how does matter know its’ boundary?

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So that if, for example, i put two cubes of copper next to each other they don’t become one

In: 1

It doesn’t.

The cubes are not perfectly smooth. They have dents and unevenness in the surface that prevent them from coming into close contact and fully bonding the way the atoms inside the material are bonded. (They also trap air molecules between them if this contact isn’t taking place in high vacuum.)

If the materials are perfectly smooth, brought into contact in a vacuum, and left there for a bit, they can indeed merge into a single object. It’s called [cold welding](https://en.wikipedia.org/wiki/Cold_welding), and it occurs precisely *because* the atoms don’t know “which piece” they’re a part of.

The copper exists in an environment with free atmospheric oxygen, which quickly forms a thin oxide layer at the surface, in addition to other trace amounts of contaminants that get in the way. Metals will spontaneously bond with each other if they’re extremely clean and kept in an vacuum.

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

wow it seems that that Richard Feymann heard you and “wrote” an answer specifically for you, from wikipedia:

>The reason for this unexpected behavior is that when the atoms in contact are all of the same kind, there is no way for the atoms to “know” that they are in different pieces of copper. When there are other atoms, in the oxides and greases and more complicated thin surface layers of contaminants in between, the atoms “know” when they are not on the same part.

— Richard Feynman, The Feynman Lectures on Physics, 12–2 Friction

Everyone is explaining cold welding because you mentioned copper. The answer to your question more generally is something called “van der Waals forces.” The ELI5 answer is that if molecules get too close, their electrons start repelling each other extremely strongly. There are other factors that go into it, but basically that is what we care about here. So why is your copper example different?

For any non-metallic object, the electrons are bound rather tightly to the nuclei or in the molecular bonds holding that molecule together. Since specific electrons more or less “belong” to atoms or bonds, they don’t have anywhere to shift and will repel other electrons. This is why generally similar materials don’t just randomly fuse together.

For metals, the bonding between atoms works a little differently and electrons are shared more or less freely between the individual atoms in the metal. In a perfect world, two copper cubes would bump into each other and the electrons would repel each other at first but they have enough wiggle room that the other cube would just kind of join together. This is essentially cold welding, the “perfect world” being a vacuum with very clean copper.

In the real world, basically all metallic objects have a thin oxide layer formed around it from the raw metal reacting with the air around it. Oxides are non-metals and have ionic bonds, rather than metallic bonds. This effectively holds surface electrons in place and the surface of the two cubes effectively repel each other. Add on the fact there’s probably dust or grease on the surface as well, and you can see why they don’t just spontaneously fuse together.

People are explaining cold welding a bit, and I think it’s worth noting that spacecraft design actually has to take this into account, there are many space mission failures which have been attributed to cold welding.

However, it also isnt a *huge* consideration. Even in the vacuum of space, the metals still almost always have enough of an oxide layer to prevent this. Depending on the mission and the design of the craft it’d be an afterthought. Iirc for the ISS they basically never take this into account, because even the *slightly* different circumstances of orbiting closer to earth instead of farther out in space is enough to prevent it.