A new form of visible light discovered, altering our fundamental understanding of its nature

As we know it, visible light is electromagnetic radiation emanating from thermal energy sources in the form of tiny energy packets called photons. Photons are some of the oddest particles in classical physics, exhibiting energy, but lacking mass, and simultaneously existing as both a particle and a wave. As abstract and difficult is this is to imagine, our veneration of light will now further skew toward the arcane; physicists from Trinity College Dublin’s School of Physics and the CRANN Institute, have discovered a new form of light, distinct enough to alter our understanding of its nature.

Commenting on the experiment, assistant professor Paul Eastham said: “We're interested in finding out how we can change the way light behaves, and how that could be useful. What I think is so exciting about this result is that even this fundamental property of light that physicists have always thought was fixed can be changed.”

Fundamental knowledge dictates that the angular momentum of light—that is, one of its measurable characteristics—would be a multiple of Plank’s constant in all forms of light. Now, Ph.D. graduate Kyle Ballantine and Professor Paul Eastham, both from Trinity College Dublin's School of Physics, along with Professor John Donegan from CRANN, have observed a new form of light in which the angular momentum of each photon occupies only half of the value. Considering that the properties of light have long been considered static, this discovery is profound.  

For clarity, light beams are categorized by the color of their wavelength, as well as their angular momentum. The latter measures how much a light beam is rotating around its axis, an event which occurs even if a wave is traveling in a straight line.

Angular momentum of light, visualized. Via Royal Publishing Society

Trinity College physicists discovered the phenomenon using conical refraction, an old technique dating back nearly two centuries to 1830 when mathematician William Rowan Hamilton and physicist Humphrey Lloyd observed that light rays passing through certain crystals became hollow cylinders. When applying this technique to a screw-like structure and analyzing the resulting beams within the theory of quantum mechanics, the team predicted that the angular momentum of the photon would be half-integer. Next, using a custom-made device capable of measuring the angular momentum flow within a beam, the team was able to actually measure the variations in the flow caused by quantum effects, revealing a minute shift approximately equal to one-half of Planck’s constant, in the angular momentum of each photon.

Bearing in mind that Planck’s constant is a static value that links the amount of energy a photon carries with the frequency of its electromagnetic wave, discovering a subtle change is massive. Since the 1980s, physicists have speculated how quantum mechanics applies to particles whose motion is limited within only two of the three dimension of space, an effect which gives rise to odd possibilities, including particles with quantum numbers are a fraction of what was expected. The Trinity College experiment is the first time these speculations are observed with visible light.

Source: Phys.org