Wikipedia:Reference desk/Archives/Science/2021 April 23

= April 23 =

Substance mixing and boundaries
Do all substances/materials, including solids, mix together or interact with their surroundings and thus have indefined boundaries, even if it's only by a tiny amount at the atomic/sub-atomic level? PaleCloudedWhite (talk) 06:53, 23 April 2021 (UTC)
 * Crystals have a defined surface, which however is probably not perfectly smooth. In crystalline materials, atoms may substitute for others to some extent, just look at minerals. However some atoms like boron or hydrogen are quite unique. Graeme Bartlett (talk) 07:32, 23 April 2021 (UTC)
 * Thanks. Does this mean that, in everyday life, most materials encountered are "impure"? If, for example, I place a dinner plate on a table, do a few atoms of the plate attach to the table, and vice versa? PaleCloudedWhite (talk) 07:57, 23 April 2021 (UTC)
 * Even materials manufactured to the highest standards of purity are never fully 100% pure. Most likely, your plate and table will exchange a few billion atoms, in particular if you are willing to interpret the notion of attachment rather loosely, such as by the Keesom force, and not only by a chemical bond. The concept of "substance" or "material" is ill defined at sub-molecular levels. Atoms and subatomic particles do interact in a variety of ways with their surroundings, all ultimately boiling down to the fundamental interactions. By the Heisenberg uncertainty principle, elementary particles do not have an extent with crisp boundaries, and this fundamental uncertainty extends in principle to all material objects. --Lambiam 08:18, 23 April 2021 (UTC)
 * Thanks. Does that mean that, over time, there are forces tending to mix everything together so that nothing distinct is discernible, and if so, is there anything that works against this tendency? PaleCloudedWhite (talk) 08:37, 23 April 2021 (UTC)
 * In the long run, some cosmological models tell us, the ultimate fate of the universe may be that nothing discernible remains – so, a fortiori, nothing distinct will then be discernible: the ultimate Nirvana. On somewhat smaller time scales, while some manifestations of the forces work in the direction of mixing (see also Second law of thermodynamics), other manifestations of these forces tend to sort and organize – otherwise nothing structured would have emerged from the almost complete uniformity of the very early universe; see Structure formation. Elementary particles combined to form atoms; clumps of hydrogen gas formed stars which ignited, cooked heavier elements through nuclear fusion and then blew apart, scattering these elements through space, where they could cluster together and form rocky bodies as well as water, consisting of chemical compounds and crystals. Some of these rocky bodies had circumstances that were amenable for self-organizing and self-replicating constellations of molecules, harvesting negative entropy, also known as "life". Some life forms then came together to build a collaborative encyclopedia, and here we are. --Lambiam 09:12, 23 April 2021 (UTC)


 * Our article about cold welding, notably the quote in the lead, might be of interest. Tigraan Click here to contact me 08:40, 23 April 2021 (UTC)


 * For amusement: the phenomenon is exaggerated to an absurd degree in the novel The Third Policeman by Flann O'Brien, in which a rural policeman has been riding a bicycle for so long that mutual exchange of their atoms has, allegedly, led to each taking on properties of the other. {The poster formerly known as 87.81.230.195} 2.219.35.136 (talk)


 * Another thing to remember here is that our concepts of solids and fluids are platonic ideals, so when we describe a solid as consisting of atoms "locked" into a fixed location, and that fluids have mobile atoms that are completely free to move and exchange places, those are both unreachable extremes on a continuum of states, all real materials have some fluidity and some level of connections between the constituent particles, even crystalline solids. As noted above, on the longest time scales everything is a fluid and all matter is diffusing into other matter; it's all going to eventually end up fully mixed.  We just live on a small enough time scale to be able to note interesting behaviors in that matter on the way to its ultimate fate.  -- Jayron 32 11:36, 23 April 2021 (UTC)
 * This whole thread forcibly reminded me of Flann O'Brien's speculations on atomics and bicycles. Deor (talk) 17:38, 23 April 2021 (UTC)
 * And I had obviously overlooked the post by The poster formerly known as ... above before piping up with this. Deor (talk) 21:51, 23 April 2021 (UTC)

Thanks for all the replies - they're really interesting. I was just curious about the edges of things, but that seems to be a question about 'life, the universe, and everything', and the answer seems to be "rather like Pig-Pen". PaleCloudedWhite (talk) 06:40, 24 April 2021 (UTC)
 * In the case of two solids, the relevant quality is hardness, for getting an idea of how much each solid will damage the other and thereby pick up some bits of it. For an illustration, consider the streak test, and see Mohs scale. The complication is that in any scenario on Earth outside of ultra-high-purity clean room conditions, everything has plenty of hitchhikers along for the ride: body oils, particulates picked up from the air, microbes, and so on. --47.155.96.47 (talk) 05:15, 25 April 2021 (UTC)
 * Something that most people don't know: if you take a softer material such as nylon or teflon and rub it against a harder material such as stainless steel the harder material wears out first. This is because small bits of fine sand/coarse dust (silicon oxide; harder than plastic or steel) get embedded in the surface of the softer material and basically turn it into a file. The exception is when one of the materials is harder than sand, Examples of this would be the jeweled bearings in a high-end mechanical wristwatch and Gorilla Glass as used in many smart phone screens. --Guy Macon (talk) 12:24, 27 April 2021 (UTC)

What does this source say on Cueva de las Manos?
What does the source: Say about Cueva de las Manos? Is it anything that's not in the Wikipedia article? Thanks! Tyrone Madera (talk) 23:28, 23 April 2021 (UTC)
 * Check your mail box. --Lambiam 09:34, 24 April 2021 (UTC)
 * , Thanks! Tyrone Madera (talk) 21:11, 24 April 2021 (UTC)
 * , can you do the same for this publication? Tyrone Madera (talk) 19:55, 30 April 2021 (UTC)
 * I had no luck finding an accessible copy. My impression from what I could see is that this may be too technical to be of much interest. --Lambiam 20:44, 30 April 2021 (UTC)