With the development of artificial intelligence, virtual reality, etc., the amount of data to be processed is increasing, but the limits of integrated circuits are approaching. So instead of gates made of transistors, quantum computers that use quantum as their operational rules are emerging as an alternative. What exactly is quantum, and what can we do with it as an alternative? A reporter who has no connection with science takes a close look at everything from quantum to quantum computers, which are a recent issue, with a learning heart.


Wave-particle duality is the property that all matter has both particle and wave properties. In classical mechanics, waves and particles have very different properties, but in quantum mechanics, the two concepts are integrated into one concept.

Wave-particle duality originated from the controversy over whether light is a particle or a wave. Experiments have proven that light has both properties. Later, it was discovered that not only light but also all other substances have both particle and wave properties.
Is light a particle or a wave?
This debate is an old debate in the scientific community. Newton, who laid the foundation of classical mechanics, claimed that light is a particle. When Newton said this, many scientists accepted that light is a particle. However, when the double-slit experiment in the 19th century discovered diffraction and interference, the idea that light is a wave became the mainstream in the scientific community. And in the early 20th century, when quantum mechanics was emerging, Einstein revived the idea that light is a particle by proposing the photon hypothesis.


Max Planck's room
Einstein's light quantum hypothesis emerged in 1900 as a development of Max Planck's view on light, but there was a significant difference of opinion between the two men on the question of the existence of light quanta and their approach to the concept of quantum discontinuity.

As we have seen above, Max Planck's quantization hypothesis was the starting point of quantum mechanics, but Planck was not fond of it. In Planck's paper, the fact that energy is expressed in terms of Planck's constant and the integer multiple of the frequency of light is not actually an 'integer multiple' such as 10, 20, 30, 40, but can also be interpreted as an 'interval' such as 15, 25, 35.

The statistical method used by Planck, when the law of energy equalization was not clearly established, is not entirely the same as what we know today. Although Planck was a pioneer who opened the door to new quantum physics, he had inherently conservative limitations.
The Beginning of a Revolution
Einstein's revolution began with Planck's reform. In 1905 and 1906, Einstein proposed the photon hypothesis, which regards light as a particle, to explain the photoelectric effect. The photoelectric effect is a phenomenon in which a substance such as a metal emits electrons when it absorbs electromagnetic waves with a shorter wavelength than its own specific wavelength. Einstein had a different view of light than Planck because he clearly discussed the law of equal distribution of energy.

Einstein applied Planck's quantization hypothesis to light, assuming that light is an energy particle with energy equal to the frequency multiplied by Planck's constant, and called this light particle a light quantum. He thought that light was not a wave, but a particle with discrete energy. Einstein's light quantum clearly presupposed the existence of discrete energy.

Einstein's light quantum hypothesis was so radical at the time that it was not accepted as is. Above all, it had problems explaining the diffraction and interference phenomena of light. To overcome this, in 1909, Einstein attempted to explain the dual nature of light by using the discussion of Brownian motion that he had discussed in 1905 along with the light quantum hypothesis and the theory of relativity.


Photon hypothesis, sees the light of the world
Until 1907, discussions on the existence of photons and quantum discontinuity were mainly confined to the field of thermal radiation. In order for the photon hypothesis to be accepted by the scientific community, the area of interest had to be expanded to areas other than thermal radiation. In his 1907 paper on specific heat, Einstein attempted to explain the problem of specific heat of solids using his quantization hypothesis. Erwin Madelung, William Sutherland, and Walter Nernst agreed with him. Nernst not only played a significant role in making Einstein known to many scientists through his work on specific heat at extremely low temperatures, but he also intervened deeply in the process of gathering opinions within the scientific community surrounding quantum issues by organizing international conferences on quantum issues.
Nernst played a key role in convening the 'First Solvay Conference' held from October 29 to November 3, 1911. At that time, most of the leading scientists involved in the quantum hypothesis participated in this conference and had a heated debate on whether to accept the quantization hypothesis. The conference played a decisive role in the acceptance of quantum theory by senior scientists.


Coincidence? Or not?
In 1911, Einstein partially suspended the light quantum hypothesis and partially accepted the wave theory interpretation of light. Tired of defending the reality of light quanta, Einstein put quanta aside and began to focus on gravity. In 1916, having completed the great work of general relativity, Einstein resumed the discussion on quanta.

In 1917, Einstein discussed spontaneous and stimulated radiation, and in the conclusion, he again discussed the reason why light quanta must exist. Einstein argued that light quanta not only have energy hν, but also have momentum hν/c* in a specific direction. That is, when light is shone on an excited molecule, the molecule emits a photon, and according to the law of conservation of momentum, a repulsive force corresponding to hν/c acts on the molecule. It has become possible to interpret photons as not only discontinuous in the form of energy, but also discontinuous in the form of momentum. (* c is the speed of light; h is Planck's constant; ν is the frequency.)

In this paper, Einstein saw the problem that light is not emitted in the form of a spherical wave that goes in all directions, but rather in a specific direction like a needle. If this were the case, molecules or atoms should move in a disorderly manner and repel each time they emit light according to the law of conservation of momentum. However, since molecules or atoms actually do not move and appear to be stationary, Einstein concluded that with the current state of quantum theory, the direction in which light is emitted can only be determined by 'chance'.

However, in 1926, Max Born, who was studying the theory of quantum collisions, developed his own statistical interpretation of quantum mechanics based on this paper by Einstein. What Einstein did not recognize as a physical reality and saw as a weakness to be overcome in his theory, Born saw as a theoretical reality based on experimental facts and proposed a statistical interpretation of quantum mechanics.

Einstein spent his entire life trying to overcome the incompleteness of his quantum theory. His later research into unifying gravity and electromagnetic forces also began in that context.

In 1923, Einstein tried to derive a system of superiorly determined differential equations based on the relativistic field equations. This new attempt maintained the continuum hypothesis and deterministic techniques, reflecting Einstein's hope that the indeterminism found in quantum theory could also be resolved by a system of higher-order determined differential equations.
Photons exist
Bohr used ideas presented in Einstein's photoelectric effect paper in his atomic model published in 1913, but he was quite skeptical of the light quantum hypothesis itself. Between 1924 and 1925, a heated debate raged among scientists, led by Bohr and Einstein, over the existence of light quanta.

In early 1924, Bohr attempted to revive the radiation theory based on wave theory, claiming that the laws of conservation of energy and momentum in the microscopic world were violated based on the concept of virtual oscillators. The concept of virtual oscillators is a hypothesis that atoms are composed of a series of virtual oscillators that interact with other virtual oscillators located far away through virtual radiation fields. It was first proposed in 1924 by John Slater, an American who was working as a doctoral researcher in Europe. Bohr combined this hypothesis with his own idea of violation of the laws of conservation of energy and momentum in the microscopic world.

Bohr, Hendrik Kramers, and Slater jointly published a new theory of radiation. Bohr argued that although Einstein's light quantum hypothesis was a good conceptual tool for explaining many problems, including the photoelectric effect, it still had many problems because it used the concepts of frequency or wavelength defined by wave theory to explain the interference or diffraction phenomena of light.
In Bohr's view, even the X-ray scattering experiment on electrons announced by Arthur Compton and Peter Debye, called the Compton scattering experiment, was not a decisive experiment to confirm Einstein's light quantum hypothesis.

In order to determine which of Bohr's new wave theory and Einstein's existing light quantum hypothesis was correct, Walter Bothe and Hans Geiger performed a rigorous and decisive experiment using the coincidence-counting method, which was an improved version of the electric counter. The coincidence-counting method was a device that made the output signal of a counter appear only when the input signals were input to both of two or more counters at the same time. Using this, they were able to determine whether the collision of light quanta and electrons was a single event or a complex event caused by multiple factors.

In April 1925, Bothe and Geiger confirmed that Einstein and Compton's assumption that the energy law and its complement, the impact law, are satisfied whenever a photon and an electron collide is correct. Subsequently, when Compton again confirmed the Compton scattering effect using a cloud chamber, the photon hypothesis was finally fully accepted among scientists.


Thoughts that are agreed with and thoughts that are rejected
Einstein actively supported the statistical mechanics discussion presented by Satyendra Bose of India in 1924, and developed Bose-Einstein statistics, one of the major statistical laws used in quantum mechanics. Bose-Einstein statistics is one of the quantum statistics applied when dealing with matter such as light, and along with Fermi-Dirac statistics applied when dealing with electrons, it developed into two typical statistical mechanics used in quantum mechanics.
Einstein actively supported Louis de Broglie's matter wave theory in 1924 and Schrödinger's wave mechanics in 1926. Schrödinger's wave equation was based on the field equation, which was a continuum physics description, and this could be connected to Einstein's idea of seeking a new continuum field theory while emphasizing causality in physical laws.

On the other hand, Einstein did not believe in the Copenhagen interpretation based on indeterminism developed by Werner Heisenberg, Born, Bohr, etc. Einstein believed until his death that the indeterminism problem of quantum mechanics would be solved if a more perfect unified field theory that could unify gravity and electromagnetism appeared. Einstein's quantum theory became the foundation of quantum mechanics, but Einstein did not believe in quantum mechanics. It can't be helped that it's ironic.

We have looked at what Einstein contributed to quantum mechanics through the light quantum hypothesis. In the next article, we will discuss the Copenhagen interpretation.

References: Pioneers of Physics 12. Einstein and the Photon Hypothesis, Kyung-Soon Lim