Neelabh Datta, Biology MSC24

What if the world is far more peculiar than we ever imagined, with its smallest components governed by laws that defy intuition and reason? This very question lies at the heart of the quantum revolution—a journey that began in the late 19th century, turning our understanding of physics on its head. For centuries, scientists believed they had a firm grasp on the workings of the universe, explaining everything from planetary motion to the behaviour of gases. Yet, as they dug deeper into the microscopic world, an unsettling truth began to emerge: the universe doesn't behave as neatly as Newtonian physics suggested. Thus entered quantum mechanics, a theory that not only rewrote the rules but did so in a way that left even the greatest minds of the time scratching their heads.

The seeds of development of quantum mechanics were sown in 1887 when Heinrich Hertz, a German physicist, set out to test James Clerk Maxwell’s electromagnetic theory. During these experiments, Hertz observed a phenomenon: when he generated and detected electromagnetic waves using a metal spark gap, the intensity of the sparks varied depending on the surrounding medium. He noticed that placing the apparatus inside a glass box weakened the sparks, whereas using a quartz enclosure preserved their strength. While Hertz himself did not specifically focus on the emission of electrons, he stumbled upon a phenomenon that challenged the very foundation of classical physics. However, it would take decades and the insights of numerous scientists to fully understand its implications.

The early years of this quantum saga saw contributions from a range of physicists, but the first truly eureka moment came in 1900 when Max Planck introduced a revolutionary but strange idea. Tasked with explaining the behaviour of heated objects, known as blackbody radiation, Planck proposed that energy is not continuous but rather emitted in discrete packets, or "quanta." This idea was utterly at odds with classical physics, which viewed energy as a smooth, unbroken flow. Planck himself was initially hesitant to embrace the full implications of his theory, treating it as a mathematical convenience rather than a radical shift in thinking. Nevertheless, his discovery of what would later be called the Planck constant (\(h\)) laid the groundwork for the quantum theory that was to follow.

While Planck opened the door to the quantum world, it was Albert Einstein who blew it wide open in 1905. Just 26 years old at the time, Einstein published a paper on the photoelectric effect that fundamentally challenged the classical view of light. He proposed that light could behave not just as a wave, as had been previously thought, but also as a particle—a quantum of energy called a photon. This was a revolutionary idea, and initially, it met with significant scepticism. Yet, Einstein's explanation provided the first convincing solution to the photoelectric effect (from Hertz’s experiments), predicting how light could eject electrons from metals based on its frequency, not its intensity. In doing so, Einstein bridged the gap between the continuous nature of electromagnetic waves and the discrete energy packets that governed the behaviour of matter at the atomic level. His work would eventually earn him a Nobel Prize, though not for his more famous theory of relativity, but for this spearheading insight into the quantum nature of light.

The quantum revolution gained further momentum in 1911 at the first Solvay Congress, a gathering of some of the most brilliant scientific minds of the day. The meeting was dominated by discussions of radiation theory and the nature of quanta, with Einstein once again playing a central role. At the time, quantum theory was still in its infancy, with many questions left unanswered. One of the central challenges at that time was how to reconcile the continuous nature of classical electromagnetic waves with the emerging evidence for quantized energy levels. This debate laid the foundation for a new way of thinking about the physical world, one that would culminate in the development of quantum mechanics in the 1920s.

The 1927 Solvay Conference. 17 of the 29 attendees were—or became—Nobel laureates later.

One of the first major steps toward this new understanding came from Niels Bohr, a Danish physicist who in 1913 introduced a model of the atom that incorporated quantum ideas. Bohr's model was based on the work of Ernest Rutherford, who had discovered that atoms consist of a dense nucleus surrounded by orbiting electrons. However, Rutherford's model couldn't explain why these orbiting electrons didn't simply spiral into the nucleus due to the attractive force of the protons. Bohr proposed a solution: electrons occupy discrete orbits around the nucleus and can only jump between these orbits by absorbing or emitting specific amounts of energy, corresponding to the difference in energy between the orbits. This was a radical departure from classical physics, but it was also incredibly successful. Bohr's model explained the spectral lines of hydrogen, which had long puzzled scientists, and provided strong evidence for the quantization of energy in atomic systems.

As Bohr's atomic model gained acceptance, the stage was set for the development of true quantum mechanics, a more comprehensive and mathematically rigorous theory. The mid-1920s saw two distinct approaches to this new theory emerge. The first came from Werner Heisenberg, a German physicist who in 1925 introduced matrix mechanics. Heisenberg's approach was based on the idea that physical quantities like position and momentum should be represented not as continuous variables, as in classical mechanics, but as matrices—mathematical arrays that encode the probabilities of different outcomes. This was a dramatic break from classical physics, which treated these quantities as deterministically knowable. Instead, Heisenberg's theory introduced an inherent uncertainty into the system, famously encapsulated in his uncertainty principle. This principle states that certain pairs of physical properties, like position and momentum, cannot both be known to arbitrary precision. The more precisely one property is known, the less precisely the other can be determined. This was a profound departure from the deterministic worldview of classical physics, replacing it with a probabilistic one.

Around the same time, Austrian physicist Erwin Schrödinger developed an alternative approach, known as wave mechanics. Inspired by Louis de Broglie's earlier proposal that particles like electrons could exhibit wave-like behaviour, Schrödinger formulated an equation that described the evolution of these matter waves over time. His wave equation, now one of the most famous in all of physics, offered a different way of thinking about quantum systems. Instead of focusing on discrete jumps between energy levels, as in Bohr's model, Schrödinger's equation described a continuous wavefunction that encapsulated all possible states of a system. This wavefunction, when squared, gave the probability of finding a particle in a particular state or position, adding another layer of indeterminacy to the quantum world.

Despite their seemingly different approaches, Heisenberg's matrix mechanics and Schrödinger's wave mechanics were soon shown to be mathematically equivalent. Both provided valid descriptions of quantum systems, though they offered different ways of visualizing the underlying processes. This duality of perspectives led to intense philosophical debates about the nature of reality itself. Were particles really waves, as Schrödinger's equation suggested? Or were they discrete entities, as Heisenberg's theory implied? The debates reached a peak with the advent of the Copenhagen Interpretation, championed by Niels Bohr and Werner Heisenberg. According to this view, quantum mechanics doesn't describe an objective reality but rather the probabilities of different outcomes when we observe a system. In other words, particles don't have definite properties like position or momentum until they are measured.

Not everyone was satisfied with this probabilistic interpretation. Einstein, in particular, remained deeply sceptical of the idea that the universe was governed by chance. He famously quipped, "God does not play dice with the universe," expressing his discomfort with the indeterminacy at the heart of quantum mechanics. Schrödinger, too, had his doubts, even though he was one of the theory's key architects. His famous thought experiment, Schrödinger's cat, was designed to illustrate the absurdity of applying quantum mechanics to everyday objects. In this scenario, a cat placed in a box with a quantum device is both alive and dead until someone opens the box and observes it, highlighting the strange implications of quantum indeterminacy.

The discovery of quantum uncertainty was what first impressed the world with the depth of the quantum revolution. German physicist Werner Heisenberg, in 1927, astounded the scientific community with the revelation that deterministic cause-and-effect physics failed when applied to atoms.

Despite these philosophical disagreements, the practical success of quantum mechanics was undeniable. By the 1930s, the theory had been used to explain a wide range of phenomena, from the structure of atoms to the behaviour of semiconductors, laying the groundwork for technologies like transistors and lasers. Quantum mechanics had finally and firmly established itself as one of the cornerstones of modern physics, even if its deeper meaning remained elusive.

And yet, as much as quantum mechanics continued to evolve after that, it raised questions about the nature of reality. What does it mean for particles to exist in a superposition of states? Can it be ever possible to reconcile the probabilistic nature of quantum mechanics with our intuitive sense of a deterministic universe? And perhaps the most interesting: if quantum mechanics challenged the very fabric of reality, what other mysteries lie hidden in the quantum world, waiting to be uncovered?