100 Years of Quantum Mechanics: A Strange Idea That Changed Everything

»What do you really mean by that?« Niels Bohr kept demanding of his bright, curious 23-year-old student. 

A hundred years ago, Danish physicist Niels Bohr and his student German physicist Werner Heisenberg walked through Dyrehaven and along the Øresund for hours, trying to explain the strange behaviour of atoms when physicists studied them in their labs.

At that time physicists believed the atom was like a tiny solar system – a dense nucleus at the center with electrons orbiting around it.

At the turn of the century, Max Planck had suggested that energy comes in tiny packets, a mathematical idea he came up with to explain his experimental results. Albert Einstein then extended the idea, arguing that light itself comes in packets, or »quanta«.

With this new perspective in hand, Niels Bohr used his intuition to shed light on the true inner workings of the atom. He did not believe that electrons are like planets orbiting the Sun — they do not move around smoothly. 

They vanish from one level and suddenly appear on another, giving off (or taking in) light when they do. This sounded absurd to physicists at the time, and no theory existed to explain such behaviour.

Therefore, Bohr and Heisenberg kept debating how to build a new theory that could account for this strange behaviour of atoms. Their long walks and intense discussions marked the beginning of what we now call quantum mechanics — a theory that celebrates its 100th anniversary this year.

The early years of quantum mechanics

In 1925, besides struggling to explain nature’s mysterious behaviour, Heisenberg was also dealing with an intense hay allergy. For these reasons, he retreated to the remote island of Helgoland in the North Sea to find some peace and focus. 

There, Heisenberg would finally lay the building blocks of a mathematical theory that could explain the laws of our microscopic world: a quantum theory. 

»I have written a crazy paper« Heisenberg allegedly said to Max Born, his supervisor in Göttingen, who helped shape Heisenberg’s new idea. Just a few months later, they created the first formulation of the new theory. 

Heisenberg’s original idea was to describe nature using only what can be seen. In this picture, you do not follow where a particle »really is« but instead work directly with what you can measure in the lab: the energy of the particle, the frequencies of light it emits, or how it affects other particles. Only facts we can see enter the theory. Reality, in this view, is built by observation.

Two sides of the same coin

Around the same time, working independently in Zurich, Erwin Schrödinger developed a different yet equivalent approach based on the fact that light behaves like a wave, offering a more intuitive picture of atomic behavior.

That is, instead of focusing only on measurable quantities, he imagined particles as smooth waves spread through space — waves we never directly see, but whose shape tells us how likely it is to find the particle in different places when we measure it.

But this raised a question: if a particle is spread out like a wave, why does a measurement always find it at a single point?

This controversy makes it clear that the two approaches go hand in hand: they describe the same reality from different angles. The wave tells us the probabilities, and the measurement tells us the result. Schrödinger’s formulation soon proved mathematically identical to Heisenberg’s, uniting two seemingly opposite perspectives of the quantum world.

What began as an abstract debate soon reshaped our understanding of nature: certainty was replaced by probabilities, particles flowed like waves, and reality became inseparable from what we can observe.

The transistor and new inventions

As unintuitive and at times controversial as quantum physics was to scientists at the beginning of the 20th century, f quantum mechanics gave scientists and engineers a tool to control materials and processes they could not previously understand. 

This led to the emergence of many new technologies, and within 25 years of Heisenberg’s discovery, the foundations for many of today's core technologies were laid. 

Most prominently, in the 1930s and 1940s, quantum mechanics helped scientists understand semiconductors – that is, materials that can be tuned to either insulate or conduct electricity.

This tunability comes from electrons acting like waves in a crystal, which lets some energies flow easily while others are blocked. Soon after, the first transistors based on semiconductors were developed and they have become the core material of modern electronics.

Lights, cameras, barcodes! 

Around the same time, scientists learned how shining a beam of small particles on a material can be used to learn about its properties, creating the first sensor (devices that are able to measure some aspects of a system) that exploits quantum mechanics. 

They would drastically increase the precision and resolution with which we can investigate nature. In particular, the field of medicine has benefited greatly, for example magnetic resonance imaging (MRI) which is used in hospitals all over the world. 

The understanding of how light interacts with different materials, together with the rise of semiconductors, led in the 1950s and 1960s to the development of two new sources of light: LEDs and lasers. Thanks to their low energy consumption, LEDs are nowadays in nearly every light bulb and screen. 

Lasers, meanwhile, proved to be a very versatile tool that are not only used to read barcodes in supermarkets but are also the backbone of our global communication network: Fiber internet has taken over the world, with messages travelling at a fraction of the speed of light.

More recently, quantum dots were invented, tiny structures that behave like artificial atoms and are used in high-quality screens and solar cells.

The innovations mentioned so far were based on a better understanding of materials and the

simultaneous control of large ensembles of atoms.

The newest applications

Since the 1990s, through some of the discoveries and inventions mentioned above, scientists have gained increasing control over single atoms, electrons, and photons.

Among the first demonstrated applications during this so-called second quantum revolution were communication channels that are secure against any undetected eavesdropping. 

National institutions, banks and medical centers that deal with sensitive data require high security standards that those communication methods could provide.

The collection of such channels is what we call a network, and today the complexity of such networks is steadily increasing and can even be extended via satellites.

Control over single atoms led to the development of new and more precise sensors primarily used in research, which for example probe the nature of gravitation more accurately. 

Yet, the question of how quantum mechanics connects with our theory of gravity remains open – a fundamental problem to which Heisenberg devoted much of his life.

In recent years, no area has captured more attention – and funding – than quantum computing. 

A quantum computer is a machine whose computational units are atoms and which exploits quantum mechanics for calculations and simulations. 

They already exist, but it will take many more years until a truly useful quantum computer is built. 

With a functional quantum computer we could for example simulate complex molecular structures that would allow us to develop new drugs or materials.

Potential for good and bad use

There are good reasons to praise the innovative power of scientists and engineers over the last century. Many of the applications they developed have drastically changed our lives and, if properly regulated, could help create a more sustainable society. 

However, it is important to keep in mind that there were also periods when misanthropic aims drove technological progress and secrecy amplified the harm. 

Just fifteen years after Heisenberg shared his discovery with physicists from all over the world, the scientific environment had changed drastically: Scientists no longer shared thought experiments and ideas, but worked on secret programs aimed at building devastating weapons. 

Heisenberg worked on the Uranium Program for the Nazis, and many other contemporary physicists were involved in the Manhattan Project at Los Alamos. 

This led to the invention of the atomic bomb. As the name indicates, it was made possible by advances in quantum physics and the understanding of the microscopic world. 

Today, with ongoing wars and rising global tensions, there is once again a reflex to isolate research fields believed to have potential for both good and bad. 

Here in Denmark and all across the world, the movement of researchers gets restricted based on their nationality, and long-lasting collaborations are being cut. 

In these times, we should remember Niels Bohr’s 1950 letter to the United Nations, where he stated that a more open world would be a more peaceful world.

A young science with many unanswered questions

This year, we celebrate the 100th year of the discovery of quantum mechanics. Yet, many of the questions that Heisenberg, Einstein, Bohr, and their contemporaries were asking back then remain unanswered. And young physicists still love to debate them between university lectures. 

What does it mean to measure something? Is there an underlying and more powerful theory to be discovered? Does God really play dice?

Quantum physics is still a relatively young science, with much left to explore and discover – and it will continue to expand our understanding of the world.