The birth of the theory that changed the world

Everything needs a story. It's how humans understand and explain the world. And the best stories are always those of its origins. First there is nothing, and then there is something. It happens with superheroes, with music groups, and, yes, also with scientific theories. The problem is that tracing the exact origin of a theory to identify it as a point in space-time is an activity that contains a good dose of innocence and another of arbitrariness. This is where the power of stories comes into play. The quality of a story is the arbiter that can decide, without any kind of innocence, the instant at which a theory is born.

In the case of quantum physics, for example, there is the moment in 1900 when Max Planck studied the electromagnetic radiation emitted by bodies when they heat up and proposed that the energy they exchange with the outside world can only take on a series of specific values; that is, it is quantized. Or the moment in 1905 when Albert Einstein explained why in some materials, when illuminated with light of a certain color, an electric current is produced, something that can only be understood if we assume that light is made up of packets or particles of energy called photons.

If UNESCO has decided that 2025 is theInternational Year of Quantum Science and Technology To celebrate, literally, "one hundred years since the initial development of quantum mechanics," it's because there's a better story in 1925 than in 1900 or 1905. This is it. Grab a nice cold drink and get comfortable.

The visitor from Helgoland

One stormy night in the spring of 1925, a twenty-two-year-old man with a deformed face knocked on the door of one of the few houses on Heligoland, a group of two bare islets subjected to the pounding of the North Sea 60 kilometers off the German coast. When she opened it and saw his reddened and swollen face, the housekeeper who had rented him the room mistook him for a drunk or a brawler whom some sailor had just beaten up. But in reality, the man was a professor at the University of Göttingen with an attack of allergic rhinitis that had swollen his face. He had come to the island to spend a few days and see if the absence of plants that rose more than a foot from the ground would help him recover. After the initial shock, when she saw that he didn't twang and spoke correctly and elegantly, the housekeeper gave up her initial impulse to call the police and let him in.

In Helgoland, without being attacked by any microscopic particle of pollen, the visitor dedicated himself to swimming between the dunes that joined the two islets, to walking along the reddish cliffs on the north face of the island while reading the Western-Eastern Divan Goethe was already working on a problem that had been worrying him for months.

The microscopic enigma

Experiments had been carried out for years to elucidate the internal structure of matter. In 1897, Joseph Thomson had discovered the electron, and in 1911, Ernest Rutherford had concluded that atoms are made up of a large concentration of positively charged mass in the center, surrounded by negatively charged particles. However, the equations that had so precisely described the orbits of the planets and the motions of springs, pulleys, and balls, the equations that had so well explained heat exchanges and steam engines, the equations that had so accurately calculated the relationships between magnetic fields and currents at the forefront of the moment, could not explain the results of experiments conducted to peer into the behavior of microscopic particles.

Furthermore, observing atoms directly was impossible. The Danish physicist Niels Bohr had said that when talking about atoms, language could only be used like poetry. As the swelling in his face subsided and he recovered, the visitor from Helgoland decided there was no point in imagining what went on inside atoms and put poetry aside to focus on observable data. Every time an atom changed its energy state, it absorbed or emitted light, which could be recorded on a photographic plate. And this was all he could know. Therefore, it was necessary to figure out its behavior from this information.

Her thinking sharpened by the fresh, clean air of Heligoland and the inspiration she found from her cliff walks, she organized all these variables into rows and columns until they formed complex matrix-like structures that she began to manipulate. She worked obsessively for twenty hours a day until, one night, at three in the morning, the solution appeared before her.

"At first, I was very frightened," she would write years later. "I had the sensation of looking into an underlying background of strange, inner beauty, and I almost felt dizzy." The discovery had thrown her into such a state of unbridled excitement that, unable to sleep, she left the boarding house and walked in the darkness, barely pierced by a sliver of moonlight, to the cliffs. There, she saw a solitary pinnacle-shaped rock jutting out of the sea, and she couldn't help but climb it. Once at the top, in an image that reproduces point by point the scene painted by Caspar David Friedrich in the painting Walking on a sea of clouds, waited for the first light of the sun to dispel the fog that enveloped the cliffs and the immensity of the sea to be glimpsed.

What the young man had discovered Werner Heisenberg Amidst that romantic exaltation, the idea was that if certain operations were performed on all that stream of variables structured in the form of matrices, the results of the experiments could be explained. However, the operations were not the usual additions, subtractions, and multiplications, but rather obeyed more abstract rules. The typical image of electrons orbiting the atomic nucleus like planets around the sun blurred and became a thick fog of parameters in which the patterns of those matrices operating among themselves with a different algebra could be distinguished. Heisenberg had done with atoms what Newton had done with the planets.

What does quantum physics mean?

Beyond the fact that strange mathematics explained the results of the experiments, the question that the scientific community quickly asked itself upon seeing Heisenberg's matrices was whether those mathematics, because they were strange, were telling us that physical reality was different from what had always been thought. "This is the big question," says Gemma de les Coves, ICREA researcher in theoretical physics at Pompeu Fabra University. "This is the most difficult question in the field of scientific outreach," considers Lluís Masanes, researcher and professor of quantum information at University College London. And like all big questions, it has no clear answer.

Since Heisenberg published his discovery, quantum theory has continued to achieve success, both scientific and technological. It has led to the construction of the Standard Model of particle physics, according to which everything we see is made of seventeen particles, as well as the development of modern electronics. And this, quite simply, has changed the world. At the same time, numerous interpretations have developed regarding the meaning of the mysterious mathematical structure that underpins the theory. De las Coves sums it up this way: "Although there has been remarkable progress, there is massive disagreement regarding how we should interpret this formalism." Masanes adds a nuance: "The scientific community is far from a consensus, but all the proposals have in common that reality is very different from what we experience in our daily lives."

From Copenhagen to the Multiverse

One of the most popular interpretations of quantum mechanics is the so-called "Copenhagen interpretation." According to this version, a system behaves as if it simultaneously had several values of any physical property until it is observed, when the value is concretized to a single value. That is, a particle can behave as if it had multiple velocities simultaneously until a measurement is made. A single value for its velocity is then found. From this arises the famous thought experiment of Schrödinger's cat, which would be simultaneously alive and dead until the box is opened and it is determined whether it is still meowing or already breeding daisies.

A natural question to ask if this interpretation is accepted is what happens to the remaining possible values not obtained in the measurement. One of the most curious answers was given by physicist Huge Everett in 1957, in a daring interpretation that has gained followers in recent decades: when a measurement is made and a value is obtained (of the speed of a particle, for example), the remaining possible values don't vanish, but rather, each one in its own universe. If we were to open the box and hear a meow, there would be another universe where the most famous feline in science would have passed away. This, then, is the famous multiverse.

Despite having achieved overwhelming success in predicting the results of experiments and having generated technological applications that have changed the world, the interpretation of Heisenberg's discovery on that North Sea islet remains a headache. Gema de las Cuevas explains that at the last conference she participated in, held in Helgoland just a few weeks ago, she stated before the entire audience, after twenty years of studying it: "I don't understand quantum mechanics." And then she explained what she didn't understand. "I thought it was the most interesting thing I could say," she says. "I think it's a lesson in humility and, certainly, it's a view that goes against science as a closed and arrogant whole, as a kind of oracle who has the answer to everything, because it's exactly the opposite," she concludes.