A Brief History Of Quantum Mechanics.

Quantum physics is more than 100 years old. Unlike relativity, the development of this theory was slow and required many minds. However, it is one of the most successful theory that the mankind has in its hands. Quantum mechanics has passed numerous tests through time. Today, let us learn how was quantum physics born.

Quantum mechanics is the theory of matter. It answers the questions: What is matter made of? How the constituents of matter behave? At the end of the 19th century, matter was clearly divided into two main forms: particles and waves. Particles were the localized lumps of stuff that flew about like little bullets. The best investigated of the fundamental particles was the electron. The other form was wavelike matter. The one well-investigated form was light or, more generally, electromagnetic waves.

Also Read: Understanding the Schrodinger Wave Equation and its importance.

We had pretty much figured out how things work around us: The ripples on a pond, the motion of planets, the projectile etc. But in science, whenever we think 'We Got It All', nature gives new problems to solve. The clear distinction between the two forms of matter was soon going to get a blow in a series of events. I will discuss 3 major events that lead to the development of this beautiful theory.

1. Max Planck: The Blackbody Radiation

At the turn of the 20th century, a few engineers approached Max Planck and asked him, "How to make light bulbs more efficient?" This was the question that began quantum mechanics. Physicists were unable to explain the nature of the blackbody radiation. A blackbody is a perfect absorber and emitter of radiation. Sun, stars and light bulbs are good approximations of a blackbody. The curve of radiation from a blackbody has a particular shape as shown below.

Blackbody Spectrum was an important milestone in birth of quantum mechanics
The typical plot of blackbody radiation

Several attempts were made to explain blackbody radiation curve. The theories by Rayleigh-Jeans and Wien, however, provided partial success. This is when Max Planck comes across this problem. He makes an assumption to which he had no logic. He assumed that the light is made up of discrete, rather than continuous, packets of energy. Each packet has energy equal to hf, where h is Planck's constant and f is the frequency of light. This solved the problem mathematically but Planck wasn't convinced if his idea had any practical implication or logic. He said, "quanta (energy packet) are just a mathematical concept." In December 1900, Planck announced his results and this was the beginning of quantum theory.

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2. Albert Einstein: The Photoelectric Effect

In 1905, when scientists like Marie Curie, Max Planck, Phillip Lenard and Wilhelm Roentgen were at the peak of their career, there was a young, rebellious and ambitious man working as a third class patent clerk at Bern, Switzerland, trying to get noticed by these giants. That man was Albert Einstein, someone who was going to change the course of history in the coming decade. Einstein recognized the beauty and importance of Planck's work, something that Planck himself could not.

Einstein and photoelectric effect

When light is shed on a metal surface, electrons are emitted from the surface. It seems convincing that if we increase the intensity of light, more energetic electrons will be emitted and vice verse. Light intensity can easily be changed by moving the source of light towards or away from the metal. The experiments on photoelectric effect had a different story to tell. It was seen that the intensity of light has no effect on the energy of electrons that were emitted from the metal. Intensity did not matter to the ability of light to produce photoelectrons. All that mattered was the frequency of the light. If light was of low frequency, it could not generate photoelectrons, even if the light were very intense. If the light had a high frequency it could produce photoelectrons, even if the light was of very low intensity.

This, Einstein observed triumphantly and linked it with Planck's quanta. If the frequency (energy) of the quanta that is shed on the metal is less, it will not have enough energy to knock out the electron. Increasing the intensity of the light did nothing more than increasing the number of light quanta showering on the cathode, all them too weak in energy to liberate a photoelectron.

Einstein won the 1921 Nobel Prize in Physics for his work on the photoelectric effect. His role in building the quantum theory is significant as the phototelectric effect actually shed more light on the particle nature of light.

3. Niels Bohr: The Atomic Spectrum

The most significant blow to the old classical physics came in the form of the atomic spectrum. When scientists passed an electric current through a tube containing gas (as shown below) and analysed the emitted light through the spectrometer (prism), they were shocked. They observed very fine spectral lines. The lines were sharp. Classical physics could not explain the sharpness of the spectral lines.

Also Read: What is EM Spectrum and why it is the most important tool for an Astrophysicist?

How was quantum physics born?
Different glow of gases when electric current is passed
EM Spectrum and birth of quantum mechanics
The fine spectral lines of hydrogen that classical physics could not explain.

In 1913, Niels Bohr, a Danish Physicist, gave an explanation to this strange phenomenon. He attributed these spectral lines to the transitions made by the electrons from one orbit to the other. Bohr's theory, however, had many flaws in it which he himself could not explain. His theory was the first leap to explain the structure of the atom, and thus an important milestone towards the development of quantum mechanics.

Also Read: An introduction to the mathematics of quantum mechanics

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