The simplest harmonic oscillator system.
Experiment and theory are like the yin and yang: opposites, but one cannot exist without the other.
This single experimental observation/idea is the basis for all of special relativity.
Special relativity is the direct result of people bending their backs to accommodate for this really weird fact.
Isomers were quite confusing for early chemists, before atomic theory was widely accepted, and people where thinking mostly in terms of proportions of equations, related: Section "Isomers suggest that atoms exist".
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