Proportionality factor in the Planck-Einstein relation between light energy and frequency.

And analogously for matter, appears in the de Broglie relations relating momentum and frequency. Also appears in the Schrodinger equation, basically as a consequence/cause of the de Broglie relations most likely.

Intuitively, the Planck constant determines at what length scale do quantum effects start to show up for a given energy scale. It is because the Plank constant is very small that we don't perceive quantum effects on everyday energy/length/time scales. On the $lim_{h→0}$, quantum mechanics disappears entirely.

A very direct way of thinking about it is to think about what would happen in a double-slit experiment. TODO think more clearly what happens there.

Defined exactly in the 2019 redefinition of the SI base units to:

$6.62607015×10_{−34}J⋅s$

- Planck's law | 118, 452, 4
- Black-body radiation experiment | 33, 502, 6
- Quantum mechanics experiments | 36, 2k, 26
- Quantum mechanics | 170, 18k, 245
- Particle physics | 137, 28k, 449
- Physics | 276, 39k, 680
- Natural science | 0, 48k, 1k
- Science | 0, 52k, 1k
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- Kilogram | 47, 52, 1
- Planck constant | 163, 258, 2