– Albert Einstein
Fast forward two thousand years later. Lord Kelvin and J.J. Thomson gave the world a simple picture of the atom that can tested by experiment. These gentlemen proposed that the atom’s structure consists of electrons embedded like raisins in a dough or cake of equally positive charge.
It didn’t take long for Ernest Rutherford in 1911 to modify the cake model of the atom due to his discovery of the atomic nucleus. From his alpha scattering experiments, it was revealed that the atom has a positive charge concentrated minutely at its center with the electrons spread out over a large region beyond it. Much like the solar system with the sun at the center and the planets revolving around the sun. It this model of the atom that persists in the minds of many people today.
Yet this planetary model of the atom has a very serious flaw. According to the classical laws of physics the electrons going around the atom will emit radiation thereby losing its energy until it will spiral off and collapse to the center of the atom in a short time. Therefore this model cannot hold matter. It is not as stable as the planetary system we are in.
Niels Bohr then proposed his famous theory of the hydrogen atom in 1913. His model restricts the electron in stable, allowed, circular orbits without emitting any radiation; thus making a robust atom. Now this idea is completely alien and so contrary to what everyone believed at that time. Nothing in James Clerk Maxwell‘s electrodynamics and Isaac Newton‘s mechanics would support Bohr’s contentions. When does the electron emit energy? It is only when the electron makes a transition from one allowed orbit to another that it radiates energy. And this energy is related to an equation postulated 13 years earlier.
Bohr’s radical proposal had its roots in the work of Max Planck who in 1900 introduced the earliest concept of the quantum of action to explain the age-old problem of radiation produced by a heated body or black body radiation.
Planck’s hypothesis is completely startling and without precedence. Light was emitted in bundles of energy where each bundle was related to the frequency of light, f, by E = hf. In this equation, h , Planck’s constant, the quantum of action was introduced for the first time into the body of physics. Planck’s hypothesis was very startling, completely unprecedented and completely in contradiction with old physics.
The quantum of action hypothesis of Planck marked the beginning of modern physics. We can say that the world was never the same after Planck.