Light Behaving Badly: Strange Beams Reveal Hitch in Quantum Mechanics

A hidden property of corkscrew, spiraled beams of light could put a hitch in quantum mechanics.

The photons, or light particles, inside these light-based Möbius strips spin with a momentum previously thought to be impossible. The findings could shake up some of the assumptions in quantum mechanics, the rules that govern the menagerie of tiny subatomic particles.

"This is a sort of fairly basic property of light, and we've shown it's not working the way people thought it would," said study co-author Paul Eastham, a physicist at Trinity College Dublin in Ireland. [Wacky Physics: The Coolest Little Particles in Nature]

Hollow beams of light

The research was spurred by findings from roughly two centuries ago, when Irish physicist and astronomer William Hamilton and his colleague Humphrey Lloyd predicted that crystals with certain internal arrangements of their atoms would create a hollow tube of light if the incident light hit the crystal at just the right angle.

In honor of the 200th anniversary of this discovery, Eastham and his colleagues decided to probe the theoretical underpinnings of this phenomenon. He began to wonder what this type of hollow light beam implied for the angular momentum, or spin, of light particles that made up the ray. As he worked through the math, he realized something strange: The photons inside the conical ray would have an angular momentum of one-half of Planck's constant, the fundamental constant that governs the relationship between energy and wavelength.

But that seemed impossible, given that the equations of quantum mechanics implied that light particles could have spins that were multiples of the fundamental constant (for instance, twice Planck's constant, negative three times Planck's constant, and so on).  

Half-spin photons

To see if his calculations would be borne out in reality, the team tested the theory. They sent a laser beam through a crystal at a precise angle, and then used a mainstay optical device called an interferometer to split the beams of light and sort them according to their spin.

Sure enough, the photons, when measured, had angular momentums equal to one-half Planck's constant and minus one-half Planck's constant, respectively, the researchers reported online April 29 in the journal Science Advances.

The findings are fascinating because they imply that light particles don't behave as they're predicted to, said study co-author Kyle Ballantine, a physicist at Trinity College Dublin.

"All particles can be divided into two fundamental groups: Bosons, including photons in all measurements to date, have integer [whole number] angular momentum; and fermions [such as electrons] have half-integer," Ballantine told Live Science in an email. "This distinction leads to very different quantum behavior. Our result shows that we can make beams of photons which behave like fermions, a completely different form of matter."

Still, the new results don't reduce the significance of Planck's constant or tear down the entire edifice of subatomic physics, Eastham said.

"We haven't broken quantum mechanics," Eastham told Live Science.

However, the results are still so new that it's not clear exactly what they suggest, Eastham said. One immediate implication: The findings could affect quantum computing and cryptography, both of which would rely on statistics regarding subatomic particles that may need to be rethought, he said.

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