Why is quantum mechanics so counter-intuitive

Fundamentals of quantum physics: quantum particles on Bohmian orbits

Since the emergence of quantum mechanics, there has been a dispute among physicists and philosophers as to how this strange theory should be understood. Chance reigns, quantum leaps bring breaks in place and time into nature, intuition is lost - and then Niels Bohr and Werner Heisenberg also introduced new definitions of the terms "measurement" and "observer". The measurement of a quantum mechanical process should therefore not be possible independently of its observer. This view, called the Copenhagen Interpretation or Interpretation of Quantum Mechanics after the place where it originated, was immediately attacked by Albert Einstein and others. They only wanted to know objective, physically definable terms in the fundamentals of physics. The reference to an observer as a physical quantity seemed wrong to them because it was too subjective.

In the famous Bohr-Einstein debate, however, Bohr was able to repel Einstein's sophisticated attacks again and again and the Copenhagen interpretation prevailed. Today it is the standard working tool of quantum physicists.

David Bohm, an American quantum physicist who conducted research in Princeton, Haifa and London, among others, wanted, like Einstein, only to see "realistic" terms in the sense of physically objective quantities in theory. And he managed to reformulate quantum mechanics in the 1950s. By introducing new equations into the theory, he succeeded in avoiding the Copenhagen postulate of the observer and thus devising a realistic interpretation of quantum mechanics. While the Copenhageners still have to draw a conceptual cut between symbolically described quantum objects and macroscopic measuring devices, Bohm's theory allows, at least in principle, to understand the dynamics of the entire universe on the basis of the quantum laws.

"Compared to the Copenhagen interpretation, what I like about the Bohmian theory is that it provides a unified picture of physics," says quantum theorist Howard Wiseman of Griffith University in Brisbane, Australia. "Everything from the inside of an atomic nucleus to the entire universe can be described in the same way."

The notorious double slit

The difference between the Copenhagen and the Bohmian point of view can best be illustrated with the help of the double slit experiment. A light particle or photon flies towards a double slit and hits a screen behind it, where it is detected. The peculiarities of quantum physics mean that the photons do not hit behind the two gaps, but rather create an interference pattern as one would actually expect from waves - like sound or water waves that can pass through both gaps. And yet each photon produces only a single, well-localized point.

"The particle moves like a tiny table tennis ball that can only go through one gap and never through two at the same time"

According to the Copenhagen perspective, the path of the photon through the apparatus is not defined at all. Only the measurement on the screen results in a real determination of the location, the probability of this being given by the wave function. More than this probability cannot be stated. The photon has wave and particle properties; one therefore speaks of the wave-particle duality. But it is neither one nor the other, but a quantum object. And, according to the Copenhageners, we can only determine its properties with the help of macroscopic measuring devices, whose classic functioning we have to assume for the measurement - these measuring devices are always in a certain state and not in a superposition of states, as is common with quantum objects is.