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Monomolecular wire weapons are a somewhat common element in science-fiction.

Including in this this fantastic image I found while looking into this: armored octopus with monomolecular whip fighting dual-dagger-wielding monkey

Is the only infeasible thing in this image that the octopus would need bones to stand like that, or is the monomolecular whip also not feasible?

Let's assume that we have the ability to construct a monomolecular wire from any real material. That uses up the hand waving allowed for this question. Which material would we use, and how effective would it be for use as a blade or whip? (I have seen blades described as either rigid or a taught wire strung in a open frame. I don't care which one is used.)

To qualify as working or feasible, I'm interested in the wire not breaking while cutting through something of various densities, like a human arm. The effort required to do so, tensile strength, sharpness, etc seem all to factor into that one metric. Limb amputation. Let's call it amputationility.

So, can I cut off an arm with a monomolecular wire made of real materials?

If no real material would work, what minimum properties of a real material would need to be modified and to what value?

Note: I considered tagging this as hard-science. But given the possible requirement of a fantasy material I've left it off. However, I want answers to be as scientific as possible. An answer of "the material needs very high tensile strength" is not satisfactory, I want to know how high the tensile strength needs to be. Numbers people, show me some numbers.

Samuel
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    Is that an armored octopus??? Allow me to be the first to give you a number: +1. – Frostfyre Jun 16 '15 at 19:26
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    @Frostfyre I know right? The image description was fun to write "armored octopus with monomolecular whip fighting dual-dagger-wielding monkey" – Samuel Jun 16 '15 at 19:31
  • As is so often the case xkcd points out a problem: https://xkcd.com/697/ – Wil Selwood Jun 16 '15 at 20:01
  • I really wanna answer this but I don't know enough bout the actual science or how to get the numbers to fulfill your note //sobs... I can only theorize... – Aify Jun 16 '15 at 20:53
  • @Aify It would be a bad theory without a scientific basis. Sounds more like conjecture. Perhaps you should research and post what you can, you may learn about the actual science in the process. – Samuel Jun 16 '15 at 21:00
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    Does the weapon have to be exactly 1 monomolecular wire, or can it be a bunch of them together? The main issue I see with having a single wire would be that it doesn't cut very well - the the van der Waals force would probably "heal" whatever you just cut... – Aify Jun 16 '15 at 21:08
  • @Aify Good question. I think that a bundle of monomolecular wires can begin to stray into simple garrote wire. But, if it can remain effective as a cutting weapon and not a simple strangling weapon then I don't see why adding a few into a bundle will not work. – Samuel Jun 16 '15 at 21:12
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    Re. the image, it also seems unlikely that the wire would be visible like that: it'd be super-thin, and in order to glow it would need some kind of current, like a light bulb filament. That brings up a host of other problems. – zeta Jun 16 '15 at 21:14
  • @sumelic Yeah, they're commonly made to glow in fictional settings because an invisible weapon is not very interesting to look at (though I think it's more terrifying). I realize it won't glow unless it's heated or made of some visible-light emitting material. – Samuel Jun 16 '15 at 21:19
  • I kinda feel like Aify towards this question :( Though I'm thinking towards laser 'strings' . Because you do want to see it (for the user) and heating or energizing it to the point where it's visible is more likely to consume whatever is so razor thin that it can cut limbs simply by it's make. But I'm writing out of my hat. – Spacemonkey Jun 16 '15 at 21:25
  • One more question: Does it have to be WIRE, or does the weapon qualify as long as the edge is only one molecule thick? – Aify Jun 16 '15 at 21:26
  • @Aify It must be wire. Single molecule edged weapons can, theoretically, be as simple as obsidian. – Samuel Jun 16 '15 at 21:33
  • The biggest issue with this monomolecular wire is that it's actually impossible to scientifically visualize (unless there's some sort of molecule that bonds with itself in a straight line that I've never heard of or forgotten about). Consider carbon dioxide for a moment. CO2 bonds in straight lines, but when you try to stack it up, it's all jumbled and not a straight line anymore. It can't retain a "1" molecule thick edge in all directions. – Aify Jun 16 '15 at 21:42
  • Now consider Graphene - that's the closest I could think of that has a 1 molecule edge, but those are sheets and they don't come in the form of wire that's 1 molecule on all sides, and since you said it MUST be wire, we can't use that either. Graphene already has the highest tensile strength (value of 130000MPa) in the world, but we can't even use that! – Aify Jun 16 '15 at 21:44
  • @Aify I think you may be overthinking this. A single carbon nanotube can be considered a monomolecular wire. If the molecules form up edge to edge, stacked rings, or spirals it still counts. I think you're limiting yourself when you're only considering molecules as little balls linking together. – Samuel Jun 16 '15 at 21:55
  • It's worth noting that the octopus wouldn't actually need bones to stand like that. In the setting that that's from, they would have been in a zero or low-g space habitat. – Saidoro Jun 17 '15 at 18:56
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    @Saidoro Actually it says in the game manual that the image came from that the "octopi have been modified to include a flexible sheathe of cartilage" combined with " A careful system of powerful muscles joined to this sheathe allows the octopus to hold itself upright." They're clearly not in zero-g in the picture, the gravity may be lower, but they still have 'bones' to get around. – Samuel Jun 17 '15 at 19:15
  • @Samuel I've added more to my answer to be more clear and to answer the mini question in your bounty. – Aify Jun 19 '15 at 16:58
  • This blog post might be of interest: https://poetryinphysics.wordpress.com/2016/11/09/a-physicist-responds-to-the-three-body-problem/ – Adam Apr 12 '18 at 12:58
  • How have I been on this site 2 years without seeing this post or that battle-octopus! +1 – kingledion Sep 07 '18 at 20:33

5 Answers5

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Graphene is what you're looking for.

With a tensile strength of 130000 MPa, it has (IIRC) the highest tensile strength in the world.

So lets make a wire-thin sword!

I envision it to probably end up looking something like this:

      >----------------------------------------------<|
|||||||                                               |
      L_______________________________________________|

where the ------- represent the blade, and the <| the tip that the other end of the blade is connected to, and the ||||> represents a handle. It's important to note that the wire is being pulled taut by the <| piece at the end of the blade. The L____| represents a structure similar to that of a hacksaw, in order to hold the wires tightly.

This is a slashing/chopping weapon.

How/Why does this work?

The "Graphene wire" is really a Graphene ribbon

Graphene itself contains elastic properties, which helps with the above concept of cutting. Even if the Graphene doesn't cut right away, the elasticity will help it to continue cutting as you swing the blade through your target. "Graphene sheets (with thicknesses of between 2 and 8 nm) had spring constants in the region of 1-5 N/m and a Young’s modulus (different to that of three-dimensional graphite) of 0.5 TPa."

Graphene also has amazing shear strength. Shear modulus of graphite was reported to be ~0.44 TPa. To give you some context, the shear strength of a carbon diamond structure is ~93 GPa. 1 TPa is 1000 GPa.

To answer your question: Yes, you can.

Unfortunately, because the human body is so variable, I can't find any actual numbers regarding how much force is required to tear off a limb - however, we should note that this blade doesn't apply force the same way a sword does.

A sword cuts and splits the target because it "wedges" it apart. In this case, however, because we have a monomolecular ribbon that's completely flat, we should be able to pass through the entire target (irrelevant of what the target is made of, but assuming you gave it a good chop with no deviation in blade angle) extremely easily, since all we're severing are molecular bonds. Forces at the molecular level are at the pico-Newton level (1pN = $10^{-12}$ N); what we exert on anything using anything at any given time exerts more force than what's required. Here's some more context: One pound of force gives us 43.62 Newtons. Even a toddler could exert one pound of force by accident - so if you gave this thing to a baby and he accidentally swung it through you, good luck.

Thank you to Samuel for pointing out some numbers for me: "the shear strength of the Graphene ribbon is maybe 4200 piconewtons / angstrom, while fibers in the skin, like collagen, have a shear strength of only 5.5 piconewtons / angstrom." These numbers show that along the same area, the ribbon has a shear strength of over 750 times that of collagen.

Skin seems easy to cut though. What about bone? Luckily for us, most of bone's elasticity comes from the collagen in it, which means we cut bone just as easily as we do skin.

For an adult? It cuts anything, and everything, better than warm butter.

Once you finish slicing, the limb will only be held on by suction and surface tension. Any movement, and it simply slides/pops off.

However, even regarding the above saying that it is possible in theory, this tool is much better suited to a hospital setting requiring quick amputations than a battle situation.

Strictly speaking, this would work as an amputation device, but would be sorely suited for battle if the opponents also had access to similar weapons. In that scenario, please refer to Ville Neimi's answer (2 to 4th paragraph) regarding why it would suck as a weapon. Note that in normal use, the Graphene should be strong enough to be reused over and over again. The hexagonal structure of the Graphene ribbon means that even if any edge atoms are lost, it doesn't matter - No matter which atoms you lose, you will always have a suitable cutting edge.

References:

R. R. Nair, M. Sepioni, I-Ling Tsai, O. Lehtinen, J. Keinonen, A. V. Krasheninnikov, T. Thomson, A. K. Geim, I. V. Grigorieva. Spin-half paramagnetism in graphene induced by point defects. Nature Physics, 2012; DOI: 10.1038/nphys2183

http://www.graphenea.com/pages/graphene-properties#.VYCbRkZ8ork

https://en.wikipedia.org/wiki/Graphene#Thermal_conductivity

http://poplab.stanford.edu/pdfs/PopVarshneyRoy-GrapheneThermal-MRSbull12.pdf

https://web.engr.illinois.edu/~aluru/Journals/APL11.pdf

https://en.wikipedia.org/wiki/Bone

Aify
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    +1 for the math and research and smart-sounding stuff. I have no idea if any of it is right, but it's really impressive nonetheless. My only qualm is that the maglev you propose would probably make the magnetic tip swing like a pendulum, rather than holding it in place relative to the orientation of the handle. That would lead to a funny defense: the defender would just swat the magnetic lump at the end, and the whole nanotube would swing back around and slice the attacker in half. – BrettFromLA Jun 16 '15 at 23:12
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    Check out carbyne! https://en.wikipedia.org/wiki/Carbyne http://phys.org/news/2013-08-carbyne-stronger-material.html Might be another possible candidate. @Samuel –  Jun 17 '15 at 09:54
  • Would this work in whip form, without the levitating tip? Obviously, it'd be incredibly dangerous to use that way, but would it be possible? Or would it short out and/or not be able to cut edge-on? – Bobson Jun 18 '15 at 19:55
  • @Aify - That makes sense. – Bobson Jun 18 '15 at 20:02
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    @Bobson Burki made an excellent observation that the whip would wrap around a limb, causing a loop that could then be pulled closed. It certainly seems like an effective way to use the whip form. – Samuel Jun 18 '15 at 20:20
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    Heck, I've seen special knives so carefully sharpened that people have accidentally cut off fingertips/fingers cleanly. With a monofilament blade? not a problem, though you'd have to worry about knicks... – Isaac Kotlicky Jun 19 '15 at 12:27
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    Found your sword... – Isaac Kotlicky Jun 19 '15 at 12:40
  • Let us continue this discussion in chat. – Samuel Jun 19 '15 at 17:31
  • This is relev6: http://phys.org/news/2015-06-graphene-bright-world-thinnest-lightbulb.html – JDługosz Jun 20 '15 at 11:43
  • I upvoted you based on the content of the answer. If I could I would upvote you again for using a scientific reference. – Englishman Bob Apr 06 '22 at 14:57
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I'll assume you mean "monomolecular" literally. In that case the answer is "No", you can't make practical melee weapons from monomolecular wire. You could build tools or missiles using monomolecular wire and those could have significant amputationility.

The basic issue is that a melee weapon needs to sustain repeatedly hitting the target and, most likely, armor, other weapons and coincidental objects. When that happens the atoms of the weapon will collide with the atoms of whatever is hit. No matter how hard your weapon is this will result in some of the atoms being ablated.

Most weapons are hard enough for the loss to be insignificant, maybe requiring occasional resharpening of the edge. A monomolecular weapon needs all of its atoms for its structure. Even if the material has some redundancy so it doesn't simply go "poof" or break on first impact it will be locally weakened. So repeated impacts will result in the weapon losing strength until it suddenly breaks. Probably just at the moment you are fighting for your life.

At this point it is simpler to add redundancy by making the impact point a composite of multiple molecules or crystals, just like conventional weapons are. A thin wire of "conventional" metal maybe reinforced with nanotubes or graphene gets the job done and is more robust and much simpler to engineer.

For practical monomolecular weapon you need something where the fragility and unpredictable robustness do not matter. A single use weapon such as a missile you throw away or shoot. A specialized tool used for assassination that unless you mess up you only use a single time and then dispose of. A weapon that can recover from being broken by simply producing more of the blade or whip.

The last is probably closest to what is wanted. A whip with an electric charge or a super science force field such as in the picture adding rigidity for pseudo inertia on impact could cause significant damage. And while it would almost certainly break on impact, it would be a simple matter for a microcontroller to detect the length of the whip was reduced from the change in capacitance and extrude enough new material to keep the length constant. A monomolecular whip has very little mass for a certain length so you'd probably run out of power before replacement material.

But even then it would be much easier to use a conventional material instead of an exotic monomolecular one. More robust and probably cheaper. As noted in comments, the practicality of even this limited class of monomolecular weapons is still much less than that of more conventional weapons due to marginal benefits over much more reliable solutions, so nobody would use them. Practical in theory, but not in practice. Which I am not sure even makes sense.

Ville Niemi
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    Could you even effectively whip a monomolecular whip in an atmosphere? It seems like it would be as bad at trying to whip someone with a thread. – Samuel Jun 19 '15 at 17:10
  • Also, as noted elsewhere, monomolecular doesn't necessarily mean a sting a single molecules. A carbon nanotube works, for instance. – Samuel Jun 19 '15 at 17:15
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    @Samuel It can't have enough mass to cut by inertia, period. Ratio of surface area to mass will be too high in both air and tissue. Essentially even if it looks like a whip, you are pushing it through using whatever "force field" you have added to it. And no, I don't really think that is practical (see first paragraph). A single use garrotte or a short range missile weapon makes more sense to me. – Ville Niemi Jun 19 '15 at 17:28
  • Ok, I like the single use garrotte wire if it's used for cutting. Can you expand more technically on how that can work? – Samuel Jun 19 '15 at 17:30
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    @Samuel Not really. It would still have the fragility issue, so nobody would actually use it. I am afraid the "makes more sense" was strictly relative term here. It is also much easier to use a braided structure of high tensile strength fibers with hard and corrosion resistant coating. And as somebody noted being too sharp actually reduces the damage. Additionally tensile strength actually increases faster than resistance when the wire gets thicker. So the optimal thickness is probably well above where "monomolecular" really makes sense even for a garrotte. – Ville Niemi Jun 19 '15 at 17:40
  • So there is no such thing as a "practical monomolecular weapon" like you describe in your answer? What does it need to be made of to achieve the desired effect? – Samuel Jun 19 '15 at 17:48
  • @Samuel Good point. "Practical" is clearly stronger than I actually meant. I'll edit. And the issue was not of material, but structure. – Ville Niemi Jun 19 '15 at 17:56
  • @Samuel: the reason thread makes a bad whip is due to it's lack of mass. The mass each part of a real whip has allows it to have the momentum to keep moving when it encounters resistance (movement through air or a target). If you have a monomolecular whip that has even a few grams of mass per inch, it will work like a conventional whip. If not using it like a whip, you could instead add a weight to the end, and then swing that weight at the end of the line. – Mark Ripley Sep 29 '16 at 07:31
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I think, just like your cool picture suggests, that a whip-like weapon is what you're looking for.
Consider a whip, i.e. a strand of something (and yes, we'll use carbon-nanotubes here, because they are really, really cool!) with a handle.
Add some barbs to the end.
Keep in mind that anything that is thin enough and does not move out of the way is actually a cutting edge.

Now, you hit your opponent. The strand of carbon nanotubes will be wound around your opponent's arm, and you pull back hard. The thinness of the strand, together with the force you apply by pulling it back, and assisted by the barbs at the end, that will get hooked into your opponent's armour, clothing or flesh, result in the loop around the limb trying to get smaller, and thus cutting tissue that is in the way.

The only question remaining is: can you pull hard enough to cut through the bone? That is where it really helps if your enemy is an armed octopus: They have no bones, which will make the limb removing business a lot easier.
The downside: your octopus still has plenty of remaining limbs to make it very, very clear what he thinks about that.

Burki
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2

As a starting point, I used How does a knife cut things at the atomic level? from Physics Stack Exchange, specifically, lemon's answer. lemon1 talked about something called nanoindentation, which is typically used as a laboratory testing technique.

For now, I'll use some of Wikipedia's equations to work this out.

The Young's modulus of the thing being cut, $E$, is related to the stiffness of the contact, $S$ and the indentation depth, $h$, by $$E=\frac{1}{\beta}\frac{\sqrt{\pi}}{2}\frac{S}{\sqrt{A(h)}}\tag{1}$$ where $$A(h)=\sum_{n=0}^{n=7}C_nh^{2^{-(n-1)}}$$ Doing some re-arranging, $$A(h)=\left(\frac{S\sqrt{\pi}}{E\beta2}\right)^2$$ Setting these two equal gives us $$\left(\frac{S\sqrt{\pi}}{E\beta2}\right)^2=\sum_{n=0}^{n=7}C_nh^{2^{-(n-1)}}$$ Let's solve for $S$: $$S=\frac{2E\beta}{\pi}\sqrt{\sum_{n=0}^{n=7}C_nh^{2^{-(n-1)}}}\tag{2}$$ If we say that $C_0=C_1=C_2=. . .=24.5$, and $h$, the thickness of the human arm, is about 0.1 meters, and $E$ is about 14, then, for one tip, I find that the stiffness needed is . . . $\approx$ 1,196,000 Newtons/meter; the force needed is 196,000 Newtons. That's only if one tip is used. Add on more tips on a smaller scale, and this could be feasible. You would get smaller tips, and so smaller identations for each one, but it could work. Perhaps.

The important thing to gain from this is that the types of tips used in nanoindentation can be quite effective. A (paywalled) study also mentioned in lemon's answer showed that the different types of nanoindenters used in the process can produce slightly different results. Fortunately, the Wikipedia page on the devices produces a nice starting point for research . . . which led me nowhere. Curses.

What was I even trying to get at? Consider a long piece of barbed wire. Now make the barbs tiny - really tiny - and lined on every piece of the wire. Then turn each barb into something like a nanoindenter. Now you've got quite the weapon. The reason I covered nanoindenters was that I wanted to see if it would be possible to pick a design such that the shape would be more important that the composition.

In any event, the resulting weapon would look like this:

        ||
--------||
        ||---x-x-x-x-x-x-x-x-x-x-x-x-x-x-x-x-x-x-x-x-x-x-x-x-x-x-x-x-x-x-x-x
--------||
        ||

The tip of each "x", though, would be in the shape of a nanoindenter.

1 Note to any potential editors: the username is all lowercase.

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HDE 226868
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  • The Young's modulus is given for bone on the linked Wikipedia article: Human Cortical Bone, GPa: 14, psi: 2.03×10^6. Where does the summation to seven come from? – Samuel Jun 19 '15 at 19:47
  • @Samuel That's embarrassing. I didn't check the list. The summation comes from the expression for $A$ given in the Wikipedia page; I put it like that for simplicity. – HDE 226868 Jun 19 '15 at 19:48
  • Wouldn't this be ineffective as a weapon? It sounds like it would be rough, and the barbs would only produce superficial cuts. I don't quite understand why multiple nanoindenters would be useful. – zeta Jun 19 '15 at 19:49
  • @sumelic Lots of tiny barbs would, I think, do quite a bit of damage. Scaling down the nanoindenters a bit more would make it more effective. – HDE 226868 Jun 19 '15 at 19:51
  • But how is the weapon used? Do you just hit people with it and wait for them to bleed out? If they're actually barbs, won't the weapon stick to the enemy? I don't get how you would deal fatal damage with lots of tiny barbs. – zeta Jun 19 '15 at 19:53
  • @sumelic Again, these things are really tiny, not like typical barbs. I'm not sure how there's much difference between them and something like a weapon described in the picture. – HDE 226868 Jun 19 '15 at 19:55
  • So it's supposed to slice like a knife? I'm not too familiar with the molecular mechanisms of that, but the Physics answer you linked seems to suggest that nano-indenters are a) stabby, not slicey and b) most useful for damaging hard and brittle materials. The examples of blades are all smooth, not barbed. A barbed blade seems like it would be less effective at slicing than a single-edged blade. – zeta Jun 19 '15 at 19:58
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    Oh, I see. It should be to 8 then. As you do now have the value for bone, can you find S and show the numbers? Perhaps one of these indenters would provide a more accurate representation. – Samuel Jun 19 '15 at 20:13
  • @Samuel I still can't find the values for the $C$s. – HDE 226868 Jun 19 '15 at 20:38
  • The C values are for the cutting tip, are they not? – Samuel Jun 19 '15 at 20:58
  • @Samuel Hold on. I've got some math coming. – HDE 226868 Jun 19 '15 at 21:17
  • @Samuel Got it. – HDE 226868 Jun 19 '15 at 21:25
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No way. The positive answer(er)s all fail to take into account a) how exceedingly small a molecule is, b) how many molecules there are in a body (and how much they interact with one another) c) how a chain has to weather the sum of all forces acting on it and d) that a force acting perpendicularly on a chain can not simply be colinearized as is, but will lead to a colinear force many times the former magnitude.

Have a breaking strength of 1TP like one answer gave for graphene: One Pascal is one Newton of force (100 grams in earth gravity) on one square meter. So 1TP means 10^12 Newtons per square meter! Yay! But consider the cross section of a molecule: Let's be generous and set it to 2nmx2nm - thats (2*10^-9)^2. So the breaking force for that single molecule is: 4*10^-6 Newtons ... the force gravity exerts on four tenths of a milligram of mass. You could lift four fruitflies with that! (Yay?) Any molecule encountered by the "whip" on it's way through flesh will need to be acted upon by a force - shear intermolecular-bonds, shove it out of the way, resist adhesive forces... and at any one time, the whip traversing something as small as the human finger would encounter (lowballing) 10^5 molecules - so any of those molecules could be acted upon by (in the mean) 4*10^-11 Newtons - that's just about ten times the force needed to break a hydrogen bond (weakest bond there is http://www.picotwist.com/index.php?content=smb&option=odg) and just a fourth of the force need to break a noncovalent bond. And we haven't even begun figuring in the multiplicators coming into play because the "whip" has these forces acting perpendicular to itself.

The "whip" will drift towards its target, strike with undetectable force, and then break at the first tug. Possibly there is a papercut along the way.

bukwyrm
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