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:
(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 Gμν 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’!), Tμν 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
Kmax = 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
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.
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.
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.
