Imagine travelling back to the turn of the 18th century and having a conversation with Newton, armed with your knowledge of modern physics, in particular, quantum theory. You try to "explain" the new theories to him: what have we observed, and how does this change our ideas of fundamental physics? My question is the following:

  • What experiments could have been performed while Newton was alive, which could illustrate the need for quantum theory? For example, I imagine that using telescopes and prisms, they could observe discrete spectral lines from celestial bodies, but they wouldn't be able to observe the photoelectric effect in the usual way.
  • What is the "minimal set of concepts" that you would have to introduce to explain the key part of the theory (I realize this is subjective, so take this to mean the postulates of the Copenhagen interpretation)? This can include concepts from mathematical analysis - for example, I imagine you would need to define the idea of a Hilbert space - but also from physics or philosophy.

Please include any other issues you think may be relevant. Thanks!

Edits after question was closed due to being opinion-based:

I would like to clarify that I am not asking a question about pedagogy, i.e. what didactic methods you would use to explain quantum theory to him. I am also not asking a question about engineering, i.e. how you would use the technology available at the time to construct experiments that would be convincing.

I am asking about what the essential elements are in the gap between the knowledge of a very advanced 18th century physicist (or, as pointed out by someone in the comments, a very intelligent high-school student today with knowledge of Newtonian mechanics) and say, a second- or third-year undergraduate student today, who although may not be familiar with more advanced aspects and subtleties of quantum theories, has an appreciation of phenomena which point to the need for quantum mechanics, as well as the mathematical formalism used in the theory (states, operators, measurements, etc.).

So in a nutshell, my question (which may need to be split into several questions) is about what is contained in the conceptual gap and the technological gap (this is the part about what experiments we would have been able to talk to him about) between Newton's time and today.

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    $\begingroup$ I think the question is asking too much for one post, but I'm curious what rules of engagement you have in mind. If we gave them suitable instructions, I imagine that people in Newton's time might have been able to build the equipment needed for a CHSH-inequality-violating experiment. Is that allowed? That experiment's results are traumatic enough to make just about any scientifically minded person receptive to the core concepts of quantum theory. $\endgroup$ Jul 18, 2021 at 19:31
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    $\begingroup$ I think explaining QM to Newton will be same as explaining to a high school student (since both are familiar with Newtonian dynamics), except that he is smart and can catch up easily. You will have to start with essentially all relevant 101 courses (which are mandatory for undergrad students), starting from Lagrangian & Hamiltonian mechanics, some introduction to linear algebra, thermodynamics etc.. If we have the facility, we should rather bring Newton in our present era and we can demonstrate all experimental evidences for QM. $\endgroup$
    – KP99
    Jul 18, 2021 at 19:56
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    $\begingroup$ Despite my finding the question very interesting, voting to close as opinion-based because the question is essentially about solving a multi-faceted pedagogy and/or engineering problem rather than about physics. $\endgroup$
    – ACat
    Jul 18, 2021 at 21:11
  • $\begingroup$ @KP99 do you think that's a minimal approach though? I think it's certainly possible to understand the basics of quantum theory without knowing too much about either Hamiltonian mechanics or Thermodynamics (apart from the very basics of course). $\endgroup$
    – Lili FN
    Jul 19, 2021 at 13:14
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    $\begingroup$ @ChiralAnomaly thanks for the example! I guess the question is then, what prerequisites are necessary to understand that experiment. Because my guess is that maybe he would need a crash course on probability to even understand what that is about. $\endgroup$
    – Lili FN
    Jul 19, 2021 at 14:01


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