Newton’s first law appears to break down in the quantum realm

Newton’s first law of motion says that particles move in straight lines unless influenced by a force but a new experiment shows that the quantum version of that assumption fails for quantum particles of light.

Newton’s first law says that objects move at constant speeds until a force affects them
Shutterstock/Peshkova

An experiment with light shows that one of the fundamental laws of motion may not always hold in the quantum realm.

Much of our understanding of how objects like tennis balls or bicycles move stems from the laws of motion, which were formulated by Isaac Newton in the late 1600s. Newton’s first law states that objects naturally move at constant speeds and along straight lines unless they encounter a force that pushes them to do otherwise.

However, for extremely small objects, Newton’s laws must be swapped for analogous rules in quantum theory. In this case, determining exactly how an object moves is challenging because of the Heisenberg uncertainty principle, which says that the object’s position and momentum can never be simultaneously measured with perfect precision.

Takafumi Ono at Kagawa University in Japan and his colleagues designed an experiment that takes this principle into account and tests whether moving along straight lines is a rule that can carry over from Newtonian physics into the quantum realm.

Their quantum object of choice was a photon, or a single unit of light, which they produced with a laser. They made it travel through several lenses and slits before it hit a detector. The researchers designed these devices so that when the photon passed through them, it emerged with momentum and position values that fell within a range of numbers allowed for by the uncertainty principle.

With this principle satisfied, they used an equation from quantum theory that mimics Newton’s first law to predict what they would read from the detector, with the assumption that the photon had been moving along a straight line. When they compared these predictions to measurements from the experiment, they found a 45 per cent discrepancy.

“What we measured could not be explained by the assumption that quantum particles move along straight lines,” says Ono.

Holger Hofmann at Hiroshima University in Japan, whose past theoretical work inspired the new experiment, says this finding is complicated by the fact that quantum laws typically only provide statistical information, such as the probability of something happening for many particles, while leaving questions about a single particle’s behaviour unanswered.

“It is difficult to tell how this straight line assumption breaks. It always works for averages, so how can it not work for an individual particle? We typically don’t even have data for that case,” he says.

For a photon, part of the solution may be to recast it as less particle-like than it seems and allow it to split or “smudge out” as it travels. But a lot more experimental work is needed to justify this perspective, says Hofmann. Photons are already a singular kind of particle as they do not have mass and travel at the fastest possible speed in the universe.

For Krister Shalm at the National Institute of Standards and Technologies in Maryland, the new experiment highlights the differences between photons and quantum particles of matter. “If you talk about particle trajectories, and you want to talk about photons as particles, you can think of them as little balls bouncing around,” he says, “but the consequence is that you’re eventually going to get the wrong answer.”

Journal reference:

Physical Review A, forthcoming