Niels Bohr and IYQ25

Pioneers of Quantum Theoretical Physics

 Part 3

 

“If quantum mechanics hasn't profoundly shocked you, you haven't understood it yet.”

-       Niels Bohr

 

UNESCO has proclaimed 2025 as the International Year of Quantum Science and Technology (IYQ). This year-long, worldwide initiative will celebrate the contributions of quantum science to technological progress over the past century, raise global awareness of its importance to sustainable development in the 21st century, and ensure that all nations have access to quantum education and opportunities.

In celebration of IYQ25, this series of articles focuses on the key personalities of quantum theoretical physics and their work – ten of the greatest, from Planck to Feynman. The first article (see here) focused on the background to IYQ25 and the advent of quantum theory through the pioneering work of Max Plack. The second one (see here) was about Albert Einstein and his contributions, primarily to quantum physics. This is the third one – on Niels Bohr, another great architect of the quantum revolution.

 

Introduction

After Max Planck and Albert Einstein had set in motion the quantum revolution through their concept of light as packets of energy (photons), the attention shifted to a study of the mechanism of energy emission or absorption, leading to deep insights into the structure of matter at its most fundamental level. Perhaps the most important figure in this endeavor was the legendary Danish physicist Niels Bohr who is the central figure in this article.

Early Picture of the Atom

The concept of the atom has evolved over centuries, with various models proposed to explain its structure and properties.

In the 5th century BCE, Democritus proposed that all matter consists of indivisible particles called atomos. A century later, Aristotle rejected the atomic theory, favoring the idea that matter was continuous and composed of four elements: earth, water, air, and fire. These ideas largely prevailed until the advent of John Dalton’s atomic theory in 1803.

Dalton revived the atomic concept, proposing that: (i) Matter is made of small, indivisible atoms, (ii) Atoms of an element are identical, but different from those of other elements, (iii) Atoms combine in fixed ratios to form compounds, and (iv) Chemical reactions involve the rearrangement of atoms.

A major breakthrough came towards the end of the last century when English physicist J J Thomson discovered the electron through his cathode ray experiments. He showed that these negatively charged particles are a constituent of all matter. In the ‘Plum Pudding Model’ that he proposed, electrons were pictured as embedded in a positively charged ‘soup’, with the atom as a whole neutrally charged.

Rutherford’s Nuclear Model

Ernest Rutherford's alpha scattering experiments, also known as the gold foil experiments, were pivotal in shaping our understanding of atomic structure. Conducted in 1909 by Hans Geiger and Ernest Marsden under Rutherford's supervision, these experiments provided evidence for the nuclear model of the atom.

In these experiments (see below for a diagrammatic description), a beam of alpha particles (positively charged helium nuclei) was directed at a thin sheet of gold foil.  A fluorescent screen was placed around the foil to detect the scattered alpha particles. Based on the prevailing "plum pudding" model of the atom, it was expected that the alpha particles would pass through the foil with minimal deflection. In the actual experiment, most alpha particles did indeed pass straight through the foil with little or no deflection. However, a small fraction of them was deflected at large angles, and some even bounced back toward the source. 

The fact that most alpha particles passed through the foil indicated that atoms are mostly empty space.  The large deflections and backscattering of some alpha particles suggested the presence of a small, dense, positively charged region within the atom. This region was later termed the nucleus.

Rutherford proposed a new model of the atom, where (i) the atom consists of a small, dense, positively charged nucleus at its center, (ii) the electrons orbit the nucleus at relatively large distances, much like planets orbiting the sun, and (iii) the nucleus contains most of the atom's mass.

Rutherford's alpha scattering experiments disproved the plum pudding model and led to the development of the nuclear model of the atom. This laid the groundwork for further discoveries in atomic and nuclear physics, including the development of quantum mechanics and the understanding of atomic spectra.

By 1913, the photon theory of light initiated by Max Planck and further developed by Albert Einstein set the stage for Bohr’s Atomic Model, which introduced quantized electron orbits to explain atomic stability and spectral lines.

Niels Bohr (1885 - 1962)

Niels Bohr, one of the greatest scientists of the 20th century, played a central role in the development of quantum mechanics and the formulation of its Copenhagen Interpretation. His pioneering work not only transformed our understanding of atomic structure and quantum phenomena but also shaped the philosophical foundations of modern physics. Bohr’s intellectual leadership, collaborative spirit, and profound insights into the nature of reality established him as a key figure in the quantum revolution. Here we explore Bohr’s life, his contributions to quantum physics, and his pivotal role in the Copenhagen Interpretation.

Early Life and Scientific Beginnings

Niels Henrik David Bohr was born on October 7, 1885, in Copenhagen, Denmark, into a family deeply rooted in academia. His father, Christian Bohr, was a prominent physiologist, and his mother, Ellen Adler Bohr, came from a wealthy and influential Jewish family. From an early age, Bohr exhibited a keen interest in science and mathematics, excelling in his studies. He enrolled at the University of Copenhagen in 1903, where he studied physics and earned his doctorate in 1911 with a thesis on the electron theory of metals.

Bohr’s early work caught the attention of the scientific community, and in 1912, he traveled to Manchester, England, to work with Ernest Rutherford (see a picture of the two below), the discoverer of the atomic nucleus. This collaboration proved to be a turning point in Bohr’s career, as he began to address the fundamental questions of atomic structure and the behavior of electrons.

The Bohr Model of the Atom

Bohr’s most famous contribution to quantum physics came in 1913 with his model of the atom, which combined classical physics with the emerging principles of quantum theory fueled by Einstein’s work in the field. Building on Rutherford’s nuclear model, Bohr proposed that electrons orbit the nucleus in discrete energy levels, or "quantized" orbits. Electrons could transition between these orbits by absorbing or emitting energy in the form of photons, with the energy of the photon corresponding to the difference between the energy levels.

Postulates of Bohr’s Model of the hydrogen-like atom:

1. Electrons move in discrete orbits (energy levels) around the nucleus. Unlike Rutherford’s model, where electrons could spiral into the nucleus, Bohr proposed that electrons move in specific, stable orbits without radiating energy.

2. Quantization of Angular Momentum. The angular momentum (L) of an electron in orbit is quantized, given by:

L = mvr = nh/2π

3. Energy of the Electron is Quantized. The energy (E_n) of an electron in the nth orbit can be shown to be: 

E_n = - 13.6 eV/n²

4. Electrons Absorb or Emit Energy During Transitions. An electron moves from a lower to a higher energy level by absorbing energy. When it falls to a lower orbit, it emits energy in the form of a photon with frequency (ν) given by:

hν = E_i - E_f

This explains the hydrogen spectrum, with the well-documented Lyman, Balmer, and Paschen series corresponding to electron transitions to different orbits.

Below is a diagrammatic representation of Bohr’s model, illustrating the quantized orbits and electron transitions.

The illustration below depicts the origin of the different spectral series - Lyman, Balmer, etc.  

The Bohr model successfully explained the spectral lines of hydrogen and provided a framework for understanding atomic structure. However, it also introduced the idea of quantization, which challenged classical physics and laid the groundwork for the development of quantum mechanics. Bohr’s model was a significant step toward reconciling the behavior of atoms with the principles of quantum theory.

Bohr’s personal life

Niels Bohr came from an intellectual and influential family in Denmark. His father Christian Bohr was a Professor of Physiology. His mother, Ellen Adler Bohr, came from a wealthy Jewish banking family. His brother Harald Bohr was a mathematician and Olympic football player. He married Margrethe Nørlund in 1912 and had six children, all sons. Despite some major personal tragedies, including the death of a son* in tragic circumstances, Bohr’s family was a contented and happy one.

[* Niels Bohr’s eldest of six sons, Christian, died tragically in a boating accident in 1934 at the age of 18 even as the father was looking on helplessly, and another, Harald, was severely mentally disabled and died at the age of about 10.]

His son Aage Bohr (1922 - 2009) became a famous physicist in his own right and studied the shapes of nuclei and the collective motion of nucleons within an atomic nucleus. This work later brought him a share of the 1975 Nobel prize for physics. Below is a memorable picture of the father-son duo, taken much before this happened. 

It is interesting to note that the father contributed crucially to the understanding of the atom and the son did the same for the understanding of the nucleus of the atom.

Incidentally, the example of Niels and Aage Bohr is one of four such instances of a father-son duo winning a Nobel Prize in physics (the other three being J J Thomson & G P Thomson, Henry Bragg & Lawrence Bragg, and Manne Siegbahn & Kai Siegbahn).

Bohr was Denmark’s most famous scientist, contributing greatly to its national pride. He was also deeply involved in public life and scientific diplomacy.

During World War II, Bohr, of partial Jewish descent, faced Nazi persecution. With help from the Danish resistance, he escaped to Sweden, and later to Britain and the U.S., contributing to the Manhattan Project that lead to the development of the atom bomb and its cataclysmic consequences.

After WWII, Bohr campaigned for peaceful use of nuclear energy and called for international cooperation on nuclear weapons control. He advised both Winston Churchill and Franklin D Roosevelt about the dangers of a nuclear arms race.

Niels Bohr also played a key role in developing science education in Denmark.

The Development of Quantum Mechanics and the Copenhagen Interpretation

In the 1920s, Bohr became a central figure in the development of quantum mechanics, working alongside physicists such as Werner Heisenberg, Max Born, and Erwin Schrödinger (as will be elaborated in future articles in this series). Heisenberg’s formulation of matrix mechanics and Schrödinger’s wave equation provided mathematical frameworks for describing quantum phenomena, but the interpretation of these theories remained a subject of intense debate.

Bohr played a key role in shaping the philosophical foundations of quantum mechanics, culminating in its historic Copenhagen Interpretation. This interpretation, developed primarily by Bohr and Heisenberg, became the dominant framework for understanding quantum theory. Its key principles include:

1. Wave-Particle Duality: Particles such as electrons exhibit both wave-like and particle-like behavior, depending on the experimental setup.

2. The Uncertainty Principle: Formulated by Heisenberg, this principle states that certain pairs of physical properties, such as position and momentum, cannot be simultaneously measured with arbitrary precision.

3. Complementarity: Bohr introduced the concept of complementarity, which holds that quantum phenomena can be understood only through mutually exclusive but complementary descriptions. For example, an electron can be described as either a particle or a wave, but not both at the same time.

4. Probabilistic Nature: Quantum mechanics provides probabilities, not deterministic predictions, for the outcomes of measurements.

5. Role of the Observer: The act of measurement affects the system being observed, and the observer plays a fundamental role in defining reality.

The Copenhagen Interpretation represented a radical departure from classical physics, emphasizing the limitations of human knowledge and the inherently probabilistic nature of the quantum world.

Bohr’s Role in the Copenhagen Interpretation

Bohr’s intellectual leadership was instrumental in the development and dissemination of the Copenhagen Interpretation. He was a master of dialogue and collaboration, fostering a spirit of open inquiry among his colleagues at the Institute for Theoretical Physics in Copenhagen (see picture below), which he founded in 1921. Bohr’s institute became a hub for quantum research, attracting some of the brightest minds in the world of physics.

Bohr’s philosophical insights, particularly his principle of complementarity, provided a coherent framework for interpreting the seemingly paradoxical nature of quantum phenomena. He argued that the wave-particle duality and the uncertainty principle were not limitations of the theory but reflections of the fundamental nature of reality. Bohr’s ability to articulate these ideas in a clear and compelling manner helped to establish the Copenhagen Interpretation as the orthodox view of quantum mechanics.

The Bohr-Einstein Debates

One of the most famous chapters in the history of quantum physics is the series of debates between Niels Bohr and Albert Einstein. Einstein, a staunch realist, was deeply skeptical of the Copenhagen Interpretation and its reliance on probability and observer-dependent phenomena. He famously declared, "God does not play dice with the universe," expressing his belief in a deterministic and objective reality.

Bohr and Einstein (see a picture of the two above) engaged in a series of thought experiments and discussions, most notably at the Solvay Conferences. Einstein sought to demonstrate the incompleteness of quantum mechanics, while Bohr defended the Copenhagen Interpretation. Their debates, though often unresolved, pushed the boundaries of quantum theory and deepened our understanding of its deeper implications.

EPR Paradox and its Consequences

One of the most significant exchanges occurred in response to the EPR paradox, a thought experiment proposed by Einstein, Podolsky, and Rosen in 1935. The EPR paradox highlighted the phenomenon of quantum entanglement, in which the properties of two particles are correlated in such a way that measuring one particle instantaneously affects the other, regardless of the distance between them. Einstein argued that this "spooky action at a distance" violated the principle of locality and demonstrated the incompleteness of quantum mechanics. Bohr, however, countered that the EPR paradox did not undermine the Copenhagen Interpretation but instead illustrated the non-classical nature of quantum reality. He argued that the entangled particles should be treated as a single system, and their properties could not be defined independently of the measurement process. Bohr’s response reinforced the principles of complementarity and the role of the observer in quantum mechanics.

The verdict of history is that Bohr won the debate hands down!

Legacy and Impact

Niels Bohr’s contributions to quantum physics and the Copenhagen Interpretation have had a profound and lasting impact on science and philosophy. His work laid the foundation for modern quantum mechanics and provided a framework for understanding the behavior of atoms and subatomic particles. The Copenhagen Interpretation, despite ongoing debates and alternative interpretations, remains one of the most influential perspectives in quantum science.

Bohr’s intellectual legacy extends beyond his scientific achievements. He was a passionate advocate for international collaboration and the peaceful use of science. During World War II, Bohr played a key role in the Manhattan Project (see his picture below, with the scientific head of the Manhattan Project, Robert Oppenheimer), though he later became a vocal proponent of nuclear disarmament. His commitment to dialogue and understanding, both in science and in global affairs, serves as a model for future generations.

Niels Bohr’s pioneering life and work in quantum physics represent a remarkable synthesis of scientific discovery and philosophical inquiry. His contributions to the Bohr model of the atom, the development of quantum mechanics, and the formulation of the Copenhagen Interpretation transformed our understanding of the physical world. Bohr’s intellectual leadership, collaborative spirit, and profound insights into the nature of reality continue to inspire scientists and philosophers alike. As we grapple with the mysteries of the quantum world, Bohr’s legacy reminds us of the enduring power of curiosity, creativity, and the pursuit of knowledge.

Honors and Recognition:

In 1947, Bohr received Denmark’s highest honor, the Order of the Elephant. His face appears on Denmark’s 500-krone banknote (see picture below). Bohr was both a scientific pioneer and a statesman, blending his work in physics with global diplomacy and national pride.

 

Epilogue

Bohr and Einstein were great friends except when they were exchanging arguments challenging each other’s interpretations of what the quantum theory really meant. Their lengthy arguments can be crisply summed up as follows:

             Einstein: God is subtle… doesn’t play dice …

   Bohr: Man, shut up and stop telling God what to do!… just calculate!

[For both, ‘God’ was a metaphor for Nature’s laws. Both reshaped how we see the fabric of ‘reality’.]

 

 

 

Einstein and IYQ25

Pioneers of Quantum Theoretical Physics

 Part 2 

"Einstein was not merely a scientist; he was a revolutionary thinker who reshaped the very foundations of our understanding of the universe."

– Michio Kaku, describing Einstein’s transformative influence.

 

 

UNESCO has proclaimed 2025 as the International Year of Quantum Science and Technology (IYQ). This year-long, worldwide initiative will celebrate the contributions of quantum science to technological progress over the past century, raise global awareness of its importance to sustainable development in the 21st century, and ensure that all nations have access to quantum education and opportunities.

In celebration of IYQ25, this series of articles focuses on the key personalities of quantum theoretical physics and their work – ten of the greatest, from Planck to Feynman. The first article (see here) focused on the background to IYQ25 and the advent of quantum theory through the pioneering work of Max Plack. This is the second one – on Einstein and his contributions to the quantum revolution.

Einstein and his place in history

In 1999, TIME magazine enshrined Albert Einstein as the Person of the Century (see picture above), recognizing his unparalleled influence on science, philosophy, and global history. His selection over other notable figures, such as Gandhi and Roosevelt, reflected his profound impact on our understanding of the universe. Einstein's contributions were deemed timeless and universal, making him the most fitting choice for the extraordinary honor.

A byword even outside the realm of science and one of the most celebrated personalities in human history, Albert Einstein (1879 – 1955) revolutionized our understanding of the most fundamental concepts of science, including matter and energy, space and time, gravitation, etc. His achievements and their impact on human knowledge are unparalleled since the times of Isaac Newton (1643 – 1727). His contributions span the development of special and general relativity, quantum theory, statistical mechanics, and cosmology. While he is most famous for the theories of relativity, his seminal work in quantum physics laid the foundation for many aspects of modern quantum science, yet he remained deeply skeptical of its philosophical implications, particularly the probabilistic and observer-dependent nature of the Copenhagen Interpretation (to be discussed in greater detail in future articles of this series).

Here we explore Einstein’s life, his contributions to quantum physics, and his unhappiness with the dominant interpretation of quantum mechanics.

First, let us merely summarize Einstein’s notable contributions to other areas of physics before focusing on his pioneering work in quantum physics.

1. Special Relativity (1905)

In his annus mirabilis (‘miracle year’) of 1905, Einstein formulated the theory of special relativity, which redefined our understanding of space and time, as also of matter and energy. It had just two key postulates that were to ‘transform our understanding of the universe’ as Michio Kaku put it – on both microscopic and macroscopic scales. They were:

    (a) The laws of physics are the same for all inertial observers.

    (b) The speed of light is constant in all inertial frames.

[An inertial frame is a frame of reference in which the laws of inertia, i.e., newtons laws of motion, are valid. The Earth as a frame of reference is an example, although not in the strictest sense of its definition.

While the first postulate seems to be all that one might expect from common sense, the second is certainly not so, leading to several counterintuitive situations that could be experimentally validated.]

They led to groundbreaking results such as time dilation, length contraction, and the famous energy-mass equivalence equation:

E = mc²

[This essentially means that a small amount of mass m can be converted into a large amount of energy E under the right conditions, such as in an atom bomb or nuclear reactor. The speed of light c has the value 2.9979 x 10^-8 m/sec.]

2. General Relativity (1915)

Einstein extended relativity to include gravitation, showing that gravity is not a force (as Newton described it) but rather the curvature of spacetime caused by mass and energy [We live in a four-dimensional world, with three dimensions of space and one of time]. This was to pave the foundation for modern Cosmology and our understanding of the macroscopic universe. Without going into any explanations, his results can be neatly summarized mathematically in the form of a tensor equation as:

Here, the Einstein Tensor G_μν represents the curvature of spacetime due to gravity, the second term is the cosmological constant introduced by Einstein to allow for a static universe (his ‘biggest blunder’!), T_μν is the energy-momentum stress tensor. G is the gravitational constant and c, the speed of light in vacuum.

[This is not the place for details of what this is and how this was arrived at, except to observe that Einstein’s Nobel Prize winning contributions on the photoelectric effect and the photon concept pale into relative insignificance compared to his theories of Special and General Relativity.]

In contrast, the much simpler, and certainly more famous, Newton’s law of gravitation can be stated as:

Here, F is the gravitational force between two objects of masses m₁ and m₂, separated by a distance r.   The gravitational constant G has the value 6.6743 x10^-11 m³ kg^-1 s^-2.

3. Brownian Motion (1905)

Einstein provided a mathematical explanation for Brownian motion (the random movement of particles in a gas or liquid caused by collisions between particles and the atoms or molecules in the fluid, observable through a microscope), offering strong evidence for the existence of atoms and molecules. This work helped confirm kinetic theory and classical statistical mechanics. Jean Perrin’s experiments on Brownian motion, which won him the Nobel Prize in Physics in 1926, provided the supporting empirical evidence.

4. Stimulated Emission and the basis for Lasers (1917)

Einstein introduced the concept of stimulated emission, a principle underlying the functioning of lasers that were operationally realized decades later. He predicted that atoms could emit photons when influenced by external electromagnetic radiation, a fundamental idea later used in laser technology.

Einstein’s Contributions to Quantum Physics

The Photoelectric Effect and the Birth of Quantum Theory

The photoelectric effect, first observed by Heinrich Hertz in 1887 and later studied in detail by Philipp Lenard* and others, involves the emission of electrons from a material when it is exposed to light. Classical physics, primarily based on Maxwell's electromagnetic theory and the wave theory of light, faced insurmountable difficulties in explaining the experimental observations of the photoelectric effect. These were ultimately resolved by Einstein's photon theory in 1905, which generalized the concept of quantized light energy first employed by Planck (see here).

[*Notwithstanding his Nobel Prize winning work that led to profound consequences in the hands of Einstein, Lenard was a rabid opponent of almost everything that Einstein did and stood for. He was also a staunch supporter of Hitler and champion of ‘Nazi German Physics’.]

Experimental Background

In experiments on the photoelectric effect, light is shone onto a metal surface (see diagram of an experimental setup below), and the properties of the emitted electrons (called photoelectrons) are measured. Key observations include:

1. Threshold Frequency: Electrons are only emitted if the light frequency exceeds a certain threshold, regardless of the light's intensity.

2. Instantaneous Emission: Electrons are emitted almost instantaneously when the light strikes the surface, with no detectable time delay.

3. Kinetic Energy of Electrons: The maximum kinetic energy of the emitted electrons depends on the frequency of the light, not its intensity. Higher-frequency light results in higher-energy electrons.

4. Intensity Dependence: The number of emitted electrons increases with the intensity of the light, but their maximum kinetic energy does not.

 

Difficulties faced by Classical Physics

Classical wave theory of light, which treats light as a continuous electromagnetic wave, could not adequately explain these observations:

1. Threshold Frequency: According to classical theory, the energy of a wave is proportional to its intensity (amplitude squared). Thus, even low-frequency light should eventually emit electrons if the intensity is high enough. However, experiments showed that no electrons are emitted below a certain frequency, regardless of intensity.

2. Instantaneous Emission: Classical theory predicted that electrons would need time to accumulate energy from the light wave before being emitted. However, experiments showed that emission occurs instantaneously, even at very low light intensities.

3. Kinetic Energy Dependence: Classical theory suggested that the energy of emitted electrons should depend on the intensity of the light, not its frequency. However, experiments showed that the kinetic energy of electrons depends on the frequency, not the intensity.

Einstein's Photon Theory

In 1905, Einstein proposed a revolutionary explanation based on an extended application of Max Planck's quantum hypothesis. He suggested that light is composed of discrete packets of energy called photons, each with energy E = hν, where h is Planck's constant and ν is the frequency of the light. This theory resolved the difficulties as follows:

1. Threshold Frequency: Electrons are emitted only if the energy of a single photon hν exceeds the work function φ of the material, which is the minimum energy required to eject an electron. This explains why light below a certain frequency cannot emit electrons, regardless of intensity.

2. Instantaneous Emission: Since energy is delivered in discrete packets (photons), an electron can be emitted immediately if it absorbs a photon with sufficient energy. No time delay is needed for energy accumulation.

3. Kinetic Energy Dependence: The maximum kinetic energy of the emitted electrons is given by K_max = hν - φ. This directly links the energy of the electrons to the frequency of the light, not its intensity.

4. Intensity Dependence: The intensity of light determines the number of photons, and thus the number of emitted electrons, but not their individual energies. This explains why increasing intensity increases the number of electrons but not their maximum kinetic energy.

Einstein's photon theory provided a complete and accurate explanation of the photoelectric effect, aligning perfectly with experimental observations. It also marked a significant step in the development of quantum mechanics, challenging the classical wave theory of light and introducing the concept of wave-particle duality. For this work, Einstein was awarded the Nobel Prize in Physics in 1921 (overlooking his vastly more important contributions through Relativity). The photoelectric effect remains a cornerstone of modern physics, demonstrating the quantized nature of light and energy.

Before discussing Einstein’s other significant contributions to quantum physics let us look at the man and the times he lived in.

Early Life

Albert Einstein was born on March 14, 1879, in Ulm, Germany, to a Jewish family. His father, Hermann Einstein, was an engineer and businessman, and his mother, Pauline Einstein, was a pianist. As a child, Einstein was curious but slow to speak, leading his parents to worry about his intelligence.

At age 5 (see his stunningly handsome picture below), young Einstein was fascinated by a pocket compass his father showed him, sparking his lifelong interest in physics and unseen forces. By age 10, he was deeply influenced by science and philosophy books, including works by Euclid and Kant. Both his looks and his academic interests stayed with him well past his most productive period in life. 

Einstein attended Catholic elementary school in Munich but struggled with its rigid, discipline-focused system.  At Luitpold Gymnasium (now Albert Einstein Gymnasium), he excelled in mathematics and physics but disliked rote learning and strict teachers.

In 1894, his family moved to Italy, but Einstein stayed behind to finish school. In 1895, at 16, he failed the entrance exam for the Swiss Federal Polytechnic School (ETH Zurich), doing well in mathematics and physics but poorly in languages and other subjects. He then attended Aarau Cantonal School (Switzerland) to improve his grades.

In 1896, at age 17, he passed the entrance exam and joined ETH Zurich, where he later studied physics and mathematics.

He disliked formal lectures and often skipped classes, preferring to study independently. His classmate Marcel Grossmann (who later helped him with advanced mathematical concepts) took notes for him.

Despite being brilliant in mathematics and physics, he was seen as a rebellious student who questioned his professors. In 1900, he graduated with a diploma in physics and mathematics, but his unconventional approach made it hard for him to secure a job in any academic position after graduation. He worked as a private tutor in mathematics and physics for students.

Unable to secure any job suited to his capabilities, Einstein began work as a lowly patent examiner (see picture below) in Bern, Switzerland. However, this job gave him opportunities and time for pursuing his academic interests and set in motion the avalanche of new ideas that were to light up the world of physics, producing some of his most groundbreaking work, including his annus mirabilis papers of 1905, which addressed the photoelectric effect, Brownian motion, and special relativity. These papers eventually established Einstein as a leading figure in theoretical physics and set the stage for his later contributions to quantum theory. 

Einstein as a patent office clerk

Einstein as a University Professor

Personal Life

Contrasting strongly with his world-famous public image, Einstein’s personal life was complex and often troubled, particularly in his relationships with his family. Einstein married Mileva Marić in 1903, a fellow physicist and one of the few women studying science at the time. Their marriage became strained due to Einstein’s increasing focus on his work and reported emotional detachment. The marriage ended in divorce in 1919, with Einstein agreeing to give Mileva his (anticipated) Nobel Prize money as part of the settlement.

Einstein and Mileva had two sons, Hans Albert and Eduard. Hans Albert Einstein became an engineer, but his relationship with his father was often distant. Eduard Einstein suffered from schizophrenia, leading to hospitalizations. Einstein deeply regretted being unable to care for him, especially after leaving for the USA in 1933.

Wife Mileva and sons

Einstein later married cousin Elsa in 1919, but their relationship was also troubled. 

Before marrying Mileva, Einstein and Mileva had a daughter, Lieserl, born in 1902. Little is known about her fate, but some reports suggest she may have died of illness or was given up for adoption. Einstein never publicly acknowledged her existence, and details emerged only through letters discovered much later.

Einstein as a public figure

Albert Einstein’s fame as a public figure extended far beyond his scientific achievements. He became one of the most recognizable and influential intellectuals of the 20th century.

An iconic picture taken on a historic occasion placing Einstein where he belongs!

Einstein’s global fame exploded in 1919 when a solar eclipse experiment confirmed his General Theory of Relativity. British astronomer Arthur Eddington led the expedition that showed light bending around the sun, proving Einstein’s General Relativity based predicts correct. Headlines like “Revolution in Science – Newtonian Ideas Overthrown” (The Times, UK) made Einstein an overnight celebrity. He became a household name, appearing on newspaper covers worldwide.

Unlike most scientists, Einstein actively engaged with the press and the public, explaining complex theories in simple, quotable terms. His wild hair, thoughtful expression, and informal attitude contributed to his public image as a genius.

Einstein speaking during a "Science and Civilization" lecture in 1933 at the Royal Albert Hall in London. Photograph by Hulton Archive, Getty Images.

Initially Einstein opposed World War II but later supported efforts to stop Nazi Germany. A Jewish scientist, he fled Germany in 1933 as Hitler rose to power. He spoke out against both fascism and anti-Semitism.

In 1939, Einstein signed a letter to US President Franklin D Roosevelt warning that Nazi Germany might develop nuclear weapons. This historic letter led to the Manhattan Project, which developed the atomic bomb and its fearful aftermath.

Einstein’s photon theory was a radical departure from classical wave theory and marked a significant step toward the development of quantum mechanics. However, Einstein’s relationship with quantum theory was complex. While he recognized its empirical success, he was deeply troubled by its philosophical implications, particularly the probabilistic and observer-dependent nature of the Copenhagen Interpretation spearheaded by his friend and intellectual rival Niels Bohr.

The Copenhagen Interpretation and Einstein’s Skepticism

The Copenhagen Interpretation, formulated by Niels Bohr and Werner Heisenberg in the 1920s, became the dominant framework for understanding quantum mechanics. It postulates that particles do not have definite properties until they are measured, and that the act of measurement itself affects the system being observed. This interpretation embraces the probabilistic nature of quantum mechanics, rejecting the prevailing deterministic worldview of classical physics (to be discussed in more detail in the next article of this series).

Einstein was deeply uncomfortable with the Copenhagen Interpretation. He famously declared, "God does not play dice with the universe," expressing his belief that the universe operates according to deterministic laws, not probability. Einstein’s skepticism was rooted in his realist worldview, which held that physical reality exists independently of observation. He could not accept the idea that the act of measurement could fundamentally alter the state of a system.

The Einstein-Bohr Debates

Einstein’s challenges to the Copenhagen Interpretation were most vividly expressed in his debates with Niels Bohr (see the picture below of the two together). These intellectual exchanges, which took place at the Solvay Conferences and other scientific gatherings, are among the most famous in the history of physics. Einstein devised a series of thought experiments to demonstrate the incompleteness of quantum mechanics, arguing that the theory failed to provide a complete description of physical reality. 

The EPR Paradox

One of Einstein’s most notable challenges was the EPR paradox, formulated in 1935 with his colleagues Boris Podolsky and Nathan Rosen. The EPR paradox highlighted the phenomenon of quantum entanglement, in which the properties of two particles are correlated in such a way that measuring one particle instantaneously affects the other, regardless of the distance between them. Einstein argued that this "spooky action at a distance" violated the principle of locality, which states that physical processes occurring at one location do not depend on the properties of objects at other locations. He concluded that quantum mechanics must be incomplete, as it could not account for these correlations without invoking non-local effects.

Bohr, however, defended the Copenhagen Interpretation, arguing that quantum mechanics provided a complete and consistent description of reality, even if it departed from classical intuitions. The EPR paradox ultimately led to the development of Bell’s theorem and experimental tests of quantum entanglement, which confirmed the non-local nature of quantum mechanics and supported the Copenhagen Interpretation. Rather paradoxically, the EPR paradox proved to be the death knell of the deterministic view that Einstein had so strongly championed.

Einstein’s Later Years and Legacy

Despite his skepticism, Einstein’s contributions to quantum physics were foundational. His work on the photoelectric effect and his insights into quantum entanglement remain central to the field. However, Einstein’s later years were marked by his pursuit of a unified field theory, which sought to reconcile quantum mechanics with general relativity. This endeavor, though ultimately unsuccessful, reflected Einstein’s unwavering commitment to a deterministic and unified understanding of the universe.

Einstein’s challenges to the Copenhagen Interpretation also had a lasting impact. His critiques spurred further research into the foundations of quantum mechanics and inspired the development of alternative interpretations, such as the many-worlds interpretation and pilot-wave theory. While the Copenhagen Interpretation remains the most widely accepted framework, Einstein’s questions continue to provoke debate and exploration.

Einstein’s Other Contributions to Quantum Physics

Wave-Particle Duality (1909-1916)

Einstein was among the first to argue that light has both wave and particle properties, foreshadowing Louis de Broglie’s wave-particle duality principle. This will be elaborated in a future article. His work helped establish the foundation of quantum field theory.

Einstein Coefficients and Quantum Transitions (1917)

Einstein introduced coefficients that describe how atoms absorb and emit radiation. These coefficients played a major role in quantum electrodynamics and the understanding of atomic transitions, besides paving way for the invention of the laser that happened much later.

Bose-Einstein Statistics and Bose-Einstein Condensate (1924-1925)

Collaborating with Satyendra Nath Bose (see my earlier article on Bose and this historic collaboration), Einstein extended quantum statistics to particles which later came to be known as bosons. This led to the prediction of Bose-Einstein condensation, where particles occupy the same quantum state at extremely low temperatures. This phenomenon was experimentally confirmed in 1995.

Today, Bose-Einstein Condensate (BEC) is recognized as a state of matter in which separate bosonic atoms or subatomic particles, cooled to near absolute zero temperature, coalesce into a single quantum mechanical entity on a near-macroscopic scale. This form of matter was predicted by Einstein in 1924 on the basis of the quantum formulations of Satyendra Nath Bose, foreshadowing the development of Bose-Einstein Statistics applicable to all bosons.

Eric A Cornell and Carl E Wieman (Nobel laureates in Physics, 2001) demonstrated the formation of the Bose-Einstein condensate in ultracold rubidium atoms in 1995. Here, a series of images show, from left to right, increasing density as those rubidium atoms begin to form a BEC. (Image credit: NIST/JILA/CU-Boulder - NIST Image)

Historical Significance of Einstein’s Quantum Contributions:

1. Advancing Quantum Mechanics

Einstein’s work on the photoelectric effect and wave-particle duality was pivotal in the early development of quantum mechanics, laying the groundwork for quantum field theory, quantum electrodynamics, and solid-state physics.

2. Quantum Technologies

His discoveries directly influenced modern technologies, including semiconductors, lasers, and quantum computing. The photoelectric effect is fundamental to solar panels, while Bose-Einstein condensation has applications in quantum simulations and superconductivity.

3. Inspiring the Quantum Revolution

Though Einstein resisted the Copenhagen interpretation, his challenges forced physicists like Bohr, Heisenberg, and Schrödinger to refine quantum mechanics, leading to quantum mechanics' probabilistic framework and further advancements in quantum field theory.

4. Quantum Entanglement and Modern Physics

Einstein’s scepticism about quantum entanglement ultimately led to its experimental verification, which underpins modern quantum information science, including quantum cryptography and quantum computing.

Conclusion

Einstein's contributions to physics transformed our understanding of the universe, from relativity to quantum mechanics. His work on the photoelectric effect launched quantum mechanics, while his debates with Bohr shaped its interpretation. Despite his discomfort with quantum mechanics' indeterminacy, his insights led to foundational developments in quantum physics and modern technology. Today, his contributions continue to influence cutting-edge research in cosmology, particle physics, and quantum computing, making him one of the most significant figures in scientific history.

The bottom-line

Einstein’s image as a "quirky genius" is still widely used in pop culture. The famous E = mc² equation became symbolic of genius itself. In this article, the focus has been on a slightly less well-known equation E = hν that opened up another face of this genius.