Light has momentum, new research confirms

Scientists have solved a 150-year-old mystery about the nature of light-matter interactions. Researchers were able to measure the force light exerts on matter.

The breakthrough confirmed light possesses momentum, an idea first proposed by the great German astronomer Johannes Kepler in 1619. Kepler surmised the pressure of the solar rays caused comets' tails to always point away from the sun.

In 1873, James Clerk Maxwell, a pioneer of mathematical physics, elaborated on the idea, claiming momentum within electromagnetic fields of light accounted for the phenomenon.

"Until now, we hadn't determined how this momentum is converted into force or movement," Kenneth Chau, a professor of engineering at the University of British Columbia, Okanagan, said in a news release. "Because the amount of momentum carried by light is very small, we haven't had equipment sensitive enough to solve this."

With the help of new technology, scientists built a sensor sensitive enough to measure the interaction between photons and matter.

First, researchers outfitted a special mirror with acoustic sensors. Next, the engineers wrapped the mirror in a heath shield to minimize interference and background noise. Then, scientists shot laser pulses at the mirror and measured the elastic waves rippling across the face of the mirror.

"We can't directly measure photon momentum, so our approach was to detect its effect on a mirror by 'listening' to the elastic waves that traveled through it," said Chau. "We were able to trace the features of those waves back to the momentum residing in the light pulse itself, which opens the door to finally defining and modelling how light momentum exists inside materials."

The new research, described this week in the journal Nature Communications, could help scientists develop a variety of light-powered technologies.

"Imagine travelling to distant stars on interstellar yachts powered by solar sails," said Chau. "Or perhaps, here on Earth, developing optical tweezers that could assemble microscopic machines."

Scientists at NASA and the Planetary Society are, in fact, working on solar sail technology.

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