When order breaks down, subatomic particles have a homing instinct, new research shows, confirming a theoretical prediction. When rigid structures have dissolved, particles gravitate back to the safety of their starting points after being nudged away.

Intuitively we think of disordered systems as being ones where objects move around a lot, as opposed to the rigid stasis or order – consider the movement in a schoolroom with and without a teacher present. Quantum physics, however, just keeps laughing at our intuitions, and in 1958 Philip Anderson showed that when disorder gets high enough electrons actually become more locally restricted, never moving far from where they were when the disorder rose.

Anderson's work helped explain poor electricity conduction in disordered systems, but it carried more within it, recognized in 2019 as the “quantum boomerang effect”. The effect sees particles revert to their initial positions when moved away in disordered systems. Sixty-four years after Anderson's paper, this has been confirmed with a new paper in Physical Review X.

Dr David Weld of the University of California, Santa Barbara, told ScienceNews the effect is actually poorly named. A boomerang will keep going past you if your catching skills are poor. Whereas in a disordered system an electron given a push is “More like a dog than a boomerang,” faithfully returning and stopping at its starting point (if that's where its person is).

In an ordered system, on the other hand, the electron will either keep moving or run into an atom and be captured.

To check the truth of the effect Weld and co-authors cooled 100,000 lithium atoms to a Bose-Einstein Condensate, observing the movements of the atoms themselves, rather than much harder to track electrons. Using the quantum association between momentum and position, made famous by Heisenberg, the team looked for restoration of movement, rather than position.

When a series of 25 laser pulses boosted the electrons' momentum, they quickly fell back to what they had before. This might sound familiar from a classical view of the world – we are used to momentum being lost to friction. However, in the quantum world that doesn't apply. Instead, the loss of momentum is an example of the atoms' wave-like behavior.

The boomerang effect was only predicted to occur in certain circumstances and the authors confirmed this, finding the effect depends on the regularity of the laser pulses, falling apart when this was varied. Co-author Professor Patrizia Vignolo of the Universite Cote d’Azur told ScienceNews the results perfectly match expectations, not a safe bet for novel quantum experiments.

The paper suggests this is only the beginning, with the potential to explore the “Boomerang phenomena in higher-dimensional systems... more exotic initial states, and the presence or absence of many-body boomerang effects in interacting systems.”