Is quantum mechanics analog

Oil droplet experiment: out for analog quantum theory

But in de Broglie's picture, and also in the jumping droplet experiments, the driving force of the entire experiment - the particle - can only be on one side of the wall. It inevitably loses contact with the pilot wave on the other side of the barrier that is perpendicular to the screen. Without contact with the particle or drop, however, the wavefront quickly runs out of breath; it comes to a standstill long before it reaches the slot.

As a result, there can be no interference pattern. The Danish researchers have now verified these arguments with computer simulations. Why did Bush keep trying with bouncing droplets anyway? "I never liked thought experiments," he says. "The nice thing about the situation is that we can carry out real experiments."

Still, the thought experiment with the partition highlights the inherent problem of de Broglie's idea: in a quantum reality driven by local interactions between a particle and a pilot wave, one loses the symmetry required for double-slit interference and other non-local quantum phenomena becomes. For both, an ethereal, non-local wave function is required, which can run unhindered on both sides of each wall. But pilot waves simply don't produce that, says Tomas Bohr: “Since one of the sides in the experiment has a particle and the other doesn't, it just doesn't work. You inevitably violate a very important symmetry in quantum mechanics. "

It's a matter of taste

Some experts believe that the simplest version of de Broglie's theory was bound to fail. When describing individual particles that are guided by pilot waves, de Broglie did not consider the possibility that several interacting particles in quantum physics can be "entangled" with one another. They can then be described by a single common non-local wave function. In this case, their properties are also correlated if the particles are light years apart.

Experiments with entangled photons since the 1970s have shown that this phenomenon is real and that quantum mechanics is therefore not tied to local determinism. De Broglie's theory, however, has a problem: a theory of on-site interactions between a particle and its pilot wave takes on a very strange shape if one wants to describe more than one particle in that way.

Until his death in 1987, de Broglie rejected such criticism of his theory. He was firmly convinced that real pilot waves were somehow capable of entangling. Some physicists who performed hopping droplet experiments have long shared this dream. But with pilot waves, which cannot even generate double-slit interference in individual particles, it collapses, just like a wave function in the Copenhagen interpretation.

Early on, in 1952, de Broglie had offered some kind of compromise: a version of his theory that he had developed together with physicist David Bohm and that is now known as Bohemian mechanics or De Broglie-Bohm theory. In this picture there is an abstract wave function that extends through the room and is similarly mysterious as the Copenhagen variant. According to the theory, however, real particles move in it - and not the wave-particle hybrid from the Copenhagen interpretation.

Evidence in the 1970s showed that the De Broglie-Bohm theory makes exactly the same predictions as standard quantum mechanics. With the solid particles, however, new challenges arise, such as how and why the wave function is connected to physical particles at certain points. "Seen from this perspective, quantum mechanics looks no less strange," says Tomas Bohr. Most physicists see it similarly. Still, it is still a matter of taste, which theory is better to find - the experimental predictions are identical.

Tomas Bohr attributes his famous grandfather's certainty that nature is inevitably bizarre on the quantum scale to Niels Bohr's most important physical research: In 1913 Bohr calculated the energy levels in the hydrogen atom. He realized that when electrons jump between orbits and release quantized packets of light, a mechanical picture no longer makes sense. For example, the energy levels of the electrons could not be related to their orbital movement around the atomic nucleus. Even the causality failed because electrons seem to know where they are going to land before jumping, and thus emit a photon of the correct energy. "He probably saw more clearly than most of the others how funny the whole thing is," explains Tomas Bohr. "Unlike most other people, he was philosophical and therefore ready to accept that nature is strange."

In the past few years, Tomas Bohr has often asked himself what his grandfather would have said about the jumping droplet experiments. “I think he would have been very interested,” says the physicist and adds with a laugh: “He would probably have known what to think of it much faster than I did. And I would certainly have been thrilled that such a system could be reproduced - after all, it comes very close to what de Broglie was talking about. "