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 the physics of matter, or more precisely, 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 first introducing 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 kind of a 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.)
Later, Albert Einstein in 1905, in order to explain the photoelectric effect of light, postulated consistently with Max Planck’s quantum hypothesis, that, light itself is made of individual quantum particles, called photons, in amounts E = hf, where h is Planck’s constant and f is the frequency of the light field. The photoelectric effect was observed upon shining light of certain wavelengths on some materials, such as metals, which caused electrons to be ejected from those materials only if the energy of light quanta was greater than the work function of the metal’s surface. Although this equation is exactly the same as Planck got in 1900, the meaning was completely different. On one side where for Planck this was the discreteness of the interaction of light with matter, for Einstein this was the quantum of light energy – whole and indivisible.
In 1909, Einstein showed that certain phenomena could only be successfully explained if we used both wave and particle view. The full explanation of the observed effect contained the sum of two terms, one a particle term, and the other a wave term. The need for both is what is called “wave-particle duality.”
Bohr’s theory of 1913 and its later elaboration gave a wonderfully rich repertoire of methods that accounted 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 theory that soon was termed as “the old quantum theory.”
Another approach, that proved equivalent and is easier to picture, was based on a supposition by de Broglie of 1923 and was later developed by Schrodinger in 1926. Einstein had shown that a wave phenomenon, light, also had particle-like properties. Could particle-like electrons also have any wave properties?
The answer is yes. A wave of a particular wavelength is related to a particle of some definite momentum.
Here is de Broglie’s formula that states 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. Some that can be listed were:
• Matrix based approaches proposed by Heisenberg, Born and Jordan.
• The matter waves of de Broglie and Schrodinger.
• 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 Newtonian physics. The best the theory can offer are probabilities. This predicament 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 did not offer much clarity on how to apply it to macroscopic bodies. Besides, tables, chairs, houses, and elephants do not obviously manifest a combination of wave and particle-like properties.
Akshat Mishra is a Physics Undergraduate at Centre for Excellence in Basic Sciences, Mumbai. He feels the need to explore the depths of the not-so-dark universe while at the same time watch the quanta in action. Electronic Music is what puts him in the thinking zone.