Researchers from Ruhr-University Bochum discovered that light can do the opposite of what we expect: instead of speeding things up, it can act as an invisible brake at the nanoscale.
Conventional physics tells us that light adds energy — it heats particles, sets them in motion, pushes them along. But a team of German and Indian scientists just caught light doing the exact opposite. Published in Nature, their study shows that fluorescent carbon nanotubes suspended in water move significantly slower when illuminated. The brighter the light, the greater the braking effect.
The key lies in a phenomenon called quantum friction. Unlike everyday friction — the bumping and grinding of two surfaces — quantum friction operates at the electron level. No physical contact is needed. When light hits the nanotubes, it creates excitons: paired particles made of an electron and a "hole" where an electron used to be. These excitons travel along the nanotube and couple with surrounding water molecules, transferring momentum away from the nanotube and effectively slowing it down.
The research team used terahertz (THz) spectroscopy, a technique that measures molecular energy and motion using electromagnetic waves, to detect the energy transfer at work. They found that when excitons were slowed down at structural defects within the nanotubes — effectively disabling the quantum friction pathway — the braking effect vanished entirely. This proved that the mobility of excitons along the nanotube is directly responsible for the deceleration.
"This discovery of light-induced quantum friction fundamentally changes our understanding of interfacial processes," said physical chemist Sebastian Kruss, who led the study with theoretical physicist Marialore Sulpizi and physical chemist Martina Havenith.
The ability to control friction with light opens up entirely new possibilities in materials science. Researchers suggest that this effect could be used to guide the movement of nanorobots through liquids with precision, or to fine-tune the conditions of chemical reactions at the molecular scale.
"What's fascinating is that this effect vanishes entirely when we use nanotubes in which the electronic excitations that lead to the fluorescence are slowed down at defects," Kruss added. "This means it is the mobility of the excitons along the nanotube that is in direct exchange with the environment and creates this decelerating effect."
The experiments also blur the boundary between solid and liquid physics at the nanoscale. It is well established that quantum weirdness takes over at the smallest scales, and this discovery is one of the clearest demonstrations yet of how differently the rules apply. As the research team notes, the findings challenge the traditional view that light always accelerates motion, and instead reveal a more complex picture where light can both energize and decelerate depending on the quantum environment.