Utility of Monomolecular/-filimant weapons?

Factual Questions
1

In science fiction literature I have often heard of “monofilimant/-molecular” weapons. These weapons (seem to typically be whips and swords), have blades, etc. that are one molecule in width. The theory is that it will easily be able to slip between molecules in the target and sever the bonds, presumably giving way to the anime moment where the hero strikes the villain, the villain laughs at the futile gesture and slowly the top half of their head slides off.

Would such a weapon be possible? If so, would it work like that?

2

Knives, even in science fiction, don’t cut the bonds within molecules (intramolecular bonds) – otherwise you’d change the chemical properties of a substance by cutting it. The bonds between molecules, also called intermolecular bonds, are much weaker than the bonds within molecules, and it’s these bonds that are broken by physical processes such as cutting.

Bonding in metals is different than in, say, butter or plastic. Metals are essentially clusters of positively-charged ions (nuclei along with some of their electrons) held together by delocalized electrons – see here, and here for a page about intermolecular bonding in other substances.

I don’t know much about how knives and swords work on a molecular level. But it would be possible to make a weapon that had a blade that was a few atoms thick, or maybe even one atom thick (if it was made of metal), or one or a few molecules thick if it was made of a non-metallic material. Since this is science fiction, probably the most likely material would be a carbon-based nanotube. It’s already possible to make structures from nanotubes that are one molecule thick, though I’m not sure if they’re ‘sharp’ or if they’re stable enough to use in weapons.

Regarding the scene you described, I’m not sure if that’s possible. For one, I assume that a certain amount of energy would be required to cut through all the cells and bone in someone’s head, no matter how sharp the weapon was. It might not be possible to simply tap a villian with a monofilament blade and watch their head slide off – you might still need to strike them with a fair amount of force to get the necessary energy to slice through.

3

I’m not sure if it’s possible, but, given some exaggeration for dramatic effect, the device probably would work like that.

The cutting is due to pressure. The molecular monofiliment would create an immense amount of pressure very easily. Assuming it’s a millionth of an inch, one pound of pressure on the handle would give a million lb/in^2 of pressure on the monofiliment, more than enough to cut through things.

4

If the bonds between molucules are so much weaker then the bonds in molecules, how exactly do you plan to make a monomolecular strand. There’s no way the forces between the molecules would hold the strand togethor when you whipped it, let alone when it cut through someon.

5

The point is that the weapon is made of only one molecule, so it’s a lot stronger than an ordinary fine thread. The site Roches linked has a further link here to a page of pictures showing various molecular structures. The tubes in the first two pics would be useful.

6

My point still stands, it would be like trying to cut through someone with a length of fishing line. Its too light, you can’t whip it through someone. I don’t believe that any of those intermolecular bonds would be strong enough to cut through someone.

7

You have two problems with a mono-weapon: it’s too light to hit hard, and it’s too weak to survive being hit hard. The first is easy enough - hang a weight on the end. (See, for example, the cloned ninja in the story Johnny Mnemonic. [Important note: Don’t see the movie!])

The second problem isn’t so easy, and I’m certainly not qualified to answer it. Maybe somebody else understands it well enough to explain.

8

I’ve got a related question:

Intermolecular bonds aren’t fixed things. Even in solids, the molecules are in constant motion, weakening a bond here, strengthening a bond there, flipping around, straightening out, so long as we’re above 0 degrees Kelvin, right? If so, then what would it mater if you temporarily interrupted the bonds with a monofilament weapon? Wouldn’t it be like slicing a blade through water (to give an easily viewed example)? The bonds just reform once the weapon passes by? I understand that even conventional weapons are seperating molecules from molecules, but conventional blades also push the molecules far enough apart from one another that the bonds couldn’t easily reform. Is this correct?

Appologies if this has been answered in one of the links. I’m so enthusiastic I can’t contain myself.

9

it would depend on the bond. Molecular forces act over a very small distance, if you broke the bond and there was a 6 inch gap, there’s no way the molecular forces would pull the strand back togethor. There’s no reason why you couldn’t reform the strand after it broke with new molecules of the same type.

10

The problem with inter-molecular bonds reforming as the monomolecular filamen passes through seems a potential deal breaker for monofillament whip weaponry. I wonder though if a monofilment heald within a solid clamp (imagine a violin bow) with the monofilament vibrated at a small amplitude,high frequency vibration could be considered. This would impart more kinetic energy to the molicules that it splits up, I susspect causing extreme local heating. The monofillament would have high kinetic energy itself, but this would not be completely analagous to heat as its motion would be ordered rather than random. I wonder what the linmits are for monofilament harmonic resonances are?

11

To give some idea of how small a distance intermolecular forces (the van der Waals force and other related forces) act within, the van der Waals force depends on the reciprocal of the distance to the sixth power. This means that if you double the distance between two molecules, the force between them is reduced to 1/64th of its original magnitude. If you change the distance from a few angstroms (say, 10 angstroms or 100 trillionths of a meter) to 6 inches, you’ll reduce the intermolecular force almost to zero.

Even if you separate the molecules by the diameter of a nanotube, that might be enough to keep them apart – remember, with an organism you’d be cutting cell membranes and slicing apart DNA and proteins. The diameter of a carbon atom, or even an iron atom, may not be enough to keep them apart, though (and I can’t be sure a nanotube would be enough either). Within an organism, molecules are moving around, rotating, translating and vibrating – not to mention the molecules themselves are distorting in various ways. A monofilament that increased the distance between the molecules of a substance by less than the distances ordinarily achieved with natural motion might not ‘cut’ the substance in the usual way.

12

One solution to the problem I’ve seen is in one of Fred Saberhagen’s Beserker stories, the monofilament extends out the cutting surfaces of an otherwise conventional sword. This allows the weilder to slice through something which he wouldn’t ordinarily be able to do.

13

Thats not really a sollution though. The sharp side of a razor blade is only a few atoms wide. What makes a single molecular fiber sharper than a razor blade? How does it improve cutting or reliability over a conventional edged weapon? Personally I think the cooler, ultra sharp cutting weapons are the Vibra-blades you hear about in starwars.

14

First things first.

Macromolecular structures are more than a single atom in thickness. Even a single walled carbon nanotube is much more than one carbon atom in thickness. It is a tube, formed of roughly hexagonally oriented networks of carbon atoms, thirty or forty atoms thick, not counting the center of the tube, which can be much larger still.

The edge of a razor blade is not “a few atoms” thick, except here and there where the irregularities of crystalline alignment allow it by sheer chance. Mostly, it is thousands to millions of atoms thick.

Other things next.

The monofilament, as a weapon is generally dependent upon the strength of the filament, as well as its thinness. The Niven stories of “Sinclair Monofilament” used as weapons depend on it being fixed at the ends to relatively rigid objects. In the cases where it is only fixed at one end, it has to have a driving force, such as a vibration, imparted by a power source. The exception to this is the Variable Knife, in the Known Universe stories, but the infamous Slaver Stasis field holds that one absolutely rigid. All of these devices depend on a level of tensile strength orders of magnitude higher than the strong nuclear force. As of this time, those are comfortably fictional.

Now, the E string on an ordinary guitar is easily “sharp” enough to inflict a cut, perhaps even a serious one, if used as a whip. Not much when compared to a short sword, of course, but much easier to smuggle into a secure environment. Fix both ends to a handle, and even this fairly old technology becomes a very effective weapon, called a garrote.

An E string is a whole lot more than a few hundred atoms wide. A bundle of fifty double walled carbon nanotubes would be much smaller in diameter, and perhaps a whole lot stronger. Now you have a garrote that cuts like a razor. String it out to a few yards, and you can stretch it across your favorite do not enter door, and you have a basically invisible knife, that would easily cut through a human sized object unless the human was wearing very good armor. Clamp it to a handle, with a nice platinum ball at the other end, and you have a swung weapon that cuts like a razor, except at the end.

On and on, you can reinvent the bolo, and its cousins, and in the end, you have something basically more cumbersome, and slightly less effective than a katana, and probably more difficult to learn to use. But a guy with M1911a is still gonna kill you. Aside from smuggling, the wire as a weapon has very few definitive advantages.

Tris

“Our friend brings us good news. If the Persians darken the sun with their arrows, we will be able to fight in the shade.” ~ Dieneces of Sparta ~

15

Another possibility, if the filament is not entirely flaccid, is to vibrate it along its axis - I saw some fascinating work a while back with a length of semi-rigid plastic - it was attached to a platform that caused it to oscillate back and forth very rapidly along its long axis - this had the odd effect of causing it to stiffen and stand out perpendicular to the vibrating platform (in fact it can act in many ways as if it is a pendulum, just not pointing downwards. It would require a good deal of comfortable fictional fancy, but allowing that, it could form the basis of a variable-sword type weapon.

16

Was that on the Royal Institution Christmas Lectures? There was one a while back where they did a fair impression of the Indian Rope Trick, although nobody climbed up the “rope”. IIRC, this behaviour was predictable in computer models of a chain of hinged rigid links, but nobody had quite figured out why it worked with a rope.

17

It may well have been the Royal Institution lectures where I saw it; I am a bit of a fan.

18

But I don’t think we’ve yet answered the question of whether the monofilament weapons as described in much science ficiton (where it’s a thread one molecule thick, not one atom thick) could actually spread the molecules of an object far enough apart to disrupt intermolecular forces and cut the object. (Appologies if we have, but I’m just not finding such). Maybe if it was a really big macro molecule? I’m too lazy and stupid to look up the math and calculate just how big a molecule would have to be to interrupt the intermolecular forces between other molecules. Is the creation of such a molecule possible given what we currently know?

I suspect that the only way a monofiliment weapon would work is if it was vibrating, as **Bippy the Beardless ** suggested, and so imparted additional kenetic energy to the substance as it passed by, rocketing the substance’s molecules farther apart from one another than they would normally be spread simply by the width of the weapon. Or, if the monofiliment part was added to the edge of a conventional weapon, creating a fine cut which is then spread apart by the body of the weapon, as Tuckerfan has pointed out. What do y’all think? Am I totally off base?

On the other hand, it is kinda stupid to judge the quality of science fiction based on what current research presents. But it’s fun to nit pick anyway :slight_smile:

19

It isn’t a Larry Niven variable sword kind of thing but it exists. I have read that chipped obsidian is sharp to almost a molecular level and is used for eye and other extremely delicate surgery.

Regards

Testy