What is old quantum theory

The enigmatic transformation of the quanta

How the transition from the quantum world to classical physics occurs is still unknown. Viennese physicists have now been able to show for the first time how quantum properties disappear from a certain number of particles.

The turn from the 19th to the 20th century was a revolutionary time. Centuries old ideas were discarded and replaced by new ones. This is also the case in physics: some of the most intelligent minds that Europe has ever produced recognized that Newton's laws, which were valid for 200 years at the time, were insufficient to describe all the properties of our world. Niels Bohr, Erwin Schrödinger, Albert Einstein and others threw so-called classical physics overboard and developed quantum physics. She was finally able to describe and predict the nature of atoms, the smallest parts of our matter.

This new physics follows some seemingly unbelievable laws. They say, among other things, that matter not only has mass, but also always has wave properties. In addition, the properties of the particles are not even clearly defined. They are only probable to a certain extent and also change depending on whether you measure them or not.

Even more than 100 years after the development of quantum physics, fundamental questions still remain open. Because although the world around us - and man himself - has these “crazy” quantum mechanical properties at the atomic level, the world around us appears quite normal. Newton's famous apple still falls to the ground despite the discovery of quantum mechanics. This discrepancy - the connection between quantum physics and classical physics - remains a mystery to this day. A research group led by ERC award winner Jörg Schmiedmayer at the Atomic Institute of the Vienna University of Technology is working on exactly this question - including in projects funded by the FWF and the EU.

Measure transition. Now, for the first time, they have been able to directly observe the formation of classical properties in a quantum mechanical system (Nature Physics, September 8). To this end, they developed special test and measurement methods in order to better understand the transition from quantum mechanical to classical states.

They use so-called atom chips - a new technology with which a major obstacle in the investigation of quantum mechanical states has been overcome: It enables the observation of a perfectly controlled small cloud of atoms with a manageable number of particles with quantum mechanical properties. The state of the gas is observed with a specially developed measuring method that describes the state of the atomic cloud using so-called interference patterns without influencing the experiment.

In the new chip, the researchers first catch a small cloud consisting of several thousand atoms in a magnetic field. They then cool this “mini gas” until the quantum mechanical properties appear close to absolute zero (minus 273.15 degrees Celsius) - the atomic cloud finally reaches a state called Bose gas, which theorists predicted 100 years ago. "Then all the atoms behave exactly in unison," explains Tim Langen, head of the current study. "A normal gas behaves differently, there is chaos when many tiny particles fly wildly around each other." But not in this moment of extreme cold, in which the atoms all have the same quantum mechanical properties and are theoretically in all places in the gas at the same time, that is, are "delocalized".

Atomic chips.
As the atoms interact with each other, disorder begins to spread at a certain speed. Where there is already disorder, the atoms lose their quantum properties. You can then assign a temperature to them, as with a classic gas. “How quickly the disorder spreads depends on the number of atoms,” says Langen. At any point in time there is a clear boundary between the range that can already be described by a classical temperature and the range in which the quantum properties are still unchanged. After a certain time, the disorder has taken hold of the entire atomic cloud. The decisive observation is that this happens without contact with the outside world solely through quantum effects - because the atoms in the gases are perfectly shielded from the classical outside world in the atom chip. “So far, such behavior could only be assumed; our experiments prove that nature actually behaves like this,” says Schmiedmayer.

It is still a mystery how the classical world emerges from the laws of quantum physics. “There is no such chaos in quantum mechanics. Nevertheless, classical systems like the air around us have a chaotic equilibrium. Nobody knows exactly how such states of equilibrium arise from quantum mechanics, ”says Langen.

Theory and practice. Although it still cannot be fully explained how the transition occurs, the Viennese researchers have now been able to describe the step from the quantum world to the classical world more precisely with their experiments. They found that the classic properties can appear at any point in the gas and spread out from there like a cone of light. In addition, the special transition state can be calculated mathematically. This could also be a theoretical bridge between the world of quanta and the Newtonian world.

In addition to providing a more detailed explanation of the laws according to which this works in principle, the new findings could also be useful for very practical things in the near future. Quantum computers, for example, operate precisely at this boundary between quantum mechanics and the classical world. A more precise knowledge of the dynamics of quantum mechanical systems could make the actual implementation of quantum computers possible and perhaps revolutionize our world again.


The quantum mechanics was developed between 1925 and 1935 by Werner Heisenberg, Erwin Schrödinger, Max Born, Wolfgang Pauli or Niels Bohr, because classical physics (and the older quantum theories) failed to describe the processes in atoms.

From theory there are many aspects that contradict conventional physics - such as Heisenberg's uncertainty principle or the entanglement of particles. How quantum theory and classical physics are related is largely unknown.

Some The peculiar quantum properties are nevertheless the basis for technical applications: from laser or X-rays to the secure transmission of information ("quantum cryptography") to the development of (very fast) quantum computers.

("Die Presse", print edition, October 6, 2013)