A Brief History to the birth of Quantum Mechanics- 17th to 20th century; Newton to Maxwell.

Prologue:

People read Quantum Mechanics (QM) and all kind of fancy ideas pop up. Multiverse, Multiple realities (Schrodinger's Cat is Dead and Alive?), Duality of nature, metaphysical and philosophical undercurrents to our understanding of reality and the Universe and a general mysticism affiliated to Modern physics- Does God really play dice with us?

And the truth of the matter is, Quantum Physics(QP) is just as mystic to general public as it is to physicists. Multiple explanations are present to explain experimental observations and phenomenon leading to multiple Quantum Mechanical (QMal) Interpretations. None of them absolute and supreme. Everyone of them probable and possible.

There are as far as 20 modern QMal Interpretations in circulation. A few fiercely debated. Others being sidelined as a mere foot-note. Some of the few mainstream interpretations (and a few favoured by me) are given as follows.

1.       The Copenhagen Interpretation aka Shut up and Calculate Interpretation

2.       Hidden Variables Interpretation aka Determinism is fundamental Interpretation

3.       Many Worlds Interpretation aka The Pop Culture Interpretation

4.       Van-Neumann-Wigner Interpretation aka Consciousness causes Collapse Interpretation

5.       De Broglie- Bohm Interpretation aka Pilot Wave Theory

6.       Quantum Darwinism aka Quantum Natural Selection Theory

There are 13 other major QMal Interpretations. Several other subdivisions to these interpretations exist as well. Many of these Interpretations have been disproven as well, in subsequent experiments and many interpretations have evolved and have been modified to adjust their predictions with the experimental outputs.

The explanations to most of these, if not all, interpretations will be given at the end of our journey. A forewarning has to be issued that although The Copenhagen Interpretation has been followed by almost all the scientific schools and Many Worlds Interpretation has been popularized amongst the masses by the mainstream media and pop culture references, none of these, along with the rest of the interpretations of QM, have any experimental backing to prove their authenticity. The main reason behind this is one of physics and metaphysics burning question- The Measurement Problem.

Courtesy: Dominic Walliman- Various Interpretations of QM
For more details into interpretation of Quantum Mechanics here's the link to a YouTube video where Dominic Walliman explains various interpretations of Quantum Mechanics-  https://www.youtube.com/watch?v=mqofuYCz9gs

 

 INTRODUCTION:

The Path of Light

In order to understand QM, one must first ask the question “What was the need to create a system of mechanics so complex and unintuitive?” Every new breakthrough in science has always been credited to a phenomenon that remained unanswered. Something inexplicable and unexplainable has almost always been the cause of a new discovery. Similar was the case in QM.

The Corpuscular Theory of Light:
In order to answer the question asked above, one must travel back almost 300 years before the actual conception of QMal postulates to 1637 CE and Rene Descartes has just given his version of corpuscular theory of light. 35 years later Sir Isaac Newton pioneered this point of view and elaborated on it in 1672, officially formulating a “Corpuscular” or “Particle” Theory of light. He stated in his Hypothesis of Light in 1675 that light was composed of corpuscles (particles of matter) which were emitted in all directions from a source. This was one of the early front runners in explaining the nature of light and successfully explained optical phenomena like reflection, polarization (qualitatively), and even refraction, although incorrectly.

Similarities between reflection of light on a reflecting surface and collision of ball on a wall.

The Wave Nature of Light:

Around the same time as Rene Descartes and Sir Isaac Newton, Robert Hooke and Christian Huygens were exploring the same question, but via a different route. Robert Hooke, in his quest to explain the existence of colours, developed a “pulse theory” which compared emission of light from a point source like spreading of water waves from a splash in water (Micrographia-“Observation IX”, 1665). 7 years later, Hooke proposed that light’s vibration could be perpendicular to the direction of propagation of light.
In 1678, Christian Huygens worked out a mathematical framework explaining the wave nature of light and published it in his Treatise on light in 1690. He proposed that light was emitted in all directions as a series of waves in a medium called the Luminiferous ether.

Wave theory heavily derived it’s predictions from the behavior of water waves in water and sound waves in air. Just like sound and water waves, light waves were supposed to interfere with each other and form an interference pattern, as well as necessarily required a medium to travel (which Huygen termed as ever-permeating, Luminiferous Ether)

The main criticism of Wave Theory of Light by Newton (who was a contemporary scientist of Huygen) was the rectilinear propagation of light (light travels in a straight line). Waves were known to bend around obstacles (as observed in the case of sound waves), while light travelled strictly in a straight line, just like a particle would. However, the phenomena of diffraction of light had been observed by Francesco Grimaldi, and Newton had to concede that a light particle could create a localised wave in the aether, in order to explain the phenomenon of diffraction.
Newton's theory was predominant for more than 100 years and took precedence over Huygens' wave theory of light, partly because of Newton's great prestige, and partly because the phenomena exhibited by light at that time were mostly explained by the corpuscular theory of light*. However, since Newton was able to explain most of the optical phenomena and also held a lot of prestige in the scientific community, his corpuscular theory became widely accepted in the scientific world. In 1704, Newton published the final version of his theory in the now famous book, Optika.

Newton’s reputation led to Corpuscular Theory holding much sway during the 18^th century until 1801, when Thomas Young demonstrated the now famous, Double-Slit Experiment(this is not the last time we will be reading about this experiment) on light to produce an interference pattern. The exhibition of Interference by light was a big blow to Newton’s Corpuscular theory and a major victory for Huygen’s Wave Nature of light. His wave mechanics were accurately able to explain and predict phenomena like interference, and refraction, correctly, while Leonard Euler in 1746, has already argued in Nova theoria lucis et colorum that wave theory of light is a better explanation for the phenomenon of diffraction of light.

Interference pattern observed by Thomas Young in his Double Slit Experiment.

The final nail in the coffin to Newton’s Corpuscular theory of light came in 1850 when Leon Foucalt’s experimentally found out the speed of light fairly accurately to make comments on the two competing theories of light. Newton's corpuscular theory implied that light would travel faster in a denser medium, while the wave theory of Huygens and others implied the opposite. Foucalt’s results supported the Wave theory of light and hence, in 1850, Newton’s corpuscular theory was finally discarded.

One of the salient features in the Wave theory of light was that it required light, just like other waves, a medium of transmission to propagate from one point to another. In the next section, we will see how this particular point led to the evolution of light as a simple wave to an “Electromagnetic” radiation.

*Since, Newton thought of light being made up of particles, he assumed that light should move faster in a denser medium as the particles must get accelerated in a denser medium where gravitational pull was greater. This change in speed led to the refraction of light. (Of course, the underlying explanation of light travelling at different speeds in different media resulting in deviation in the path of light at the point of incidence forms the basic reasoning behind refraction of light but the explanation given by Newton was way off target here. As we know now, light travels slower in an optically denser medium). 

 

The Electromagnetic Theory:

Just as developments in the study of optics were being carried out leading to eventual discarding of the Newtonian Corpuscular Theory of Light, a general consensus has developed around the mid 19^th century that light was indeed a wave. But what kind of a wave?

As discussed earlier that the traditional scientific definition of a wave necessarily required a medium of transmission. However, light travels from Sun to Earth without the presence of any medium in between. In order to solve this conundrum, Huygen had introduced an invisible, omnipresent medium known as Luminiferous Ether and this kind of stop-gap solution was generally accepted, although with a lot of doubt.

However, this was not the only theory doing rounds in the scientific circles of mid 19^th century. A certain Michael Faraday, in 1845, demonstrated a phenomenon which is now known as the Faraday Rotation. This phenomenon was a direct experimental evidence given by Faraday which established a dependence between magnetic field lines and light, hinting an underlying relationship between magnetism (and by Faraday’s own laws of electromagnetic induction, electricity too) and light. While in 1846, Faraday just speculated about light being a disturbance in the magnetic field lines, by 1847, he had proposed that light is in-fact a high-frequency electromagnetic vibration, which could propagate even in the absence of a medium such as the ether. However Michael Faraday had no formal education and had no background of mathematics and hence could not give a mathematical framework to back his proposal.

It is here that a certain James Clerk Maxwell enters into the frame and through his ingenious mathematical mind, came up with a set of 4 differential equations, now known as the famous Maxwell’s equations, which laid the foundation of Electrical Engineering and ushered the world into the era of modern science.

Faraday's work in electromagnetic induction and electromagnetism inspired James Clerk Maxwell to study electromagnetic radiation and light. Around 1862, while lecturing at King's College, Maxwell calculated that the speed of propagation of an electromagnetic field is approximately that of the speed of light. He considered this to be more than just a coincidence, commenting,

"We can scarcely avoid the conclusion that light consists in the transverse undulations of the same medium which is the cause of electric and magnetic phenomena.”

 By 1873, Maxwell in his seminal work, A Treatise on Electricity and Magnetism, had given a complete mathematical description of the behavior of electric and magnetic fields. Using the 4 differential equations (Maxwell’s equations), Maxwell proved that light is propagated as an oscillating electric and magnetic field and hence concluded that light was therefore a form of electromagnetic radiation. Soon after, Heinrich Hertz confirmed Maxwell's theory experimentally by generating and detecting radio waves in the laboratory, and demonstrating that these waves behaved exactly like visible light. Light was still considered to be a wave, just one that doesn’t necessarily needs a medium to propagate.

James Clerk Maxwell and his famous Maxwell's Equations and Heinrich Hertz.

An Electromagnetic radiation. The arrow points to the direction of propagation. The Blue sinusoidal wave depicts the oscillating Electric Field while the Red sine wave depicts the oscillating Magnetic Field.

This is, however, not the end of the story. In 1900, a certain Max Planck will mathematically solve a then unsolvable problem of physics, eventually giving birth to Quantum Physics.

That's for the next segment.