Physicists study holes in light by tying light beams in knots

"From a maths point of view, it isn't the knot that's interesting, it's the space around it," physicist Mark Dennis said.

By Brooks Hays
Physicists study holes in light by tying light beams in knots
Scientists used holographic technology to twist polarized light into complex knots. Photo by University of Bristol

Aug. 1 (UPI) -- To better understand the way light energy flows through space, scientists are tying light beams in knots.

Though a laser beam may appear as a single stream of focused light, it's actually a vibrating electromagnetic field. Inside the field are photons spinning in different directions. Polarization describes light's multidirectional properties.


To tie light in knots, scientists deployed the same holographic technology utilized by polarized sunglass lenses. By spinning polarized filters, scientists were able to twist light into knots.

"We are all familiar with tying knots in tangible substances such as shoelaces or ribbon," Mark Dennis, a physicist at the University of Bristol, said in a news release. "A branch of mathematics called 'knot theory' can be used to analyze such knots by counting their loops and crossings."

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"With light, however, things get a little more complex," Dennis said. "It isn't just a single thread-like beam being knotted, but the whole of the space or 'field' in which it moves."

The loops and crossings -- and the 'holes' -- are what scientists are really interested in.

"From a maths point of view, it isn't the knot that's interesting, it's the space around it," Dennis said. "The geometric and spatial properties of the field are known as its topology."

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To study these unique topological properties, scientists created what are known as pollination singularities, a point of circular polarization featuring other intertwining polarizations. Using new manipulation techniques, scientists were able to create more complex polarization singularities than were previously possible.

In three-dimensions, these complex polarization singularities appear like knots. In addition to the standard torus knot, scientists were also able to generate a figure eight knot.

In creating these unique knots, scientists were able to compare their real world observations with the predictions offered by knot theory formulas. Researchers detailed the comparisons in the journal Nature Physics.

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"One of the purposes of topology is to talk about showing data in terms of lines and surfaces," Dennis said. "The real-world surfaces have a lot more holes than the maths predicted."

Put simply, light moves in much more complex ways than scientists thought.

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