A remarkable way to measure the fine structure constant

A remarkable way to measure the fine structure constant

A light source (left) sends a beam of light through a special material, which changes the direction of polarization – through an angle that is given by the fine structure constant. Credit: Tatiana Lysenko / TU Wien

The fine structure constant is a fundamental constant of nature and its measurement is crucial in physics. Recently, researchers at TU Wien discovered a unique way of measuring.

The value of one over 137, also known as the fine structure constant, is considered a crucial number in physics. It plays a significant role in atomic and particle physics.

While traditionally the fine structure constant is measured indirectly through calculations and measurements of other physical quantities, researchers at TU Wien have developed an experiment that allows the direct measurement of the fine structure constant in the form of an angle.

1/137 — the secret code of the universe

The fine structure constant describes the strength of the electromagnetic interaction. It indicates how strongly charged particles such as electrons react to electromagnetic fields. If the fine structure constant had a different value, our universe would look completely different – atoms would be a different size, so all chemistry would work differently, and nuclear fusion in stars would also be completely different.

A much-discussed question is whether the fine structure constant is actually constant, or whether it could have changed its value slightly over billions of years.

Direct measurements instead of calculations

“The most important physical constants have a specific unit – for example, the speed of light, which can be given in the unit of meters per second,” says Professor Andrei Pimenov from the Institute of Solid State Physics at TU Wien. “It’s different with the fine structure constant. It has no unit, it is simply a number – it is dimensionless.”

But usually, when measuring the fine structure, different quantities with different physical units must be measured, and then the value of the fine structure constant is deduced from these results. “In our experiment, on the other hand, the constant of the fine structure itself becomes directly visible,” says Andrei Pimenov.

A thin film that rotates light

A laser beam is linearly polarized – the light oscillates exactly in the vertical direction. Then the beam hits a layer of a special material that is only a few nanometers thick. This material has the property of changing the direction of polarization of light.

“A material that rotates the polarization of a laser beam is, in itself, nothing unusual. Different materials can do this; the thicker the material layer, the more the laser polarization is rotated. But here we are dealing with a completely different effect”, explains Andrei Pimenov. “In our case, the polarization is not rotated continuously, but jumps.”

When passing through the thin film, the direction of polarization of the light makes a quantum leap. After the passage, the light wave oscillates in a different direction than before. And when the size of this jump is calculated, an amazing result appears: the amount of this angular change is exactly the fine structure constant.

“We thus have direct access to something quite unusual: a quantum of rotation,” says Andrei Pimenov. “The constancy of the fine structure becomes immediately visible as an angle.”

Reference: “Universal spin gauge by quantum anomalous Hall effect” by Alexey Shuvaev, Lei Pan, Lixuan Tai, Peng Zhang, Kang L. Wang and Andrei Pimenov, 7 November 2022, Applied Physics Letters.
DOI: 10.1063/5.0105159

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