It appears to me that experiments such as " Wigner's friend" or "Schrödinger's cat" cannot be done because the contents of the lab or box is always known for an outside observer in the form of gravitational, electric and magnetic fields. As far as I know such fields cannot be "blocked" by any material. Therefore I question the relevance of those experiments for the understanding of quantum mechanics.

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    $\begingroup$ It may well be known for an outside observer, but if they don't tell the participant, so what... $\endgroup$ – user207455 Jul 3 '19 at 6:09
  • $\begingroup$ If the state of the cat is known to be "alive" or "dead" for both observers (the one inside the box and the other outside), the box becomes irrelevant and there is no difference between what those observers experience. Both would agree in every aspect. The so-called paradox dissolves. $\endgroup$ – Andrei Jul 3 '19 at 7:17

As per the correct comments, you do not define what you mean in this case by isolation. For the purposes of the experiment, the info just cannot get out of the lab. That is why I am assuming you need a isolation that prevents info from getting out of the lab. Though, this way info still could get into the lab from outside. But that does not count as observation (since no info gets out). For the purposes of this experiment, the whole BH, that is, inside the event horizon should be the lab.

These experiments are thought experiments, but the main ideas are sound and work perfectly well with QM. The reason we use the word box, is to analogously simulate the case of a quantum variable. We can only get information about the certain quantum variable, whenever we observe it. Between measurements, the box acts like a quantum system, where no info can get out of.

You are asking whether we could prepare the lab so that is it completely isolated from the outside world.

In our world, there is no way as you say to shield completely against gravitational and EM fields. So we cannot make sure that no info is getting out of the lab. Just think about neutrinos, even if you could shield against these fields, neutrinos could easily get out.

The answer is that the lab has to be inside the event horizon of a black hole. If one wants to do a measurement, that person will have to go beyond the horizon (and never return). Contrary to popular belief, the lab that would fly inside the horizon would not be spaghettified right away, it might be intact as well (it depends on the characteristics of thee black hole). It might not be practically doable, but this is the only way to make sure the lab is isolated.

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    $\begingroup$ This answer seems to make exactly zero sense. How does being within the event horizon of a black hole isolate the lab from the rest of the universe? $\endgroup$ – WillO Jul 4 '19 at 11:49
  • $\begingroup$ Arpad, you are saying that the gravitational field cannot be completely shielded. Can you at least partially shield it? $\endgroup$ – Andrei Jul 4 '19 at 12:26
  • $\begingroup$ @WillO the EH is the definition of a potential barrier, and no info can get out of it. The question was for info getting outside. Anything can get in, but nothing gets out. I will edit my answer because you are right about the question not being clear whether it wants an isolation for both directions of info transfer or not. $\endgroup$ – Árpád Szendrei Jul 4 '19 at 12:26
  • $\begingroup$ @Andrei currently, gravity is best described by GR. QM does not yet give a detailed explanation about gravity, and the gravitons are only hypothetical. Though, gravitons are so weakly interacting with matter, and yet we have not been able to produce gravitons in an experiment. There is no way currently to give even the slightest shielding against gravity. $\endgroup$ – Árpád Szendrei Jul 4 '19 at 12:30
  • $\begingroup$ So....because the lab is inside the event horizon and nothing can leave the event horizon, it follows that nothing can leave the lab? I suppose then, that because my house is in the universe and nothing can leave the universe, it follows that nothing can leave my house. Yet somehow I manage to go to work every day! $\endgroup$ – WillO Jul 4 '19 at 12:55

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