Light Does a Half-Twist off the Planck

by Equinox May 24, 2016

Research revealing ‘a new property of light’ has been widely reported during the past week. In essence, physicists at Trinity College, Dublin (Ireland) have discovered that spinning photons don’t always spin as expected.  In the Abstract of their research article published in Science Advances April 29, 2016, authors Kyle E. Ballantine, John F. Donegan and Paul R. Eastham state: 

In the usual three-dimensional setting, the angular momentum quantum numbers of the photon are integers, in units of the Planck constant ħ. We show that, in reduced dimensions, photons can have a half-integer total angular momentum. We identify a new form of total angular momentum, carried by beams of light, comprising an unequal mixture of spin and orbital contributions. We demonstrate the half-integer quantization of this total angular momentum using noise measurements. We conclude that for light, as is known for electrons, reduced dimensionality allows new forms of quantization.

So what the heck are they even talking about, and why has this bit of recondite research garnered so much attention from the mass media? The Multiverse Today breaks it all down for you here: 

A photon is the elementary particle of light. It is a discrete bundle (or ‘quantum’) of electromagnetic energy. Photons are devoid of mass or electrical charge, and while considered to be particles, they simultaneously display characteristics of being waves. The study of photons, their properties and behavior, is what led to the field of physics known as quantum mechanics through which we learned that energy can be measured in discrete units on the quantum scale, and that E = mc². 

Photons are always in motion, spinning on an axis that is parallel to the direction of their linear trajectory. Note, however, that while a photon’s spin is parallel to the direction of movement, the relative direction of its spin can be either backwards or forwards, depending on the photon’s spin identity. Furthermore, a beam of photons will also display an orbital twist in which the beam’s wavefront rotates about its propagation axis like an atomic corkscrew spiraling through space. 

The net force of all this spinning and rotating is mathematically expressed in terms of angular momentum. It had previously been thought that light’s angular momentum would always be a full integer multiple (+ or -) of Planck’s constant (h), or more accurately, integer multiples of the ‘reduced Planck constant’ which is designated as ħ (pronounced ‘bar-h’) where:   

Inspired by the work of their Trinity College, Dublin predecessors, William Rowan Hamilton and Humphrey Lloyd who, back in the 1830’s, showed that a beam of light passing through a biaxial crystal takes on the shape of a hollow cylinder, the team of contemporary physicists were able to demonstrate that light from a laser beam passing through biaxial crystals (and thereby deprived of its normal three-dimensionality) can be coerced into acquiring a fractional value of angular momentum. Specifically, a ½ integer value. 

Paul Eastham, leader of the Trinity College research team, was kind enough to provide this further elucidation to The Multiverse Today:

In this context ‘reduced dimensionality’ refers to the use of beams which have a special direction, the direction the beam is going in. The reason this matters is that angular momentum tells you how the wave varies with angle, or equivalently, how it looks if you rotate it. And you can only make the right sort of variation around a single direction in space. Actually this is not quite enough to allow for the right sort of variation, you also need the hollow in the centre. What all this means is that for a hollow beam you can tie the light waves together around the axis of the beam, in a way that you cannot do otherwise.

And in a May 2016 interview with Euan McKirdy for CNN, Eastham said: 

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.

Speculation that this new realm of fundamental research may eventually lead to practical benefits in such fields as digital communications, fiber optics, and quantum computing has sparked widespread interest from public and commercial sectors.

Indeed, we at The Multiverse Today wonder if the future ramifications of their discoveries may eventually earn the Trinity College team a place of historical honor on par with Sir Isaac Newton and his experiments with light and prisms nearly 350 years ago.

 Information in this article was drawn primarily from the following sources:

There are many ways to spin a photon: Half-quantization of a total optical angular momentum;  By Kyle E. Ballantine, John F. Donegan and Paul R. Eastham,   Science Advances   02016-04-29. 

Photons with half-integer angular momentum are the latest twist on light; physicsworld.com  02016-05-16

Researchers illuminate the hidden properties of light;
By Euan McKirdy, CNN  02016-05-18