Researchers shed light on a 150-year old mystery of Light

Understanding of light momentum.


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Light is composed of photons and photons have momentum. But, how light interacts with the matter has remained a mystery for close to 150 years. Now a new study by the UBC‘s Okanagan campus may have uncovered the key to one of the darkest secrets of light.

Study co-author and UBC Okanagan engineering professor Kenneth Chau said, “Johannes Kepler, famed German astronomer, and mathematician, first suggested in 1619 that pressure from sunlight could be responsible for a comet’s tail always pointing away from the Sun. It wasn’t until 1873 that James Clerk Maxwell predicted that this radiation pressure was due to the momentum residing within the electromagnetic fields of light itself.”

“Until now, we hadn’t determined how this momentum is converted into force or movement. Because the amount of momentum carried by light is very small, we haven’t had equipment sensitive enough to solve this.”

CAPTION Kenneth Chau is an associate professor of engineering at UBC's Okanagan campus.
Kenneth Chau is an associate professor of engineering at UBC’s Okanagan campus.
UBC Okanagan

Now technology has shed light on this mystery. In order to measure these extremely weak interactions between light photons, scientists constructed a special mirror that is equipped with acoustic sensors and heat shielding to keep interference and background noise to a minimum. They at that point shot laser beats at the mirror and utilized the sound sensors to distinguish elastic waves as they moved over the surface of the mirror, such as watching ripples on a pond.

Chau said, “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. 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 modeling how light momentum exists inside materials.”

“Imagine traveling to distant stars on interstellar yachts powered by solar sails. Or perhaps, here on Earth, developing optical tweezers that could assemble microscopic machines.”

The discovery is important in advancing our fundamental understanding of light, but Chau also points to practical applications of radiation pressure.

The study is published in Nature Communications.