To many, quantum physics, or quantum mechanics, may seem an obscure subject, with little application for everyday life, but its principles and laws form the basis for explanations of how matter and light work on the atomic and subatomic scale. If you want to understand how electrons move through a computer chip, how photons of light travel in a solar panel or amplify themselves in a laser, or even why the sun keeps burning, you will need to use quantum mechanics.

## What is quantum physics?

Quantum mechanics is the branch of physics relating to the elementary components of nature, it is the study of the interactions that take place between subatomic forces. Quantum mechanics was developed because many of the equations of classical mechanics, which describe interactions at larger sizes and speeds, cease to be useful or predictive when trying to explain the forces of nature that work on the atomic scale.

Quantum mechanics, and the math that underlies it, is not based on a single theory, but on a series of theories inspired by new experimental results, theoretical insights, and mathematical methods which were elucidated beginning in the first half of the 20th century, and together create a theoretical system whose predictive power has made it one of the most successful scientific models created.

**Discoveries, principles, and theories that led to quantum mechanics**

The story of quantum mechanics can be said to begin in 1859, a full 32 years before the discovery of the electron. Many physicists were concerned with a puzzling phenomenon: no matter what an object is made of, if it can survive being heated to a given temperature, the spectrum of light it emits is exactly the same as for any other substance.

In 1859, physicist Gustav Kirchhoff proposed a solution when he demonstrated that the energy emitted by a blackbody object depends on the temperature and the frequency of the emitted energy, i.e

A blackbody is a perfect emitter - an idealized object that absorbs all the energy that falls on it (because it reflects no light, it would appear black to an observer). Kirchhoff challenged physicists to find the function J, which would allow the energy emitted by light to be described for all wavelengths.

In the years following, a number of physicists would work on this problem. One of these was Heinrich Rubens, who worked to measure the energy of black-body radiation. In 1900, Rubens visited fellow physicist Max Planck and explained his results to him. Within a few hours of Rubens leaving Planck's house, Planck had come up with an answer to Kirchoff's function which fitted the experimental evidence.

Planck sought to use the equation to explain the distribution of colors emitted over the spectrum in the glow of red-hot and white-hot objects. However, when doing this, Planck realized the equation implied that only combinations of certain colors were emitted, and in integer multiples of a small constant (which became known as Plank's Constant) times the frequency of the light.

This was unexpected because, at the time, light was believed to act as a wave, which meant that the values of color emitted should be a continuous spectrum. However, Planck realized that his solution gave different values at different wavelengths.

In order to explain how atoms were being prevented from producing certain colors, Planck made a novel assumption - that atoms absorb and emit energy in the form of indistinguishable energy units - what came to be called quanta.

At the time, Planck regarded quantization as a mathematical trick to make his theory work. However, a few years later, physicists proved that classical electromagnetism could never account for the observed spectrum. These proofs helped to convince physicists that Planck's notion of quantized energy levels may in fact be more than a mathematical "trick". ^{}

One of the proofs was given by Einstein, who published a paper in 1905 in which he envisioned light traveling not as a wave, but as a packet of "energy quanta" which could be absorbed or generated when an atom "jumps" between quantized vibration rates. In this model, the quanta contained the energy difference of the jump; when divided by Planck’s constant, that energy difference determined the wavelength of light given off by those quanta.

In 1913 Niels Bohr applied Planck's hypothesis of quantization to Ernest Rutherford's 1911 "planetary" model of the atom. This model, which came to be called the Rutherford-Bohr model, postulated that electrons orbited the nucleus in a similar way to how planets orbit the sun. Bohr proposed that electrons could only orbit at certain distances from the nucleau, and could "jump" between the orbits; doing so would give off energy at certain wavelengths of light, which could be observed as spectral lines.

It now appeared that light could act as a wave and as a particle. However, what about the matter?

In 1924, French physicist Louis de Broglie used the equations of Einstein's theory of special relativity to show that particles can exhibit wave-like characteristics, and vice-versa.

German physicist Werner Heisenberg met with Neils Bohr at the University of Copenhagen in 1925, and after this meeting, he applied de Broglie's reasoning to understand the spectrum intensity of an electron. At the same time, Austrian physicist Erwin Schrödinger, working independently, also used de Broglie's reasoning to explain how electrons moved around in atoms. The following year, Schrödinger demonstrated that the two approaches were equivalent.

In 1927, Heisenberg reasoned that if matter can act as a wave, there must be a limit to how precisely we can know some properties, such as an electron's position and speed. In what would later be called "Heisenberg's uncertainty principle," he reasoned that the more precisely an electron's position is known, the less precisely its speed can be known, and vice versa. The proved an important piece of the quantum puzzle.

In the Heisenberg-Schrödinger quantum mechanical model of the atom, each electron acts as a wave, or "cloud") around the nucleus of an atom, with the ability to measure only the speed or position of an electron to a particular probability. This model replaced the Rutherford-Bohr model.

All these revelations regarding quantum theory revolutionized the world of physics and revealed important details about universal actions at atomic and subatomic levels.

Quantum mechanics further combined with other phenomena in physics such as relativity, gravitation, electromagnetism, etc. also increased our understanding of the physical world and how construction and destruction occur within it.

For their exceptional contributions, Planck, Einstein, Bohr, Heisenberg, and Schrödinger were awarded the Nobel Prize in Physics in 1918, 1921, 1922, 1932, and 1933 respectively.

## The Bohr-Einstein debates

While it may seem as though quantum mechanics progressed in a fairly straightforward series of theoretical leaps, in reality, there was a lot of disagreement among physicists over its relevance.

These disagreements reached a peak at the 1927 Solvay Conference in Brussels, where 29 of the world's most brilliant scientists gathered to discuss the many seemingly contradictory observations in quantum theory that could not be reconciled. One major point of contention had to do with the theory that, until they are observed, the location and speed of entities such as electrons, can only exist as a "probability".

Bohr, in particular, emphasized that quantum predictions founded on probability are able to accurately describe physical actions in the real world. In what later came to be called the Copenhagen interpretation, he proposed that while wave equations described the probability of where entities like electrons could be found, these entities didn't actually exist as particles unless they were observed. In Bohr's words, they had no "independent reality" in the ordinary physical sense.

He described that the events that take place on atomic levels can alter the outcome of quantum interaction. According to Bohr, a system behaves as a wave or a particle depending on context, but you cannot predict what it will do.

Einstein, in contrast, argued that an electron was an electron, even if no one was looking at it, that particles like electrons had independent reality, and prompting his famous claim that “God does not play dice with the universe”.

Einstein and Bohr would debate their views until Einstein's death three decades later, but remained colleagues and good friends.

Einstein argued that the Copenhagen interpretation was incomplete. He theorized that there might be hidden variables or processes underlying quantum phenomena.

In 1935, Einstein, along with fellow physicists Boris Podolsky and Nathan Rosen published a paper on what would be known as the Einstein-Boris-Podolsky (EPR) paradox. The EPR paradox described in the paper again raised doubts on the quantum theory.

The EPR paper featured predetermined values of momentum and particle velocity and suggested that the description of physical reality provided by the wave function in quantum theory is incomplete, and therefore, physical reality can not be derived from the wave function or in the context of quantum-mechanical theory.

The same year, Bohr replied to the claims made by Einstein. In his response, published in the *Physical Review, *Bohr proved that the predetermined values of the second particle’s velocity and momentum, as per the EPR paradox were incorrect. He also argued that the paradox failed to justify the inability of quantum mechanics to explain physical reality.

## Do we live in a quantum world?

The understanding of elementary particles and their behavior helped to create groundbreaking innovations in healthcare, communication, electronics, and various other fields. Moreover, there are numerous modern technologies that operate on the principles mentioned in quantum physics.

**Laser-based equipment**

Laser technology involves equipment that emits light by the means of a process called optical amplification. Laser equipment work on the principle of photon emission and they release the light with a well-defined wavelength in a very narrow beam. Hence, the laser beams function in alignment with theories (such as the photoelectric effect) mentioned in quantum mechanics.

A report published in 2009 reveals that extreme ultraviolet lasers when hit a metal surface can cause electrons to move out of the atom, this outcome is said to further extend Einstein’s photoelectric effect in the context of super-intense lasers.

**Electronic Devices and Machines**

From flash memory storage devices like USB drives to complex lab equipment such as electron microscopes, an understanding of quantum mechanics led to countless modern-day inventions. Light-emitting diodes, electric switches, transistors, quantum computers, etc are examples of some highly useful devices that resulted from the advent of quantum physics.

Let us understand this from the example of Magnetic Resonance Imaging (MRI) machine, this medical equipment is very useful in diagnosing the brain and other body organs. MRI works on the principle of electromagnetism, it has a strong magnetic field that uses the spin of protons in hydrogen atoms to analyze the composition of different tissues.

MRI aligns all the protons in the body as per their spin, due to the magnetic field, the protons absorb energy and emit the same (quantum theory), the MRI scanner uses the emitted energy signals received from all the water molecules to deliver a detailed image of the internal body parts.

**X-Rays**

Used in medical diagnosis, border inspection, industrial tomography, cancer treatment, and for many other purposes, X-rays are a form of electromagnetic radiation. While the discovery of X-rays predates quantum mechanics, quantum mechanical theory has allowed the use of X-rays in a practical way.

A beam of X-rays can be regarded as consisting of a stream of quanta. These quanta are projected out from the target of the X-ray tube, and, on penetrating tissue, there is an effect produced that is proportional to the number of the quanta multiplied by the energy carried by each quantum.

The emitted electrons also emit photons, which are able to penetrate the matter and form its image on the X-ray screen. Therefore, the elementary particles mentioned in quantum mechanics interact with X-ray energy to deliver the inside look of an object.

**Fluorescence-based Applications **

Fluorescence is referred to the emission of light under UV exposure that takes place when an electron achieves a higher quantum state and emits photons, fluorescent lamps and spectrometers work on basis of quantum theory. Various minerals such as Aragonit, Calcite, and Fluorite are also known to exhibit fluorescence.

Fluorescence is also used to lit synthetic gems and diamonds, jewelry manufacturers use this phenomenon to create artificial imitation stones that look brighter and more beautiful than the naturally occurring original stones.

Apart from these applications, quantum mechanics has contributed to our understanding of many areas of technology, biological systems, and cosmic forces and bodies. While there are several important questions remaining in quantum physics, the core concepts, which define the behavior of energy, particles, and matter have continued to hold constant.