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I just got into a flame war with a non-physicist who insists that the quantum-mechanical idea that the process of observation affects the outcome of an experiment is the fundamental reason why the actions of one human are affected by whether or not there is another human on hand who is watching.

How do you disabuse someone of such nonsense, when he or she knows nothing about quantum mechanics?

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    $\begingroup$ What if someone's being observed but don't realize they're being observed. If they act the same as they would otherwise, this would defeat that theory. You could say people behave differently if they're aware they're being observed, but that's just psychology, not quantum mechanics. $\endgroup$ – barrycarter Jun 5 at 18:09
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    $\begingroup$ You cannot disabuse someone of such nonsense if they are unwilling to observe what you are saying ;-). We are not quantum systems, we exist in the large N limit. perhaps this is a key to the argument. However, let's assume that your adversary was pondering the philosophical consequences of QM. The statement does deserve discussion and not being able to convince an open minded person that it nonsense may be meaningful. After all, physics isn't finished. $\endgroup$ – ggcg Jun 5 at 18:20
  • $\begingroup$ Another point is that the non-physicist's statement may be altogether wrong. Is it even true that "the actions of one human are affected by whether or not there is another human on hand who is watching"? That is, is it always true to a measurable degree? Perhaps a novice piano player is scared of an audience but a pro is not. This would contradict the premise that our actions are affected by observation (in a predictable, deterministic way). Keep in mind QM is deterministic, we always know what the wavefunction will do. $\endgroup$ – ggcg Jun 5 at 18:23
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    $\begingroup$ I have found it quite impossible to disabuse someone of ignorance which they are intent on keeping, FWIW. $\endgroup$ – J. Murray Jun 5 at 18:42
  • $\begingroup$ Impossible to tell from this description how they are thinking. Is the problem that they think observer participation effects among humans are due to 'consciousness collapsing the wavefunction'? Or is the problem that they think quantum uncertainty is not fundamental, and arises merely from physicists probing the atom to measure it, analogous to certain aspects of participant observation on the human level? $\endgroup$ – Mitchell Porter Jun 5 at 18:59
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It sounds like this is a problem in psychology and how one person influences another by persuasion. Maybe the following would help. All I am going to do is propose that one can eliminate the human element entirely from discussions of measurement and detection.

We can present quantum theory without the need to mention the word "observer" by providing a model of the kind of physical process which is ordinarily involved in measurement-like interactions. That would be a process involving thermodynamic irreversibility, but not necessarily involving consciousness or reasoning beings. So, for example, the photomultiplier tube "clicks" whether or not anyone hears it click. So then the statement would be not that the act of observation disturbs the experiment, but something more mundane but which does capture correctly what happens: it is the physical interaction between the quantum system and the measuring apparatus which disturbs the quantum system. Of course it does! Why wouldn't it?

With this in hand, the point which makes quantum different from classical is merely that you can't take a limit in which this interaction tends to zero. When the interaction tends to zero, so does the degree to which any information passes from one system to another.

So now I would say to my non-physicist friend that there is not much more to it than that, at least for the practical purpose of everyday calculations and predictions. Of course I am aware that not everyone will be satisfied with my simple model, in that it sweeps under the carpet the exact sense in which a process can be irreversible when quantum theory seems to say that everything is unitary. But for practical purposes the notion of irreversibility is all we need in order to say that a process happened, whether or not any conscious or reasoning being was aware that it happened. Schrodinger's cat is either alive or dead well before anyone takes a peek.

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