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I've just found the following paper in arXiv written by a Spanish politician who is also a physicist. In it he claims that you can setup a sequential Stern-Gerlach-type experiment in such a way that the particles get energy from the measurement process, hence the title "heating the coffee by looking at it".

https://arxiv.org/abs/1402.3190

He admits that the energy has to come from somewhere and that somewhere is the S-G magnets, however he doesn't give a hint of how that could happen. I have forgotten a lot of the QM I learned from my undergrad days, although I can follow the paper more or less and it sounds a little fishy to me.

I think there is something very odd in having a background magnetic field and then the S-G magnets on top of it. For example, I don't understand how you can have states with spins perpendicular to the uniform background magnetic field right after the second S-G magnet.

EDIT: My question is, in a setup as the one proposed in the paper, is it correct to say that after the second S-G magnet the particles will have spins that are perpendicular to the background magnetic field?

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    $\begingroup$ Hi and welcome to physics.SE. Currently, this question isn't really a good fit for the site, since it is effectively asking us to review a paper ("is [there] some mistake [in] the equations?"). Please edit it to ask a self-contained question about physics. It's okay to refer to a paper for background or motivation, but you should ask a question that is more than "is this paper correct or not?", since that's for the academic review process to decide, not for us. $\endgroup$ – ACuriousMind Oct 24 '16 at 21:01
  • $\begingroup$ Note that as the whole system before measurements is closed and conservative, by a mere change in the choice where to put detectors you could decrease the total energy as easily as the author increases it: just "detect away" the more energetic particles instead of the less energetic ones. $\endgroup$ – Ruslan Oct 25 '16 at 9:52
  • $\begingroup$ Is it published? $\endgroup$ – Qmechanic Oct 25 '16 at 10:28
  • $\begingroup$ @Ruslan, I believe that its usually assumed that you can make the Stern-Gerlach sensors as "delicate" as you like. I think that a version of the setup can be designed in such a way that the energy that enters or exists the system are as small as we want. But this is my non-expert opinion. The author seems to agree with you (while, at the same time, make it the point of the paper as it if were something unusual.) $\endgroup$ – Hackenbush Oct 25 '16 at 11:45
  • $\begingroup$ @Qmechanic, I think it has been "published" only in arXiv, I don't think a real journal would accept it. At least from a formal point of view (language, quality of citations...) $\endgroup$ – Hackenbush Oct 25 '16 at 11:46

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