Many "applications" are linked to quantum physics - let it be the laser, LED's, transistors, MRI scans, atomic clocks, electron microscope or CCD-detectors in digital cameras. I wonder wether these applications really only got developed starting from quantum theory, or wether quantum mechanics only is able to describe them, with the applications having already been there before quantum physics was elaborated as a theory.
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1$\begingroup$ All of the technologies you mention were invented after the theory of quantum mechanics was formulated, although the theory has developed and grown in tandem, especially solid state physics $\endgroup$– RC_23Commented Oct 12, 2022 at 16:44
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$\begingroup$ Thank you. Is there a source (book, paper) that elaborates on this that you could recommend? $\endgroup$– manuel459Commented Oct 12, 2022 at 16:48
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$\begingroup$ This translated talk might be of interest: nature.com/articles/121580a0 . It's from Niehls Bohr just as Heisenberg/Schrodinger formulated their contributions. This can also provide kind of a timeline to center on. You're looking more at "triode", "x-rays" than anything else. $\endgroup$– meltynessCommented Oct 12, 2022 at 18:34
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3$\begingroup$ One thing that did exist before QM, and indeed became the motivating case for creating QM, is the light bulb. Max Planck was asked to find the optimum temperature to operate a filament at, to produce the most visible light for the least energy. The spectrum (intensity vs. frequency) of light could not be explained by classical EM theory -- see "Ultraviolet Catastrophe." Planck's result for the real spectrum, plus other results like the photoelectric effect, laid the foundation for QM. $\endgroup$– RC_23Commented Oct 12, 2022 at 22:21
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I guess that it depends on what exactly an "application" is. Quantum effects (e.g. rectifying semiconductor junctions, radioactivity, atomic spectrum, etc.) were certainly discovered before their quantum explanation. Arguably, that's what prompted the development of quantum mechanics!
If an "application" is the use of a quantum effect (i.e. an effect that cannot be explained by classical physics --- that's actually a slippery definition*), then there were many. Off the top of my head: crystal detectors (a type of semiconductor diode), gas-discharge lamps (including neon signs), and radium self-luminous paints.
People regularly tinkered with stuff they could not explain. In fact, for most of human history, that's all inventors did!
*Arguably, most (all?) chemistry is quantum effects, and arguably any chemical reaction is a quantum application. People have used chemical reactions for as long as there have been people.
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$\begingroup$ I would suggest as a definition, a classically measurable relationship between classically measurable states that contradicts the predictions of classical physics, but can be correctly predicted by QM. $\endgroup$– g sCommented Oct 14, 2022 at 19:14
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$\begingroup$ How would that apply to situations where classical physics makes no prediction? What's the classical prediction for atomic spectra (classically measurable with a prism) or most things related to chemical reactions? $\endgroup$– lnmaurerCommented Oct 14, 2022 at 19:52
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$\begingroup$ @inmaurer no prediction mapping from state 1 to state 2 is the same as a prediction that state 2 will not regularly follow from state 1. Most of chemistry, however, can be done by treating all quantum interactions as taking place in a black box, and measuring macroscopic qualities of systems, such as mass, temperature, and enthalpy, provides all you need to know to predict the behavior of chemical systems using classical thermodynamics, often trivial thermodynamics. $\endgroup$– g sCommented Oct 15, 2022 at 14:24
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$\begingroup$ I'm not sure whether atomic spectra would qualify as a classically measurable quantum effect or not under my definition, or as another unexplained quality of a material we can identify by its macroscopic qualities, but for whose inner workings we have no model. $\endgroup$– g sCommented Oct 15, 2022 at 14:39
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$\begingroup$ “Predicting” (the word in your definition) and “measuring macroscopic qualities” are two very different things. How does classical physics predict, say, the heat capacity of matter (a quantity frequently needed for calorimetry)? You can certainly measure it in various ways, but that’s not a prediction. (Boltzmann’s classical model didn’t produce that accurate results and was arguably proto-QM to boot. Einstein and Debye’s models were certainly QM.) $\endgroup$– lnmaurerCommented Oct 15, 2022 at 19:04