Breaking Newton's Law: oscillation instead of free fall

A ripe apple falling from a tree has inspired Sir Isaac Newton to formulate a theory that describes the motion of objects subject to a force. Newton’s equations of motion tell us that a moving body keeps on moving on a straight line unless any disturbing force may change its path.

The impact of Newton’s laws is ubiquitous in our everyday experience, ranging from a skydiver falling in the earth's gravitational field, over the inertia one feels in an accelerating airplane, to the earth orbiting around the sun.

In the quantum world, however, our intuition for the motion of objects is strongly challenged and may sometimes even completely fail. In the current issue of Science an international team of physicists from Innsbruck, Munich, Paris and Cambridge (USA) describes a quantum particle that shows a completely unexpected behavior.

In a quantum gas the particle does not move like the famous falling apple, but it oscillates. At the heart of this surprising behavior is what physicists call ‘quantum interference’, the fact that quantum mechanics allows particles to behave like waves, which can add up or cancel each other.

To observe the quantum particle oscillating back and forth the team had to cool a gas of Cesium atoms just above absolute zero temperature and to confine it to an arrangement of very thin “tubes” realised by high-power laser beams. With a special trick, the atoms were made to interact strongly with each other.

At such extreme conditions the atoms form a quantum fluid whose motion is restricted to the direction of the tubes. The physicists then accelerated an impurity atom, which is an atom in a different spin state, through the gas. In our everyday world this corresponds to the apple falling from the tree.

The scientists, however, observed that the quantum wave of the atom was scattered by the other atoms and reflected back again. The result is a striking oscillatory movement. The experiment demonstrates that Newton's laws cannot be used in the quantum realm.

The fact that a quantum-wave may get reflected into certain directions has been known since the early days of the development of the theory of quantum mechanics. For example, electrons reflect at the regular pattern of solid crystals, such as a piece of metal. This effect is termed ‘Bragg-scattering’.

However, the surprise in the experiment performed in Innsbruck was that no such crystal was present for the impurity to reflect off. Instead, it was the gas of atoms itself that provided a type of hidden order in its arrangement, a property that physicist dub ‘correlations’.

The publication has demonstrated how these correlations in combination with the wave-nature of matter determine the motion of particles in the quantum world and lead to novel and exciting phenomena that counteract the experiences from our daily life.

“Understanding the oddity of quantum mechanics is also relevant in a broader scope,” says Michael Knap, Professor for Collective Quantum Dynamics at the Technical University of Munich. “It might help to understand and optimise fundamental processes in electronics components, or even transport processes in complex biological systems.”