Quantum
Quantum (plural quanta) means the smallest amount or unit of something, especially energy.[1] In physics, energy is said to be quantized in some systems. This means the system can have only certain amounts of energy, and not any amount in between.[2] A textbook compares this to a car that could only travel at certain speeds.[3] Most people know that matter comes in small lumps such as atoms, but fewer people know that energy can come in lumps too.[4]
Planck's quanta
In 1900 Max Planck solved a physics puzzle known as the blackbody radiation problem.[5] He assumed that tiny oscillators inside the walls of a cavity could only have certain separate amounts of energy. He called these separate energy values quanta.[6] Planck said that the energy (E) could only take the values E = nhf, where n is a whole number (1, 2, 3 and so on), f is the frequency, and h is a number called Planck's constant.[7]
At first, other physicists did not take the idea seriously, because it did not follow from any accepted theory at the time. Even Planck himself saw it as a useful mathematical trick.[8] His work was the first sign that energy is sometimes quantized on a small scale, and he was awarded the Nobel Prize in Physics in 1918.[9]
Quanta of light
In 1905 Albert Einstein published an explanation of the photoelectric effect. In it he gave Planck's energy quantum a new meaning: a particle of light.[10] These particles of light are called photons, and the energy of each photon is proportional to its frequency.[11]
Quantized energy in atoms
In 1913 Niels Bohr used Planck's ideas of quantization and Einstein's ideas about photons in his description of the atom.[12] Instead of allowing an electron in an atom to have any amount of energy, Bohr assumed that the energies of its orbits were quantized.[13] However, Bohr's model still used the idea of precise orbits from classical mechanics. This idea was later found not to work for very small things, once a proper model of quantum mechanics was developed.[14]
Related pages
References
- ↑ "quantum". Cambridge Dictionary. Cambridge University Press. Retrieved 2026-10-18.
the smallest amount or unit of something, especially energy
- ↑ "29.1 Quantization of Energy". College Physics 2e. OpenStax. Retrieved 2026-10-18.
Energy is quantized in some systems, meaning that the system can have only certain energies and not a continuum of energies, unlike the classical case.
- ↑ "29.1 Quantization of Energy". College Physics 2e. OpenStax. Retrieved 2026-10-18.
This would be like having only certain speeds at which a car can travel because its kinetic energy can have only certain values.
- ↑ "29.1 Quantization of Energy". College Physics 2e. OpenStax. Retrieved 2026-10-18.
While most of us are familiar with the quantization of matter into lumps called atoms, molecules, and the like, we are less aware that energy, too, can be quantized.
- ↑ "6.1 Blackbody Radiation". University Physics Volume 3. OpenStax. Retrieved 2026-10-18.
The blackbody radiation problem was solved in 1900 by Max Planck .
- ↑ "6.1 Blackbody Radiation". University Physics Volume 3. OpenStax. Retrieved 2026-10-18.
Planck's hypothesis of discrete energy values, which he called quanta , assumes that the oscillators inside the cavity walls have quantized energies .
- ↑ "6.1 Blackbody Radiation". University Physics Volume 3. OpenStax. Retrieved 2026-10-18.
Planck assumed that the energy of an oscillator ( E n E n ) can have only discrete, or quantized, values: E n = n h f , where n = 1 , 2 , 3
- ↑ "6.1 Blackbody Radiation". University Physics Volume 3. OpenStax. Retrieved 2026-10-18.
When Planck first published his result, the hypothesis of energy quanta was not taken seriously by the physics community because it did not follow from any established physics theory at that time. It was perceived, even by Planck himself, as a useful mathematical trick
- ↑ "29.1 Quantization of Energy". College Physics 2e. OpenStax. Retrieved 2026-10-18.
This was the first indication that energy is sometimes quantized on a small scale and earned him the Nobel Prize in Physics in 1918.
- ↑ "6.1 Blackbody Radiation". University Physics Volume 3. OpenStax. Retrieved 2026-10-18.
This perception was changed in 1905 when Einstein published his explanation of the photoelectric effect, in which he gave Planck's energy quantum a new meaning: that of a particle of light.
- ↑ "6.2 The Bohr Model". Chemistry 2e. OpenStax. Retrieved 2026-10-18.
Planck's ideas of quantization and Einstein's finding that light consists of photons whose energy is proportional to their frequency
- ↑ "6.2 The Bohr Model". Chemistry 2e. OpenStax. Retrieved 2026-10-18.
In 1913, Niels Bohr attempted to resolve the atomic paradox by ignoring classical electromagnetism's prediction that the orbiting electron in hydrogen would continuously emit light. Instead, he incorporated into the classical mechanics description of the atom Planck's ideas of quantization and Einstein's finding that light consists of photons whose energy is proportional to their frequency.
- ↑ "6.2 The Bohr Model". Chemistry 2e. OpenStax. Retrieved 2026-10-18.
Instead of allowing for continuous values of energy, Bohr assumed the energies of these electron orbitals were quantized
- ↑ "6.2 The Bohr Model". Chemistry 2e. OpenStax. Retrieved 2026-10-18.
Bohr's model was severely flawed, since it was still based on the classical mechanics notion of precise orbits, a concept that was later found to be untenable in the microscopic domain, when a proper model of quantum mechanics was developed to supersede classical mechanics.