August 23, 2018
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Scientists unveil new technique to measure the momentum of LIGHT for the first time
Scientists have long suspected that light has momentum, a property measured by the combination of mass and velocity. But, photons – or particles of light – are said to be massless, and just how their momentum can exert force on matter has largely remained a mystery. A new technique could finally help to solve the 150-year-old conundrum, using acoustic sensors to ‘listen’ to the elastic waves of laser pulses traveling across a mirror.
German astronomer Johannes Kepler first suggested back in 1619 that pressure from sunlight could be responsible for the position of a comet’s tail, which always points away from the sun. Then, more than 200 years later, James Clerk Maxwell predicted the radiation pressure was the result of momentum in the electromagnetic fields of light. But, in the time since, scientists have struggled to explain how the phenomenon works.
‘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,’ said Kenneth Chau, an engineering professor at the University of British Columbia, Okanagan Campus.
To get to the bottom of it, the researcher and colleagues from Slovenia and Brazil devised a new setup to measure the weak interactions between light photons. The team built a mirror equipped with acoustic sensors and heat shielding to cut down on interference and background noise. Then, they shot laser pulses at the surface, and used the sound sensors to detect elastic waves as they traveled. The effect, the researchers note, is much like watching ripples across a pond.
Chau says: ‘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 travelled 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 modelling how light momentum exists inside materials.’
The new discovery could finally be a step in the direction of understanding light momentum, the researchers say. And, understanding radiation pressure could have a number of applications.
Chau said: ‘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. 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.’
Extracted from: www.dailymail.co.uk
