system
Given the following post, respond as if you're commenting on Reddit:
human
Can you fuse any two pieces of the same material in space just like metal?

We know that on earth here when we cut two pieces of metal the freshly cut surface oxidizes preventing the metal from joining back together, 

Though in space the metal does not oxidize and is able to fuse back together into a single piece of metal, 

My question is, is this the case for all material? For example wood, (I don’t know why we can’t fuse that back together on earth) if we took a piece of wood to the vacuum of space could we fuse it back together just through pressure? 

And if we can’t, can someone explain why we can’t?
gpt
For this kind of cold-welding process to work, you need a few things to happen:
1. The two surfaces have to come into atomically close contact, with relatively few gaps between the materials, to form a strong bond at the interface to be joined.  In other words, the atoms or molecules have to be in close enough contact to form a chemical bond.
2. Once in close contact, a large fraction of the atoms have to be chemically capable of forming bonds with each other.

Metals often satisfy these two conditions because they behave something like atoms floating in a “sea” of electrons, with a lot of flexibility in the spatial arrangement of the atoms.  This makes the metals malleable (edit: and more importantly, ductile), so the interfaces can be smushed together to remove most gaps or imperfections at the interface, especially if there is no air in the way.  Can’t do that with wood, which has a more rigid structure (made mostly of organic molecules that are much more specific in their spatial bonding geometries) that can’t be deformed much from its equilibrium structure.

Also, since metal atoms can easily form metallic bonds with many neighbors, which are often the same kind of atom, one part of the surface will look much like any other, and you don’t have to “match” the surfaces perfectly with respect to binding partners.  Every atom at one surface could pretty much bind any atom of the other surface, with little reorganization needed.  Wood, on the other hand, is a complex mixture of organic molecules comprising carbon, oxygen, nitrogen, hydrogen, phosphorus, and other elements that bond with each other in rather specific ways.  For any given atom on a surface, it’s unlikely it’ll find itself next to an atom on the other surface that is capable of binding to it without significant input of energy (and probably catalysis of some kind).

So, with wood, there will be significant gaps throughout much of the interface, and most of the parts that do touch won’t be reactive with each other.  So, no bond will form.