Pushing Particles Forwards Might Make Them Go Backwards Because Quantum Physics Is Bonkers

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You are very lucky that you ended up about the size that you are today, somewhere between one and ten feet tall and weighing somewhere between one and one thousand pounds. This is a very good size. Not to body shame, but if you were, say, a quadrillion times shorter and weighed a nonillion times less (that’s one followed by 30 zeros), that would be very inconvenient for you. Everything would be very inconvenient for you.

One thing you take for granted as a human-sized thing, for example, is that when you push things, they move forward. But a team of researchers realized that this is not necessarily the case if you zoom into the quantum world, where particles might decide to go backwards, no matter what kind of outside force you put on them.

“We wanted to show this is a universal quantum mechanical effect,” study author Daniela Cadamuro from the Technical University of Munich in Germany told Gizmodo. “In the presence or absence of a force, the particle will always have a probability to move backward, even if there is a positive momentum.”


One of quantum mechanics’ core tenets is that the smallest particles act like dots and flowing waves at the same time. That’s demonstrated by a quintessential experiment: If you shoot particles individually through parallel pairs of slits, they appear like dots on the wall behind them. But shoot enough particles and they make a pattern on the wall as if a wave had passed through—I use the same example here.

But that means scientists’ understanding of individual particles requires using the mathematics of probability, tweaked to describe quantum mechanics. This is something that might make sense on paper, but doesn’t make intuitive sense when you try and apply it to moving things—so you end up with an effect called “backflow.” It is not the same as plumbing backflow.


Jonathan Halliwell, professor in theoretical physics at Imperial College London who was not involved in the research, told Gizmodo you can understand backflow as follows:

Suppose I have a very large room full of people and I instruct them all to move towards the door and leave the room. 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.


Some consider this a consequence of those tweaks to the regular rules of probability that I mentioned above when applied to a quantum world. Each particle comes with a special equation, from which you can get a list of its allowed properties, alongside their given probabilities. But the tweaks sometimes let the probability values become negative, which is a crazy sounding thing. You’d never say there’s a negative fifty percent chance that a flipped coin will land on heads. In this case, it’s like there’s a chance for someone to wind up back inside a room even if they’re leaving the room.


Study author Henning Bostelmann from the University of York in the United Kingdom explained that the paper, published last week in Physical Review A, is a mathematical result generalizing this backflow effect to any kind of external force that could act on a particle. But, explained Cadamuro, their math only works for particles in one dimension. That’s as if the people in Halliwell’s example could only walk forward or backward. The paper also doesn’t take into account the specific properties of particles aside from their momentum.

The effect hasn’t been tested in a lab yet, and people are actively working on creating an appropriate setup—one team proposed using Bose-Einstein condensates, special kinds of cold atomic arrangements that experience quantum mechanical effects in larger systems. But this result is important in its own right. “It’s a great test of the foundations of quantum mechanics,” said Cadamuro. She’s more interested in mathematics, but said there could also be some important implications for quantum computing.


Halliwell didn’t see any limitations to the team’s paper aside from the ones that they listed. He believes the backflow phenomenon is real. But now it’s time for some real-world physical proof.

“The main issue is to find a convincing experimental test and then persuade someone to do it!”


[Physical Review A]