**Why there is such thing called Quantum Mechanics ?**

Since the classical Newtonian physics fails while considering behavior of a system; that is very small, that travels very fast, that subjects very strong gravity etc. in early 1900’s German physicist Max Planck publishes his study “the effect of radiation on a blackbody substance” and introduced the world to “Quantum Mechanics”.

Due to classical physics, energy is a continuous wave-like phenomenon that is independent of the characteristics of physical matter. Yet, due his theoretical work, Planck verified that energy, in certain situations, may exhibit characteristics of physical matter. His theory was about the formation of the radiant (shining or glowing brightly) energy that is consisted of particle-like components, called “quanta” (meaning “cost” in Latin). Moreover, he was rewarded the Nobel prize for his work on blackbody radiation, in 1918.

With the contributions of other scientists, such as, Albert Einstein, Niels Bohr, Louis de Broglie, Erwin Schrödinger, and Paul M. Dirac, improved Planck’s theory and made possible the development of Quantum Mechanics (energy is both matter and a wave, depending on certain variables). Quantum Mechanics and Special Relativity (Einstein) formed the today’s modern physics.

Basically, Quantum Theory is the theory of the matter. Since matter is consisted of atoms and atoms are consisted of particles such as electrons, protons and neutrons, Quantum theory provides us our best explanation of these particles. Yet, we can also consider the matter in form of radiation such as light. The duality of light states light consists both light waves and particle-like photons, where the notation of photons from the statement comes from Quantum Theory (and introduced by Einstein, in 1905 as “light quanta”).

**What is the Nature of Light ?**

Thomas Young, an English physicist, showed that light has the properties of a wave in an experiment called Young’s Interference Experiment, in 1807. This experiment showed that lights (waves) passing through two slits (i.e. double-slit) where waves add together or cancel each other and then interference fringes appear. This phenomenon cannot be explained unless light is considered as a wave.

In 1905, Albert Einstein succeeded in explaining the photoelectric effect, that had never been explainable since light considered as a wave, he asserted that light is a particle containing energy corresponding to their wavelength. The photoelectric effect is a phenomenon where irradiating a blue light on metal emits electrons from it. However, red light does not cause electron emission from metal no matter how long or how intense the light is applied. To understand this effect, you should think of photon as (clusters of) particles containing energy.

**Relief of Einstein**

When we look into history of science specifically the history of quantum mechanics we immediately observe that photoelectric effect is one of the most important phenomenons which explained by Planck’s “quanta” idea. Although Einstein won his Nobel Prize for his work on photoelectric effect, his personal relief that make him comfortable about the discrete nature of nature came from the work of his former teacher Heinrich Weber. Weber studied the specific heat(Amount of heat per unit mass required to raise the temperature by one degree Celsius.) of diamond which defers from the classical theory. This problem remained an anomaly until Einstein. Using Planck’s idea he found an expression for the average energy of an atom in a crystal vibrating in 3 direction with the same frequency. By comparing his formula for the heat capacity with the data obtained by Weber, Einstein was able to fit the vibration frequency in a such way that agrees with the experiment.

Einstein’s work was important for him to convince himself but beside that it was also important to attracting interest into Quantum Theory. By that interest we have an enormous improvements in the field along years. With the next articles in this series we will explore the fruits of this bizarre theory and we will learn about why it is so useful and why it is so unnatural for our macroscopic brains. Join us in this journey and take care yourself until the next article.

**U. Karadeniz, O.S. Tapsin **

Resources: https://www.pitt.edu/~jdnorton/teaching/HPS_0410/chapters/quantum_theory_origins/, John D. Norton — https://www.history.com/this-day-in-history/the-birth-of-quantum-theory — https://photonterrace.net/en/photon/duality/ — Quantum Generations: A History of Physics in The Twentieth Century, H. Kragh — https://www.ucl.ac.uk/~ucapahh/teaching/3C25/Lecture11s.pdf —