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I was told the following basic facts:

  • Objects made of the same metal don’t fuse in the atmosphere because they form an oxide layer. They do fuse in space.
  • Gold doesn’t oxidize.

Yet, gold nuggets held together also don’t fuse (I can pull them apart without resistance). How does each gold atom “know” which object it belongs to?


marked as duplicate by Kyle Oman, Kyle Kanos, Brandon Enright, DavePhD, Jim Jun 20 '14 at 17:35

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  • $\begingroup$ Similar question about plastics by the same author - good idea, since the answers probably differ. $\endgroup$ – Roman Starkov Jun 20 '14 at 12:29
  • $\begingroup$ Great question, but sadly @jinawee's duplicate asks the same (and has good answers). Upvoting your plastic question though! $\endgroup$ – Kyle Oman Jun 20 '14 at 14:41

The gold objects you are pushing together are not sufficiently flat to stick, at the atomic level. I imagine that the contact at those points that actually touch may well be sufficient for fusion, but at the atomic scale there is so little contact that you can simply separate the objects apart like nothing happened.

Experiment: if you hammer two pieces of rust-free iron together, they stick a bit, because hammering them together improves how much of the metal actually touches at the atomic level. But the contact is still not very good, especially since iron has the oxide layer you mention.

Cold welding makes use of the idea that if you make the metals truly contact each other without any contamination, they will actually stick together. Quote from none other than Richard Feynmann, answering your question literally:

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.

In particular, gold does actually stick together exceptionally well in the right conditions, the right conditions being nanoscale flatness, aligned crystalline structures, gold purity and clean surfaces:

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

  • $\begingroup$ Add to "right conditions": purity and clean surface. Anything you touch with your hands will have oils on them. And I believe that gold does form a monolayer oxide in air, but I may need to be corrected on that. $\endgroup$ – garyp Jun 20 '14 at 13:53
  • $\begingroup$ @garyp thanks, updated. A comment on this answer says that gold forms a monolayer of oxygen "and other stuff", rather than oxide, so I can imagine that being relevant too, yeah. $\endgroup$ – Roman Starkov Jun 20 '14 at 14:12

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