Kicking a rolling ball towards a net will cause it to go into that net (hopefully), as physical objects move based on any force that is applied to them.

However, that isn’t necessarily the case in quantum physics, as a new study confirmed that quantum particles can move in the opposite direction of a force applied to them.

Known as ‘backflow’ – this principle had been shown previously, but only in “free” quantum particles, which do not have force is acting on them. The new study, published in the journal *Physical Review A*, expands on the previous finding.

“The backflow effect in quantum mechanics has been known for quite a while, but it has always been discussed in regards to ‘free’ quantum particles, i.e., no external forces are acting on the particle,” study author Daniela Cadamuro, a researcher at the Technical University of Munich, said in a press release.

Using a highly complex mathematical analysis, the study team was able to estimate the magnitude of backflow as a force. They also showed the phenomenon affects all quantum particles, is a very weak force and difficult to measure, which could explain why it’s flown under the radar until now.

Simply put, the team found that quantum particles have a probability of moving backwards, against their momentum.

“This new theoretical analysis into quantum mechanical particles shows that this ‘backflow’ effect is ubiquitous in quantum physics,” said Henning Bostelmann, a mathematics researcher at the University of York. “We have shown that backflow can always occur, even if a force is acting on the quantum particle while it travels.”

Bostelmann noted that the discovery is based on the wave-particle duality of quantum particles. This principle has been shown in a classic quantum physics experiment involving particles being fired at two slits in a wall: The particles appear as particles behind the wall, but end up creating a pattern on the wall that appears to have been made by a wave.

The new discovery is also based on probability mathematics. The team’s calculations may not make intuitive sense – the idea that something can move against a force applied to it – but they do make theoretical sense.

The team reached their conclusion by adjusting probability math to describe quantum mechanics. Those adjustments combined with the unique equations for quantum particles equalled probability values that were negative – an odd result that essentially confirmed the existence of backflow.

“Forces can of course make a particle go backwards - that is, they can reflect it, and this naturally leads to increased backflow,” said Gandalf Lechner, a mathematics researcher at Cardiff University. “But we could show that even in a completely reflection-free medium, backflow occurs. In the presence of reflection, on the other hand, we found that backflow remains a small effect, and estimated its magnitude."

Cadamuro noted that the study expanded upon work done with ‘free’ quantum particles, which have no forces acting upon them.

“As ‘free’ quantum particles are an idealised, perhaps unrealistic situation, we have shown that backflow still occurs when external forces are present,” she said. “This means that external forces don't destroy the backflow effect, which is an exciting new discovery.

“These new findings allow us to find out the optimal configuration of a quantum particle that exhibits the maximal amount of backflow, which is important for future experimental verification.”

Researchers are currently focusing on producing a suitable experimental setup. One group has proposed utilizing Bose-Einstein condensates, unique kinds of cold atomic arrangements that can model quantum mechanics on a larger scale. These experiments could eventually lead to quantum technology research, including quantum computing and encryption technologies.

Source: University of York

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