Is it true that if two pieces of metal touch in space

[!Jon Snow!]

they are permanently stuck?

 

[Nugatory]

Yes, if the vacuum is hard enough and the surfaces are clean and smooth enough and we bring them together in the right way. Google for "cold welding"

 

[AlephZero]

Yes, if the vacuum is hard enough and the surfaces are clean and smooth enough and we bring them together in the right way. Google for "cold welding"

If the right situation you don't need a vacuum or even special cleaning for cold welding.

https://www.youtube.com/watch?v=8SSOKppwpAE

This was a standard method for assembling electronic circuits, until surface-mount components killed it. Some pictures here: http://en.wikipedia.org/wiki/Wire_wrap

 

[!Jon Snow!]

Unlike cold welding process at macro-scale which normally requires large applied pressures, scientists discovered that single-crystalline ultrathin gold nanowires (diameters less than 10 nm) can be cold-welded together within seconds by mechanical contact alone, and under remarkably low applied pressures.[2] High-resolution transmission electron microscopy and in-situ measurements reveal that the welds are nearly perfect, with the same crystal orientation, strength and electrical conductivity as the rest of the nanowire. The high quality of the welds is attributed to the nanoscale sample dimensions, oriented-attachment mechanisms and mechanically assisted fast surface diffusion. Nanoscale welds were also demonstrated between gold and silver, and silver and silver, indicating that the phenomenon may be generally applicable and therefore offer an atomistic view of the initial stages of macroscopic cold welding for either bulk metals or metallic thin film.[2]

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