I am not asking about a boundary between classical and QM objects. There are many questions on this site about such a boundary. I am simply asking about an object being able to be at rest (in the lab frame).

Matter around us is built up of quarks and electrons. Both quarks and electrons are QM objects, and they cannot be at rest (have zero momentum, HUP).

With this answer I am trying to stress that at the level of nanometers and below, the atomic and molecular dimensions, the concept of "at rest" is a classical concept that has no meaning in a quantum mechanical system. The classical framework has to be abandoned once the dimensions and momenta are constrained by the HUP.

What is the reason behind why a quantum particle cannot be at rest?

Does rest exist?

Although quarks and electrons both possess rest mass, this is just invariant mass (the length of the four vector), and has nothing to do with the electrons or quarks being at rest.

Objects though, that are being built up by electrons and quarks, around us, can be at rest (relative to us).

If I start by building quarks (protons and neutrons) and electrons together, then bind more atoms via covalent bonding, I will have to reach a level at which the object will be able to stay at rest in the lab frame.

I have not found any such experiment, and I do not know if this has anything to do with Avogadro's number.


I do believe that the true underlying world is QM, and all objects are QM, but I would like to know if there is a level at which the ability of being at rest (relative to the lab frame) becomes apparent.


  1. How many elementary particles (or atoms) do I have to bind together to create an object that is able to stay at rest (in the lab frame)?
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    $\begingroup$ Are the things on your desk not subject to the HUP? $\endgroup$ – user253751 Feb 24 '20 at 16:38
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    $\begingroup$ "I am not asking about the boundary between classical and QM objects" This appears to be exactly what you are asking about $\endgroup$ – By Symmetry Feb 24 '20 at 16:41
  • $\begingroup$ @BySymmetry I do believe that the true underlying framework is QM, and all objects are QM objects. I am simply asking when the ability of rest (relative to the lab frame) becomes appearent in our experiments. $\endgroup$ – Árpád Szendrei Feb 24 '20 at 16:50
  • $\begingroup$ @BySymmetry thank you I edited tomake that clear. $\endgroup$ – Árpád Szendrei Feb 24 '20 at 16:55
  • $\begingroup$ A single atom can be at rest, sort of. ;) See en.wikipedia.org/wiki/IBM_(atoms) $\endgroup$ – PM 2Ring Feb 24 '20 at 17:16

Every object obeys the HUP, regardless of how many particles make it up. There is no clear point at which it is no longer followed, it just becomes less important for more massive objects because the uncertainties become negligable compared to the dimensions of the object and its momentum. If you have an object made of a million atoms, it still has a small uncertainty in its position but it isn't noteworthy. This uncertainty never goes away, just gets smaller as more mass is added.

  • $\begingroup$ thank you, so basically, the zigzag movement (zitterbewegung) or the QM uncertainty in position (or momentum) becomes smaller and smaller as the object gets bigger and bigger, and this uncertainty becomes relatively small relative to the size of the object? $\endgroup$ – Árpád Szendrei Feb 24 '20 at 20:11
  • $\begingroup$ @Árpád Szendrei yeah thats the gist of it, the effect is always present just less noticable on a classical scale $\endgroup$ – bemjanim Feb 24 '20 at 20:17

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