100 years of quantum mechanics: With this experiment, the theory of atoms and electrons became philosophical

There are experiments that have proven to be key moments in the development of modern physics. One is the double-slit experiment. In this experiment, particles—for example, electrons—are shot at a wall with two slits, and behind them they hit a detector screen. If the particles behaved classically, two stripes would be observed on the screen, where most of the electrons hit.

In fact, however, one observes an interference pattern similar to that observed in waves. For some reason, the electrons behave like waves. Note: This does not refer to a collective property of many electrons. The individual electron exhibits wave characteristics. Thus, one can shoot one particle after another at the wall with the double slit, and after a sufficient number of impacts, one observes an interference pattern.

In his famous lectures on quantum theory, physicist and Nobel Prize winner Richard Feynman describes the double-slit experiment as the "heart of quantum mechanics." It contains its "only secret." What is this secret?

As a classical particle, an electron moves along a definite trajectory, from the electron source through one of the slits to the screen. In reality, however, the electron—mind you, the single electron—behaves as if it were passing through both slits simultaneously and interacting with itself: totally crazy!

The history of quantum theory is also a history of this madness. Feynman again, in his book "QED: The Strange Theory of Light and Matter": "Nature, as described by quantum electrodynamics, seems absurd to common sense. Yet theory and experiment agree. And so I hope you can accept nature as it is—absurd."

"Nature as it is" – it is precisely this expression that proves to be extremely misleading. And the double-slit experiment reveals its problematic nature. The expression accentuates the fundamental difference between two modes of description.

We throw a tennis ball. Classical physics describes its clearly defined trajectory. At any given moment, it is in a state: for example, it has a location x and a velocity v. And we use the laws of motion to calculate how the state changes over time. Quantum theory also describes the motion of particles, but it does so differently, using a so-called state function.

And this is where the fundamental difference becomes apparent. A classical state clearly defines the properties of an object. A quantum state only defines the probability that an object has those properties. In other words: classical physics speaks about nature in the indicative—as it is—while quantum physics speaks in the subjunctive—as it could be.

This creates a completely different picture of the particle trajectory in the double-slit experiment: The electron could pass through slit A with probability a, or through slit B with probability b. Physicists express this indecision as follows: The particle is in a superposition of two states. This has something ghostly about it (as Albert Einstein saw it, although he was never convinced by this interpretation). It completely contradicts our classical understanding of reality. Regardless of whether we observe it or not, the electron clearly "in reality" passed through a slit.

But in physics, "in reality" means that something has been observed. Observation itself is also a physical process, namely the interaction of the object being studied with a measuring device.

Such an apparatus could look like this: We mount a detector behind slit A that registers whether the electron passes by. If the answer is "no" and we observe an impact on the screen, this means that the electron has passed through slit B. Such a device thus determines the unambiguous path of an electron without disturbing it.

What we also notice is that the two-line pattern appears on the screen. Now we turn off the detector. So, we don't know which slit the electron passes through. And lo and behold: the interference pattern reappears. This isn't just a thought experiment. Numerous versions of this type have been conducted in real life.

A strange connection emerges. The detector provides us with information. Turning it on means: We want the information about which slit the particle passed through. Turning it off means: We forgo that information. Simply turning the detector on and off "causes" the different patterns on the screen. In the slogan of quantum physicists: Don't look, then wave – look, then particle. It seems as if the mere fact of observing the electron exerts an influence on its behavior. That's hard to swallow.

The double-slit experiment brings an old philosophical question into physics: Does something exist independently of whether we observe it? George Berkeley posed this question at the beginning of the 18th century and condensed the answer into the well-known formula: exist = be observed ("esse est percipi"). Quantum theory seems to prove him right. Does this mean that the electron particle only exists when we turn on the detector? Does it change from particle to wave when we turn it off?

Niels Bohr, one of the fathers of quantum mechanics, saw such questions as pseudo-problems. "There is no quantum world," he wrote, "there is only an abstract physical description. It is wrong to think that the task of physics is to discover what nature is. Physics is concerned with what we can say about nature."

Bohr didn't deny the reality of the measured effects; he simply admonished physicists: Don't ask what an electron is; ask how it behaves in a specific measuring device. Using the laws of quantum theory, one can calculate and predict the results very precisely when electrons interact with a measuring device. Hence the infamous advice: "Shut up and calculate!"

It's by physicist David Mermin. And one usually forgets his postscript: "And I won't shut up." Good thing! Because the code of silence is a form of censorship on thinking about the problem. That sort of thing doesn't go down well with physicists. They never tire of debating the problem of the double-slit experiment, in numerous, often extremely elaborate versions – from Schrödinger's cat to Wigner's friend, Everett's many worlds, Wheeler's delayed election, and even Roger Penrose's collapse theory.

This is an unmistakable sign that quantum theory is alive. Physicists don't just want to calculate with it; they want it to provide insights into what holds the world together at its core. The mysterious heart of quantum theory still beats. And it continues to drive questions. I dare to propose the following thesis: It is a philosophical heart.