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There is a part of my setting surrounded by a 100-foot-tall, four-sided, indestructible invisible wall. The wall is made of all of the molecules that were in contact with the area that became the wall, whether those molecules were dirt, rock, metal, water or air, permanently petrified and rendered solid and immovable. And it is thin. Extremely thin. Somewhere between 1 to 3 molecules thick (it varies since I know molecules aren't perfectly aligned like little voxels and the number of molecules in the area that became the wall would have varied throughout its length).

And being that it is both extremely thin and cannot bend, break or move in any way, that sounds like it would have an incredible and dangerous amount of cutting power for anything that makes contact with the top of the wall while trying to climb out of it, and that if anything fell on top of it, whether it be flesh, stone or steel, the mere force of gravity would be enough to slice clean through the object like a hot knife through warm butter.

Is this impression correct? Are there any limits to the cutting power of the top of this wall, and if so, what sort of material would it take to make contact with the sharp top of this wall and not be sliced in half?

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  • $\begingroup$ Does the wall have flat, rough, or sharp edges? Being thin does not necessarily make something sharp....are your walls like sheets of paper, because I know those are sharp! $\endgroup$ – Alendyias Jun 10 at 20:29
  • $\begingroup$ @Alendyias The edge has a similar variance in its surface as the sides of the walls do, for the same reasons. Does that answer the question, or were you looking for something different? $\endgroup$ – Jason Clyde Jun 10 at 20:33
  • $\begingroup$ Well, it tells me the edges are relatively rough, and I can see where you're coming from, but with my limited understanding of physics my guess is the same as yours. Sorry.... $\endgroup$ – Alendyias Jun 10 at 20:35
  • $\begingroup$ @Alendyias Does a molecule or two really make such a difference? Then how would any surface be smooth? $\endgroup$ – Jason Clyde Jun 10 at 20:36
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    $\begingroup$ P.S. The phrase "cutting power" does not have any well understood meaning... $\endgroup$ – AlexP Jun 10 at 20:48
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If the substance can be cut it will be cut by this.

What you are describing is commonly referred to as a Monomolecular blade. The blade is so thin that it is either one molecule or less than three molecules wide. Most attempts to make a blade like this fail since any blade that thin is inherently very fragile, but you side step that problem entirely. Essentially if the walls are vertical, the tops of the wall are all monomolecular blades at the very top.

Sure but anything?

Theoretically, monomolecular swords can cut through molecular bonds if they are thinner than the bonds. However, there are cases where the wall will be thicker than individual bonds. In this case we use traditional methods of determining how effective it is at cutting. To determine if a material can be cut by a blade you need to know the strength of the material and the pressure applied by the blade to the material, as well as how much the blade bends. Since the walls are immovable there is no give to the wall so we can calculate the pressure in an ideal scenario without taking into account bending. Next we get the pressure. If an object rests on the wall, the force applied to the object in one G gravity is 9.8*Mass Newtons. Assuming that the atoms in the wall can get up to 1.5 nanometer wide for large rock molecules, making the maximum width about 4.5 nanometers. So the area the pressure is applied over is the length of the item across the wall time 4.5 nanometers. This will make the pressure incredibly high and let the wall cut through basically anything.

Example

Diamond is very tough. It has a density of 3.51 g/cm^3. Placing a cube 1 cm on a side on the wall would give it a weight of 3.51 g and a length of 1.41 cm along the edge of the wall if we put opposite edges on the wall. This means the force on the cube is 9.81 * 0.00351 newtons or 0.0344 newtons and the area of the force is 4.5 *01^-9 * 0.0141 meters or 6.345 * 10^-11 meters squared. Since pressure is force/area the pressure is 3.44 * 10^-2/6.345 * 10^-11 pascals, or 5.42 * 10^8. This pressure is comparable to a high end water cutter without the use of any energy aside from the gravity applied to the material. Water cutters can cut through diamonds. Therefore, the wall will cut through the diamond.

Can it cut through large objects?

In most cases yes, but in some cases no. The wall will support some of the weight of the item with the edge of the blade through the pressure on the blade. If the item then is put on the wall in a way that it can use friction on the wall to support it it can reduce the weight centered on the blade edge, reducing the pressure to a point where the blade won't cut anymore, or at least without more energy put into it. he larger the object, the higher the chance this will occur before it cuts all the way through. If you cut through butter for example, cutting straight through is easy, but if you turn the knife as you cut the butter becomes harder to cut through since you have to push the butter aside with the knife. With harder materials this will become more noticeable.

More information

The 4.5 nanometers is not a hard number, The wall would likely be thinner that that in most places, increasing the pressure up to 40 times if the wall is a single hydrogen atom wide. While on average the average might be only 2 nanometers, this doubles the pressure. Any item more vertical than it is horizontal will also have increased pressure. Any item that puts more than one G of force on the wall will have more pressure, if an item falls unevenly on the wall since the wall does not bend there will be more pressure on the one spot that hits the wall. Even after hitting the wall the item might not slow down as the wall can only impart a very small amount of force as the object passes due to the low surface area, leading to the rest of the object to hit he wall at a higher speed than a wide cutting implement.

Exceptions

Simply touching an object o the wall won't cut it (literally and figuratively). Some force must be applied for the cut to occur. If you put steel on the wall with less than one G of force under certain conditions it will not cut since the pressure is not sufficient. However, since simply letting an object rest on the wall will cut most materials this is not a huge problem.

Since you have already stated the existence of immovable objects, there is something this wall can't cut. Armor made out of immovable objects. The wall is immovable, but the wall is actually moving all the time. The planet moves under it, and the solar system moves around the galaxy. The wall is immovable in relation to something. So if you make a small immovable wall that is immovable in relation to your arm you have a perfect armor.

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  • $\begingroup$ Friction is an interesting thing you brought up. I wonder if there'd be any way to determine the friction of my magically-petrified wall beyond just making it up, given that the state the molecules and atoms have been put in is blatantly impossible. $\endgroup$ – Jason Clyde Jun 11 at 0:36
  • $\begingroup$ This raises an interesting point about the definition of "molecule". A bowling ball made of bonded polymers can be a single molecule, and the term isn't well as well-defined for a metallic lattice. Also, molecules in air are rather more diffuse than solids, so you may end up with a wall which can be pushed through in places, with the "fixed" air molecules tearing rents due to nonuniform resistance. These regions would likely be permeable to gaseous helium, if the wall-effect can't capture additional atoms. $\endgroup$ – Anon Jun 11 at 3:32
  • $\begingroup$ I presume for this question that any large molecules over four atoms wide get ripped apart after the wall is set. $\endgroup$ – Charlie Hershberger Jun 11 at 3:34
  • $\begingroup$ @JasonClyde the magically-petrified wall will have the exact friction properties of the molecules that where there before, but with even more friction as the petrified molecules can't give way and reduce friction overall. Water or oils will be interesting, as it might actually have a high friction coefficient if it can't move out of the way. $\endgroup$ – Trioxidane Jun 11 at 7:58
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Indestructable does the trick here. Indestructable blades - of any thickness - will cut anything, when the force applied will be large enough.

In RL, a working monomolecular (or triple-molecular) knife blade cannot be engineered. Any angle not exactly matching 90 degrees will damage the blade locally. Surfaces you would want to cut, will have local gradient angles far off 90 degrees. On many places, the knife will break and these breaches will propagate. The thinner the blade, the more you need to approach an exact 90 degrees on surface normal vector anywhere irt the blade angle of movement. So either you'll need a perfectly smooth and straight surface to cut, or you'll need to create an infinitely small knife, to penetrate at 90 degrees on a single point, without damage. A monomolecular needle could penetrate anything.. a monomolecular blade will just disintegrate on first contact with anything solid.. Grapheme will not cut things, it has to be attached to a surface to keep it intact.

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A 3 molecules wide, straight effectively 2-D surface of infinite hardness?

It will cut anything except atoms themselves.
You will even get a faint misting of carbon soot and ozone along the edge, as CO2 molecules run into the edge and sever themselves into component atoms! The simple thermal energy of air is enough to achieve this, with a fine enough blade.

On the other hand....
1 to 3 molecules wide, ONLY?
Your wall will leak like a sieve! Worse!

Unless the magic field that traps the wall also incorporates other atoms that pass through its domain, thus continuously increasing the density of the wall, it will at first pose virtually no impediment to passage.

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