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Why are nucleons, atoms and molecules considered identical particles (bosons or fermions) even if they can be distinguished by the state of their most elementary components?

Also, what size does a molecule have to be so we can we cease to consider two molecules of that specie identical?

user140255
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  • Buckyballs (C$_{60}$ molecules) have been shown to create an interference pattern. –  Feb 13 '18 at 21:15
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    @Pieter I think you need to explain why an interference pattern is significant in this context. – StephenG - Help Ukraine Feb 13 '18 at 21:18
  • @undead, you are wrong. nucleons, atoms and molecules are not identical particles. Also, all molecules of one particular substance are identical. – niels nielsen Feb 13 '18 at 22:53
  • Related: https://physics.stackexchange.com/questions/275363/are-there-fields-corresponding-to-the-composite-particles-e-g-hydrogen-atom-fi – dmckee --- ex-moderator kitten Feb 13 '18 at 23:10
  • Are you asking why a neutron is considered identical to a neutron, and a water molecule identical to a water molecule? Or are you thinking that a neutron is identical to a water molecule, which is false? –  Feb 13 '18 at 23:31
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    OP referred to "molecules of that specie", and that is quite clear. –  Feb 13 '18 at 23:41
  • @Ben Crowell, I know that neutrons and water molecules are not identical, this is not my question... My point is that neutrons can eventually be distinguished from one another by the internal states of their quarks. So it seems that considering them identical is an approximation. My question is at which point this approximation breaks up. – user140255 Feb 14 '18 at 14:35
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    This explanation is pretty good: https://en.wikipedia.org/wiki/Identical_particles#Distinguishing_between_particles. Notice that whereas in principle you can never distinguish identical particles, in some cases you can. See here: https://physics.stackexchange.com/questions/377078/making-indistinguishable-particles-distinguishable/377270#377270. Also related: https://physics.stackexchange.com/questions/283682/theoretically-could-there-be-different-types-of-protons-and-electrons – valerio Feb 14 '18 at 14:45
  • @nielsnielsen - only if all the atoms in the molecules are the same isotopes of course... – Jon Custer Feb 14 '18 at 17:31
  • @jon custer, thanks, I get that, but the content of OP's question didn't seem to go there. – niels nielsen Feb 14 '18 at 18:13
  • @valerie thank you for the references, these are instructive, but still not do not answer my specific question. Should the question be made clearer in some way? Because right now I don't understand why people are voting to close it down – user140255 Feb 15 '18 at 01:28

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