Quantum physics began in 1900 when the German physicist Max Planck determined the scale of quantum phenomena. In 1905, Albert Einstein extended this idea by proposing that light itself is made up of discrete packets of energy, or “quanta.” Building on these ideas, in the first decades of the twentieth century, the Danish physicist Niels Bohr played a critical role in establishing the foundations of quantum mechanics.
A theory full of paradoxes, ‘quantum theory’ postulates that particles behave like waves and objects can exist in multiple states at once. Crucially, as Bohr argued, the act of observation appears to change what is observed. This strange reality, better understood thanks to Bohr and his colleagues, has rendered quantum mechanics to be one of the most revolutionary, successful, and useful frameworks in the history of science.
What Role Did Niels Bohr Play in the Quantum Revolution?
Born and educated in Copenhagen, Denmark, Niels Henrik David Bohr (1885-1962) began his life as a physicist working on the structure of atoms. He earned his doctorate from the University of Copenhagen in 1911, and soon after, traveled to England to work with physicist J.J. Thomson (1856-1940), who had discovered the electron. He later collaborated with Ernest Rutherford (1871-1937) and developed his now-famous Bohr model of the atom, which proposed that electrons ‘leap’ up and down energy levels and keep specific distances from a central nucleus. The ‘Bohr Atomic Model’ was revolutionary work and earned him a Nobel Prize in 1922. It became an early cornerstone of quantum mechanics (Heilbron, 2020).
In 1921, Bohr returned to Denmark and established the Institute for Theoretical Physics in Copenhagen. The institute became a global centre for cutting-edge research, attracting the star scientific minds of the day, such as Max Born (1882-1970), Werner Heisenberg (1901-1976), and Paul Dirac (1902-1984). Hitherto best known for his Atomic Model, his work leading the Copenhagen Institute surpassed all expectations, leading to a theoretical legacy – the ‘Copenhagen Interpretation’ of quantum mechanics – that went on to conjure a new, somewhat unsettling view of physical reality itself.
What Is the Copenhagen Interpretation and Why Is It Important?
Formulated collectively by Bohr and his colleagues in the 1920s, the Copenhagen Interpretation of quantum mechanics was the first coherent, general attempt to understand the world of atoms as represented by quantum mechanics. It remains to this day one of the most widely accepted views of the quantum world. For Bohr, nature is not deterministic: the quantum world is full of uncertainty and paradox. Particles are simultaneously waves; they can exist in multiple states at once, while observation affects reality.
Thus, at its core, the Copenhagen Interpretation holds that quantum systems do not possess definite properties until they interact and are observed. Until that moment, a particle exists in a ‘state’ of quantum superposition – a cloud of all possible outcomes. When observed, this superposition “collapses” into a singular, definite outcome.
The Copenhagen Interpretation is important because it provides a conceptual framework for understanding the unpredictable and non-deterministic nature of quantum phenomena. The collective power of Bohr’s atomic model, Heisenberg’s equations, and Dirac’s mathematics opened the way to our modern understanding of why the theory of quantum mechanics yields results that match lab-based experiments with such stunning precision.
How Can We Understand Bohr’s Physics through Schrödinger’s Cat?
The clearest explanation of the Copenhagen Interpretation is the famous thought experiment, Schrödinger’s cat, proposed in 1935 by the Austrian-Irish physicist Erwin Schrödinger, as a critique of Bohr’s ideas. Imagine taking a cat and placing it in a sealed box. Inside the box is a device containing a tiny amount of radioactive material, with a 50% chance of decaying over the next hour. If the material decays, a Geiger counter triggers a hammer that shatters a vial of poison, killing the cat. If the material does not decay, that cat will remain alive.
According to classical logic, after an hour, the cat is either dead or alive – the probability is 50/50. But according to the Copenhagen Interpretation, until the box is opened and it is observed, the cat is simultaneously equal parts alive and dead. Only when the box is opened does the fate of the cat “collapse” into a single definite outcome (Orzel, 2014).
Schrödinger’s point was to show the absurdity of extending the new quantum rules to everyday objects. Yet, at the subatomic level, quantum superposition is not just theoretical, it is real – an experimentally observed phenomenon. In this regard, the Copenhagen Interpretation suggested that quantum mechanics was counterintuitive in the extreme and called for a radical rethinking of our understanding of reality.
Bohr vs. Einstein: The Great Debate?
As the promise and possibilities presented by quantum mechanics gained traction in the 1920s, Niels Bohr and his colleagues at Copenhagen led the way. However, not everyone, even its pioneers, was keen to embrace its implications. Despite being one of the founders of the theory, Albert Einstein famously rejected the indeterminacy at the heart of Bohr’s interpretation.
Believing that a complete theory of nature must be deterministic and predict outcomes with certainty, his famous protest that “God does not play dice with the universe” was aimed at Bohr’s assertion that nature is at its core probabilistic. Bohr responded that Einstein should “stop telling God what to do” (Kumar, 2009).
The intellectual clash of the two giants of physics endured for decades, through public lectures, academic papers, and personal correspondence. Einstein repeatedly challenged Bohr with thought experiments designed to expose the flaw in the Copenhagen Interpretation. Bohr, in turn, patiently explained the new ideas to Einstein and defended their consistency and experimental success.
Although Einstein never fully accepted the full implications of quantum mechanics – he maintained that there must be something else “behind it” – he was ultimately forced by Bohr to concede that it was a remarkably accurate theory. Bohr, for his part, pushed to concede that his ideas were not as simple as he first thought, improved and refined them through Einstein’s sustained challenge.
When Einstein died in 1955, Bohr offered a moving, heartfelt tribute to his genius. When Bohr died in 1962, a chalk drawing of Einstein’s ‘box of light’ thought experiment – devised years earlier to challenge Bohr – was still on his blackboard (Rovelli, 2017). The image stands as a fitting testament to Bohr’s genius – his lifelong dedication to the pursuit of a deeper understanding of the strange, and often counterintuitive reality of nature.
