All of us are more or less familiar with the Classical Newtonian Physics. However, it fails, when we consider very small systems like individual atoms and even particles out of which they are made off. This is where the Quantum Theory came into the field of physics and changed its dimensions. Quantum theory gives us our best account of nature in the very small. The standard quantum theory makes no changes to the ideas of space and time of relativity theory. Some versions of quantum theory are set within the spacetimes of general relativity.

Quantum theory is a theory of matter, or more precisely it is a theory of the small components that comprise familiar matter. The ordinary matter is made up of particles, like electrons, protons, and neutrons. Quantum Theory also provides an account of the matter in the form of radiation, such as light. It is commonly known that light somehow consists both of light waves and also particle-like photons. The notion of these photons comes from quantum theory (and from Einstein directly, who first introduced them in 1905 as “light quanta”).

The given neat division of matter into particle-like and wave-like would not persist. The story of the coming of quantum theory is the story of the breakdown of this division.

## The History of Quantum Theory

The **history of quantum mechanics** is a fundamental part of the modern physics. The 1859–60 winter statement of the black-body radiation. The discovery of the photoelectric effect in 1887. And the 1900 quantum’s hypothesis by Max Planck that any energy-radiating atomic system can theoretically be divided into a number of discrete “energy elements” *ε* (epsilon). Such that each of these energy elements is proportional to the frequency *ν* with which each of them individually radiates energy, as defined by the following formula: (where *h* is a numerical value called Planck’s constant.)

Then, Albert Einstein in 1905, in order to explain the photoelectric effect of light, postulated consistently with Max Planck’s quantum hypothesis. Stating that, most important, light itself is made of individual quantum particles, called photons. The photoelectric effect was observed upon shining light of particular wavelengths on certain materials, such as metals, which caused electrons to be ejected from those materials only if the light quanta energy was greater than the work function of the metal’s surface.

## Wave-Particle Duality

In 1909, Einstein showed that certain phenomena could only be successfully explained if we used both wave and particle view. The full observed effect came from the sum of two terms, one a particle term, the other a wave term. The need for both is sometimes called “wave-particle duality.”

## Matter Waves

Bohr’s theory of 1913 and its later elaboration gave a wonderfully rich repertoire of methods for accounting for atomic spectra. They depended on a contradictory mix of classical and non-classical notions. By the early 1920s, the limits of this system began to show and theorists also turned to the task of making some coherent sense of this body of theory that soon came to known as “the old quantum theory.”

Another approach proved equivalent and is easier to picture. It was based on a supposition by de Broglie of 1923 and developed by Schroedinger in 1926. Einstein had shown that a wave phenomenon, light, also had particle-like properties. Might the reverse be also true? Might particle like electrons also have wave properties?

The hypothesis answered yes. It associated a wave of a particular wavelength with a particle of some definite momentum.

Here is de Broglie’s formula that tells us which wavelength goes with which momentum:

Momentum = h / Wavelength

## The New Quantum Theory

In the later part of the 1920s, however, all these ideas coalesced into what was called the “new quantum theory,” to distinguish it from the “old quantum theory” of the decades before. There were

• matrix based approaches proposed by Heisenberg, Born and Jordan; and

• the matter waves of de Broglie and Schroedinger; and

• Dirac’s classical “c-numbers” and quantum “q-numbers.”

A matrix from Born and Jordan’s paper on the new quantum mechanics of 1925.

First, the new theory introduced an element of probability that was unknown in classical physics. The best the theory can offer are probabilities. This circumstance proved deeply troubling to many thinkers of the era, including Einstein. They found it repugnant to think that the fundamental laws of the universe might be probabilistic and described the difficulty as a breakdown of “causality.” There were deeper problems.

Second, the new quantum theory worked very well for small particles. However, it was far less clear how it should be applied to macroscopic bodies. Besides, tables, chairs, houses, and elephants do not obviously manifest a combination of wave and particle-like properties.

## What should we know?

- What theories of matter looked like at the end of the nineteenth century.
- How Planck’s analysis of heat radiation generated a problem for classical physics.
- What is contained in Einstein’s 1905 proposal of the light quanta.
- How Bohr used atomic spectra to infer to a new and strange model of the atom.
- How the proposal of matter waves started to make sense of Bohr’s proposal.

Video Courtesy – ” * Best0fScience* “

Reference – Origins of Quantum Theory by John D. Norton

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