Breaking Newton's Law: Intriguing oscillatory back-and-forth motion of a quantum particle

June 1, 2017
Innsbruck physicists have observed an intriguing oscillatory back-and-forth motion of a quantum particle in a one-dimensional atomic gas. Credit: Florian Meinert

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 , however, our intuition for the motion of objects is strongly challenged and may sometimes even completely fail. What about imagining a marble falling through water oscillating up and down rather than just moving straight downwards? Sounds strange. Yet, that's what experimental physicist from Innsbruck in collaboration with theorists from Munich, Paris and Cambridge have discovered for a quantum particle. At the heart of this surprising behavior is what physicists call 'quantum interference', the fact that allows particles to behave like waves, which can add up or cancel each other.

Approaching absolute zero temperature

To observe the quantum particle oscillating back and forth the team had to cool a gas of Cesium atoms just above and to confine it to an arrangement of very thin tubes realized by high-power laser beams. By means of 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. As this moved, it was observed to scatter off the gas particles and to reflect backwards. This led to an oscillatory motion, in contrast to what a marble would do when falling in water. The experiment demonstrates that Newton's laws cannot be used in the quantum realm.

Quantum fluids sometimes act like crystals

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 Innsbruck work has demonstrated how these correlations in combination with the wave-nature of matter determine the 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 may also be relevant in a broader scope, and help to understand and optimize fundamental processes in electronics components, or even transport processes in .

The study is published in the journal Science.

Explore further: Bell correlations measured in half a million atoms

More information: "Bloch oscillations in the absence of a lattice" Science (2017).

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1 / 5 (1) Jun 01, 2017
Umm, I thought everyone already knew this?
5 / 5 (3) Jun 01, 2017
Umm, I thought everyone already knew this?

Huh, guess we don't need to test our theories anymore if we already 'know' them theoretically. You just saved the world a lot of money, good job.
not rated yet Jun 02, 2017
"Newton's laws cannot be used in the quantum realm"

Umm, I thought everyone already knew this.
1 / 5 (1) Jun 05, 2017
This phenomenon fits the original Planck electronic model of the atom. In fact, my article "Analyzing Atoms Using the SPICE Computer Program" illustrates the driving forces acting of atoms in the near field. Century old theory re-visited.
Da Schneib
5 / 5 (2) Jun 05, 2017
Just to reinforce @somefingguy's amusing snark, this was known from theory but never shown by experiment. Experiments are important in science; they test theories and hypotheses. It's unfortunately unsurprising that the usual suspects don't get the difference between theory and experiment.

Hey, wait, aren't you the guys who are always handwaving at, "It's all a bunch of theory?" Yeah, seems to me that's one of the standard crank arguments. This time it's not theory; it's fact. See how that works?
5 / 5 (1) Jun 05, 2017
Wonder what the "special trick" is..
Da Schneib
5 / 5 (1) Jun 05, 2017
Wonder what the "special trick" is..
Making things that ordinarily only happen in a crystal lattice happen in a tube of cesium atoms so the things can be measured directly.

You can figure it out from the abstract if you are familiar with Bragg reflections and Bloch oscillations. But this is pretty esoteric stuff. The article above doesn't do a good job on this.

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