Planck discovers quanta Science

The answer is “how much?”

Black body curves of Planck for various temperatures and comparison with classical theory.   | Photo Credit: Wikimedia Commons

When quantum mechanics is mentioned, names such as Albert Einstein, Niels Bohr, Louis de Broglie, Erwin Schrodinger and Paul M. Dirac are the ones that readily come to mind. But at its foundation lies Max Planck, whose idea on the basis of energy quantisation led to the formulation of modern physics.

Planck, who opted to study physics having entered the University of Munich at age 16, received his doctorate with a thesis on the second law of thermodynamics at the age of 21. His thesis work became the basis of his research which led Planck to discover quanta.

The problem pertained to a black body - an idealised physical body that absorbs all electromagnetic radiation. Theoretically, therefore, it should radiate light of all frequencies equally when heated. In real life, however, the distribution of energy radiated did not match up with the predictions made by classical physics.

Beyond what was known

Even though Planck was steeped in traditional physics, he was willing to look beyond it. He guessed that his solution lie in combining the dependences in the low-frequency region and the high-frequency region in the simplest possible manner. He began working on transforming such a result into a formula relating energy to the frequency of the radiation.

  On October 19, 1900, he presented his formula at a meeting of the German Physical Society and it was accepted as correct. Planck though saw it merely as a lucky guess and went to work on deriving his result from basic principles.

By December 14, 1900, he had achieved this, but only after changing one basic assumption: that energy, instead of being continuous, comes in distinct particles. Thus, the oscillators making up the black body and re-emitting the energy that falls upon them do not absorb energy continuously, but only do so in discrete amounts. These discrete quantities came to be known as “quanta”, from the Latin for “how much?”.

Far reaching impact

Though Planck’s radiation law was readily accepted, the importance of the discovery and its far reaching impact on classical physics was not realised at first. Even Planck himself wasn’t convinced if it was anything more than a little mathematics that helped solve his particular problem.

By 1905, however, Einstein had used the theory of quanta to describe photoelectric effect accurately. In 1913, Bohr had incorporated Planck’s idea into his revised model of the atom. In 1918, his work on black body radiation earned Planck the Nobel Prize in Physics.

Slowly but steadily, quantum mechanics - the mathematical application of the quantum theory that conforms to the duality principle (energy exhibits properties of both wave and particles) - made its way. And along with Einstein’s theory of relativity, quantum mechanics now dictates our understanding of modern physics.


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Printable version | Oct 18, 2021 6:50:53 PM | https://www.thehindu.com/in-school/sh-science/the-answer-is-how-much/article7983395.ece

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