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It's a common little tidbit in most fantastical magic novels to have little men and women in funny hats flinging ice cubes(albeit a little more dangerous than ice cubes but it's practically the same) at each other.

I'm not questioning their choice of projectile. If anything is shot with enough force it will cause damage, for example, an indestructible teddy bear.

Rather I would like to know what advantages a melee weapon(spear,sword, knives etc,etc,etc) that is close to the temperature of Absolute zero which is −273.15°Celsius / −459.67°Fahrenheit would have.

This is provided that my rabbits can hold the weapon properly and use it properly. Also assume that the weapon is always at the temperature of absolute zero with the help of handwavium, the pointy part, not the handle. And assume that the weapon still functions as a weapon, we can't have it breaking in the middle of a duel.

Now, regarding the weapons structure/material. I'm not sure whether this would affect the question but I'll state it anyway. I would assume most metals would shatter like glass at such temperatures so I would assume it would be made of ice or solid nitrogen or solid helium. But since its starting to sound like a stupid idea and the temperatures are already infeasible, assume that the weapon is made of unobtanium and handwavium.

What advantages would my Winterwrath wielding rabbits have on the battlefield or duels? And before I forget, it's in a medieval-esque world. Firearms are still superior and bringing a knife to a gunfight is stupid unless you run out of bullets. Oh, the rabbits don't matter much, it's just more fun to have rabbits instead of humans.

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    $\begingroup$ The swords would be perpetually covered in solid air. $\endgroup$ – SMS von der Tann Oct 3 '16 at 12:29
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    $\begingroup$ On the bright side, they can throw it at their enemies :D $\endgroup$ – Skye Oct 3 '16 at 12:32
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    $\begingroup$ Note that answers seem to assume that "Absolute Zero" in this case is basically the same as having a melee weapon which is incredibly close to Absolute Zero. This is because as far as real physics is concerned, trying to explain the properties of "Absolute Zero" is completely theoretical, like trying to explain the properties of something moving faster than the speed of light. $\endgroup$ – DoubleDouble Oct 3 '16 at 19:19
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    $\begingroup$ To make this less painful for those who are more familiar with thermodynamics, can you change this to be "almost absolute zero?" You're going to run into a horrible amount of thermodynamic trouble already with this weapon, but making it exactly absolute zero is going to induce all sorts of really undesirable side effects. You'll avoid a lot of answers that wont help you if you change the temperature just a little. (I recommend having it be 4 kelvin, because that'll behave virtually identically, but will be above cosmic background radiation, which makes me comfortable) $\endgroup$ – Cort Ammon Oct 3 '16 at 19:28
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    $\begingroup$ It can keep your beer cold in a hot day $\endgroup$ – jean Oct 3 '16 at 19:55
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They could set their enemies on fire, and power their artillery.

Hear me out. A perfect heatsink of that kind would liquefy the air around it. It would solidify if you left it in one place for long enough, but liquid will do. 21% or so of the atmosphere is oxygen, which means that these creatures would have an unlimited supply of liquid oxygen available to them. The nitrogen is almost irrelevant; all they have to do is stand their swords on end and 'stir' them above a bucket; the air around would condense and drip off and they could rapidly have a gallon or so of LOx to soak rag balls in.

Being cut by something cold is really not that much worse than being cut by something room temperature, and the accretion of ice on the blade (remember, there's a lot of water vapour in the air) would make them heavy, blunt and unwieldy. I suspect that this technology would mostly be used to make really, really big explosive devices and rocket fuel. The fog previously mentioned would also be handy, both militarily and as a permanent source of fresh water condensed from the air.

It would also revolutionise logistics, since transporting food long distances while keeping it fresh would be trivial; no small thing for a medieval society.

(I would agree about superconductors, but nothing has been said that implies these creatures have electricity. Or, indeed, physics. What we in the 21st century could do with a material at 0K which exhibited an infinite heat capacity is almost beyond imagination.)

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    $\begingroup$ I don't think you can split oxygen from nitrogen just by stirring them with a cold stick – their boiling point is quite near to each other, so you would get a liquid mix of both. (I also don't really get how your liquid oxygen would give you explosives, that might need a bit of elaboration.) $\endgroup$ – Paŭlo Ebermann Oct 3 '16 at 23:17
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    $\begingroup$ @PaŭloEbermann Wikipedia mentions turning things into unstable bombs by soaking them in LOx. $\endgroup$ – Nelson Oct 4 '16 at 3:03
  • $\begingroup$ Per Wikipedia: "Liquid nitrogen has a lower boiling point at −196 °C (77 K) than oxygen's −183 °C (90 K)" $\endgroup$ – Roger Lipscombe Oct 4 '16 at 10:01
  • $\begingroup$ if you left it in one place for long enough - this "long enough" is as close to "instantly" as the weapon's temperature is close to absolute zero. The ice on the blade would actually be frozen air, not just the water vapour. $\endgroup$ – Nick Volynkin Oct 4 '16 at 10:35
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tl;dr: You now have a universe-destroying weapon.

We need to clarify what Absolute Zero really is. You can treat it as a temperature, but it's really more of a concept: matter having zero energy (theoretically, it still won't, because of the uncertainty principle and the zero-point energy that comes from that). So for your weapon to maintain a "temperature" of absolute zero, it must be somehow discarding all energy which comes into it from normal heat transfer from its surroundings. Physically, it's regarded to be unattainable because of this. Let's say that you can magically hand-wave away that, and this weapon can always maintain absolute zero. To come up with a rough analogy, you now have a thermal "hole" in the universe, into which all heat will eventually flow and disappear.

Temperature change relates to the difference in temperature between two bodies. The fun bit here is that one of your bodies will never heat up, so everything around it, in contact with it, must always be cooling due to convection. Well, it will always be losing heat to the weapon, it could still gain heat from some other surface it is also in contact with. But that other surface would then be transferring heat energy to it (and losing that energy in the process), which would cool the other surface down, and eventually both would still be losing heat to the awesome power of the Absolute Zero Weapon.

And that would continue to happen. All energy would keep flowing in the direction of the Absolute Zero Weapon, and it would slowly siphon all the heat from the universe.

If you put some kind of magical restriction in place, stopping heat transfer beyond a certain radius, it would remove all the heat energy from that radius. No matter what, you're creating something that is capable of completely, totally, and absolutely destroying anything it comes in contact with (up to and including the universe itself).

Advantages: guaranteed to destroy your enemy.

Disadvantages: will destroy you, too .

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    $\begingroup$ While this is technically true, I'm guessing it would be at least a few billion years before anything outside the earth's orbit would be noticeably affected. (For instance, viewed from another planet, cooling the Earth to absolute zero is almost identical to replacing the earth with empty space. The only difference outside of eclipses would be a tiny, tiny patch of sky with a background radiation of 0 K instead of 4 K.) $\endgroup$ – Charles Staats Oct 4 '16 at 2:03
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    $\begingroup$ This isn't entirely true. While, on the one hand, you're right that the handwavium is violating the laws on physics on some level, you still need to consider the fact that it does so only on a small scale. It's only sucking the heat out of the universe at a slow rate and is incapable of freezing larger objects faster than they warm up due to conductivity (e.g. things reach an equilibrium). You can also avoid great thermodynamic issues by having that heat appear elsewhere, say, anti-handwavium hot weapons. $\endgroup$ – Danikov Oct 4 '16 at 9:06
  • $\begingroup$ True, if you can keep from violating conservation laws by having heat-creating weapons as well (well, you're still violating conservation laws in a sense, but it's better), you're mostly fine. It wouldn't be incapable of freezing larger objects, though, it would just take a very long time, unless you had the handwavium heat-creating weapons. $\endgroup$ – IfSentient Oct 4 '16 at 11:52
  • $\begingroup$ Make weapons 3-part: hot part, cold part and input. Basically magic refrigerator: you input power and hot part gets hot, cold part gets cold. Thermodynamics are in tact. And, since it's magic, you don't need those parts to be connected by something tangible. $\endgroup$ – Daerdemandt Oct 4 '16 at 14:04
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As a melee weapon, this actually doesn't really confer much effect. Unlike hot temperatures which can go to the extreme and create almost instant incinerating effects, cooling is a lot harder to do and requires a mixture of prolonged contact and/or a cascade of coolant (e.g. immersing in a stream of liquid nitrogen rather than a pool) to achieve a rapid effect.

At best your blades might have a cauterizing effect that may make their wounds less severe. Arrowheads or bullets, on the other hand, if they continued to suck the heat out of their target by simply resting inside them, could be very dangerous indeed. A handwavium splinter-based weapon, even more so.

Your rabbits might have more luck turning their cold handwavium to superconductors. Temperatures like that enable fun magnetic manipulation that could lead to giant electromagnets for countering metallic armour (especially in a siege situation), maybe primitive coil guns, or if you want to be fancy, multi-piece blades that are held together with magnetic fields or telescopic spears.

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Fog

Your bunnies would shield themselves in a cool fog and hide from their enemies. It's a good thing that they have fur because they'd get pretty cold standing/hopping around in that supercooled air.

Stabbing wise, the wounds would freeze-dry the flesh, so slashing flesh wounds would not bleed and also wouldn't hurt that much (nerves would freeze, so cuts would be numb).

Rabbits would be better off with traditional edged weapons at room temperature, I think.

Unless they like frozen sushi.

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    $\begingroup$ Actually I was thinking it might cause instant frostbite, so any nicks or scratches could be damaging. $\endgroup$ – Skye Oct 3 '16 at 12:33
  • $\begingroup$ Frostbite takes a while to set in after the flesh is frozen enough for the tissue to die. Like gangrene, it takes time for the dead flesh to rot. $\endgroup$ – Snow Oct 3 '16 at 12:37
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    $\begingroup$ My point, you don't have to kill your enemies then. A injured soldier would retreat with a thawing wound that would be bound to fester all manner of diseases. $\endgroup$ – Skye Oct 3 '16 at 12:41
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    $\begingroup$ This answer seems to be forgetting that no matter how cold something is, it won't have infinite heat conductivity nor will the materials you use it on. $\endgroup$ – Shufflepants Oct 3 '16 at 15:01
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Cold burn

Otherwise known as frostbite can occur. See https://en.wikipedia.org/wiki/Frostbite for more details. It is more likely that the solid air on your or weapon would be left behind on wounds, this would result in death of the surrounding skin cells and potential freezing of blood and tissue fluid. This means that attacks from this weapon could be deadly, it would have substantial intimidation effects after enemies see its effects. And who expects a spear to shoot out sleets of solid air.

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    $\begingroup$ Maybe if you're leaving the blade in them for several seconds. But I feel like if your opponent has time to just leave a cold blade in you for a few seconds, you've got bigger problems. $\endgroup$ – Shufflepants Oct 3 '16 at 15:10
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"Science-based": they could work as a bombs.

(Un)fortunately there are many problems with such swords, as they completely disobey laws of physics, yet you still want us to somehow use them. They are terrible in the sense that you can't really apply logic and real world for them, and even if you could(e.g. just very cold sword), this wouldn't help much anyway. Let's ignore how stupid such idea is and try to use it anyway.

The temperature is so cold that it would freeze the air itself. This would kill our bunnies real quick, so let's assume that they have a scabbard that protects outside world from freezing.

Bunnies would be quite fit. They would have even fitter commandos that would go to the enemy sites unnoticed, and take out their swords all at once. This would cause something like an explosion(I believe it'd be called implosion?) that would make everything around freeze, starting from air.

Send this commando into enemy base, and in no time you have it frozen. This is also terrifying as you have virtually no means to undo it, so reclaiming such ground or rescuing people isn't an option.

The only problem is that theoretically, after some time, the cold would get to you. But I'm sure some handwavium or magic could trap or limit the swords power.

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How cold can you make something? 0K is cold, but it doesn't describe how cold it feels.

Something with 0 heat capacity but 0K would not cool down anything. It would be a perfect mirror-finish and be a perfect insulator.

Suppose we made a 0K object that was the opposite of a perfect insulator. A perfect heat-suck.

Every particle that touches it loses almost all kinetic energy. The air turns to solid. Maybe to make it useful, it isn't quite zero, and it actually pushes the resulting solids away so they don't stick or somesuch.

How far away is does this energy-sucking aura go? The blade isn't really stuff at this point (as even stuff isn't stuff). We could imagine a blade that cooled things off rapidly within a range of a few mm or cm, unless contained by handwavium. The hilt and scabbard would contain said handwavium, preventing the rabbit from being frozen.

Swung through a target, it would freeze the flesh. This would prevent immediate bleeding, but effectively kill everything near the blade. If we add in the repulsion thing, where it prevents solids from being solid, it could cut through the frozen flesh as if it wasn't there.

So now you have a weapon that is an "ice blade". It generates flakes of frost when exposed to air. When it cuts into something, it leaves a clean cut with frozen edges. It behaves a lot like you'd expect a "frost blade" to behave like.

The blade cutting strait though flesh is most of the damage, but the whatever radius of leftover frozen flesh both increases the long-term damage and reduces the short-term damage (as the target doesn't bleed out ... until it defrosts).

The weapon is an infinite black with frozen flakes of "snow" falling off it. This "snow" mostly frozen air. The rabbits have a long handle and a large guard. Their scabbards have a "funnel" of handwavium at the entrance to keep the blade away from their flesh. "Grabbing" the blade leaves you with a frozen hand that dies as it thaws out.

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The very cold temperature will froze everything around, including air. Assuming the blade material is nonstick, the frozen air would be projected in the direction of the blade trajectory, allowing some kind of distance fighting ( your character could make some nice choreography and iced air would cut the enemy a few meter further.

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  • $\begingroup$ We might have a dripping air problem, but I'm sure it's fine... $\endgroup$ – Skye Oct 3 '16 at 12:36
  • $\begingroup$ Probably yes, but you may assume that due to fast motion, the low pressure behind the blade make the solid to sublime and the liquid state is only visible on the front side (in direction of the enemy), dealing additional damage over short time. $\endgroup$ – Adrian Maire Oct 3 '16 at 12:42
  • $\begingroup$ At best you'll leave a trail of rapid subliming dust behind the blade, not a sheet of solid air. The blade might be cooling the air, but the rest of the air and everything else around (via thermal radiation) is warming it up a lot more. $\endgroup$ – Shufflepants Oct 3 '16 at 15:09
  • $\begingroup$ In real world, sure! I am not even sure that a fast moving blade could absorb enough energy to solidify any air around. $\endgroup$ – Adrian Maire Oct 3 '16 at 15:32
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I think that the sword could easily penetrate most armors and cut through most things like a knife through butter, because most metals become brittle when super cooled and everything organic just becomes extremely easily breakable.

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    $\begingroup$ That would require prolonged contact. $\endgroup$ – Skye Oct 3 '16 at 12:47
  • $\begingroup$ Well, I would say if you parry the enemies attack once his sword is done for. $\endgroup$ – HopefullyHelpful Oct 3 '16 at 14:47
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    $\begingroup$ @HopefullyHelpful You'd be wrong. Even if you were to fiat that the cold sword itself had infinite heat conductivity, your opponent's sword most certainly will not. It's heat can only leave it so quickly and contact for a tiny fraction of a second won't be nearly enough surface area contact or enough time. I haven't done the math, but I wouldn't be surprised if the amount of heat generated by the friction of the sword contact were greater than heat lost due to conductive cooling. It could easily be that you sword could be slightly warmer after such a brief contact. $\endgroup$ – Shufflepants Oct 3 '16 at 15:06

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