A new study published in Physical Review A suggests that, somewhat counterintuitively, quantum particles have a chance of moving backwards, even when pushed forwards.

The study was carried out by scientists from the Universities of York, Munich, and Cardiff. They were investigating the idea of “backflow”, that quantum particles can move oppositely to their momentum.

“We have shown that backflow can always occur, even if a force is acting on the quantum particle while it travels,” lead author Dr Henning Bostelmann from the University of York said in a statement. “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.”

This idea has been touted for a while, but this is the first time it has been discussed with regards to particles that are experiencing a force. Previous models had looked only at free particles, those that do not have an external force acting on them.

“This means that external forces don't destroy the backflow effect, which is an exciting new discovery,” study co-author Dr Daniela Cadamuro from the Technical University of Munich said in the statement.

Quantum particles are basically those that operate on the smallest scale possible. At these scales, some pretty weird physics takes place, including the idea that light particles can act both as a wave and a particle.

This study shows a further weird effect. A quantum particle can reflect a force acting upon it, leading to increased backflow. This is basically the movement of particles back towards the direction they were coming from.

“Suppose I have a very large room full of people and I instruct them all to move towards the door and leave the room,” Jonathan Halliwell from Imperial College London, who was not involved in the research, told Gizmodo. “Classically, the total mass of people in the room would steadily decrease. But in quantum mechanics, the total mass of people in the room could INCREASE, even though each person has a positive outward velocity.”

The study doesn’t say that quantum particles will always move backwards. Rather, it says they always have a probability of doing so. We just won’t always know which. But the effect was found to be very small, which is why we haven’t noticed it before.

While this was essentially only a thought experiment, it’s possible it could be tested in the laboratory in the future. It’ll also allow researchers to figure out what configuration of quantum particle experiences the maximum amount of backflow.