An odd phenomenon in quantum mechanics called backflow has just gotten a little weirder with new research showing not only can particles seem to ignore momentum and leap backwards, they are able to do so while a force urges them on.

This is the first time the property has been theoretically mapped out to show how it persists even when particles aren't isolated. While the discovery could potentially have some application in future quantum technologies, for now it's just another dot point in the weird stuff tiny particles do.

A team of mathematicians from the Universities of York, Munich, and Cardiff combined analytical and computational techniques to study whether backflow remains stable when kinetic energy is taken into account.

To get the 101 on backflow, we have to go back to the good ol' uncertainty principle. In basic terms, this is the limit imposed by the Universe on the extent of measurements on certain paired properties.

One of the most famous pairings is momentum and position – the better we know a particle's momentum, the more vague its position is. And vice versa.

It's this ambiguity that causes quantum tunnelling, another counter-intuitive act of physics where a particle can potentially pass through a barrier by virtue of its imprecise position.

Backflow is just another consequence of nature's lack of certainty. To get technical, as a particle moves in one direction with a known momentum, the probability of it being found further back from where you'd expect increases with time.

It's tempting to try to picture this using cars or baseballs or people spontaneously moonwalking, but remember, quantum mechanics is not about solid balls or ocean waves rolling around. It's all probability.

But if it helps, it's like pushing a kid on a swing blindfolded. You know how heavy they are and how hard you're pushing, but at some point their mass is further back than you might predict, as if they suddenly swung backwards for a split moment.

"The backflow effect is the result of wave-particle duality and the probabilistic nature of quantum mechanics, and it is already well understood in an idealised case of force-free motion," says researcher Henning Bostelmann from University of York.

This consequence of quantum uncertainty might be well understood for particles zipping around on their own in a theoretical vacuum, but would it still stand if they bumped into something that imparts a force?

Apparently so.

"We have shown that backflow can always occur, even if a force is acting on the quantum particle while it travels," says Bostelmann.

The backflow effect is incredibly tiny. What the analysis shows is even when met with a kinetic force, it persists.

"This means that external forces don't destroy the backflow effect, which is an exciting new discovery," says co-researcher Daniela Cadamuro from Technical University of Munich..

Even as we shove little quantum junior, there is a small chance their mass will not move forward with our hands.

So far the mathematics only stands for particles moving in one direction, like our hypothetical quantum child on a swing. It has also only been demonstrated with respect to momentum, without taking into account the effect's impact on other quantum properties.

Next step will be to see if backflow effects can be observed under laboratory conditions.

In a world where miniaturisation of technology means quantum oddities are becoming physical obstacles, it's important to understand how particles behave.

Practical applications aside, it's just one more example of how weird and wonderful the Universe is on the tiniest scale.

This research was published in *Physical Review A*.