The value of the fine structure constant – perhaps the most important constant in nature as it dictates the strength of electromagnetism – has been measured directly by researchers in Austria and the US. The technique they used involves measuring how much the polarization of light rotates as it passes through a magnetic topological insulator, and while it is not as accurate as other methods, the researchers believe its directness could lead to cleaner tests of whether this supposed constant varies over time.
The fine structure constant, denoted α, is a dimensionless number with a physical interpretation that has evolved alongside physicists’ understanding of electromagnetism. When Arnold Sommerfeld introduced it in 1916, it was the velocity of an electron in the first circular orbit of the Bohr model of the atom, divided by the speed of light in vacuum. In quantum electrodynamics, it is the coupling constant that determines the strength of interactions between electrons and photons. What remains undisputed, however, is its centrality to physics, and the fact that it cannot be calculated theoretically – it is a free parameter that must be inserted into the Standard Model of particle physics. Its value is around 1/137, and if it were even slightly different – perhaps just 1/138 – it would rewrite the rules of chemistry and change stellar nuclear fusion so much that life could not exist.
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