The application that immediately popped into my head would be the ability to give your target brain freeze at will. While not enough pain to disable an attacker, it's certainly enough of a distraction to cause them to miss an attack or to be slower to react to your attack.

Your teeth contain very sensitive nerves and tissues that do not like extreme temperatures. These structures are (in normal teeth) insulated by the tooth's outer layers and only receive indirect stimulus. Your weapon appears to be able to bypass that insulation and thermally shock the sensitive nerves within. This would be an extremely effective disabling weapon. Based on personal experience with a cracked tooth (where the tooth pulp would sometimes receive direct stimulus), I can tell you that thermally shocking the inside of a tooth can cause a disabling, drop-you-to-your-knees level of pain. I don't want to imagine what it would be like if you hit a whole mouth full of teeth at the same time. You'd leave no actual damage, but could completely incapacitate an opponent for a few moments.

Along that same line, your weapon allows you to bypass the body's natural insulation and provide direct thermal stimulus to all sorts of nerves that are not designed to be directly stimulated. This could make the cold significantly more painful or impactful than if the same temperature were applied to the outside of the body. The body's insulation would slow the rate at which the affected area could warm back up, potentially allowing you to keep an opponent crippled in pain through repeated applications.

The human body has a number of automatic responses to thermal shock. Some of these can lead to hyperventilation or to a heart attack. Even the sudden temperature change of stepping out of a hot bath into a cold room can alter your blood pressure enough to cause a fatal cardiac event. It's much less likely to kill in a healthy individual with no underlying heart problems, but it's still a possibility.

For a less painful use case, wait until the target blinks and freeze their eyes shut. Their body heat will eventually melt the ice and restore their sight, but for a short while they'll be completely blind.

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    $\begingroup$ "For a less painful use case, wait until the target blinks and freeze their eyes shut." If your speed and precision allows you to hit a target the size of an eyelid literally in a blink of an eye then why don't you use a piece of metal for an effect that lasts a lifetime? $\endgroup$
    – zovits
    Aug 13 '20 at 9:15
  • $\begingroup$ @zovits Because you don't want something with permanent effects, just a temporary disabling condition that wears off and leaves no trace. $\endgroup$
    – bta
    Aug 14 '20 at 1:48
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It would be as deadly as today's sniper rifle - i.e. one good hit = death. The only difference is that rifle works at long range and is mostly useless in melee and your weapon would be the opposite.

And just like sniper rifle it would be absolutely useless in warfare, rather than assassination. Even more useless than a rifle, actually, considering difference mentioned. When your position is stormed by 100 soldiers, you'll hardly have chance and time to approach and slash each of them with your vorpalfreeze sword, while rifle would allow you to have at least some kills while opposing force is closing up.

That was true for initial "50C reduction" version of question - we're 60%+ water and our usual body temperature is around 36C, so it guaranteed cooling of water inside body below its freezing point. You could just stick your sword pretty much anywhere and have affected part literally implode and rupture almost every every cell of affected area. Even hitting just limbs could be deadly thanks to great shock and clogged blood vessels.

Your newer "15C reduction" version is much less deadly as it now requires precise hit into vital spot or cold enough environment to make at least outer layers of body cold enough to still reach water freezing point with "help" of your sword.

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I'm going to disagree with the answers here and say it wouldn't do much, with caveats below. The main problem is that you are cooling a small volume (a 5cm diameter cylinder moving through the body). Let's say your target is 40cm thick front to back and you're slicing through them across the chest side to side, let's say 50 cm. Based on your edit, let's also say the cooling is magically instantaneous. So you've got a 3cm x 40cm x 50cm slice of the body that is at 22C instead of 37C. That's less than 5L of body volume (less than 10% of an average human) at a temperature that's at worst lukewarm or room temp, spread out across a wide-ish region and sandwiched with warm human flesh above and below. Someone with a better recollection of calorific equations can calculate the energy required to raise the temperature of 5L of water by 15C but I think it wouldn't be particularly vexing. Remember we can easily handle ice-cold liquids in our stomachs - in my pint chugging days, I could down a fridge-cold (~4C) pint in seconds (and more than one if needed). And we can breathe in big lungfuls of cold air - our body warms it up by surrounding it by warm blood and tissues, as would happen here. It's even worse in a fight, when muscle activity is probably producing a lot of excess heat, and the body is overall in "heat credit".

The only caveat is the "magically instantaneous" part. There's a reason instantaneous isn't really allowed in physics. The human body deals well with gradual temperature changes of a much larger overall magnitude, but it's true (as others have pointed out) that you only find 22C internal organs in dead people. There may be some cellular changes (especially in organs that are normally well shielded from temperature changes) that occur at 22C that we don't know of - the heart's conduction may fritz, the brain may "freeze", you may have all sort of weird and unpleasant nerve stimuli as suggested by bta above. However keep in mind that donated human organs are cooled to lower temperatures than that (albeit with special preservatives/protective chemicals) and they work just fine. Also, hibernating mammals can reach ridiculously low body temperatures and wake up ok.

On a worldbuilding level, it also wouldn't be hard for people to start carrying heat pads, hot cocoa flasks, exothermic reactions, fireball spells or what have you - hypothermia is a slow process, which is why it's often associated with exposure (lack of shelter and sustained heat loss), and your spell is brief. If you want to use it as a weapon, it needs to cool down to freezing (expansion, lots of shattered little cell walls, tissue goes insta-necrotic when it thaws) or slam the whole body at once.

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    $\begingroup$ It used to be -50C. 5cm diameter - i.e. 50mm caliber hole of dead stuff that is no longer flesh, organs and vessels, but an inert and alien chunk of ICE anywhere in body is pretty much guaranteed instadeath. $\endgroup$ Aug 13 '20 at 16:32
  • $\begingroup$ The previous version had the same problem, actually much worse, because it lacked the "instantaneous" provision. 50C/s seems a lot, but if we envision a sweeping sword-like movement (let's say 1m/s, which is a very gentle sweep), each human voxel in the path only experiences 0.05 m / 1 m/s = 0.05 s of cooling, or 2.5C. It would probably be pleasantly refreshing. It would only work if you held the "sword" quite still (it's long and narrow, so small changes in angle would add up) for a non-trivial amount of time (seconds) on an immobile target, making it entirely unsuitable for combat. $\endgroup$
    – Guest
    Aug 14 '20 at 17:38
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Normal body temperature is 37 degrees Celsius. Hypothermia severe enough to cause incapacitation sets in around 28 degrees, only a 9 degree temperature loss.

So on the face of it, a weapon capable of cooling anything by 15 degrees would seem at least capable of causing hypothermia severe enough to cause incapacitation.

However, hypothermia is caused by a temperature loss in the entire body. Cooling just part of the body doesn't necessarily cause hypothermia, because blood flow and conduction will restore thermal equilibrium throughout the body.

So if your aim is to cause incapacitating hypothermia, think about the total energy loss required, not the temperature change. A temperature change of 9 degrees over the entire body is equivalent to a change of 15 degrees over 40% of the body. Meaning, in the 3 seconds of activation time, the attacker must sweep the weapon through at least 40% of the target's body mass.

Seems a little...awkward, especially if the target is resisting. Perhaps with a surprise attack from behind, it could be done.

Furthermore, hypothermia happens slowly, in situations where the cold exposure is constant. Comparing the instant and temporary cooling effect of a magical weapon to hypothermia is questionable. I don't know how one could medically test this, but I suspect even with an instant cooling of the entire body to hypothermia temperatures, it takes at least a minute for the symptoms like unconscious to set in, especially when the target has just received a good dose of adrenaline from the surprise of being attacked. In that time, it's likely an otherwise healthy opponent's temperature will have significantly recovered, so you may not ever be able to render the target unconscious.

At best, the weapon may cause symptoms similar to cold shock. It would be surprising, and perhaps confusing if it's not a common item. And chilling of the muscles will certainly impair the opponent's coordination and speed.

But it seems to me there are non-magical weapons that are far more effective at incapacitating an opponent with a surprise attack. Like, a rock to the head. Furthermore, rocks, knives, and crowbars are not subject to any "recharging period" and they are easier to obtain.

In short, seems like a pretty silly weapon to me, unless your objective is specifically non-lethal. Even then, you'll need a reason why it would be preferable to existing non-magical weapons like tasers, baton rounds, or pepper spray which are easier to use, have longer range, etc.

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  • $\begingroup$ Thank you for the answer! The part about the cold shock is pretty useful - lots of answers focus on lethality, but other ways this can impair someone is great to know too. In fact I'm specifically trying to find that sweet spot of nonlethal but still incapacitating enough - simply killing with magic isn't exactly difficult, the real flex of magical expertise is being able to finely adjust the power output to reach that sweet spot ;) $\endgroup$
    – votbear
    Aug 13 '20 at 19:17
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If we go by the high number of -50 C/s and a maximum time of 3s, we get a temperature change of around -150C.

The human body has huge parts which are more or less liquid. Such as the blood.

Lets look at blood first. Blood freezes at around -2 to -3 degrees C. So it becomes solid. And as everything, the blood will expand. But for that, we have to look at the whole body. Almost everything in the path of the wand will solidify pretty much instantly. And thus expand.

There isn't really much to say about what will happen. Everything that freezes expands to, depending on what it is, a few times its original volume.

So my best guess is that the affected area would just sort of "explode". Even if you just apply the force for a sec or so, everything is just cooled down so fast that there is really no different outcome.

And I hate to be that guy, but this is a really OP weapon.

Lets think about it, you can break evey lock open by just using humid air. You can disable large vehicles such as tanks or helicopters by freezing parts of their engine, thus blowing it up. There is no armor against it as it will just sort of blow up.

What is so OP about the weapon is the speed. 3 sec is not long enough to react. If you made it 3 min or so to cool down to -150 C, it would be fairish. But still really OP.

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  • $\begingroup$ Yeah, that's true. The first step is definitely to reduce the rate of cooldown, since currently it's just far too much. I'm also thinking of drastically reducing the duration it's enabled, or maybe give it a longer duration but making the cooling effect much slower like you said. $\endgroup$
    – votbear
    Aug 12 '20 at 10:40
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    $\begingroup$ decreasing a temperature (including freezing) reduces size rather than increasing for all materials other than water (H2O) so your answer doesn't seem valid to me. Water expands indeed while freezing, but only near the freezing point and it is not so huge expansion (I don't remember details, but it's something in the range of 10-20% of volume). So everything about the explosive nature of the process is incorrect. $\endgroup$
    – Ister
    Aug 12 '20 at 12:00
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    $\begingroup$ Uhm, no ? Water is just about everywhere inside the body. Sure, some Materials dont expend when frozen. But Water does. Blood is 92% or so water. Flesh has anywhere from 30 to 80% Water in it. So if you shock freze the whole thing, everything expands at once, but stuff can expand into itself so it has to go outwards. Thus a sort of Explosion. $\endgroup$
    – Erik Hall
    Aug 12 '20 at 12:22
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    $\begingroup$ Have you ever seen someone put a vegetable or fruit into a vat of liquid nitrogen (usually to shatter it with a hammer a few seconds later)? Those are mostly water. They do not spontaneously explode. The expansion of water as it freezes is simply not enough to cause an explosion. It'll rupture cells and cause tissue damage, but that's just frostbite. $\endgroup$
    – BBeast
    Aug 13 '20 at 6:02
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    $\begingroup$ "Everything that frezzes expands to, depending on what it is, a few times its original Volume." Umm, no it doesn't. Water is rare in that it expands about 9% (according to a quick Googling) when it turns to solid at freezing point, and if you make it colder, it shrinks a bit like any solid. 9% is not going to be any kind of a problem for non-frozen tissue either. $\endgroup$
    – hyde
    Aug 13 '20 at 6:22
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It would be discomforting.

15C drop will barely even be noticeable. Even 60C will do almost nothing. A quick search tells me a 16oz bottle of water takes 1.5 hours to freeze in a typical freezer. My freezer typically shows a temperature of -20C, giving us around a 60C difference. I can hold that frozen bottle of water with no damage for more than enough time to retrieve a weapon and fight off an attacker. More evidence that it will not be effective comes from a search on times for hypothermia to set in. In freezing water it can take 30 minutes to 90 minutes for hypothermia to set in.

https://youtu.be/jnsDdJwW-KU This video shows a man holding liquid nitrogen in his hand till a small spot freezes. What was interesting, is that it only takes a second or two to return pretty much to normal. Liquid is -195C. For your weapon to do anything, you will need it to be a much greater differential than 60C.

As the question is stated, it currently cools by 15C in a small area and is inactive a considerable percentage of time. In the inactive time, the body will heat up a bit. Since it is a swinging weapon I would picture this being used against an active combatant. If they are not, why not just directly target a critical body part rather than concentrating the cold into a weapon. I picture a person with a sword being able to slay the person with the ice lightsaber since it will take too long to kill. You won't freeze any part since that would take multiple strikes at the same location and in active combat the body will be warming itself.

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  • $\begingroup$ There's quite a difference between making contact with something cold and actually removing heat from the body. You can take stuff from your freezer safely, because the heat transfer from your hands into the bottle is quite slow. The effect OP describes is more like if you have held the frozen bottle for longer time (say, 15 minutes), but in an instant. The skin in the video unfreezes almost instantly, because it's been affected very shortly. The -195C temperature is irrelevant, if you keep it that short, the actual energy transfer is what counts. $\endgroup$
    – Frax
    Aug 13 '20 at 8:54
  • $\begingroup$ The question clearly explains the magical effect instantly cools the target, unlike putting something in a freezer, or liquid nitrogen. $\endgroup$
    – Phil Frost
    Aug 13 '20 at 15:39
  • $\begingroup$ Right, it's the energy transfer and a 60C difference is insufficient to do real damage in 3 seconds. To make this work you have to hand wave things. The only advantage this has over a sword is it's incorporeal. Flash freezing may not even do any damage, there is a whole industry based on the idea you can be completely frozen and thawed back out. Flash freezing doesn't rupture cells. To actually do damage you need to freeze something slow enough to cause I've crystals to form and rupture the cells.... $\endgroup$ Aug 13 '20 at 16:36
  • $\begingroup$ The premise of the question even backs me up. It's an air conditioning spell that instead of cooling a whole house cools a sword sized space just won't have the power to damage. $\endgroup$ Aug 13 '20 at 16:39
  • $\begingroup$ Just reread the question it contradicts your instantly cools. The question reads "Anything touched by the blade's area is rapidly cooled down, say a 15C temperature drop.". As the question currently reads, this weapon does effectively no damage. $\endgroup$ Aug 13 '20 at 19:13
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Another angle to look at might be frostbite. It happens at about -4 C at which it's effectively destroyed. Body temperature starts at about 36 C, so that takes about 3 seconds of exposure. A typical sword seems more effective if targeted at limbs. It should cause serious organ damage, maybe better than a stab to the brain or lungs would.

The difference is how cooling one portion would affect the rest of the body, but I suppose the effect would be similar to bleeding. It seems like something to use as anti-armor rather than a conventional weapon.

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It depends on the part of the body that's suddenly cooled. Several of the other answers consider what would happen if the external temperature was lowered by 15 °C, but the weapon is able to cool down the internal tissues - almost instantly delivering the effect of being plunged in ice-cold water for several minutes.

On the one hand the effect is not as bad because the cooled volume is way smaller. On the other hand, the organism has had no time at all to adapt. You have warm tissues next to cold tissues, "expecting" the cold tissues to perform when they just won't be able to.

White muscles (bowels, stomach, etc.) - not very much damage happens. The cold will trigger the normal pain reflex, which in the case of the abdomen can be crippling. A seasoned warrior might be able to withstand it.

Striated muscles (arms, legs, shoulders, belly, back): the muscle will seize, possibly damaging itself in the process. Severe muscle pain. Massive vasoconstriction, followed by very painful restoration of function.

Heart: don't know, but arhythmia is a definite possibility, and I wouldn't rule out a heart attack. It's one thing for a transplanted heart at rest to be slowly cooled down; it's a completely different thing for a heart in the heat of the battle.

Brain and, more than that, the blood vessels therein: immediate loss of consciousness, very likely instant death or loss of brain function (possibly recoverable, possibly not. This would be the equivalent of diffuse axonal damage). The vasoconstriction/dilation effect would be painful in the extreme, similar to hemicrania - you get a taste of that if you cool down the sphenopalatine ganglia with ice or very cold drinks: as soon as the vasodilation starts, the whole meningeal network starts signaling pain. This happens with a localized cooling down of about 8-10 °C and a significant thermal mass behind. With a cooling down of double that and a cooled down volume behind to reduce the overall thermal mass, the pain might be enough to cause loss of consciousness by itself.

Lungs: oxygen solubility in affected tissues would change by as much as 30%, which would likely briefly reduce breathing capacity. Water would also condense in the lungs, stimulating a coughing fit. All in all, no great mischief here.

Eyes: immediate sight loss, probably recoverable. Again, excruciating pain from the facial nerves. In fact, I believe a hit to the face would be immediately incapacitating at best, killing at worst, since it would likely also involve the brain.

Chances of blood clotting are low, but blood viscosity will locally increase by as much as 25% and this might cause perfusion differentials. Little damage, but temporary loss of function, especially in already stressed tissues (e.g. muscles in battle).

As I see things, the first cold strike would noticeably hamper a swordsman, rendering them vulnerable to a second and better aimed strike. After that, the victim is very much a goner, as the third strike would find a defenseless and very likely unconscious opponent.

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