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Exactly what could an obsidian blade do, if it were unbreakable?

Knapped obsidian holds an edge one molecule thick. It's sharp enough to cut between human cells, and is even used in surgeries today.

One of the conceits of this setting is that items of mythic power can only be broken or altered under specific circumstances. For all other practical purposes, a relic won't burn, shatter, warp, etc.

So an obsidian blade relic wouldn't be encountering issues of chipping or snapping off when directed at plate armor, stone doors, or the like. No, breaking the blade isn't an issue - though perhaps friction or binding would be. What other concerns and limitations would a wielder realize, with the use of such a blade? Would it get stuck in things, despite its unbreakable and peerlessly sharp edge?

Feel free to use hard science or common sense to describe answers. I've a basic understanding of tensile strength, shear strength, etc., but I'm looking for a way to describe this to readers in simple terms.

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  • $\begingroup$ Hello, you who I know I don't know. Welcome to worldbuilding.SE, get lost and never find out again :p I like your first question and hope you have a good time here. Is your relic absolutely undeformable? A wooden knife will never cut steel, because it is much softer. When sawing long enough (without destroying the knife), you might remove a splinter, but this is like waiting for it to rust. Long and not satisfying. Likewise your obsidian relic wouldn't be able to do some things because a normal obsidian <whatever> couldn't do it. $\endgroup$ – J_F_B_M Mar 28 '16 at 5:50
  • $\begingroup$ Is your blade one molecule thick along the whole blade, or is that the width at the edge? Also what do you mean by binding? Spontaneous fusion to the material it cuts (aka cold welding)? $\endgroup$ – J_F_B_M Mar 28 '16 at 6:43
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    $\begingroup$ There's a Scholastic book series called Everworld by K. A. Applegate where there's a mythological race of advanced metallurgists (that supposedly look like Groucho Marx) that created an unbreakable knife that can cut through anything for the main characters (they were expecting a bigger weapon, too). They jokingly nicknamed it "Excalibur." Good book series, highly recommended. $\endgroup$ – The Anathema Mar 28 '16 at 15:29
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  • $\begingroup$ If the blade's edge was really one molecule, the edge would be closer to 0.3nm than 3nm, given a length of 0.16nm for the Si-O bond, and an average distance between the two oxygen atoms of 0.227nm (which is by far, far, far the largest contribution to the molecule's size). Wait, now I can't find who said "3nm" any more. Where did I see that? $\endgroup$ – Damon Mar 29 '16 at 10:26
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So you got an indestructible obsidian blade, now what?

It is not the ultimate weapon of all-cutting. While being useful against soft materials even in combat situations, harder materials might need more strength to achieve the same result in the same time. Ultimately you should be able to cut almost anything with it, but it will take time.

And yes, you still can get stuck. It will be easier to free than a steel blade, but ultimately an obsidian surface still experiences friction.


Usage as weapon

You will still have a hard time cutting any solid surface. For this matter, skin is not really solid*.

To cut through for example a metal plate you have to first break the binding between the metal-atoms, which is pretty strong. Luckily your obsidian blade focuses all the strength you put in on a single-atom width line, so you can probably cut the first layer of atoms quite easily.
For the next layer you not only have to cut it, but also push the first layer a bit to the side, as blades are usually getting a bit wider away from the edge (otherwise you had a blade that would be only one atom in width). For every** deeper layer you have to deform all the layers above.

This means you probably can not simply cut through the steel armor of your enemy, unless you hit him really hard. You may possibly not even cut his sword in half, though you could leave some nice cuts along the edge. It would be better for your enemy to replace the sword after the fight, but that isn't helpful to you now.

You're biggest problem is not that your sword might not be able to cut it, but that it might miss the cutting power. Weapons made from Iron do not only cut, they also crush due to their weight. So while an iron sword may not be able to cut through a particular leather-armor on first strike, due to the weight and the inertia it might still carry on where the obsidian blade due to it's smaller mass simply can't.


Can it get stuck?

Sure, put an anvil on it and it won't move an inch anymore. A blade can cut only at the edge, if something blocks the blade it is the same af if you put a sheet of metal (for example) between the two blocking objects. The only advantage you have over metal is that the surface of your obsidian can be much smoother, allowing you to pull it out easier thanks to a lower friction. But you still have friction, and you still have to overcome it.

Can it get stuck when chopping wood?

Yes again. Penetrating the wood might be easy, but because it is wet the wood will expand slightly after cutting it, as there now is a gap of air. Then you have a whole tree pushing on your blade. You can likely pull it out in a second or two, but you should still pay attention when fighting (between) trees.


FAQ

If I drop the blade (without the handle), will it drop to the core?

No, not likely. You still have to break apart the bonds in the material you want to cut, and the blade is not heavy enough for that. Also, the core is really dense, so it probably float on it's surface.

If the blade could cut everything like hot butter, would it be the ultimate weapon?

Just because you can cut everything doesn't mean everything waits to be cut. Enemies may take you out from afar, or dodge your blow to land their own. Also cutting a tunnel through a mountain might be a bad idea, due to cave-ins. Last but not least, having a good blade doesn't make you a good blade-wielder.



* As we are talking about a 30 Angstrom blade and semi-free moving lipids, you shouldn't have a problem cutting through skin.

** Technically once you made a cut deeper than your blade is high you don't have to completely account for the upper layers anymore.

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  • $\begingroup$ @sumelic Brain.exe not working. I was looking for a heavy metal object, thought of an Anvil and forgot to translate it to English. $\endgroup$ – J_F_B_M Mar 28 '16 at 11:38
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Another answer has already given the basics:

  • Piercing is only part of cutting
  • The blade behind the edge is still subject to friction

There are a few additional points specific to blades made of real-life materials, however, that may be of value in thinking out what your superlative weapon can and cannot do.

Terminology

To do this clearly, we need some basic terminology.

Edge: The edge is formed where the two bevels meet.

Bevel: The bevels are the flat parts that extend directly back from the edge at an angle.

Centerline: If you a draw a line from the edge to the spine, this forms a single line. The angle of a bevel is measured from the centerline with the vertex at the edge.

Flat: The portion of a blade that runs more or less parallel to the centerline, often incorrectly called a bevel (or, in reverse, bevels are mislabeled as flats).

Friction is an issue along the edge while dragging, and along the bevels and flats when deep into a cut.

Teeth

All blades have teeth, like saws. These can be large or small, evenly or irregularly spaced, consistently or inconsistently sized. They can be perfectly in a line, splayed, or in multiple parallel lines.

When you try to cut something, you normally slide somewhat along the teeth. This is much the most efficient method. Each tooth penetrates in sequence, splitting the gap further open as you go along.

Because obsidian is volcanic glass, the teeth may or may not be parallel or in a line. Because the material has to be napped, the teeth will tend to be irregular in size, shape, and spacing.

This means that if the blade was originally just traditionally-napped obsidian your cuts will always be jagged. No matter what you do, your blade is always going to act like a serrated bread-knife or steak-knife.

Shape (Profile)

With a real knife, if you had ultra-tough-and-hard materials, you'd polish the edge to bring all your teeth to a highly regular pattern of consistent size, but this blade can't be sharpened. And that's a problem.

If you have materials that just won't break, the obvious thing is to grind the bevels to as narrow a total included angle (the angle of both bevels added together) as you can manage, then refine and polish the teeth until they're super-small and perfectly even. This is how a razor is made, for instance. Normally, you don't do this with a weapon, because as soon as you hit something it'll crush, but that's not a problem here.

This pattern is ideal, because your knife won't wedge. That is, it has very narrow shoulders, because the bevels are extremely thin. When you cut, the friction of the cut material against the bevels is a significant part of what stops you from sliding through easily.

Unfortunately, you can't sharpen this thing. Since it's impossible to break, it's impossible to abrade. So you're stuck with whatever profile you started with.

Cutting

Once you focus on the fact that your blade is always a saw, you'll realize that the usual smash-with-the-edge approach we associate with swords is grossly inefficient. The best thing is to drag the edge as you hit. So how well will your obsidian weapon cut?

The more regularly-spaced, narrow-beveled, and small the teeth--and the more they are all precisely in line along the centerline--the better it will cut.

If nothing can crack those teeth despite a gross lack of supporting material (very narrow bevels), then the thickness of the blade at the spine can be ridiculously thin. This will dramatically reduce friction. It will also make your blade weigh far less. Normally weight in swords is a serious problem, since steel is very heavy, but a very thin blade won't have this problem.

Conclusions

You need first to decide what this thing looks like. Does it look like a thin, elegant sort of straight katana, but black as night? Or is it a massive, primitive chunk of death, with big irregular fangs? That will tell you what the tooth pattern should be.

Next, decide how heavy it is. Obsidian's volumetric mass is about 1/3 that of steel, for comparison. Bear in mind, if you make comparisons to extant swords, that many swords are not solid steel precisely because it's so darn heavy (katana are the most obvious example here). From a rough weight--say, that of a baseball bat?--you can work out the volume of the blade.

Once you know that, you can decide how thick you want the blade to be at the flat, and how wide you want it from edge to spine (or if it's two-edged, from edge to edge across the spine).

With all that in hand, you can work out quite easily how the thing is most efficiently used. Probably the best is going to be to hit somewhere around the lower third of the blade and then, while continuing to press as hard as possible, to drag the haft backwards. If the teeth are small and regular, it'll glide through anything; if they're large and irregular, it'll go clunk-clunk-clunk and make a nasty gash. If the bevels are narrow and the spine thin, it'll soon be a very, very deep cut; if wide and thick, not so much.

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You said the blade cannot be broken or altered, but is that in the long-term or at any given instant? If the blade cannot be altered at all, ever, it has infinite hardness, which would cause a whole lot of physical issues on a whole lot of levels - for instance, since its molecules could not vibrate, it would be perpetually at absolute zero temperatures. It isn't really 'magically enhanced obsidian' at that point, but an adamantine-like material that is completely unlike anything that can exist according to physics.

Since this is presumably not what you are looking for, it seems that the most logical interpretation of 'unbreakable' is that relics can be deformed in the short term if a force is applied to them which would break or deform their natural materials, but if they are deformed, they immediately spring back into their original shape once pressure is released (presumably through magic). In other words, relics have effectively infinite elasticity, instead of hardness. This could also cause weird effects (for example, if you stretched it to the point where it became a line of single atoms several million miles long) but that would require forces not ordinarily encountered in a typical fantasy setting so would be less problematic than the 'infinite hardness' interpretation.

Such a relic, even if sharpened to a single molecule, would not really be an 'ultimate' weapon by any means. Obsidian blades are sharp, but brittle, and not particularly hard compared to most weapon and armor quality materials (which is why they can be knapped so well). Your blade would cut through flesh, cloth, leather, and soft plant matter like nobody's business, but trying to penetrate steel or any other material that would shatter or blunt an ordinary obsidian blade would be like hitting a stone wall with a Nerf foam sword - it might not break your weapon, but you clearly won't be getting anywhere.

An unbreakable obsidian knife would be useful for leatherworkers, surgeons, butchers, chefs, jungle explorers, hunters, theives, and backstabbing assassins. For fighting armed and armored opponents, you're better off going with conventional steel.

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    $\begingroup$ I assume it can be broken. "Thus may this blade born from the fire and the earth serve you in slaying your archnemesis and all his foes." Nobody told the hero it would shatter when entering the archnemesis, ending it's life and the blade. Come on, everyone knows how relics work. As long as you not test them they serve you well, but god beware you try to abuse their indestructibility... $\endgroup$ – J_F_B_M Mar 29 '16 at 13:31
  • $\begingroup$ I thought it might be similar to Dwarf Fortress artifacts, which never accumulate damage or wear but in every other sense operate like high-quality mundane items of the same material, and are similarly unable to penetrate armor harder than themselves. This would be a reasonable way of explaining how that could work. $\endgroup$ – IndigoFenix Mar 29 '16 at 13:54
  • $\begingroup$ Now I know why adamantine rang a bell... You already had !FUN! today? Anyway, weaker materials are able to damage harder materials, though usually you may need multiple tries. So no, you won't cut that stone door open anytime soon, but you may leave scratches (and, with usual blades, lots of obsidian-shards). $\endgroup$ – J_F_B_M Mar 29 '16 at 14:04
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Considering that it can split cells and it sharp enough to cut through anything, it should never be able to get stuck. It should be able to move in a straight line wherever the blade it pointed, so long as the hilt can fit. It can't get stuck because it should be able to move back along the path it has already cut.

If only the cutting edge is that sharp, then it reverts back to the basic physics of normal swords, with the special property that it can easily create the first pierce of any material. This does give it better cutting and slashing powers, but it doesn't help the sept cut through stone unless enough force is applied to create a cut wide enough for the rest of the blade.

For basic application, it just becomes a super weapon and possible a very useful tool for crafting. You can cut anything to shape and, potentially, make items extremely flat.

This is just a rough presentation of my thoughts. If you could clarify if the entire blade is only 1 molecule thick or otherwise, I'll come back and make some edits

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    $\begingroup$ Ah, 5 minutes to much research. You have a good point, if the whole blade is only one atom wide, you should always be able to pull it out again, but if it is double-edged and even slightly wider in the middle you might already get problems. $\endgroup$ – J_F_B_M Mar 28 '16 at 6:39
  • $\begingroup$ There is no justification, I'm afraid, for thinking that the faces of the blade are frictionless, which is what your claim requires. While soft tissue won't be much of a problem, hard materials such as bone and armor will squeeze the blade from the sides, and that may well jam it in place. $\endgroup$ – WhatRoughBeast Mar 28 '16 at 14:27
  • $\begingroup$ @whatroughbeast that is very true. I didn't take material types to be too important. My post was mainly a quick sketch of my thoughts before I had to sleep, and it was too long for a comment. I'll be back with some numbers later. Thanks! EDIT: actually CAgrippa above has done that already. I'll leave it it him. $\endgroup$ – ChronoD Mar 28 '16 at 16:17

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