Four hundred years ago, German astronomer Johannes Kepler was the first to theorize that sunlight must exert some kind of pressure on matter, as a comet’s tail always points away from the sun. Fast forward to 1873: Scottish mathematical physicist James Clerk Maxwell predicted that this radiation pressure comes from the electromagnetic field of light, writing a series of equations to describe light that incorporate its momentum.
Today we use light momentum to drive solar sails for spacecraft. We also rely on it to create optical tweezers and traps that build microscopic machines.
But even though the momentum of light was described and predicted 150 years ago, exactly how this force is imparted to matter as movement remained a mystery until an international team of researchers were able to record the sound that light makes when it hits an object.
Co-author Kenneth Chau, professor of engineering at the University of British Columbia Okanagan, worked with researchers in Slovenia and Brazil to detect the incredibly small force of a laser light illuminating a mirror. The study was published in Nature Communications.
One of the most challenging obstacles was figuring out how to block out all the noise to isolate the sound of light momentum alone. The heat energy of light is so much stronger than its momentum that it can dwarf the signal the team was actually looking for.
The team constructed a special low loss mirror, one that is nearly perfectly reflective at the wavelength of light used in the experiment to prevent light absorption, and at the same time is less responsive to heat than a regular mirror.
Laser light was fired in pulses at the mirror, producing elastic waves that rippled across its surface like ripples in a pond. The movement triggered sound waves that were recorded using sensitive acoustic sensors, and the sound was then mapped back to the initial movements in 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 in a statement. “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 sound waves from light striking objects are so quiet that we can’t hear them with our ears, but the force of light momentum has real-world applications. This improved understanding of how light can apply force to objects can help design more efficient solar sails for space travel. It can also be applied to building even more complex microscopic machines. And it illuminates a centuries-old mystery about energy and forces that are all around us.