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Could an 18th century or earlier scientist have come across phenomena which require quantum theories to explain them, given the apparatus available at the time?

I'll choose 1805 as the cut-off date, because that's when Maudslay's micrometer revolutionised precision in instruments.

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  1. Christian Huygens discovered in 1690 polarized light - this is the first quantum effect ever observed. The transformation behavior of rays of completely polarized light was first described by Etienne-Louis Malus 1809 (who coined the name ”polarization”), and that of partially polarized light by George Stokes 1852.
    In modern terminology, the behavior described by Malus (resp. Stokes) is identical to that of a qubit in a pure (resp. mixed) state. Stokes 1852 paper contains all modern quantum phenomena for a single qubit, discussed in classical terms. (For details, see my lecture
    The transverse nature of polarization was discovered by Augustin Fresnel 1866, and the description in terms of (what is now called) the Bloch sphere by Henri Poincare 1892. In modern terminology, polarization is a manifestation of the massless spin 1 nature of the unitary representation of the Poincare group defining photons.

  2. The second oldest observed quantum effect are spectral lines, apparently first discussed in 1802 by William Hyde Wollaston. (For the history of spectroscopy, see

Both phenomena require quantum physics for their explanation (though polarization can also be explained by a statistical version of classical electrodynamics).
But, of course, before 1900 nobody considered these to be quantum effects. Spectral lines were first described as a quantum effect in 1913 by Niels Bohr. Polarization was first described as a quantum effect in 1930 by Norbert Wiener.

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Wiener wrote a quantum paper? In 1930? I got it, I think you mean Eugene Wigner. I don't like the idea that polarization is quantum--- it's only quantum if you have photons. If not, it's just a field which transforms under rotation--- an E and B field. – Ron Maimon Jun 26 '12 at 3:54
@RonMaimon: References to the original papers by Norbert (!) Wiener and Stokes are in the slides of my lecture cited. Read the paper by Stokes and you'll see that he describes precisely and in almost modern terms all the properties of a qubit! Thus there is no doubt that he interpreted a quantum effect. – Arnold Neumaier Jun 26 '12 at 11:01
@RonMaimon: That polarizationn is not just an electromagnetic field can be seen from the phenomenon of partial polarization, which is impossible in a deterministic Maxwell field, but needs a statistical interpretation. But even when calculated classically, the results are identical with the quantum statistics of a single photon. – Arnold Neumaier Jun 26 '12 at 15:39
I think that this is not true. You can do polarization simply by having a classical model of electrons on beads tilted with respect to the E-axis of the electromagnetic radiation. Polarization isn't quantum unless it is polarization of individual photons. If you can't resolve photons, you can't argue that polarization is quantum, because you get partial transmission and partial absorption with frictional charges with oscillations allowed only in certain directions – Ron Maimon Jun 26 '12 at 19:27
@RonMaimon: There is no need to resolve individual photons to claim a quantum effect. For example, laser light is definitely quantum although one usually doesn't resolve the photons involved. – Arnold Neumaier Jun 27 '12 at 12:10

Very little in the field of chemistry makes any sense in detail without quantum mechanics. If you want "phenomena which require quantum theories to explain them", just look around ... why is wood brown and leaves green and iodine yellow? What chemicals are stable versus unstable, why do different elements react different ways, why do salt crystals form cubes while ice forms hexagons?

None of these questions can be answered in a correct and consistent way except in the framework of quantum chemistry (and its consequences like orbital hybridization, delocalized electrons, resonance stabilization, Pauli exclusion principle, electron orbitals, the relation of light absorption and emission to electronic structure, etc. etc.)

These sorts of things are not usually discussed as motivation for quantum mechanics because it is a long and difficult road from the basic principles of quantum mechanics to explaining facts in chemistry like why ice is hexagonal. One might imagine that there is an alternative explanation of all the facts of chemistry that does not require quantum mechanics ... well, there isn't, but there is no particularly easy and pedagogical way to convince people of that. By comparison, there is a relatively simple path from basic principles of quantum mechanics to the two-slit experiment.

For the exact same reason, the conceptual breakthroughs in quantum mechanics did not historically arise from the attempt to explain why iodine is yellow. The path is too indirect. It required years of work AFTER quantum mechanics was established to understand how quantum mechanics was the only sensible explanation of almost everything in chemistry.

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The problem is that chemistry requires detecting atoms. – Ron Maimon Jun 25 '12 at 20:00
Chemistry requires detecting atoms?? Are you serious?? You don't need to detect atoms to know that ice crystals are hexagonal or that methane flames are blue or that nitrogen gas is less flammable than oxygen. You don't even have to be human to see that grass is a different color than rock! But if you want to explain these facts in a self-consistent and detailed framework, that framework HAS to be based on quantum mechanics. – Steve B Jun 25 '12 at 22:14
The issue is that the parts of chemistry which demonstrate quantum mechanics require detecting atoms. The blue flash could come from plum-pudding. Without alpha particle scattering from an individual atom, you don't know if it's quantum down there. – Ron Maimon Jun 26 '12 at 1:41
If you want to correctly explain why ice crystals are hexagonal, you need to invoke the Pauli exclusion principle, quantum superpositions, delocalized electrons, electron spin, etc. etc. Are these not part of quantum mechanics??? The original question asked for "phenomena which require QM to explain them". There is no theory besides quantum mechanics that can correctly and consistently predict the shape of every crystal. – Steve B Jun 26 '12 at 2:53
You can explain this by saying water molecules are little hexagon shaped tinkertoys. This is what pre-quantum atomists supposed. Without a mechanism to peer inside atoms, you don't need quantum mechanics for chemistry at room temperature, but you would for specific heats of cold molecules (the disappearing degrees of freedom) and for thermal blackbody light. – Ron Maimon Jun 26 '12 at 3:19

using an analogy between optics and PHYSICS ROWAN HAMILTON, could have discovered quantum mechanics by means of the Eikonal Equation $ (\nabla S)^{2}=N $

however there were no empirical evidences of Hamilton's discovery by this era so Hamilton rejected the idea of a 'wave mechanics' it was XIX century

it is a pity, science could have advance more than 50 years if Hamilton and othere had taken their ideas seriosly :(

for more

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Hamilton's work was in 1833, thus later than the time limit set by the OP. – Arnold Neumaier Jun 25 '12 at 15:20
But Hamilton's equations could have been formulated by Leibnitz. The issue is that conservation of energy was only appreciated during the industrial revolution, since it was the heat engines that drove the analysis of entropy that established that there was a conserved energy at all. – Ron Maimon Jun 25 '12 at 20:00

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