It can be measured from the Josephson effect and was independently confirmed by the quantum Hall effect.
Freeman Dyson pointed out that when it is negative, QED is unstable, so a perturbative expansion in it must be asymptotic rather than convergent. It can be measured from the Josephson effect and was independently confirmed by the quantum Hall effect. Attempts to propose formulas for its infrared value are regarded as numerology since it runs logarithmically. It was first introduced by Sommerfeld to account for small deviations from the Bohr model in hydrogen. FTP, identify this constant which is defined as e squared over h-bar c and often approximated as one over one hundred thirty-seven.
Freeman Dyson pointed out that when it is negative, QED is unstable, so a perturbative expansion in it must be asymptotic rather than convergent.
fine-structure constant (accept Sommerfeld constant until he is mentioned; accept alpha)