A unity value above 137, also known as the fine structure constant, is considered a key number in physics. It plays an important role in the physics of atoms and elementary particles. While traditionally the fine structure constant has been measured indirectly through the calculation and measurement of other physical quantities, researchers at TU Wien have developed an experiment that allows direct measurement of the fine structure constant in the form of an angle.

1/137 is the secret code of the universe

The fine structure constant describes the strength of electromagnetic interaction. It shows how highly charged particles such as electrons respond to electromagnetic fields. If the fine structure constant had a different value, our universe would look very different; atoms would be different sizes, so all chemistry would work differently, and nuclear fusion in stars would also be very different.

A frequently debated question is whether the fine-structure constant is really constant or may 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 expressed in meters per second,” says Professor Andrii Pimenov of the Institute of Solid State Physics at the Vienna University of Technology. “It’s a different matter with the fine structure constant. It has no units of measure, it’s just a number – it’s dimensionless.”

Usually, however, when measuring fineness, different quantities need to be measured in different physical units, and then the value of the fineness constant is derived from these results. “On the other hand, the permanent thin structure itself becomes directly visible in our experiment,” says Andriy Pimenov.

A thin film that rotates light

The laser beam is linearly polarized – the light oscillates exactly in the vertical direction. The beam then falls on a special layer of material only a few nanometers thick. This material has the ability to change the polarization direction of light.

“The material that rotates the polarization of the laser beam is not unusual in itself. Different materials can do this, the thicker the material layer, the more the laser polarization rotates. But here we are dealing with a completely different effect,” explains Andriy Pimenov. “In our case, the polarity isn’t constantly spinning – it’s bouncing.”

The polarization direction of light passing through a thin film makes a quantum jump. After the light wave has passed, it oscillates in a different direction than before. And when the size of this jump is calculated, a surprising result emerges: The quantum of this angular change is exactly a constant of fine structure.