Light might have no mass, but it can still push things around. This is known as radiation pressure. Light particles (photons) carry a momentum with them, but how this momentum is transferred is not exactly clear. However, new research has come up with a way to actually study these interactions between light and matter.
An international team constructed a very special experiment to study the momentum of light. Photons carry a tiny momentum and their effect can only be studied cumulatively. Still, there were no devices sensitive enough to measure the effect. This is why it has been so difficult to study how radiation pressure is converted into force or movement.
As reported in Nature Communications, the team built a mirror fitted with acoustic sensors. They shot laser pulses at the mirror and studied the effects.
The sensors recorded the vibration generated by the photons. The elastic waves moved across the mirror like ripples on the surface of a pond. The observations finally confirm predictions regarding the transfer of momentum.
"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," study co-author Professor Kenneth Chau, from the University of British Columbia, said 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 modeling how light momentum exists inside materials."
Understanding light is not just important in itself, it opens doors to actually employing radiation pressure in engineering. The potentially interesting applications range from astronautics, where it could be used to propel spacecraft, all the way down to nanotechnology, where we could use it to produce handy tractor beams.
"Imagine traveling 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. We're not there yet, but the discovery in this work is an important step and I'm excited to see where it takes us next."
The manipulation of light is a key part of modern technology. The ability to use its momentum would give us a lot more control over what we can do with it.