High Quality Content by WIKIPEDIA articles! Quantum electrodynamics (QED), a relativistic quantum field theory of electrodynamics, is among the most stringently tested theories in physics. Precision tests of QED consist of measurements of the electromagnetic fine structure constant, ?, in different physical systems. Checking the consistency of such measurements tests the theory. Tests of a theory are normally carried out by comparing experimental results to theoretical predictions. In QED, there is some subtlety in this comparison, because theoretical predictions require as input an extremely precise value of ?, which can only be obtained from another precision QED experiment. Because of this, the comparisons between theory and experiment are usually quoted as independent determinations of ?. QED is then confirmed to the extent that these measurements of ? from different physical sources agree with each other.