## What If? Follow-up: Can a Wire Cut a Person in Half?

Credit: Soerfm (Wikipedia).

In my last post, I determined that it is not possible for a human being to be cut in half by running into a cheese-cutting wire that is stretched across a road. The wire will break long before the human will. (I caution that this is the case for simply running at the wire only. A wire saw is very real and can cut through many materials by sawing action.)

But science fiction frequently features the monomolecular wire, a wire only one molecule thick that can cut through anything because it’s so sharp. In modern works this usually means carbon nanotubes. Could this work where the steel cheese-cutting wire couldn’t? No again. A carbon nanotube wire wouldn’t be supernaturally sharp. A piece of broken glass can approach its sharpness, and obsidian (volcanic glass) is sometimes used to make surgical scalpels for this reason. And even if a carbon nanotube wire was a thousand molecules thick, it would still be as delicate as spider silk–too fragile to cut through things.

Okay, but there’s another option. Could a thicker carbon nanotube wire, possibly still hair-thin, be strong enough to cut through living tissue. The answer to that one is actually yes. See my previous post for the physics and mathematics details, but here’s the bottom line. Given some reasonable assumptions about carbon nanotubes, a wire with a diameter of about 0.2 millimeters could cut someone in half just by running into it. 0.2 millimeters is very thin, similar to the thinnest fishing line you can buy. It’s not invisible, but a single line strung across a road could be from a distance.

How fast would you have to hit this super-wire to meet a grisly fate? There are a lot of variables, but I’m getting about 13 miles per hour, depending on how far it can stretch–a nice sprinting speed for a lot of people. There are probably some other factors involved, but it’s almost certainly possible, and if you’re on a motorcycle, you’re definitely doomed.

Hmm…does this bode poorly for our real-life future, if such a weapon can be created? Many sci-fi authors have suggested so, but I prefer to be an optimist. I suspect that in the future, with all the robots around, it will be rather difficult to lay a trip wire like this across a road without being noticed. Now, constant surveillance is a whole other problem in itself, but that’s a matter for another post. In the meantime, David Brin has some interesting thoughts about it.