Planck’s Constant

6.62606957(29) × 10^-34 joule seconds

Episode #4 of the course “Most important numbers in the world”

At the turn of the 20th century, physics was taking a new direction in studying the properties of electromagnetic fields. An electromagnetic field, or a “quantum,” was understood to function in terms of waves, but was later understood to be a measurement of the energy of particles, which came to be termed “photons.” First theorized and published by Max Planck in 1900 as a constant proportion between the energy of a charged atomic oscillator and the frequency of its electromagnetic wave, Planck understood that actions took place over small, incremental changes that appeared seamless from a macrocosmic perspective.

Classic mechanics could not explain how energy was maintained between an atom’s nucleus and its electrons, so Planck theorized that this change must be so small it needed an incremental measurement. His initial assertions may have been incorrect, but in less than a decade, Planck’s constant had been refined into one of the fundamental principles of quantum mechanics. Signified as “h,” Planck’s constant is essentially a measurement of a rate of angular momentum or the spin of an electron around a nucleus. It is theoretically the rate of energy expended over time and distance as energy is transferred between moving electrons and their atomic nuclei.

Planck’s value creates a constant scale used to measure light and matter and the movements of these subatomic particles and their electromagnetic fields. It is such an incredibly small constant that it is adapted to the scale of human perceptibility, but it is so important that it establishes the basis of other subatomic measurements and scales. Planck’s work was essential to later discoveries in quantum physics, including those made by Albert Einstein. Because of Planck’s constant, Einstein theorized and calculated how light photons functioned in an electromagnetic field.

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